commercialisation de ces organismes puisse être envisagée (Barré et Delathière, 2010). De même, l'identification de substances natu- relles ayant des propriétés acaricides est actuellement en plein essor et un programme de recherche est développé localement pour valoriser la biodiversité néo-calédonienne (Hüe et al., 2014a ; Lebouvier et al., 2013). Même si les substances identifiées devront être soumises à différents tests pharmaceutiques avant de pouvoir être mises sur le marché, l'exemple de la commercialisation par le passé d'acaricides issus de la biodiversité (par exemple pyréthri- noïdes, avermectines, isoxazolines) laisse espérer que de nouvelles molécules sont encore à découvrir.

## ■ CONCLUSION

La présente étude indique qu'à ce jour près d'un quart des ele- veurs calédoniens dont les tiques *R. microplus* ont été analysées ne peuvent plus compter sur l'amitraze pour contrôler de manière durable les infestations par les tiques. Cette molécule étant mainte- nant utilisée depuis plus de 18 ans sur le territoire, il est à craindre que cette proportion ne cesse d'augmenter alors que la sensibilité à la deltaméthrine n'a pas retrouvé un niveau suffisant pour espérer sa réutilisation, dix ans après l'arrêt de sa distribution sur le ter- toire. A l'avenir, à l'exception, sans doute transitoire, des mélanges de plusieurs molécules dont l'association potentialise l'efficacité, les éleveurs ne disposeront plus que des lactones macrocy- cliques ou du fluazuron pour contrôler les populations de tiques ; or l'usage de ces produits a des conséquences importantes pour la conduite des troupeaux compte tenu des temps d'attente élevés qui leur sont associés.

Il est crucial aujourd'hui d'utiliser de façon correcte et raisonnée ces dernières molécules. L'implication des professionnels dans la définition des mesures à mettre en œuvre est un élément majeur à développer. Le transfert de la gestion de la lutte contre la tique du Gouvernement de la Nouvelle-Calédonie vers le Groupement de défense sanitaire s'inscrit dans cette stratégie et représente un tour- nant majeur pour le territoire. La mise en place de mesures alterna- tives à la lutte chimique représente le prochain défi à relever pour la filière afin de garantir la pérennité des élevages bovins.

## Remerciements

Les auteurs remercient le personnel de l'abattoir pour sa collabora- tion et la société Anowo pour avoir fourni gracieusement le papier nylon utilisé dans les tests.

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Summary

Hüe T., Petermann J., Hurlin J.-C., Gaia H., Cauquil L. Resist-
ance of cattle tick *Rhipicephalus (Boophilus) microplus* (Can-
estrini) to deltamethrin, amitraz and moxidectin in New Cale-
donia: Review of the situation and perspectives for tick control

Cattle tick *Rhipicephalus microplus*, introduced in New Cal-
edonia in 1942, has a significant impact on cattle health and
farm profitability. The control of this parasite, based to date
on the use of acaricides, led to the development of resist-
ance to the chemicals successively available on the market.
To assess the status of resistance to the latest products, a sur-
vey based on resistance tests to deltamethrin, amitraz and
moxidectin was implemented between October 2013 and
September 2014. The aims were to establish the efficiency of
amitraz which had been in use for 18 years in New Caledo-
nia, to assess a possible re-use of deltamethrin 10 years after
discontinuing its distribution, and to monitor the emergence
of a possible resistance to macrocyclic lactone whose use is
currently restricted. According to the selected resistance cri-
teria, the resistance prevalence, or an intermediate status, to
deltamethrin and amitraz was 25.8 and 23.0 %, respectively.
No resistance to moxidectin was found. In the context of the
progressive development of resistance to amitraz, tick-con-
trol management was transferred to a health defense associa-
tion (Groupement de défense sanitaire), which was expected
among other missions to develop integrated parasite-control
programs. Based on the observed results, this article presents
the possible change in tick-control management in New Cal-
edonia, which has to evolve from an exclusive and intensive
use of chemicals to a set of complementary measures within
the framework of an integrated control led by professionals.

Keywords: Rhipicephalus microplus, resistance to acari-
cides, amitraz, deltamethrin, milbemycin, integrated control,
New-Caledonia

Resumen

Hüe T., Petermann J., Hurlin J.-C., Gaia H., Cauquil L. Resistencia de garrapata *Rhipicephalus microplus* (*Boophilus* (Canestrini) a la deltametrina, amitraz y moxidectina en Nueva Caledonia: Examen de la situación y perspectivas para el control de garrapatas

La garrapata del ganado *Rhipicephalus microplus*, introducida en Nueva Caledonia en 1942, tiene un impacto significativo en la salud del ganado y la rentabilidad de la explotación. El control de este parásito, basado hasta la fecha en el uso de acaricidas, condujo al desarrollo de la resistencia a los productos químicos sucesivamente disponibles en el mercado. Para evaluar el estado de resistencia a los últimos productos, entre octubre de 2013 y septiembre de 2014, se llevó a cabo una encuesta basada en las pruebas de resistencia a deltametrina, amitraz y moxidectina. Los objetivos fueron establecer la eficiencia de amitraz, en uso durante 18 años en Nueva Caledonia, evaluar una posible reutilización de deltametrina 10 años después de la interrupción de su distribución, y para vigilar la aparición de una posible resistencia a la lactona macrocíclica, cuyo uso está restringido actualmente. Según los criterios de resistencia seleccionados, la prevalencia de resistencia, o un estado intermedio, a la deltametrina y amitraz fue 25,8 y 23,0%, respectivamente. No se encontró resistencia a la moxidectina. En el contexto del desarrollo progresivo de la resistencia al amitraz, la gestión del control de la garrapata fue transferida a una asociación de defensa de la salud (Agrupación de Defensa sanitaria), la cual se esperaba, que entre otras misiones, desarrollase programas integrados de control de parásitos. En base a los resultados observados, este artículo presenta el posible cambio en la gestión del control de las garrapatas en Nueva Caledonia, que tiene que evolucionar a partir de un uso exclusivo e intensivo de productos químicos hacia un conjunto de medidas complementarias en el marco de un control integrado, dirigido por profesionales.

Palabras clave: *Rhipicephalus microplus*, resistencia à los aca-
ricidas, amitraz, deltametrín, milbemicina, lucha integrada,
Nueva Caledonia
<figure><img src="image_29.png" /></figure>

ELSEVIER

<figure><img src="image_30.png" /></figure>

Research paper

# Survey of cattle tick, *Rhipicephalus (Boophilus) microplus*, resistance to amitraz and deltamethrin in New Caledonia

J. Petermann, L. Cauquil, J.C. Hurlin, H. Gaia, T. Hüe*

Institut Agronomique néo-Calédonien, Connaissance et amélioration des agrosystèmes, Laboratoire de Parasitologie, BP 73, 98890 Paita, New Caledonia

<figure><img src="image_31.png" /></figure>

CrossMark

## ARTICLE INFO

**Article history:**

Received 19 March 2015

Received in revised form 1 December 2015

Accepted 12 December 2015

**Keywords:**

Resistance

Reversion

*Rhipicephalus (Boophilus) microplus*

Amitraz

Deltamethrin

New Caledonia

## ABSTRACT

The evolution of tick resistance to acaricides in New Caledonia was followed in two ways: through two large scale surveys in 1998 and 2014 and through the routine analysis of samples of ticks with suspected resistance. The results of the two approaches were qualitatively similar though analysis of ticks with suspected resistance gave higher frequencies of resistance, as expected of a biased sample.

Resistance tests using a larval packet test have been conducted since 1993 for deltamethrin and 2003 for amitraz. Deltamethrin was used in country-wide control from 1986 to 2003 and amitraz since 1996. This study analyzed the variation of resistance parameters such as lethal concentration 50 (LC50) and the resistance factor over 21 years for deltamethrin and LC99 over 11 years for amitraz. There was an increase in deltamethrin's annual mean LC50 from 1993 to 2004 when it reached 2.9 g/l and then a progressive decrease, reaching a level consistently below 1 g/l since 2007. Even though breeders stopped using deltamethrin in New Caledonia in 2003, the percentage of susceptible strains has remained below 30% since 1998. Amitraz's LC99 mean increased significantly from 0.31 g/l in 2003 to 2.96 g/l in 2014. Whereas all tested strains in 2003 were susceptible, only 40% of strains tested at the request of farmers were susceptible in 2014. The recent territory-wide survey showed that 76.7% of strains are still susceptible. This study established that resistance to amitraz has developed slowly in New Caledonia. A reversion phenomenon may have occurred concerning deltamethrin resistance, visible through the decrease of LC50 mean and the decreased proportion of very resistant strains; however the proportion of susceptible strains remains at a low level and there is anecdotal evidence that high resistance can re-emerge rapidly.

© 2015 Elsevier B.V. All rights reserved.

## 1. Introduction

*Rhipicephalus (Boophilus) microplus* is an important cattle tick widely distributed in most countries with a tropical or subtropical climate (Cutullé et al., 2009; Estrada-Peña et al., 2006). This tick was introduced into New Caledonia in 1942 via the import of animals from Australia (Rageau and Vervent, 1959) and is nowadays the main ectoparasite of Caledonian cattle (Bianchi et al., 2003). Blood loss and reduction in weight gain resulting from tick feeding are of the major factors that affect cattle production in the territory (Daynes and Gutierrez, 1980).

In New Caledonia, the extensive use of acaricides has led to resistance to the successive classes of acaricide and the regular introduction of another class of chemicals. Pyrethroid acaricides were introduced in late 1986 to control organophosphorus resis-

tant ticks (Beugnet, 1995). In New Caledonia, the only pyrethroid used was deltamethrin as a dip. Farmers have never used either pour on formulations or other pyrethroids such as flumethrin. After only 5 years of use, the first case of pyrethroid resistance was detected in 1991 (Beugnet and Chardonnet, 1995; Brun, 1992), Amitraz (Taktic®) was used for the first time in 1996 to control pyrethroid-resistant ticks and the first case of amitraz resistance was reported in 2003 (Ducornez et al., 2005).

For a long time, the local strategy of chemical use was decided by the Caledonian government, which ordered, paid for and distributed acaricides to the cattle owners, in consultation with farmers' associations. Until now, the chemicals are free of charge, except for macrocyclic lactones. As farmers have to pay for macrocyclic lactones and not for other products, they used the former only once or twice a year. Macro cyclic lactones were used for the first time in 2003 in New Caledonia. Long lasting acaricides like long acting ivermectin and insect growth regulator (fluazuron) are now used in an integrated pest management program controlled by governmental structures. Fipronil has never been used in New Caledonia as the authorities consider it causes too much damage to the

<footnote>* Corresponding author at: Institut Agronomique néo-Calédonien, Laboratoire de Parasitologie, BP 73, Paita, 98890, New Caledonia.
E-mail address: hue@iac.nc (T. Hüe).</footnote>
environment and its use was not strictly necessary at the moment
in New Caledonia as other products remain efficient. Due to exten-
sive nature of cattle production, acaricides have to be easy to use
by dipping or spraying. A class of chemical is used until resistances
appear and is then replaced by a new one throughout the territory
when resistance becomes widespread.

Historically herds were dipped every 3 to 4 weeks to prevent tick infestations but since the beginning of the 2000s, recommendations changed and farmers were asked to treat animals only when ticks were observed.

As New Caledonia has a tropical climate, ticks are present all year around and depending on weather conditions and herd management, breeders can dip animals up to 10 times a year.

The objective of the present study was to analyze the evolu-
tion of tick resistance over 21 years for deltamethrin including the
10 years following the discontinuation of its use. The evolution of
amitraz use was followed for 11 years. These results are used to
define the local strategy of tick control by governmental officers
and associations of farmers.

## 2. Materials and methods

2.1. Area description

New Caledonia is a French island in the South Pacific Ocean with a dry tropical climate, 1200 km to the west of Australia and 2000 km to the north of New Zealand, approximately 400 km long and 50 km wide, with about 600 farms running about 80,000 cattle under extensive conditions. The main breed, Limousin cattle, was introduced in 1905 but its breeding and performance are compromised by its high susceptibility to tick infestation and the development of acaricide resistance.

## 2.2. Tick collection

Ticks that were to undergo resistance tests were collected in two ways. Firstly, two large surveys were conducted, one in 1998 to assess the resistance level to deltamethrin after it had been used for 10 years and one in 2014 to assess the resistance level to amitraz after 11 years of exclusive use. For these surveys, ticks were collected directly on farms or at the slaughterhouse. About 20% of Caledonian farms were sampled during each survey. Secondly, breeders may ask for a resistance test if they observe a decrease in the efficiency of a treatment. In this case, they send the ticks directly to the laboratory. These resistance tests are referred to as diagnostic tests. The number of tests recorded each year depended on requests from breeders. Interpretation of the results depended on the origin of the tick samples. During surveys, the farms were chosen using random selection, whereas when tests were conducted at the request of farmers, there was already a suspicion of resistance and the prevalence of resistant strains was expected to be higher.

## 2.3. Bioassays

2.3.1. Deltamethrin test

Routine tests to determine resistance to deltamethrin were established in New Caledonia in 1993 (Beugnet and Chardonnet, 1995). The modified larval packet test (LPT) (Stone and Haydock, Stone and Haydock, 1962) was used to test the *in vitro* efficiency of this acaricide on *R. microplus* populations. Briefly, a commercial deltamethrin solution (50g/l) (Butox 50%®, Intervet) was diluted in olive oil to generate a solution of 12.8 g/l deltamethrin. Serial dilutions in two parts acetone and one part olive oil generated the testing doses. Eleven doses, including the control (diluents only) were prepared for each bioassay and each dose had two replicates

(Mendes et al., 2011; Stone and Haydock, 1962). Packets were pre-
pared by placing 670 µl of testing dose on a 7.5cm x 8.5 cm piece
of filter paper (Whatman #541, Techmed) before allowing acetone
evaporation for one hour in an extractor hood. Treated papers were
folded in half and sealed on the sides with clips to form pack-
ets. Approximately 100 R. microplus larvae were placed into each
treated filter paper packet, which was then sealed with additional
clips and placed in an incubator (27 °C and 85% of relative humidity
(RH)) for 24 h. After that time, mortality was determined.

For each test the concentrations (LC) causing 50, 90 and 99%
mortality were calculated. The LC50 of each sample was compared
to the LC50 of a reference susceptible strain to determine its level
of resistance. The reference strain used was Yeerongpilly, with an
LC50 of 0.044 g/l. A resistance factor (RF), which is the ratio between
the LC50 of each tested strain and the reference strain, was calcu-
lated. If RF was below 3, the strain was classified as susceptible,
between 3 and 5, the strain was intermediate, between 5 and 50,
the strain was resistant and when RF was above 50, very resistant
(Bianchi et al., 2003). The LC50 concentrations corresponding to
each RF threshold were 0.132 g/l, 0.22 g/l, 2.2 g/l respectively.

2.3.2. Amitraz test

Tests to determine resistance to amitraz were set up in New
Caledonia in 2003 (Ducornez et al., 2005). The modified LPT was
conducted following the procedures described in Miller et al.
(2002). In this test, nylon fabric (Type 2320, Cerex Advanced Fab-
rics, Pensacola, FL) was used as a substrate. Serial dilutions from a
maximum dose of amitraz (10 g/l) were made using a 2:1 acetone
and olive oil diluent. Formulated amitraz (Taktic ND 12.5% EC, Inter-
vet) was used. Twelve doses, including the control (diluents only)
were prepared for each bioassay and each dose had three replicates
(Miller et al., 2002). A volume of 670 µl of each dilution was applied
to a piece of nylon fabric (7.5 cm × 8.5 cm). After 2 h in an extrac-
tor hood, the acetone evaporated and pockets were made with the
treated fabrics. Approximately 100 R. microplus larvae were placed
into each treated filter paper packet, which was then sealed with
additional clips and placed in an incubator (27 °C and 85% RH) for
24 h. After incubation, the live and dead larvae were counted in
each packet to determine mortality.

As there are no official criteria to define resistance, LC99 is used
to define the level of resistance to amitraz in this study (Miller
et al., 2002). Limits of LC99 used in this study where those pro-
posed by Barré (2006). When LC99 is below 1.0 g/l, the strain was
classified as susceptible. If LC99 was between 1.0 and 2.0 g/l, the
strain was intermediate and when LC99 was over 2.0 g/l, the strain
was considered to be resistant to amitraz.

2.3.3. Statistical analysis

Probit analysis was run on the bioassay results using Polo-Plus
(LeOra Software, 1987). The log-probit model estimated by Polo-
Plus gives regression line representing the relationship between
the percentage of larval mortality and the acaricide dose. Deviation
from linearity of log-probit transformed data was assessed through
a chi-square test (Robertson and Preisler, 1992). For each test the
following parameters were estimated: LC50, 90, 99 and the slope
of the regression line.

Differences between years concerning the number of resistant,
intermediate or susceptible strains were tested with a chi-square
test. The difference in mean of LC50 or LC99 between years was
tested with a non-parametric Wilcoxon test. Statistical analyzes
were done using R® software.

3. Results

Since the beginning of resistance tests to deltamethrin, 963 tests
have been conducted: 114 tests during the survey of 1998, 104 dur-
<caption>**Table 1**</caption>

Number of resistance tests registered each year for deltamethrin and amitraz at the laboratory of parasitology of New Caledonia (for 1998 and 2014, the number of tests includes tests done for the survey).

<table><thead><tr><th>Number of test for deltamethrin<br>Year</th><th>12<br>1993</th><th>2<br>1994</th><th>7<br>1995</th><th>10<br>1996</th><th>51<br>1997</th><th>206<br>1998</th><th>72<br>1999</th><th>96<br>2000</th><th>58<br>2001</th><th>30<br>2002</th></tr><tr><th>Year</th><th>2003</th><th>2004</th><th>2005</th><th>2006</th><th>2007</th><th>2008</th><th>2009</th><th>2010</th><th>2011</th><th>2012</th><th>2013</th><th>2014</th></tr></thead><tbody><tr><th>Number of test for deltamethrin</th><td>26</td><td>4</td><td>24</td><td>25</td><td>28</td><td>36</td><td>27</td><td>45</td><td>43</td><td>25</td><td>32</td><td>104</td></tr><tr><th>Number of test for amitraz</th><td>53</td><td>13</td><td>51</td><td>60</td><td>71</td><td>47</td><td>30</td><td>46</td><td>56</td><td>31</td><td>33</td><td>123</td></tr></tbody></table>

ing the last study of 2014 and 772 tests at the request of breeders. From 1993 to the end of its extensive use in 2003, routine tests only targeted this acaricide as more and more resistance was observed in the field. Even after the use of deltamethrin was discontinued in Caledonian farms in 2003, resistant tests to deltamethrin were maintained routinely to follow a potential reversion phenomenon (Table 1).

In 2004, when deltamethrin was no longer used, the first cases of resistance to amitraz were recorded. Since then, requests from breeders were more targeted to putative resistance to amitraz. Since 2003, 454 tests for amitraz resistance have been conducted: 364 were done at the request of farmers and 90 were done during the 2014 survey (Table 1).

### 3.1. Resistance to deltamethrin

As the resistance factor is based on LC50 for deltamethrin, we choose to follow this parameter to study the evolution of deltamethrin resistance. The evolution of the LC50 mean between 1993 and 2014 is presented in Fig. 1. The LC50 mean reached its highest level in 2004 with a value of 2.98 g/l. It rapidly decreased over the following years to reach a low point of 0.33 g/l in 2008. The difference between 2006 (2.36 g/l) and 2007 (0.81 g/l) is significant (*p*<0.001). Then it remained between 0.34 and 0.91 g/l from 2008 to 2014. The distribution of strains per class and per year is presented in Fig. 2.

In the population of ticks collected by breeders, the proportion of strains resistant or very resistant to deltamethrin increased from 1994 to a prevalence of over 96.5% in 2001 and 2002. This population is biased as these ticks were sampled on farms having problems with acaricide, so this proportion may be higher than that of the overall population of ticks on the territory. Indeed, in 1998, the proportion of resistant or very resistant strains in diagnosis tests was 87.0% whereas it was 45.6% in the territorial survey and the distribution of tick strains between each class was significantly different (*p*<0.001) between the diagnosis tests and the survey. The proportion of very resistant strains reached a peak at 75.0% in 2004, but these results should be interpreted with caution as there were only 4 tests conducted during that year. However, this proportion was 63.8% in 2001, but quickly decreased after 2004 under 10% in 2009 where it remains. The proportion of susceptible or intermediate strains regularly decreased in diagnostic tests from 1993 to 2001 and was 54.4% in the survey of 1998. This proportion varies after deltamethrin use was discontinued but never reached the level of 1998 during the last survey of 2014.

Although the difference of LC50 is not significant between the surveys of 1998 and 2014, the distribution of strains between the classes is significantly different for the 1998 survey and the one of 2014 which shows more intermediate and resistant strains but only 0.03% very resistant strains.

### 3.2. Resistance to amitraz

Since initiating resistance tests to amitraz in 2003, the mean of LC99 increased steadily and reached 2.9 g/l in 2014; with a significant increase (*p*<0.001) of the LC99 mean between 2003 (0.32 g/l) and 2014 (2.96 g/l). The evolution of this mean is presented in Fig. 3.

Considering the parameters of LC99 used in this study, the tested strains are distributed in three classes and the results over 11 years are presented in Fig. 4.

The proportion of tick strains resistant to amitraz in diagnosis tests began to increase in 2005 and varied from year to year to a maximum of 53.3% in 2014. However, a large difference (*p*<0.001) is observed between results obtained through diagnostic tests and the survey. While only 40.0% of strains tested at the request of farmers were considered susceptible, the territorial survey indicated that 76.7% of tested strains were still susceptible to amitraz.

## 4. Discussion

Deltamethrin was first introduced in 1986 in New Caledonia when resistance to organophosphorus was widespread. The first resistance was observed in New Caledonia in 1991 (Brun, 1992), after 5 years of use and resistance to deltamethrin rapidly spread over the whole territory. In a widespread survey in 1998, Bianchi et al. (2003) found that 57% of tested strains were no longer susceptible to this acaricide. The proportion of susceptible strains fell under 5% in diagnosis tests between 2001 and 2003. The low prevalence during these 3 years is probably an overestimate as only tick strains coming from farms with suspicion of acaricide resistance were tested. This is confirmed by the difference between proportion of resistant and very resistant strains in 1998 during the survey and routine diagnostic tests, which were respectively 45.6% and 87.0%. But resistance to deltamethrin became more and more problematic in farms. In 2003 it was decided to cease use of this acaricide, 17 years after its introduction, and the replace it with amitraz and macrocyclic lactones. These results are consistent with other studies that highlighted the quick spread of synthetic pyrethrinoid (SP) resistant strains 10 to 15 years after its first use in a country. Fernández-Salas et al. (2012) observed that 90.6% of tested strains in the state of Veracruz (Mexico) were resistant to cypermethrin 18 years after the introduction of SP in this country. Mendes et al. (2011) found that resistance to deltamethrin in the state of Sao Paulo (Brazil) reached 86.4% after more than 20 years of use.

As deltamethrin has not been used since 2003, tick strains tested for diagnosis after 2003 did not come specifically from farms facing trouble with this acaricide. However, the LC50 mean stayed constant for two years and then began to decrease in 2006 to quickly drop in 2007. Since then, the proportion of resistant strains stayed at a relatively high level: between 47.2% to 81.4% depending on the year, without any clear trend. In contrast, the proportion of very resistant strains decreased significantly in 2007 to below 10% in 2008, where it has stayed. As deltamethrin was removed from farms between 1996 and 2003 when resistance phenomena were diagnosed, the decrease of resistance is, in this study, observed 4 to 11 years later, equivalent to 16 to 44 generations of ticks in New Caledonia (De Meeûs et al., 2010).

Nevertheless, this reversion is limited as the proportion of resistant strains remains at a constant high level 11 years after
<figure><img src="image_32.png" /><figcaption>**Fig. 1.** Evolution of mean LC50 (g/L) between 1993 and 2014 for deltamethrin ("1998d": bioassays done at the request of farmers, "1998s": bioassays done for the survey). Dotted lines indicate the values of resistance thresholds (susceptible/intermediate/resistant/very resistant).</figcaption></figure>

<figure><img src="image_33.png" /><figcaption>**Fig. 2.** Evolution of deltamethrin resistant classes (S: susceptible, I: intermediate, R: resistant, VR: very resistant) between 1993 and 2014 in New Caledonia ("1998d": bioassays done on the request of farmer, "1998s": bioassays done for the survey). The dotted line indicates the end of the use of deltamethrin in New Caledonia.</figcaption></figure>

the cessation of use of this acaricide. During the survey of 2014, more than three quarters of tested strains were still resistant to deltamethrin. When compared to the survey of 1998, recent results indicate that the level of resistance is higher 11 years after the removal of this acaricide than 12 years after its first use on the territory and is now at a stable level.

The phenomenon of reversion has been mentioned in several publications but rarely described. Stone (1972) reported that reversion of resistance to DDT appeared in 13 generations of ticks to fall from 100% to 55% resistance but there was no further decline for at least another 18 generations. This reversion was inconsistent with organophosphates as, depending on pesticide, important reversion was recorded for a BHC-dieldrin resistant strain after 24 generations but no changes were observed after 5 generations for one organophosphosphate-resistant strain.

Rodríguez-Vivas et al. (2011) observed that, over a period of 2 years, the SP resistant traits remain fixed and concluded that acquisition of resistance is faster than any loss in resistance.

An important practical question is whether deltamethrin could be used at all without rapidly increasing the prevalence of very resistant populations. A potential strategy might be to use deltamethrin once or twice a year, alternating with amitraz and long lasting acaricides in an integrated tick management program, given that it would not be possible to use this acaricide as sole means of chemical control. Resistance gene frequency is known to increase rapidly when selection pressure is reinstated (Stone, 1972).

In New Caledonia, in some farm experiencing amitraz resistance and with a resistance factor to deltamethrin lower than 5, breeders started to re-use deltamethrin. The product seemed quite effective
<figure><img src="image_34.png" /><figcaption>**Fig. 3.** Evolution of LC99 means (g/L) from 2003 to 2014 for amitraz ("2014d": bioassays done at the request of farmers, "2014s": bioassays done for the survey). Dotted lines indicate the values of resistance thresholds.</figcaption></figure>

<figure><img src="image_35.png" /><figcaption>**Fig. 4.** Evolution of amitraz resistant classes (S:susceptible, I:intermediate, R:resistant) between 2003 and 2014 in New Caledonia ("2014d": bioassays done at the request of farmers, "2014s": bioassays done for the survey).</figcaption></figure>

initially then lost efficacy again after only three treatments with this acaricide (unpublished data). While this has not been assessed on a large scale, the anecdotal results are not encouraging.

Amitraz was used for the first time in New Caledonia in 1996 as a reaction to deltamethrin resistance. As the use of deltamethrin was discontinued in 2003, amitraz has since been the main acaricide used by breeders. This is the first large survey on amitraz resistance on the island, and 18 years after its introduction and 11 years after its exclusive use prevalence of resistant strains remains low at 14.4%. Tick resistance to amitraz is documented in Australia and in Central and South America. Several studies assess the onset of resistance development with diverging results between countries. In Mexico, Rosado-Aguilar et al. (2008) found that resistance to this acaricide developed after only 15 treatments and 5 generations of

ticks. In the same country, Rodriguez-Vivas et al. (2006) observed a prevalence of 19.4% of resistant strains after about 10 years of use alternating with other acaricides and, Fernández-Salas et al. (2012) reported that 54.7% of tested strains in the same country were resistant to amitraz eighteen years after its first use. In Brazil, amitraz has been available since 1977 (Martins et al., 2008). Mendes et al. (2001) studied the efficacy of amitraz in the Sao Paulo state in Brazil. Resistance varied from 74.4% to 95.8%, whereas Machado et al. (2014) recorded that amitraz efficacy was 54.1% in the state of Rio Grande do Sul in South of Brazil.

On the other hand, different Australian surveys indicated that resistance prevalence stayed relatively low. Jonsson et al. (2000) pointed out a prevalence of farms with tick resistant strains of 10% in 1997 after more than 20 years of use of this molecule, and Jonsson
and Hope (2007) related that this prevalence was still at 10.8% 10 years later, in the same country.

These differences may be partly due to the conditions of the surveys and the parameters selected to estimate resistance level. There are no officially defined parameters to classify tick strains as susceptible or resistant. Rodríguez-Vivas et al. (2006) and Fernández-Salas et al. (2012) used the larval immersion test and mortality at a discriminating dose whereas Rosado-Aguilar et al. (2008) used the LC50 with the same test and Ducornez et al. (2005) performed a larval packet test and selected LC50, the slope and mortality at a discriminating dose to define a resistant strain.

Our results are closer to the Australian ones and our study confirms that the development of resistance to amitraz in R. microplus populations has been slower than that to previous pesticides used as control measures (Li et al., 2005).

All these investigations are important in order to define the future strategy of tick control. Development of amitraz resistance increases slowly but is becoming more and more problematic for Caledonian farmers.

Nowadays, there are 10% of farms with ticks resistant to both deltamethrin and amitraz. The use of amitraz began when a large majority of farms were resistant to deltamethrin. So, the development of amitraz resistance happened in farms with resistance to deltamethrin. It is very likely that this situation was not due to the development of cross resistance between deltamethrin and amitraz which has not been described yet. But, for breeders with multi-resistant tick strains, treatment has become a challenge. These farms have still some tick control possibilities with long lasting acaricides and with genetics and agronomic options.

Long lasting acaricides, such as long lasting ivermectin or an insect growth regulator like fluzuron are now used in amitraz resistant farms under the control of government agencies. However long withholding periods complicate herd management and resistance to these products is already known in other countries and will inevitably develop within few years in the territory. Resistance to ivermectin was already reported in Brazil in 2001 (Martins and Furlong, 2001) and more recently in Mexico (Perez-Cogollo et al., 2010). A first case of resistance to fluzuron was also recorded in Brazil in 2014 (Reck et al., 2014).

Apart from farmers who have enough technical and financial support to maintain susceptible cattle breeds, such as Limousine or Charolais cattle, in these tick infested areas, local breeders are increasingly encouraged toward genetic transition, using tick resistant breeds and hence reducing the impact of ticks on their animals. Over the past decades the control of tick infestation in order to maintain populations of susceptible breeds of cattle was based on acaricide use alone; however in the future for those wishing to maintain susceptible breeds, tick control will need to combine different approaches in an integrated pest management strategy. There are some other ways to manage tick control like the elimination of more sensible animals in susceptible cattle breeds (Frisch et al., 2000), the use of vaccination, natural plant extracts or entomopathogenic species (Borges et al., 2011; De la Fuente et al., 2007; Samish et al., 2004). Vaccination has been used in Australia and is still used in South America with a BM86 based vaccine and there are some research projects in order to find other tick protein suitable for vaccination (Jonsson, 2004). Some molecules extracted from plants have showed efficacy in lab test (Jonsson, 2004). Some treatments with these molecules may now be developed. It is a new challenge for breeders to combine this possibility in order to control cattle ticks infestation.

This study demonstrates the slow but patent development of amitraz resistance and the possibility of reversion for deltamethrin resistance. These results are important to the design of strategies for tick management on a territorial scale and points to the need for regular surveys of acaricide resistance.

Tick resistance to acaricides is based on multiple mechanisms and molecular tools to examine these mechanisms are not yet routinely available (Rosario-Cruz et al., 2009). Once they are, it will be interesting to study the development of resistance and the reversion phenomena and their association with the evolution of resistant gene frequency in tick populations.

## Acknowledgements

The authors thank the farmers and the slaughterhouse workers as well as the staff of the Laboratory of Parasitology of Port-Laguerre (IAC) for their help in the collecting of tick populations and their participation to the resistance tests, Anowo Ltd., for providing nylon paper and Katherine Knight and Peter Willadsen for correcting English.

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# CHAPITRE III - Mise au point d'une méthode de lutte agronomique

Cette partie s'appuie sur les articles suivants présentés pages 94 à 100, pages 101 à 110 et pages 111 à 119 :

Hüe, T., Fontfreyde, C. (2019b) Development of a new approach of pasture management to control *Rhipicephalus microplus* infestation. *Tropical Animal Health and Production*, 51:1989-1995. doi: 10.1007/s11250-019-01899-x

Hüe, T., Berger, A., Wang, H. H., Grant, W. E., Teel, P. D., de León, A. A. P. (2021a). Integrated control of the cattle tick, *Rhipicephalus australis* (Acari: Ixodidae), in New Caledonia through the Pasture and Cattle Management method. *Parasitology Research*, 120(8), 2749-2758, doi: 10.1007/s00436-021-07235-3

Hüe, T., Fontfreyde, C., Wang, H. H., Grant, W. E., Teel, P. D., de León, A. A. P. (2021b). Optimizing long-acting acaricide use for integrated tick management of *Rhipicephalus australis*-infesting cattle in New Caledonia. *Tropical Animal Health and Production*, 53(3), 1-9.

## 1. Développement de la méthode

### 1.1 Genèse de la méthode

La méthode de la lutte agronomique proposée dans cette partie est née du suivi d'une exploitation entre 2014 et 2016 (Hüe, données non publiées). L'exploitation étudiée hébergeait 22 bovins pâturant sur 20 ha scindés en 8 parcelles. Lors de ce suivi, tous les déplacements des animaux étaient enregistrés par l'éleveur. En parallèle, 3 à 5 transects de 100m de long par 1m de large étaient réalisés (Salomon et al., 2020) pour avoir une estimation des populations de larves de tiques présentes dans les différentes parcelles, dans les couloirs fréquentés par les animaux entre les parcelles, entre une parcelle et le carré de rassemblement, et dans l'annexe du carré (appelé localement « stockyard »). La surface restreinte de l'exploitation permettait d'avoir un relevé précis des populations de larves lors de chaque suivi.

Pour compléter les collectes de larves, l'éleveur relevait de manière semi-quantitative l'infestation parasitaire des animaux (faible, moyenne, élevée) au cours du temps pour mettre en relation les populations de larves de tiques observées dans les parcelles et la pression parasitaire sur les animaux.

Après deux années de suivi, il s'est avéré que les observations de tiques sur les animaux étaient corrélées aux collectes de larves réalisées dans les parcelles où étaient les animaux 3 semaines avant. De même, la présence de larves de tiques dans les parcelles et/ou dans les couloirs et/ou annexe était corrélée à l'infestation des animaux qui y pâturaient les semaines précédentes (Hüe, données non
publiées). Suite à ce suivi, nous avons fait l'hypothèse qu'il était possible de prédire où et quand des parcelles seraient contaminées.

## 1.2 Principes de la méthode de gestion agronomique

La méthode développée vise à diminuer les populations de tiques dans un élevage, en :

1) réduisant leur introduction depuis l'extérieur,

2) limitant leur dispersion au sein de l'exploitation,

3) réduisant au maximum les contacts entre les larves de tiques et les troupeaux pour réduire les infestations,

4) utilisant les acaricides à bon escient et à des périodes précises.

Cette nouvelle approche a été développée selon une méthode d'**analyse de risques** en identifiant les éléments clés de la biologie et de l'écologie de la tique impliqués dans l'infestation des pâturages et des troupeaux.

Les points d'action identifiés sont :

1/ éviter d'introduire des tiques sur l'exploitation. Cela implique qu'une quarantaine soit réalisée lors de l'achat d'animaux et qu'un traitement acaricide avec un produit dont l'efficacité est connue soit effectué sur les animaux introduits. Il faut également s'assurer que les barrières entourant l'exploitation empêchent l'introduction de tiques via des mouvements d'animaux entre l'extérieur et l'intérieur de l'exploitation. Ces barrières périphériques vont également éviter que des cervidés puissent s'introduire sur l'exploitation et y amener des tiques.

2/ savoir si l'acaricide utilisé est efficace ou si une résistance à ce produit existe sur l'exploitation. Un test de résistance des tiques vis-à-vis de l'amitraze (acaricide le plus utilisé sur le territoire) est donc préconisé.

3/ s'assurer que les apports alimentaires des animaux sont satisfaisants. Des animaux sous-alimentés sont 5 fois plus sensibles aux tiques que des animaux bien alimentés (Sutherst et al., 1983). De plus, si la ressource fourragère est insuffisante, les animaux auront tendance à casser les barrières pour aller chercher à manger, notamment en saison sèche, ce qui favorisera la dispersion des tiques au sein de l'exploitation. La densité de bovins ou charge à l'hectare (nombre de bovins à l'hectare) va donc avoir un rôle très important et doit être adaptée à la ressource alimentaire.

4/ limiter la dissémination des tiques par le troupeau et réduire le contact hôtes-parasites. Les tiques se déplacent très peu par elles-mêmes (Barré et Delathière, 2010a). Si le troupeau est changé de
parcelle entre le moment où les animaux sont infestés par les larves et le moment où les tiques femelles gorgées tombent, ce sont les bovins qui transportent les tiques d'une parcelle à une autre. Ils disséminent ainsi les tiques au sein de la propriété au gré de leurs déplacements. En sachant où sont les animaux quand ils ont des tiques et en identifiant les zones qu'ils fréquentent ou traversent (parcelles, couloirs, annexes*...), il est possible de prévoir où les femelles gorgées vont potentiellement être déposées. À partir de là, il est possible d'estimer où et quand les larves de tiques seront présentes dans les semaines suivantes.

Les mesures proposées dans la méthode développée reposent en partie sur la connaissance de la biologie et de l'écologie des tiques concernant les stades non parasitaires, de la chute de la femelle adulte gorgée à la survie des larves. Le premier point est de savoir où tombent les tiques femelles gorgées quand les animaux sont infestés pour savoir où seront les larves. Hitchcock (1955) a enregistré que le pic de détachement des tiques gorgées se situait le matin entre 6h et 10h avec la chute de 36,0% des femelles gorgées sur cette période quand les animaux étaient maintenus en stabulations. Bianchi et Barré (2003a) ont montré que le déplacement des animaux le matin accentuait la chute des tiques puisque – à infestation initiale égale – les bovins déplacés le matin des pâturages vers les stabulations présentaient 40 fois moins de tiques gorgées une fois arrivés à destination que les animaux maintenus sur la même période en stabulations. Si les animaux sont déplacés, notamment pour aller vers les installations de traitements acaricides, il faut alors éviter de déplacer le troupeau le matin pour éviter qu'il ne dissémine des tiques tout au long du trajet. Si les animaux divaguent sur l'exploitation, traversent des parcelles, empruntent des couloirs ou fréquentent l'annexe du stockyard* pendant les heures où les tiques se détachent, il est important de collecter ces informations pour gérer l'émergence des larves de tiques par la suite.

Une fois que l'on a déterminé où étaient tombées les tiques gorgées, le second facteur est de savoir d'estimer quand les larves vont émerger et pendant combien de temps elles vont rester infestantes. Comme mentionné dans le 1er chapitre, en Nouvelle-Calédonie, Desquesnes et Vignon (1987) ont estimé qu'il faut compter de 6 à 10 semaines entre le détachement des tiques gorgées et l'éclosion des œufs selon la saison. Ces auteurs considèrent que les larves de tiques peuvent survivre de 10 à 14 semaines mais que 80% de ces larves sont mortes après 6 semaines en été et 10 semaines en hiver. Les 20% de larves restantes ne sont pas assez actives pour représenter une source d'infestation importante pour le bétail. Ces informations ont été utiles pour le développement de calendriers prévisionnels rotatifs de gestion des pâturages
Les différentes mesures mises en œuvre de la lutte agronomique et leur rôle dans l'introduction, le maintien et le développement des populations parasitaires au sein d'une exploitation sont présentés dans l'annexe I.

## 1.3 Développement d'outils de gestion

La méthode a nécessité la création ou l'adaptation de 3 outils : le calendrier de pâturage, le calendrier prévisionnel des tiques et la cartographie de l'exploitation.

* **Calendrier de pâturage**

Pour savoir quelles vont être les parcelles infestées, il faut avant tout connaître les rotations des animaux. Le premier outil mis en place dans les élevages fut donc un calendrier de pâturage. Ce calendrier est un outil dont l'usage est initialement dédié à la gestion des parcelles. Les éleveurs y indiquent les rotations des animaux mais également les passages de gyrobroyeurs, les amendements, la réalisation de coupes fourragères... Cet outil enregistre les rotations des animaux mais permet également de noter les observations de tiques sur les animaux. À partir du moment où l'éleveur sait dans quelle parcelle étaient les animaux quand il a vu des tiques, il peut savoir où et quand seront présentes les larves grâce à un calendrier prévisionnel.

* **Calendrier prévisionnel de présence de larves de tiques**

Cet outil sert à visualiser quand les larves vont émerger et pendant combien de temps elles vont présenter un risque pour les animaux. Les connaissances sur la biologie de la tique développées au point 5.1. du 1er chapitre permettent de savoir, en fonction de la saison : 1/ combien de temps s'écoule entre la chute des tiques gorgées dans la parcelle et l'éclosion des larves et 2/ pendant combien de temps celles-ci resteront suffisamment actives pour présenter un danger pour les animaux. À partir de la date d'observation des tiques sur les animaux, le calendrier prévisionnel de présence des larves de tiques indique automatiquement la période de risque de présence des larves sur une parcelle (figure 11).
<figure><img src="image_36.png" /></figure>

Figure 11 : Calendrier prévisionnel de présence de larves de tiques sur les parcelles, abrégé en « calendrier prévisionnel des tiques ».

L'information recueillie est reportée sur le calendrier de pâturage, ce qui permet de visualiser sur un
seul document les parcelles qui présenteront des larves dans les semaines ou mois à venir. Un exemple
de calendrier de rotation couplé au « calendrier prévisionnel des tiques » est présenté en annexe II.

* Cartographie de l'exploitation

La cartographie est le 3ème pilier de cette méthode. Il permet d'identifier les différentes parcelles attribuées à chaque troupeau et permet de recenser tous les trajets des animaux et toutes les zones communes (points d'eau, couloirs, annexes...). Les tiques femelles gorgées vont être disséminées là où se déplacent les animaux et notamment quand ils sont déplacés le matin.

Un exemple de cartographie d'une exploitation suivie est présenté dans la figure 12. Cette illustration mentionne par un code couleur les parcelles dédiées aux différents troupeaux ainsi que les trajets empruntés lors des déplacements des animaux.
<figure><img src="image_37.png" /><figcaption>Figure 12 : Exemple de cartographie d'une exploitation dans le secteur de Kaala-Gomen pour le suivi d'un troupeau reproducteur Limousin.</figcaption></figure>
## 1.4 Traitements acaricides

Dans la méthode de lutte agronomique, les traitements acaricides sont utilisés à des périodes précises pour optimiser leur efficacité et limiter leur usage.

* **Traitements dit « stratégiques »**

Quand un troupeau est amené à pâturer sur une parcelle suspectée d'être infestée par les larves, il est conseillé à l'éleveur de contrôler les bovins 2 semaines après l'introduction sur cette parcelle, de préférence par palpation pour sentir les tiques immatures sous le pelage. L'éleveur peut alors décider de traiter ses animaux, avec un acaricide non rémanent, avant que les parasites aient atteint le stade final de gorgement afin de casser le cycle parasitaire. Ces traitements sont appelés « traitements stratégiques ».

* **Traitements à effets rémanents**

Les acaricides rémanents vont, quant à eux, être préconisés à des périodes clés, déterminées en fonction de l'infestation des parcelles. Ces acaricides vont en effet tuer les larves qui vont se fixer sur les animaux traités et ce pendant environ 6 semaines après le traitement (Citroni et al., 1999; Ferlat, 2004). Quand il n'est plus possible de réaliser des rotations sans risquer de contaminer les animaux, un traitement avec un acaricide rémanent est planifié et un planning de rotations post traitement est défini sur 6 semaines, afin de passer dans un maximum de parcelles où les larves sont présentes. L'objectif de ce traitement est 1) de protéger les animaux et 2) de réduire l'infestation des parcelles. Un exemple d'utilisation de ces produits est présenté en annexe III.

Nous avons vu au chapitre 2 les contraintes et les inconvénients de ces traitements rémanents et pour ces raisons, il convient donc de les utiliser le moins souvent possible et d'optimiser leur efficacité en les utilisant aux périodes les plus adaptées.

L'alternance entre les deux traitements rémanents employés sur le territoire (Ivomec Gold® et Acatak®) est un facteur pris en compte pour retarder le développement de résistance à ces produits. Enfin, étant donné que l'efficacité de l'Acatak® peut être réduite s'il pleut dans la demi-journée suivant son application, l'injection d'Ivomec Gold® sera préférée si la prévision météorologique le nécessite (Beltrán Zapa et al., 2020).
## 2. Évaluation de la méthode développée

### 2.1 Essai initial sur 2 exploitations

#### 2.1.1 Matériels et méthodes

La méthode développée a initialement été évaluée dans 2 exploitations bovines entre décembre 2015 et novembre 2017.

Ces 2 élevages ont été sélectionnés selon les critères suivants :

* Infestations parasitaires fréquentes des troupeaux,

* Présence d'un troupeau d'animaux de race sensible à la tique. Ce fut ici deux troupeaux de vaches de race charolaise,

* Présence d'un second troupeau « témoin » géré sans modification de la conduite d'élevage,

* Possibilité de pouvoir faire pâturer les deux troupeaux sur des parcelles distinctes afin d'éviter la contamination croisée de parcelles.

Les caractéristiques des troupeaux de ces 2 exploitations sont présentées dans le tableau 2. Les 2 exploitations réalisait les traitements à l'aide d'un couloir d'aspersion. Les tiques testées dans les 2 élevages étaient résistantes à l'amitraze mais sensibles aux lactones macrocycliques.

<table><caption>Tableau 2 : Caractéristiques des exploitations et des troupeaux de l'étude.</caption><thead><tr><th rowspan="2"></th><th colspan="2">Elevage A</th><th colspan="2">Elevage B</th></tr><tr><th>Troupeau expérimental</th><th>Troupeau témoin</th><th>Troupeau expérimental</th><th>Troupeau témoin</th></tr></thead><tbody><tr><td>Race des vaches présentes</td><td>Charolais</td><td>Charolais</td><td>Charolais</td><td>Croisée (Taurus indicus)</td></tr><tr><td>Race du taureau</td><td>Blonde d'Aquitaine</td><td>Blonde d'Aquitaine</td><td>Brahman</td><td>Croisée</td></tr><tr><td>Nombre moyen de bovins par troupeau</td><td>7</td><td>23</td><td>86</td><td>341</td></tr><tr><td>Nombre de parcelles par troupeau</td><td>3</td><td>3</td><td>5</td><td>9</td></tr><tr><td>Surface totale des parcelles par troupeau</td><td>14 ha</td><td>24 ha</td><td>81 ha</td><td>183 ha</td></tr></tbody></table>

### Mesures mises en place

Un calendrier de pâturage et la cartographie des exploitations ont été mis en place. Au cours du premier semestre 2016, les pratiques des éleveurs ont été identifiées et enregistrées, telles que définies au point 1.2. de ce chapitre. Des mesures « correctrices » ont ensuite été proposées, discutées avec les éleveurs, et mises en place pour les troupeaux « expérimentaux » pendant le second semestre 2016 et en 2017.
La première mesure a consisté à dédier les parcelles à chaque troupeau pour éviter les contaminations croisées entre les troupeaux. Dans l'élevage B, le troupeau « témoin » traversait une parcelle utilisée par le troupeau « expérimental » à chaque fois qu'il devait aller au « stockyard » pour un traitement contre les tiques. Il a été convenu d'un nouveau trajet pour contourner cette parcelle lorsque le troupeau « témoin » était infesté.

La seconde mesure a été de préconiser un déplacement des animaux uniquement l'après-midi lorsqu'ils nécessitaient d'être menés jusqu'au « stockyard » pour y être déparasités afin d'éviter une dissémination des tiques le long du trajet. Selon les possibilités, le troupeau était alors soit relâché immédiatement après le traitement, soit gardé la nuit dans l'annexe du stockyard et traité le lendemain matin. Dans ce second cas, l'annexe ne devait pas être enherbée pour éviter qu'elle devienne un endroit favorable au développement des tiques. Plusieurs options ont été envisagées pour la gestion des annexes, comme : l'utilisation de zones désherbées, l'élevage de poules pour consommer les tiques gorgées tombant des animaux ou l'emploi de deux annexes, une dédiée au passage des animaux lors d'infestations parasitaires et une seconde pour le regroupement des animaux lors d'autres interventions (identification des animaux par la pose de boucles d'identification, pesées...).

La troisième mesure fut de prédire le risque de contamination larvaire des parcelles en fonction des précédentes observations de tiques sur les animaux et selon la saison. Les calendriers développés ont permis de gérer les troupeaux en conséquence. Différentes options étaient alors envisageables : 1) si la ressource fourragère était présente en quantité suffisante sur les autres parcelles, les parcelles contaminées étaient sorties de la rotation, 2) si la ressource fourragère n'était pas suffisante sur les parcelles non contaminées, il était préconisé de mettre le troupeau dans la ou les parcelle(s) infestée(s) et de réaliser, si besoin, un traitement « stratégique ».

## Indicateurs suivis

L'efficacité de la méthode a été évaluée en comparant le nombre de traitements acaricides entre la première et la seconde année de suivi, sur le troupeau « témoin » et sur le troupeau « expérimental ». Afin de comparer les différents traitements (amitraze et traitement rémanent), il a été considéré qu'un traitement rémanent 6 semaines équivalait à 3 traitements à l'amitraze espacés de 3 semaines (Citroni et al., 1999; Ferlat, 2004; Gomes et al., 2015). Un test de Chi-deux a été utilisé pour comparer le
nombre de traitement entre les troupeaux et entre les périodes (décembre 2015-novembre 2016 et
décembre 2016-novembre 2017). Le seuil de significativité a été fixé à $\alpha = 0,05$.

2.1.2 Résultats

L'évolution des traitements anti-tiques entre les troupeaux témoins et les troupeaux expérimentaux avant (décembre 2015 - novembre 2016) et après (décembre 2016 - novembre 2017) est présentée dans le tableau 3.

<table><caption>Tableau 3 : Comparaison du nombre de traitements acaricides entre décembre 2015 - novembre 2016 et décembre 2016 - novembre 2017 pour les troupeaux témoins et expérimentaux. ACA : acaricide courte action. ALA : acaricide longue action. Total : ACA + 3 ALA.</caption><thead><tr><th rowspan="2">Traitements acaricides</th><th colspan="3">Déc 2015 - Nov 2016</th><th colspan="3">Déc 2016 - Nov 2017</th><th rowspan="2">Résultat</th></tr><tr><th>ACA</th><th>ALA (équivalent ACA)</th><th>Total</th><th>ACA</th><th>ALA (équivalent ACA)</th><th>Total</th></tr></thead><tbody><tr><td>Elevage A Troupeau témoin</td><td>10</td><td>1 (3)</td><td><strong>13</strong></td><td>5</td><td>2 (6)</td><td><strong>11</strong></td><td>-15,4 %</td></tr><tr><td>Elevage A Troupeau expérimental</td><td>7</td><td>1 (3)</td><td><strong>10</strong></td><td>1</td><td>0</td><td><strong>1</strong></td><td>-90,0 %</td></tr><tr><td>Elevage B Troupeau témoin</td><td>9</td><td>0</td><td><strong>9</strong></td><td>7</td><td>0</td><td><strong>7</strong></td><td>-22,2 %</td></tr><tr><td>Elevage B Troupeau expérimental</td><td>11</td><td>2 (6)</td><td><strong>17</strong></td><td>4</td><td>0</td><td><strong>4</strong></td><td>-76,5 %</td></tr></tbody></table>

La réduction du nombre de traitements dans les troupeaux expérimentaux des élevages A et B a été calculée en prenant en compte la réduction du nombre de traitements dans les troupeaux témoins par la formule :

$$100 - 100 \times \frac{\text{Réduction du nombre de traitements dans le troupeau témoin}}{\text{Réduction du nombre de traitements dans le troupeau expérimental}}$$
La réduction du nombre de traitements acaricides dans les élevages A et B est ainsi respectivement de 82,9% et 70,9%. La différence de la réduction des traitements est statistiquement significative entre les troupeaux témoins et expérimentaux (p=0,008).

## 2.2 Mises en place de la méthode de lutte agronomique à plus grande échelle

Les résultats obtenus lors de la première étude ont été jugés satisfaisants par les acteurs de la filière et la méthode a été transférée au Groupement de Défense Sanitaire Animale (GDS-A) en 2017. Cette structure a alors accompagné progressivement 32 éleveurs dans la mise en place de cette méthode. Le suivi de ces éleveurs a cependant été allégé pour des raisons logistiques. Nous nous sommes appuyés sur ces éleveurs suivis par le GDS-A pour réaliser deux études rétrospectives.

### 2.2.1 Première étude rétrospective

#### Matériels et méthodes

Cette étude a été menée en 2019 auprès de 21 éleveurs ayant appliqué la méthode depuis plus d'un an. Certains éleveurs possédant plusieurs troupeaux pouvant être gérés différemment, nous avons mené cette étude à l'échelle de chaque troupeau. Au total, cette enquête a porté sur 37 troupeaux. Pour chaque troupeau, un questionnaire a porté sur les pratiques d'élevage et le nombre de traitements acaricides annuels avant la mise en place de la méthode développée puis un second questionnaire a porté sur les mêmes éléments après la mise en place des mesures préconisées. L'objectif de l'étude était d'une part, d'évaluer la réduction des traitements acaricides suite à la mise en place des mesures de lutte agronomique et d'autre part, d'identifier quels étaient les paramètres pouvant expliquer le nombre de traitements acaricides annuels. Trois paramètres supplémentaires ont été ajoutés à ceux déjà listés en annexe I : la pluviométrie annuelle moyenne, le nombre d'années depuis la mise en place de la méthode de lutte ainsi que la stratégie de traitement. En effet, certains éleveurs traitaient dès qu'ils voyaient les premières tiques alors que d'autres toléraient une faible infestation. Le nombre de traitements acaricides pour chaque troupeau a été relevé avant et après la mise en place des mesures de lutte agronomique préconisées.

Tous ces paramètres collectés dans le questionnaire sont présentés dans le tableau 4.
<table><caption>Tableau 4 : Description et modalité de réponses des variables inclues dans le questionnaire d'enquête.
Les 12 variables suivis d'un astérisque (*) ont été retenues pour la construction du modèle.</caption><thead><tr><th>Variable</th><th>Modalité de réponse</th></tr></thead><tbody><tr><td colspan="2"><strong>Description de l'exploitation</strong></td></tr><tr><td>Pluviométrie (mm) *</td><td>Continue</td></tr><tr><td>Barrières périphériques</td><td>Oui/Non</td></tr><tr><td>Barrières internes *</td><td>Oui/Non</td></tr><tr><td>Nombre de parcelles par troupeau *</td><td>Continue</td></tr><tr><td>Proportion d'animaux génétiquement résistants à la tique au sein du troupeau *</td><td>Continue</td></tr><tr><td>Degré de résistance des populations de tique à l'acaricide utilisé *</td><td>Continue</td></tr><tr><td>Fréquence d'observation des troupeaux (&gt; 2 fois par semaine) *</td><td>Oui/Non</td></tr><tr><td>Observation de cerfs dans les pâturages (&gt; 20 têtes)</td><td>Oui/Non</td></tr><tr><td colspan="2"><strong>Alimentation</strong></td></tr><tr><td>Présence de vaches suitées pendant la saison sèche *</td><td>Oui/Non</td></tr><tr><td>Chargement</td><td>Continue</td></tr><tr><td colspan="2"><strong>Gestion des pâturages et des troupeaux</strong></td></tr><tr><td>Traitement dès les premières tiques observées *</td><td>Oui/Non</td></tr><tr><td>Tenue d'un calendrier de rotation de pâtures et prise en compte des parcelles infestées pour les rotations</td><td>Oui/Non</td></tr><tr><td>Parcelles dédiées à chaque troupeau quand plusieurs troupeaux sont présents sur l'exploitation *</td><td>Oui/Non</td></tr><tr><td>Zones communes à plusieurs troupeaux (points d'abreuvement, couloirs...)</td><td>Oui/Non</td></tr><tr><td>Déplacement des animaux l'après-midi</td><td>Oui/Non</td></tr><tr><td>Traversée des parcelles pour aller au stockyard</td><td>Oui/Non</td></tr><tr><td>Ouverture des barrières pendant la sécheresse</td><td>Oui/Non</td></tr><tr><td>Tiques observées dans le mois précédent un traitement rémanent *</td><td>Oui/Non</td></tr><tr><td>Traitements rémanents planifiés selon le calendrier de rotation</td><td>Oui/Non</td></tr><tr><td>Rotations post traitement rémanent planifiées selon le calendrier de rotation</td><td>Oui/Non</td></tr><tr><td>Ancienneté de la mise en place des mesures de lutte agronomique *</td><td>Continue</td></tr><tr><td>Réalisation de « bains stratégiques » *</td><td>Oui/Non</td></tr><tr><td>Annexe et zones de déplacement (couloirs, ...) favorables aux tiques (enherbées)</td><td>Oui/Non</td></tr><tr><td>Animaux laissés au stockyard pendant une demi-journée après le traitement acaricide</td><td>Oui/Non</td></tr><tr><td>Élimination des animaux les plus régulièrement infestés</td><td>Oui/Non</td></tr></tbody></table>

## Méthodes statistiques

Un test t de Student, avec un risque α=5%, a été utilisé pour comparer la moyenne du nombre de traitements acaricides annuels avant et après la mise en place de la lutte agronomique. Un modèle statistique a été utilisé pour identifier les principaux facteurs en lien avec le nombre de traitements acaricides. Une collaboration a été initiée avec le département d'Entomologie et le département d'Écologie et de Biologie de la Conservation de l'Université de Texas (USA) pour le développement du
modèle. Ce modèle, créé dans R 3.6.0 (R Core Team, 2017) en utilisant le package « gbm » version 1.5-7 (Ridgeway, 2006), est dit à « arbres de régression boostés ». Il est présenté en détails dans Hüe et al. (2021a).

## Résultats

Les premiers résultats portent sur la réduction du nombre de traitements réalisés sur les 37 troupeaux étudiés. Alors que la moyenne du nombre de traitements annuels dans ces troupeaux était de 7,9±4,1 avant la mise en place des mesures de gestion agronomique, elle diminue à 5,3±3,6 après l'application du programme de lutte agronomique développé. Cela représente une réduction significative de 32,9% (p=0,002).

L'analyse des 12 variables retenus dans la construction du modèle statistique permet d'identifier que 5 variables représentent à elles seules 78,4% de l'ensemble des contributions à ce modèle. Ces 5 variables sont : la pluviométrie (19,3%), la durée du suivi (17,1%), le pourcentage d'animaux génétiquement résistants à la tique au sein du troupeau (15,9%), le niveau de résistance des tiques à l'amitraze (13,2%), et l'observation de tiques dans le mois précédant l'application du traitement rémanent (13,1%).

Ainsi, le nombre de traitements acaricides annuel diminue quand :

- la pluviométrie est inférieure à 850 mm,

- l'exploitation est suivie depuis plus de 6 mois,

- la proportion d'animaux résistants à la tique dans le troupeau est supérieure à 70%,

- la résistance à l'amitraze est élevée (DL99 > 0,33),

- les tiques n'ont pas été observées sur les animaux dans le mois précédent le traitement rémanent.

### 2.2.2 Seconde étude rétrospective

#### Matériels et méthodes

Une seconde étude rétrospective a été réalisée pour analyser les données collectées au cours de 85 traitements rémanents effectués sur 39 troupeaux, entre janvier 2015 et décembre 2019. Cette étude a été menée pour évaluer l'efficacité des antiparasitaires et les facteurs pouvant permettre d'optimiser ces traitements.

##### Indicateurs analysés dans cette étude

Le premier indicateur était le **délai minimal entre l'application du traitement rémanent et le traitement à l'amitraze suivant**. L'éleveur était libre de réaliser un traitement acaricide à l'amitraze en
fonction de l'observation des tiques sur ses animaux dans les semaines suivant ce traitement rémanent, sans qu'il y ait de seuil d'infestation défini pour cela.

Le second indicateur était le **délai moyen entre l'application de l'acaricide rémanent et le traitement acaricide suivant**. Ce délai fut analysé en fonction de deux paramètres : le suivi des préconisations pour les rotations post-traitement et l'observation de tiques le jour de l'application de l'acaricide rémanent.

Un troisième indicateur s'est intéressé à déterminer **combien d'animaux et pendant combien de temps** il fallait les laisser sur une parcelle après un traitement rémanent pour éliminer un maximum de larves et donc éviter une infestation du troupeau quand celui-ci reviendrait sur cette même parcelle. Nous avons utilisé pour cela la notion de chargement, ou charge animale. Le chargement est égal au rapport entre le nombre d'animaux et une surface donnée. Il est donné en nombre de bovins par hectare.

## Analyses statistiques

En raison de la faible taille des échantillons et d'une distribution non normale des données, le test non paramétrique de Wilcoxon-Mann-Whitney a été utilisé pour comparer : 1) la période minimale entre le traitement rémanent et le traitement suivant selon l'emploi de l'Acatak® ou de l'Ivomec Gold®, 2) le délai entre le traitement rémanent et le traitement suivant selon que les rotations préconisées aient été ou non respectées, 3) ce même délai selon que des tiques aient été ou non observées le jour du traitement rémanent, et 4) l'effet de différentes charges animales et durées de pâturages sur la « décontamination » des parcelles.

Pour étudier ce dernier point, la durée de séjour par parcelle a été divisée par 7 pour obtenir un « équivalent-semaine ». La charge animale présente pendant une semaine sur une parcelle a ensuite été calculé par la formule suivante :

$$ \text{Charge animale par semaine} = \frac{(\text{nombre d'animaux / surface en hectare})}{(\text{temps de présence en jour} / 7)} $$

L'observation des données collectées nous a incité à agréger cette valeur en 4 classes : >3, >4, >5 et >6 têtes/ha/semaine. Selon qu'un traitement a été nécessaire ou non lors du second passage sur la parcelle, la charge animale par semaine a été considérée comme suffisante ou non. Le pourcentage de pâturage et charge animale suffisant ou insuffisant a ensuite été calculé pour chaque classe.

Pour tous les tests statistiques, le seuil de significativité a été fixé à $\alpha = 0,05$.
# Résultats

## Période minimale entre le traitement ALA et le traitement suivant.

Sur un total de 85 traitements analysés, 38 concernaient une application d'Aatak® et 47 un traitement à l'Ivomec Gold®. Les délais minimaux observés entre un traitement avec l'Aatak® et l'Ivomec Gold® et le prochain traitement acaricide à l'amitraze furent respectivement de 6 et 7 semaines (figure 13). Comme le développement de R. australis sur les bovins, de la larve à l'adulte, nécessite environ 3 semaines, on a supposé que l'infestation par les larves avaient eu lieu 3 semaines plus tôt. Ainsi, les périodes minimales entre un traitement avec l'Aatak® et l'Ivomec Gold® et l'infestation suivante par les larves ont été estimées à 3 semaines et 4 semaines, respectivement. Il est intéressant de noter que, dans les 9 semaines suivant le traitement initial avec un acaricide rémanent, un traitement acaricide a été nécessaire dans 21 % des cas après l'application d'Aatak® et dans 38 % des cas après un traitement d'Ivomec Gold®. Cependant, cette différence observée entre les deux produits n'est pas significative (test U de Mann-Whitney, U = 55, z-score = 0,4539, p= 0,326).

<figure><img src="image_38.png" /><figcaption>Figure 13 : Distribution de la durée minimale (en semaines) entre le traitement initial avec un acaricide longue action et le traitement acaricide suivant.</figcaption></figure>
## Impact du suivi des recommandations et de l'observation de tiques le jour du traitement rémanent

À partir de 2018, des conseils techniques écrits avec des rotations de pâturages recommandées ont été fournis aux éleveurs et comparés à *posteriori* avec les rotations effectivement réalisées pour 29 traitements avec un ALA. Les conseils ont été suivis dans 62,1 % des traitements (18 traitements sur 29). La période moyenne entre l'application d'un ALA et le traitement suivant contre les tiques était de 11,0 ± 5,3 semaines lorsque les conseils n'étaient pas suivis, et de 17,7 ± 12,6 lorsque les conseils étaient suivis (figure 14). Le fait de suivre les recommandations concernant les rotations après une application d'ALA a augmenté de manière significative la période avant le traitement suivant contre les tiques (test U de Mann-Whitney, U = 51,5, z-score = 2,1125, p= 0,017). Ce conseil technique est ainsi intéressant pour prolonger de plus de 6 semaines la période moyenne avant le prochain traitement.

<figure><img src="image_39.png" /></figure>

**Figure 14 :** Durée (en semaines) entre l'application de l'ALA et le traitement suivant en considérant ou non les conseils techniques fournis. Dans ce « boxplot », la croix représente la moyenne, la barre horizontale dans la boite représente la médiane, les limites inférieures et supérieures de la boite représentent le 1er et le 3ème quartile (Q1 et Q3) et les barres extérieures représentent 1.5 fois la valeur interquartile (Q3-Q1). Les points représentent les valeurs extrêmes, ne rentrant pas dans les barres extérieures.
Sur les 34 traitements avec un ALA pour lesquels des informations étaient disponibles, des tiques ont été observées le jour du traitement dans 27 cas (79,4%) alors, qu'aucune tique n'a été signalée pour 7 traitements (20,6%). La période entre le traitement avec un ALA et le traitement suivant contre les tiques était de 11,2 ± 5,3 semaines si des tiques étaient observées le jour du traitement et de 18,5 ± 11,7 semaines si aucune tique n'avait été observée (figure 15). La différence entre les deux périodes était statistiquement significative (test U de Mann-Whitney, U = 37,5, z-score = -2,4064, p= 0,008).

<figure><img src="image_40.png" /><figcaption>Figure 15 : Durée (en semaines) entre l'application de l'ALA et le traitement suivant en considérant l'observation de tiques sur les animaux le jour du traitement.</figcaption></figure>

### Effet de la charge animale sur l'impact des ALA

L'analyse de 15 traitements avec un ALA semble indiquer que les charges animales à l'hectare plus élevées sont plus efficaces pour éliminer de façon optimale les larves de tiques dans les parcelles infestées comparativement à des chargements plus faibles, mais la différence n'est pas statistiquement significative (test U de Mann-Whitney, U = 16, z-score = 1,2374, p= 0,108) (tableau 5).

Tableau 5 : Impact de la charge animale à l'hectare hebdomadaire sur la collecte des larves de R. australis après un traitement avec un ALA dans un pâturage infesté. Les chiffres entre parenthèses indiquent le nombre de situation. Pour chaque chargement étudié, le total est égal à 15.
<table><thead><tr><td>Chargement par semaine supérieur à :</td><td>Efficacité de l'élimination des larves dans les parcelles</td><td>Chargement par semaine inférieur à :</td><td>Efficacité de l'élimination des larves dans les parcelles</td></tr></thead><tbody><tr><td>3 (14)</td><td>57% (8)</td><td>3 (1)</td><td>100% (1)</td></tr><tr><td>4 (9)</td><td>67% (6)</td><td>4 (6)</td><td>50% (3)</td></tr><tr><td>5 (7)</td><td>86% (6)</td><td>5 (8)</td><td>38% (3)</td></tr><tr><td>6 (2)</td><td>100% (2)</td><td>6 (13)</td><td>54% (7)</td></tr></tbody></table>

Les observations réalisées à partir de ces 15 traitements indiquent qu'un chargement de 5 animaux/ha pendant une semaine semble être le plus pertinent pour décontaminer les parcelles avec un troupeau traité avec un ALA.

## Ce qu'il faut retenir :

La méthode développée autour de la gestion des pâturages et des troupeaux a prouvé son efficacité lors d'un essai incluant 2 élevages avec une réduction de 70,9% à 82,9% des traitements acaricides mais également lorsqu'elle était appliquée à une plus large échelle, avec une réduction d'environ 33% des traitements.

Une enquête menée auprès des éleveurs utilisateurs de cette méthode a permis d'identifier les principaux paramètres en lien avec le nombre de traitements acaricides par an : la pluviométrie, la durée du suivi, le pourcentage d'animaux résistants à la tique dans le troupeau, le niveau de résistance des tiques à l'amitraze et l'observation de tiques dans le mois précédant l'application du traitement rémanent.

Enfin l'analyse des observations réalisées dans ces élevages a permis d'améliorer les connaissances sur l'utilisation des traitements rémanents. Il est ainsi apparu que :

- l'activité larvicide de l'Acatak® semble plus longue que celle de l'Ivomec Gold®,

- il est important de programmer l'application de ces traitements rémanents au moment opportun pour intervenir avant qu'il n'y ait des tiques sur les animaux afin de retarder les futures infestations parasitaires,

- il est intéressant d'établir un planning de rotations après le traitement rémanent en fonction des parcelles infestées pour optimiser l'élimination des larves de tiques présentes dans les pâturages.

- laisser 5 bovins traités avec un acaricide rémanent par hectare pendant une semaine permettait de décontaminer efficacement une parcelle.
## 3. Discussion

Les résultats obtenus dans ces trois études apportent des avancées intéressantes pour contrôler les tiques tout en optimisant la ressource fourragère. Ils permettent également d'améliorer l'utilisation des traitements rémanents. Cependant certains biais doivent être pris en considération.

La limite de ces travaux porte essentiellement sur la collecte des données. Que ce soit pour l'optimisation de l'usage des acaricides rémanents ou l'identification des facteurs impliqués dans les traitements acaricides, le relevé d'informations s'est fait à dire d'éleveurs. Chaque éleveur a sa propre sensibilité vis-à-vis des infestations parasitaires et aucun seuil n'avait été défini pour déclencher un traitement acaricide. Il est donc probable que certains éleveurs aient traité leurs animaux alors que d'autres ne l'auraient pas fait ou auraient décalé le traitement de quelques jours. Par ailleurs, le modèle établi pour cette méthode de lutte agronomique met en évidence les paramètres explicatifs du nombre de traitements acaricides annuels mais il n'identifie pas les paramètres à l'origine d'une réduction de ces traitements lors de la mise en place du plan de lutte. Enfin, cette méthode de gestion des pâturages n'a été testée que dans deux exploitations. Toutefois, les résultats obtenus ultérieurement avec un panel plus large d'élevages ont confirmé l'efficacité de cette nouvelle approche.

### 3.1 Valoriser la ressource fourragère

La lutte agronomique contre la tique préconisait jusqu'à tout récemment de laisser les parcelles au repos pour permettre une mortalité naturelle des larves de tiques (Wilkinson, 1957; Harley et Wilkinson, 1964; Wilkinson, 1964; Wharton et al., 1969). Les périodes de mises au repos recommandées étaient ainsi de 60 à 130 jours selon les saisons (Desquesnes et Vignon, 1987). Or, une mise en défens des pâturages aussi longue n'est pas compatible avec une bonne gestion du fourrage sur le territoire. En effet, un cycle de rotation du pâturage de 25 à 35 jours permet une utilisation appropriée des fourrages tropicaux (Salas et al., 1994; Corrêa et Santos, 2003).

Un des apports majeurs de la méthode de gestion agronomique développée ici réside dans le fait que les éleveurs peuvent adapter leurs rotations de pâturages en considérant à la fois la production de fourrage et le risque d'infestation par les tiques dans chaque parcelle. En effet, si, selon les données enregistrées, une parcelle ne semble pas infestée, l'éleveur peut y mettre ses animaux au meilleur moment du développement fourrager. S'il sait qu'une parcelle est infestée par des larves, il peut l'utiliser si c'est la meilleure période en termes de production de fourrage, mais il devra contrôler les animaux 2 semaines plus tard et les traiter si nécessaire avant que les tiques ne terminent leur cycle, évitant ainsi une nouvelle contamination des parcelles. Si l'éleveur veut et peut éviter d'utiliser une
parcelle infestée, l'herbe pourrait potentiellement être récoltée pour être utilisée pendant la saison sèche sous forme de foin ou d'ensilage. Si l'éleveur fait du foin sur une parcelle infestée par des larves, il devra cependant prendre soin d'attendre 4 mois entre la fauche et la distribution du foin pour éviter que des larves présentes dans le foin n'infestent les animaux.

## 3.2 Optimiser les traitements acaricides

L'application d'acaricides rémanents est particulièrement intéressante pour éliminer les larves de tiques dans les pâturages. Mais pour optimiser leur utilisation, encore faut-il avoir des informations fiables sur la rémanence de leur activité larvicide. Alors que la notice de l'Ivomec Gold® indique une protection contre la tique R. australis pendant 11 semaines, l'analyse des données collectées dans notre étude indique que 38% des traitements réalisés avec ce produit ont été suivis d'un second traitement acaricide au bout de 9 semaines. Notre résultat conforte les conclusions apportées dans d'autres études qui avancent une période d'efficacité moindre que ce que le fabricant indique (Davey et al., 2010; Gomes et al., 2015). L'Acatak® semble avoir un effet rémanent plus prolongé puisque 21% des traitements avec ce produit ont nécessité un traitement acaricide dans les 9 semaines qui ont suivi. Ce résultat va dans le même sens que celui de Mendonça (2010) qui rapporte un effet résiduel supérieur à 90% pendant 13 semaines après un traitement avec ce produit. Ces informations doivent être prises en compte pour adapter les rotations suivant le traitement acaricide pour pâturer sur l'ensemble des parcelles infestées pendant la période d'efficacité larvicide de ces produits. Nous avons en effet démontré que si les éleveurs suivaient les rotations préconisées après ces traitements, ils décalaient de façon significative la date du traitement acaricide suivant.

L'analyse des conditions d'utilisation des traitements rémanents a également mis en exergue l'impact de la présence de tiques sur les animaux le jour d'un traitement (Hüe et al., 2021b). L'observation de tiques le jour de l'application d'un traitement indique que certaines tiques sont déjà tombées dans la parcelle. Nous avons précédemment vu que la durée entre la chute des tiques gorgées et l'éclosion des œufs varie dans les conditions locales de six semaines pendant la saison chaude à dix semaines pendant l'hiver (Desquesnes et Vignon, 1987). Donc si des tiques gorgées sont tombées dans la parcelle le jour du traitement, l'acaricide ne sera plus efficace lorsque les larves vont émerger et les animaux risquent alors de se réinfester rapidement quand ils reviendront sur cette parcelle.

Le dernier point à considérer dans l'usage des acaricides rémanents est le temps qu'il faut laisser les animaux dans une parcelle pour impacter la population de larves présentes. Les résultats que nous avons obtenus permettent d'estimer qu'il faut laisser l'équivalent de 5 animaux par hectare pendant une semaine pour arriver à cet objectif. Ce résultat est cohérent avec les observations de Sutherst et
al. (1978) qui indiquent qu'un chargement de 5 bovins par hectare ramassait entre 50 et 85 % des larves de tiques en une semaine, contre 30 à 70 % avec deux animaux par hectare.

## Conclusions

L'objectif de la politique actuelle en Nouvelle-Calédonie n'est pas l'éradication de la tique du bétail, ce qui ne serait pas réaliste, mais le maintien de la population de tiques à un niveau suffisamment bas pour permettre l'élevage des bovins, notamment de races sensibles, dans des conditions correctes. La méthode de gestion agronomique développée ici s'inscrit dans cette politique de gestion de la tique. Elle requiert une surveillance régulière des animaux et un enregistrement des données pour comprendre la dynamique parasitaire dans l'exploitation et identifier les points critiques à l'origine de la contamination des pâturages et des troupeaux. Cependant, pour abaisser les populations parasitaires dans les pâturages, nous avons développé un vaccin permettant de réduire les capacités de reproduction des tiques afin de diminuer la production d'œufs et de larves.
<figure><img src="image_41.png" /></figure>

# Development of a new approach of pasture management to control *Rhipicephalus microplus* infestation

Thomas Hüe<fnref n="1" /> · Chloé Fontfreyde<fnref n="2" />

Received: 6 January 2019 / Accepted: 11 April 2019 / Published online: 10 May 2019
© Springer Nature B.V. 2019

## Abstract

Despite several decades of chemical control, the cattle tick *Rhipicephalus microplus* remains an important constraint for cattle farmers. The regular use of chemicals has led to the development of tick strains that are multi-resistant to acaricides. New methods of tick control are being developed and combined in integrated tick management programs. Grazing management is one part of these control methods. However, until now, grazing management consisted essentially of resting pastures for 4 to 5 months. This amount of time is generally sufficient to allow for natural tick larvae mortality to occur, but this method often compromises the quality of forages, which is less than optimal at the time of arrival of the animals in the paddock. In this paper, we propose a new approach to pasture management-based tick control that optimizes forage production. It is based on tick development biological parameters, herd management and strategic treatments. This approach was tested for 2 years on two farms raising tick-susceptible European breeds of cattle under tropical conditions. The number of chemical treatments decreased respectively by 82.9% and 70.9%. This cost-effective approach may prove useful in decreasing the number of acaricidal treatments on farms facing high tick loads.

**Keywords** *Rhipicephalus microplus* · Pasture management · Integrated tick control

## Introduction

As for many tropical cattle breeding countries, New Caledonia is plagued by the cattle tick *Rhipicephalus microplus* even though the tick-borne diseases babesiosis and anaplasmosis are absent. In addition to the reduction in weight by animals due to tick's attachment and blood feeding, heavy infestations punctually lead to cattle mortality. Tick control programs have been based on acaricide treatments since their introduction to New Caledonia in 1942. Frequent acaricide treatments have led to the development of acaricide resistances in New Caledonia as

in other tropical beef producer countries (Petermann et al. 2016). Since the 2000s, cattle farmers mainly apply amitraz for tick treatments all year long and macrocyclic lactone anti-parasitic drugs, such as non-residual avermectin and moxidectin, are punctually used on average once or twice a year. Contrary to macrocyclic lactones that are paying, amitraz is totally free for farmers. Until the 2000s, they treated cattle systematically every 4 weeks. Today, to limit the development of acaricide resistances, Caledonian farmers commonly treat opportunistically, only when they observe ticks on cattle. There is locally no consensus on threshold for tick treatment, so depending on each farmer and the frequency of herd observation, treatments occur sometimes at early stage but often with engorged ticks. Thereby, with this method of control, pastures are regularly reinfested by ticks as invariably some engorged adult females will already have been fallen to the ground before the treatment. Farmers rearing tick susceptible breeds also have limited access to long-lasting acaricides (Ivermectin, Ivomec Gold®, Boehringer Ingelheim or Fluazuron, Acatak®, Novartis). These acaricides are applied exclusively by a technician and the number of treatments is limited to one or two per year.

Current tick control programs are increasingly focused on sustainable integrated tick management and decreased acaricide use. This change is due in part not only to the

**Electronic supplementary material** The online version of this article (https://doi.org/10.1007/s11250-019-01899-x) contains supplementary material, which is available to authorized users.

<figure><img src="image_42.png" /></figure>

Thomas Hüe
hue@iac.nc

<figure><img src="image_43.png" /></figure>

<footnote n="1">¹ Institut Agronomique néo-Calédonien, Equipe ARBOREAL, BP 73, 98890 Païta, New Caledonia</footnote>

<footnote n="2">² Groupement de Défense Sanitaire Animal, Chambre d'Agriculture de Nouvelle Calédonie, BP 111, 98845 Nouméa Cedex, New Caledonia</footnote>
development of resistances but also to environmental and so-
cial considerations such as reducing soil and water pollution
with pesticides, reducing farmers’ exposition to chemicals,
and the increasing demand for greener produced food-stuffs.

Several alternative methods to acaricide treatment are
now available to integrated tick programs: such as the use
of a tick vaccine tick, pasture management, or the use of
resistant breeds. Historically, the tick-susceptible cattle
breeds Limousin and Charolais were reared on New
Caledonia. Due to the wide development of acaricide re-
sistances, Brahman, Senepol, Droughtmaster, and
Belmont Red were introduced in the 2000s and are now
bred either in pure breed or crossed with Limousin or
Charolais cattle (Hüe et al. 2014). However, there is a
strong will to maintain Limousin and Charolais breeds
in New Caledonia for their patrimonial value but especial-
ly to increase meat productivity and fertility in cross-
breeding with tick resistant breeds. Furthermore, a tick
vaccine in development in New Caledonia (Hüe et al.
2017) is now being tested on farms. Pasture management
is another way to control ticks. Until now, New
Caledonian cattle farmers were advised to avoid tick-
infested pastures for 3.5 to 4.5 months to allow for natural
tick larvae mortality (Barré and Delathière 2010). This
measure is efficient but incompatible with a good forage
management. Indeed, optimal tropical grass quality and
quantity are obtained after 4 and 6 weeks of plant growth
(Chambre d'Agriculture de Nouvelle-Calédonie 1989).
So, farmers were faced with two conflicting messages:
enhance forage quality with rapid pasture rotations or
leave pastures unused for several months to decrease tick
loads.

This paper describes a new approach to pasture manage-
ment based on knowledge of tick biology that optimizes for-
age production and tick management.

Materials and methods

Experimental sites and animals

New Caledonia is a French island in the South Pacific Ocean with an oceanic tropical climate, 1200 km to the East of Australia and 2000 km to the North of New Zealand. Approximately 80,000 head of cattle are raised under extensive conditions by about 650 farmers. The hot and wet season is usually from late December to April. It is followed by a cold season from June to August with possible rain, and the dry season is from September to December. Mean temperatures and rainfall from January 2015 to December 2017 in both areas, Tontouta and La Foa, concerned by this study are presented in Fig. 1. An unusual important rainy phenomenon was observed in November 2016.

Experiment farms

Two farms, with regular high tick pressure and frequent acar-
icide use were followed over 2 years. Both farms have two
herds with separate pastures for each. Information concerning
the different herds is summarized in Table 1.

Acaricide resistance testing was conducted regularly on both farms in our lab. Tick strains have been resistant to amitraz since 2011 on farm A and since 2015 on farm B. Ticks on both farms are susceptible to macrocyclic lactones. Amitraz is still used in these two farms and treatments are efficient enough to control tick infestations, and as this acaricide is free for farmers, it is preferred to macrocyclic lactones for tick treatment.

Recommended measures of pasture management were ap-
plied for one herd on each farm; these were the experimental
batch. The other herd on each farm that was managed with no
changes, was considered as the control batch.

Plot of pasture use and treatments for each herd were re-
corded systematically on a pasture calendar during the study.

Tools developed

A provisional calendar was developed to know when cattle were to be exposed to tick larvae in plot of pasture. The tick *Rhipicephalus microplus* is a monoxenous parasite. It spends around 18 to 23 days on the same cattle to end its development and become a fully engorged female, that will drop off early in the morning (Hitchcock 1955a; Wharton 1970). If this female finds favorable conditions on the ground, she will lay eggs which will hatch after few weeks. The average time from drop off of engorged ticks to egg hatching on New Caledonia were determined by Desquenes and Vignon in 1987 (Table 2).

So, considering the period of the year, when engorged ticks
are observed on cattle, it is possible to estimate when larvae
will be active in the plot of pasture where cattle were.

Different tools based on local biological tick parameters were
developed during this study. Farm A recorded herd movements
by computer. He was provided with a spreadsheet calendar that
automatically added hatching and larvae survival periods. A
hardback rotating provisional calendar (Cf picture 1) was devel-
oped for farmer B to predict the period of larval presence based
on the date of engorged adult tick observation.

Picture 1: Hardback rotating provisional calendar

Correctives measures

During the first half of 2016, all farmers’ practices were re-
corded to understand their herd and pasture management and
the impact on tick infestations: pastures used by each herd,
pasture rotations, date of acaricide treatments, tracks used to
go to the dipping installation (pastures crossed or
corridors)....
<figure><img src="image_44.png" /><figcaption>**Fig. 1** Mean temperatures and rainfall at A Tontouta (Farm A) and B La Foa (Farm B) in 2015 (black), 2016 (light gray), and 2017 (dark gray)</figcaption></figure>

Following corrective measures were then recommended and implemented in the two farms during the second half of 2016 and 2017.

The first corrective measure was to strictly dedicate pastures to each herd in order to avoid pasture re-infestation by another herd. In Farm B, the control herd crossed a pasture used by the experimental herd each time it was mustered for acaricide treatment. Engorged ticks were shed in transit, thereby re-infesting the crossed pasture. Animals from the experimental herd were thereby regularly infested when they stayed here. The corrective measure was to find a new track to avoid this pasture.

As previously explained, engorged ticks drop off from cattle in the morning. The second measure was therefore to only move herds in the afternoon when they needed to be treated

against ticks, thereby limiting the spreading of engorged ticks en route to the dipping installation. Depending on facilities, cattle were either treated in the afternoon and released immediately after or kept in the stock-yard at night and treated the next morning. In this last case, the area for the night should not be a grass area, considered as favorable environment for ticks' survival, at the risk of seeing cattle reinfested few months after by crossing this area. Various options were identified to respond that, as the use of a without-grass area, or in case of grass areas, the use of two areas, one dedicated to cattle infested by ticks before a treatment, and one to cattle using the stock-yard for other occasions.

The third measure was to predict the risk of larvae in a pasture based on previous tick observations on animals, season and tick survival parameters. With the provisional
<table><caption>**Table 1** Presentation of the two farms and herds</caption><thead><tr><th rowspan="2"></th><th colspan="2">Farm A</th><th colspan="2">Farm B</th></tr><tr><th>Experimental herd</th><th>Control herd</th><th>Experimental herd</th><th>Control herd</th></tr></thead><tbody><tr><td>Breed of cows</td><td>Charolais</td><td>Charolais</td><td>Charolais</td><td>Crossbreed (taurus x indicus)</td></tr><tr><td>Breed of bull(s)</td><td>Blonde d'Aquitaine</td><td>Blonde d'Aquitaine</td><td>Brahman</td><td>Crossbreed</td></tr><tr><td>Average number of beasts per herd</td><td>7</td><td>23</td><td>86</td><td>341</td></tr><tr><td>Number of pastures per herd</td><td>3</td><td>3</td><td>5</td><td>9</td></tr><tr><td>Total surface of pastures per herd</td><td>14 ha</td><td>24 ha</td><td>81 ha</td><td>183 ha</td></tr><tr><td>Treatment</td><td>Spray</td><td></td><td>Spray</td><td></td></tr><tr><td>Status to amitraz</td><td>Resistant</td><td></td><td>Resistant</td><td></td></tr><tr><td>Status to macrocyclic lactones</td><td>Susceptible</td><td></td><td>Susceptible</td><td></td></tr></tbody></table>

calendar, we were able to predict the presence or not of tick larvae for each pasture prior to the introduction of animals. Several options were available: (1) if eggs had not yet hatched in a plot of pasture, it could be used freely; (2) if a plot was infested and enough forage resources were available in other plot, it could be rested for the length of larval activity (1.5 to 2.5 months depending on the season, cf. Table 2) even though the forage quality would decrease as previously mentioned; or (3) if the plot was infested and the forage was needed, then animals were introduced but closely monitored between 2 and 2.5 weeks later to estimate tick infestation and decide whether an acaricide treatment would be necessary or not. It is safer to control animals visually — and even manually if needed to feel small ticks in the coat — in a corridor as ticks are still small at this stage and might not be seen in field. It was essential to control, and treat if needed, before 3 weeks to avoid a new generation of engorged ticks that will reinfest pastures. We define here that these treatments are strategic treatments, that means treatments are based on a plan determined by the knowledge of the ecology of the ticks to beak their cycle. Cattle were not systematically treated when moving from a plot to another as we know the risk of tick infestation for the plot from where they come out. So, we only check when the herd goes out a risky plot and/or 2 to 2.5 weeks after animals come into a risky plot. If required, long lasting acaricides were scheduled during the period when larvae were active in most plots of pasture. These products are active during 5 to 6 weeks and tick larvae fixed on animals during this period will be killed before reaching adult stage. So, the rotations were planned to maximize larvae captured in pastures by cattle.

On the control herd, farmers continued to manage animals and treat as usual. Considering that the first effects were observed at the end of 2016 and that farmer A reorganized his herds in December 2017, the monitoring periods were defined from December 2015 to November 2016 and from December 2016 to November 2017.

## Indicators

The development of number of acaricidal treatments in experimental and control herds was compared to assess the efficacy of this new pasture management method. In order to compare the number of treatments, we consider the number of treatments with short-lasting acaricide — including amitraz and macrocyclic lactones with short residual period — and the number of treatments with long-lasting acaricides. To compare both of these treatments, we consider that in terms of acaricide protection period, a long-lasting acaricide treatment is equivalent to three short-lasting acaricides. Indeed, the residual activity of short- and long-lasting products is null and 6 weeks, respectively (Gomes et al. 2015; Barré and Delathière 2010). With an average of 3 weeks for the development of ticks on cattle, minimum delays to retreat after a short- or long-lasting acaricide treatments are respectively 3 and 9 weeks.

A Student test was used to compare the number of treatments on control herds and experimental herds between December and November of the following year in 2015 and 2016, with a risk $\alpha = 5\%$.

<table><caption>**Table 2** Biological parameters of free-living stages in New Caledonia (in months)</caption><thead><tr><th></th><th>December to March</th><th>April to May</th><th>June to September</th><th>October to November</th></tr></thead><tbody><tr><td>From drop off to egg hatch = eggs in the parcel</td><td>1.5</td><td>2</td><td>2.5</td><td>2</td></tr><tr><td>Survival of 80% of larvae = larvae in the parcel: danger*</td><td>1.5</td><td>2</td><td>2.5</td><td>2</td></tr><tr><td>Total</td><td>3</td><td>4</td><td>5</td><td>4</td></tr></tbody></table>

<footnote>*The remaining 20% were not active enough to represent a source of infestation for cattle (Desquesnes and Vignon 1987)</footnote>
## Results

The development of tick treatments per herd in control and experimental herd before (December 2015–November 2016) and after (December 2016–November 2017) the application of corrective measures is presented in Table 3.

Considering the development of the number of tick treatments in control herds in each farm, the reduction of tick treatments for the Farmers 1 and 2, in the experimental herds, are respectively 82.9% and 70.9%.

For control herds, the Student test shows no significant difference between the average number of treatments before and after the application of corrective measures (p value = 0.55). Regarding experimental herds, the p value is very slightly superior to alpha (p value = 0.051).

## Discussion

The approach developed in these two farms decreased acaricide treatments by 82.9% and 70.9%. The initial difference concerning the number of acaricidal treatments for the Farmer B between the two herds is due to the herd composition. In this farm, the control herd is mainly composed of cross-breed cows *Bos taurus x Bos indicus*, whereas the experimental herd is composed of Charolais cows much more tick susceptible (cf. Table 1).

The number of farms included in this study is too low to obtain robust results but it was difficult to identify farmers agree to change their pasture management and above all to keep a control herd exposed to tick infestations during this 2-year period.

The reduction of the number of acaricide treatments is not significant (p value = 0.051), but it has to be interpreted more in details. Indeed, before and during 2016, farmers were treating when they observed ticks on cattle and heavy infestations with several hundreds of engorged ticks are common in New Caledonia. Farmer B reported such an infestation in

March 2016 in his Charolais herd, when two cows died from tick infestation. From 2017, it was recommended to control experimental herds 2.5 weeks after introduction on an at-risk pasture and to treat if needed. During these controls, tick counts were achieved and no engorged ticks were observed. Thus, in 2016, acaricide treatments were in response to tick observations in all herds, whereas in 2017, they were mainly done to prevent tick infestations in experimental herds.

Corrective measures are based on knowledge of tick biology concerning the non-parasitic stages, from engorged adult female drop off to hatching and taking account the larval survival period. Hitchcock (1955a) recorded that the majority of engorged tick dropped off between 6:00 and 10:00 am. Wharton (1970) confirmed that the great majority of engorged females had fallen by 9:00 am to 9:30 am depending on the season but noticed that they can begin to fall down during the night.

In New Caledonia, Bianchi and Barré (2003) studied the factors involved in the detachment of engorged ticks in the morning, after the night rest. The movement of animals is one of these factors and the longer this movement is, the higher the proportion of engorged tick detachment. This highlights the rule of cattle to spread ticks within the farm, if the herd is moved in the morning to go to the dipping installation, especially if it needs to cross several pastures to go to these installations.

Duration of preoviposition, oviposition, egg development, and survival of tick larvae depend on temperature and humidity and vary between studied areas. The period from detachment to hatching can take from 3 to 4 weeks in ideal conditions to 6 months (Legg 1930; Harley 1966; Hitchcock 1955b). Tick larvae can survive up to 8 months in laboratory conditions (Hitchcock 1955b) but field observations indicate a maximal survival period of 6.5 months (Legg 1930; Harley 1966). In New Caledonia, Desquesnes and Vignon (1987) studied these different periods of non-parasitic stages of *R. microplus* and estimated that it takes between 6 and 10 weeks from the detachment of engorged ticks to egg

<table><caption>**Table 3** Comparison of the number of acaricidal treatments between December 2015–November 2016 and December 2016–November 2017 for experimental and control herds</caption><thead><tr><th rowspan="2">Years:</th><th colspan="3">Dec 2015-Nov 2016</th><th colspan="3">Dec 2016-Nov 2017</th><th rowspan="2">Trend (%)</th></tr><tr><th>SLA</th><th>LLA</th><th>Total</th><th>SLA</th><th>LLA</th><th>Total</th></tr></thead><tbody><tr><td>Acaricidal treatments:</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Farmer A</td><td>10</td><td>1</td><td><strong>13</strong></td><td>5</td><td>2</td><td><strong>11</strong></td><td>-15.4</td></tr><tr><td>Control herd</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Farmer A</td><td>7</td><td>1</td><td><strong>10</strong></td><td>1</td><td>0</td><td><strong>1</strong></td><td>-90.0</td></tr><tr><td>Experimental herd</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Farmer B</td><td>9</td><td>0</td><td><strong>9</strong></td><td>7</td><td>0</td><td><strong>7</strong></td><td>-22.2</td></tr><tr><td>Control herd</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Farmer B</td><td>11</td><td>2</td><td><strong>17</strong></td><td>4</td><td>0</td><td><strong>4</strong></td><td>-76.5</td></tr><tr><td>Experimental herd</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr></tbody></table>

SLA short-lasting acaricide, LLA long-lasting acaricide

Total: SLA + 3 LLA
hatching depending on the season. In the same study, they observed that tick larvae can survive from 10 to 14 weeks but 80% of these ticks died after 6 weeks in summer and 10 weeks in winter and the remaining 20% larvae are not active enough to represent a source of infestation for cattle.

In previous studies, the knowledge about survival of larvae was used to justify pasture rest time (Harley and Wilkinson 1964; Wharton et al. 1969; Wilkinson 1957; Wilkinson 1964). This period could be up to 4.5 months (Wharton et al. 1969) which is inconsistent with quality forages especially under New Caledonian climatic conditions. The innovation of this study consists in the fact that farmers can adapt their rotation plan, herd and pastures management considering both forage production and the risk of tick infestation in each plot of pasture. They can use an “at risk” plot if it is the best period in terms of forage production, and control and treat animals if needed before ticks end their cycle, avoiding a new contamination of plots. If farmers could avoid using a pasture with tick larvae — during the rainy season for example — grass could potentially be cut and/or transformed (silage, hay...) for use during the dry season.

The development of easy-to-use automatic or hardback rotating provisional calendar makes this planning easy to define by farmers themselves.

Knowing when to use long-lasting acaricides is also a crucial advantage of this method. Cattle treated with long-lasting acaricides are very useful for collecting and killing tick larvae in pastures but the use of these products should be limited as they are supposed to promote the development of tick resistance (Kunz and Kemp 1994). As they are active against larval stages, knowledge of the presence of tick larvae in paddocks helps to determine when the use of long-lasting acaricide will be the most efficient and what will be the best plan of rotation to collect larvae and decrease tick pressure in pastures. Therefore, the new tick management method presented in this study should be systematically utilized on farms using long-lasting acaricides in order to decrease larvae in plots of pasture. The proper length of stay per paddock has to be determined for each pasture and is mainly dependent on stocking rate. Sutherst et al. (1977) estimated, with experimental and theoretical studies, that with a stocking rate of about two beasts per hectare, cattle picked up between 30 and 70% of the larvae in a week, compared with 50 to 85% with five beasts per hectare. Further studies are required to validate these results.

The aim of this study and the current policy in New Caledonia is not the eradication of cattle tick but the control of tick population at a low level. This approach is due to the failure of several decades of systematic treatments and the absence of tick-borne disease that is compatible with the presence of few ticks. With this method, a small population of ticks can survive in pastures and a constant monitoring by the farmer must be

maintained to ensure the effectiveness of this approach. This point is even more important in these two farms, as tick strains began to be resistant to amitraz allowing the potential survival of ticks during strategic treatment.

Beyond these first results, an important change is the vision of tick infestation by farmers. From the moment they understand how tick populations evolve within and between paddocks with visual tools, they move from a passive position where they observe and suffer from tick infestation on cattle to an active role where they can anticipate and manage future infestations. Moreover, until now, they were treating when they observed ticks and most of the time, animals were already impacted and pastures reinfested. They can now break tick cycles and avoid pasture reinfestation by timing treatments strategically.

Besides the knowledge of tick biology, a key of this approach is sharp understanding of herd management on each farm that implements this tick control strategy. It is essential to take time to identify all the points where tick dispersion can occur — i.e., disposition of pastures, livestock movements within the farm and between plots — in order to adapt corrective measures to each farm. These measures may require investment but, in this study, these results are obtained without any substantial financial investment for the farmers and are based only on the registration of herd rotations and observations of tick infestations, strategic acaricide use and minor adaptations of herd management. Changing for non-chemical methods to control ticks is always a great challenge, especially when acaricides are free and still efficient. These alternative methods are always more time-consuming and more constraining. In this study, these two farmers are deeply attached to this tick-susceptible breed, acaricide resistances are more and more extended in Caledonian farms, and a beginning of amitraz resistance was observed in these two farms — even if it can still be used and mainly efficient. They thereby considered this method as a last chance to maintain this breed. Caledonian farmers are now divided into two categories: those who have changed for tick-resistant cattle and those who want to keep their tick-susceptible breeds, leave to change their cattle and pasture managements.

However, this method faces the challenge of farmers' autonomy in collecting and analyzing data. Farmers should be supported by technical services at least every 1 or 2 months during 2 years. It would be interesting to develop farmers' groups to increase their autonomy and comprehension, and beneficiate from economies of scale regarding the financing of technical support. Today, this method was transferred to the technical services in charge of tick control. In December 2018, 32 farmers were already involved with this strategy.

This approach needs to be tested now in other livestock rearing conditions, intensive vs extensive, dairy farms ..., and be adapted if needed.
**Acknowledgements** The authors would like to thank Frédéric Stachurski, Ann Demburg, and Chloé Lafleur for their critical reading of the text.

## Compliance with ethical standards

**Conflict of interest** The authors declare that they have no conflict of interest.

## References

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Harley, K.L.S., 1966. Studies on the survival of the non-parasitic stages of the cattle tick *Boophilus microplus* in three climatically dissimilar districts of north Queensland. Australian Journal of Agriculture Research, 17(3), 387–410.

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<figure><img src="image_45.png" /></figure>
# Integrated control of the cattle tick, *Rhipicephalus australis* (Acari: Ixodidae), in New Caledonia through the Pasture and Cattle Management method

Thomas Hüe<fnref n="1" /> · Anna Berger<fnref n="1" /> · Hsiao-Hsuan Wang<fnref n="2" /> · William E. Grant<fnref n="2" /> · Pete D. Teel<fnref n="3" /> · Adalberto A. Pérez de León<fnref n="4" />,<fnref n="5" />

Received: 25 April 2021 / Accepted: 22 June 2021
© The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature 2021

## Abstract

Development of the Pasture and Cattle Management (PCM) method is a priority to control the cattle tick, *Rhipicephalus australis*, in New Caledonia. The PCM method provides the foundation for sustainable integrated tick control because approximately 95% of cattle ticks in infested pastures are off the host in the non-parasitic life stages, and the practice of treating cattle intensely with chemical acaricides is a risk for the emergence of resistance to these active ingredients in commercial acaricidal products available for veterinary use. Here, we report the findings of an assessment survey to document the utility of the PCM method. Analyses of questionnaire data provided by 21 beef cattle producers describing their management of 37 herds informed how to (1) assess the ability of PCM to reduce acaricide use and (2) prioritize best practices and define recommendations to breeders promoting efficient tick control with minimum acaricide use. Boosted regression tree analysis showed a significant (p=0.002) reduction of ≈33% in the number of acaricide treatments from 7.9 to 5.3 per year by using PCM. Of the 24 factors identified as potentially affecting acaricide use, six factors accounted for ≈86% of the variability in number of acaricide treatments applied annually. The six most influential factors involved farm characteristics as well as pasture and herd management recommendations. These results demonstrated the usefulness of PCM for integrated control of *R. australis* infestations while reducing acaricide use to improve cattle production in New Caledonia.

**Keywords** Cattle tick · *Rhipicephalus australis* · Pasture and Cattle Management method · Integrated tick control

## Introduction

Intense acaricide treatment schedules in New Caledonian cattle farms resulted in the detection of *R. australis* populations resistant to pyrethroids (Beugnet and Chardonnet

1995) and amitraz by the end of the last decade (Chevillon et al. 2007). By 2014, 23.3% of beef cattle producers were faced with tick strains resistant to amitraz (Petermann et al. 2016). Historically, the French cattle breeds Limousine and Charolais have been the most commonly raised for beef production in New Caledonia (Bianchi and Barre

Section Editor: Boris R. Krasnov

<figure><img src="image_46.png" /></figure>

Thomas Hüe
hue@iac.nc

Hsiao-Hsuan Wang
hsuan006@tamu.edu

William E. Grant
wegrant@tamu.edu

Pete D. Teel
ptee1@tamu.edu

Adalberto A. Pérez de León
beto.perezdeleon@usda.gov

<footnote n="1">¹ Laboratoire de Parasitologie Animale, Institut Agronomique néo-Calédonien (IAC), BP73 Païta, New Caledonia</footnote>

<footnote n="2">² Ecological Systems Laboratory, Department of Ecology and Conservation Biology, Texas A&M University, College Station, TX, USA</footnote>

<footnote n="3">³ Department of Entomology, Texas A&M AgriLife Research, College Station, TX, USA</footnote>

<footnote n="4">⁴ Agricultural Research Service, Knipling-Bushland U.S. Livestock Insects Research Laboratory and Veterinary Pest Genomics Center, USDA, 2700 Fredericksburg Rd., Kerrville, TX 78028, USA</footnote>

<footnote n="5">⁵ USDA-ARS San Joaquin Valley Agricultural Sciences Center, 9611 S. Riverbend Av., Parlier, CA 93648, USA</footnote>
2003). Since the beginning of this century, it has been rec-
ommended to treat cattle when they are first observed to be
infested with R. australis in efforts to reduce the number of
acaricide treatments per year (Barre and Delathiere 2010).
However, Limousine and Charolais cattle are susceptible to
heavy parasitism by R. australis, which requires that herds
of these breeds in New Caledonia be treated frequently with
acaricides to which the ticks are susceptible. Strategies that
integrate other management methods are needed for sustain-
able control of this cattle tick species.

In New Caledonia, the livestock producer association Groupement de Défense Sanitaire Animal (GDSA) has oversight of tick management since 2010. Cattle producers currently use primarily amitraz (Taktic EC®, Virbac or Amitik WP®, Coopers Animal Health, Ltd.) for tick treatments applied throughout the year. Short-acting macrocyclic lactones, with less than a week of residual activity, are used once or twice a year when they want to drench animals. Producers also have access to long-acting acaricides (LAA) (Ivermectin, Ivomec Gold®, Boehringer Ingelheim or Fluazuron, Acatak®, Novartis), but treatments using these chemicals are applied by the GDSA and the number of treatments with these products is limited to one or two per year. Long-acting products have an acaricide activity against tick larvae for several weeks and are particularly useful to collect and kill larvae in pastures (Hüe, 2019).

Integrated tick management involves the combination of methods that can reduce acaricide treatments (Pérez de León et al. 2014). Alternatives for acaricide use in integrated cattle tick management include the use of an anti-tick vaccine, pasture management, and the use of tick-resistant cattle breeds. Cattle can be selected and bred for tick-resistant traits, which has been attempted by selecting and breeding tick-resistant bulls, such as Brahman or Brahman crossbreed, with susceptible cows (More et al. 2019). However, in New Caledonia, some beef cattle producers are reluctant to change their cattle as Brahman cattle or crossbreeds are known to be less fertile (Frisch et al. 1987). Additionally, genetic variability in the resistance of crossbred cattle can result in tick infestations that do not meet expected control levels without the use of acaricides (Utech et al. 1978; Sutherst et al. 1983). An anti-R. australis vaccine developed locally is being tested in New Caledonian beef cattle farms (Hüe et al. 2017). Consideration of pasture management as part of integrated tick control strategies requires attention because approximately 95% of cattle ticks in infested pastures are off the host in the non-parasitic life stages including gravid females about to oviposit, eggs, and host-seeking larvae (Leal et al. 2018).

Removal of bovines from an infested pasture long
enough, usually 4 to 6 months, for the resident ticks to
expire is among the methods that can be used for tick
control (Harley and Wilkinson 1964; Pound et al. 2010;
Wang et al. 2020a) but it is inconsistent with a good

forage quality management. The optimum in terms of
quality and quantity of tropical grasses under good condi-
tions is indeed between 4 and 6 weeks (Salas et al. 1994).
Advances in knowledge of tick developmental biology
combine to information about rotations of herds among
pastures in New Caledonia were applied to develop the
PCM method with the objective of minimizing the use of
acaricides by applying treatments strategically and opti-
mize the forage resources (Hüe and Fontreyde 2019).
Considering the preliminary results from a pilot study
involving two cattle farms where application of the PCM
method decreased the number of acaricide treatments by
as much as 82.9% (Hüe 2019), the method was transferred
to the GDSA technical services team for further testing in
a larger number of farms. Here, we present results of the
scale-up study that aimed to (1) assess the ability of PCM
to reduce acaricide use and (2) prioritize best practices and
define recommendations to breeders promoting efficient
tick control with minimum acaricide use.

**Materials and methods**

Area description

New Caledonia is a French island in the South Pacific Ocean with an oceanic tropical climate. Cattle-producing operations in the West Coast of the island, where annual rainfall ranges from 600 to 1200 mm, were monitored in this study. Daily mean temperatures range between 27 and 28 °C during the hot season, and around 20-22 °C during the cold season.

Adoption of the pasture and cattle management
method

The PCM method developed in 2015 by the Agronomic
Institute of New-Caledonia is used in farms exposed to
heavy tick infestations (Hüe and Fontreyde 2019). This
method requires an initial investigation of the farm to iden-
tify the main high-risk management practices contributing
to tick infestations. Once this diagnosis is completed, cor-
rective measures to decrease tick pressure in pastures, and
thereby on animals, are proposed and discussed with the pro-
ducers. A farm-specific PCM plan is developed with correc-
tive measures allowing minimal strategic use of acaricides
based upon knowledge of tick developmental biology, for-
age management considerations, and feasibility of moving
herds among pastures (Hüe 2019). Cattle and pastures are
regularly monitored and adjustments made throughout the
year considering observations on ticks and forage resources.

<figure><img src="image_47.png" /></figure>
Assessment survey

The PCM assessment survey was conducted between April and June 2019. Twenty-one producers who had used the PCM method for more than 1 year were included. Producers with multiple herds completed an assessment for each herd, which resulted in a total of 37 PCM assessments. For each herd, producers filled out two questionnaires, one before the implementation of the method, and one after. On the first questionnaire, producers reported the number of acaricide treatments applied during the year before the PCM began. Depending on the farm, the PCM could have been in place for up to 4 years. On the second questionnaire, producers reported the number of acaricide treatments applied during the last 12 months (May 2018–April 2019).

Questionnaire

Questions were divided into three groups, including questions related to (1) the main characteristics of the farm (F); (2) herd nutrition (N) and risk of starvation for cattle during the dry season; and (3) recommendations given by GDSA regarding pasture and herd management (M) (Table 1).

The efficacy protection period of long-acting acaricides equivalent to three short-lasting acaricides was considered to compare the number of treatments between short- and long-acting acaricides for purposes of this study. This consideration was based on the residual activity of short- and long-acting acaricidal products as null and 6 weeks, respectively (Gomes et al. 2015; Barré and Delathière 2010). We also accounted for an average of 3 weeks for the development of ticks on cattle estimating minimum delays to retreatment

<table><caption>**Table 1** Codes, descriptions, and response modes of variables included in the survey questionnaires as potential factors affecting the number of acaricide treatments applied annually. Double asterisks (**) indicate the six most influential factors and single asterisks (*) indicate factors also included in the final boosted regression tree model</caption><thead><tr><th>Code</th><th>Variable</th><th>Variable response modes</th></tr></thead><tbody><tr><td colspan="3"><strong>Farm description</strong></td></tr><tr><td>F1**</td><td>Rainfall (mm)</td><td>Continuous</td></tr><tr><td>F2</td><td>Good external fences</td><td>Yes/no</td></tr><tr><td>F3**</td><td>Good internal fences</td><td>Yes/no</td></tr><tr><td>F4*</td><td>Number of parcels available to herd</td><td>Continuous</td></tr><tr><td>F5**</td><td>Percentage of tick-resistant cattle in herd (more than 50% of indicus blood per animal)</td><td>Continuous</td></tr><tr><td>F6**</td><td>Status of amitraz resistance (lethal dose for 99% kill; LD<sup>99</sup>)</td><td>Continuous</td></tr><tr><td>F7*</td><td>Frequency of herd observation > 2 times per week</td><td>Yes/no</td></tr><tr><td>F8</td><td>Observation of wild deer in cattle pastures (&gt; 20 head)</td><td>Yes/no</td></tr><tr><td colspan="3"><strong>Herd nutrition</strong></td></tr><tr><td>N1*</td><td>Cows with calves during dry period (October–December)</td><td>Yes/no</td></tr><tr><td>N2</td><td>Stocking rate</td><td>Continuous</td></tr><tr><td colspan="3"><strong>Pasture and herd management recommendations</strong></td></tr><tr><td>M1*</td><td>Acaricide treatment as soon as producer discovers a tick</td><td>Yes/o</td></tr><tr><td>M2</td><td>Account for tick-infested parcels for the pasture rotation</td><td>Yes/no</td></tr><tr><td>Code</td><td>Variable</td><td>Variable response modes</td></tr><tr><td>M3*</td><td>Parcels dedicated to each herd when several herds on farm</td><td>Yes/no</td></tr><tr><td>M4</td><td>Areas common to several herds (water point, corridors...)</td><td>Yes/no</td></tr><tr><td>M5</td><td>Herd movement in the afternoon</td><td>Yes/no</td></tr><tr><td>M6</td><td>Crossing parcels to go to the stock-yard</td><td>Yes/no</td></tr><tr><td>M7</td><td>Fences opened during dry period</td><td>Yes/no</td></tr><tr><td>M8**</td><td>Ticks observed during the month before long-acting acaricide (LAA) treatment</td><td>Yes/no</td></tr><tr><td>M9</td><td>LAA planned according to the rotation calendar</td><td>Yes/no</td></tr><tr><td>M10</td><td>Rotations after the LAA planned according to the rotation calendar</td><td>Yes/no</td></tr><tr><td>M11**</td><td>Duration of monitoring efforts</td><td>Continuous</td></tr><tr><td>M12*</td><td>Use of "strategic" treatments</td><td>Yes/no</td></tr><tr><td>M13</td><td>Corridors and mustering point favorable to ticks (grassy areas)</td><td>Yes/no</td></tr><tr><td>M14</td><td>Cattle left in the stock-yard during half a day after dipping</td><td>Yes/no</td></tr><tr><td>M15</td><td>Regularly infested animals culled</td><td>Yes/no</td></tr></tbody></table>
after short- and long-acting acaricide treatments to be 3 and
9 weeks, respectively.

Analyses of questionnaire data

Boosted regression trees were used for analyses of the poten-
tial factors affecting the number of acaricide treatments
applied annually (Table 1). This approach was combined
with decision trees and a boosting algorithm with a Pois-
son distribution (Friedman, 2002). For boosted regression
trees, the number of acaricide treatments (μ) for each herd
with the potential variables (X', including three groups of
variables) is given by logμ = α + βX'. We fitted our model
in R 3.6.0 (R Core Team 2017) using the gbm package
version 1.5-7 (Ridgeway 2006). The optimal model was
determined following the recommendations of Elith et al.
(2008) by altering the learning rate and tree complexity,
i.e., the number of split nodes in a tree, until the predictive
deviance was minimized without over-fitting, and by limit-
ing our choice of the final model to those that contained at
least 1,000 trees. Once the optimal combination of learn-
ing rate and tree complexity was found, model performance
was evaluated using a ten-fold cross-validation procedure
with re-substitution. For each cross-validation trial, 60%
of the dataset was randomly selected for model fitting and
the excluded 40% was used for testing. We calculated the
response variance explained and the area under the receiver
operator characteristic curve (AUC). We evaluated the relia-
bility and validity of our models as fair (0.50 < AUC ≤ 0.75),
good (0.75 < AUC ≤ 0.92), very good (0.92 < AUC ≤ 0.97),
or excellent (0.97 < AUC ≤ 1.00) based on the value of
AUC (Hosmer and Lemeshow 2000). We then used the
gbm library to derive the relative influence of each potential
explanatory variable in the model and constructed partial
dependency plots for the most influential variables.

Results

The mean number of tick treatments was 7.9 during the year before implementation of PCM. Tick treatments decreased to 5.3 during the 12 months covered by the assessment survey. This represents a statistically significant (p = 0.002) decrease of 32.9%.

Analyses of 500 combinations of tree complexity (rang-
ing from 3 to 7) and learning rates (ranging from 0.001 to
0.01) produced models including between 300 and 4450
trees whose values of predictive deviance ranged from
1.757 to 1.964. The optimal model had a tree complex-
ity of 5, a learning rate of 0.001, and a total of 1950 trees.
Model predictive deviance was 1.814 ± 0.292 with 85.3%
of the total response variance explained. The AUC score
was 0.845 ± 0.0732 ("good" ability to estimate number

of acaricide treatments). Overall accuracy was 86.61%.
Recursive feature elimination tests showed that thirteen of
the variables in Table 1 could be removed from the model
before the resulting predictive deviance exceeded the initial
predictive deviance of the model with all variables.

Twelve variables were included in the final model (Fig. 1). Examination of the relative contribution of the predictor variables indicated that the top six accounted for approximately 85.50% of the contribution in the overall model. Of the six most influential model variables, four represented farm characteristics (F1, 5, 6, and 3, in order of declining influence) contributing 55.37%; two represented recommendations given regarding pasture and herd management (M11 and 8, in order) contributing 30.13%.

Partial dependency plots for the twelve most influential
variables are depicted in Fig. 2. Lower numbers of acari-
cide treatments were associated most with farm character-
istics representing (1) local rainfall (F1) below 850 mm;
(2) the duration of monitoring efforts (M11) between 0.5
and 2.5; (3) proportion of tick-resistant cattle in the herd
(F5) > 70%; (4) status of amitraz resistance (F6) > 0.33; (5)
no ticks observed during the month before LAA treatment
(M8); (6) good internal fences (F3); (7) number of parcels
available to herd (F4) > 7; (8) acaricide treatment as soon as
producer detects a tick (M1); (9) parcels dedicated to each
herd when several herds on farm (M3); (10) use of "stra-
tegic" treatments (M12); (11) no cows with calves during
dry period (N1); and (12) frequency of herd observation
per week (F7) > 2.

Discussion

In this study, we analyzed 24 variables that may affect lev-
els of acaricide use by cattle producers in New Caledonia.
Except for rainfall (F1) and acaricide resistance (F6), all
these variables can be managed through informed decisions

<figure><img src="image_48.png" /><figcaption>**Fig. 1** Relative contributions (%) of the twelve most influential vari-
ables included in the final model. See Table 1 for variable codes and
descriptions</figcaption></figure>

<figure><img src="image_49.png" /></figure>
<figure><img src="image_50.png" /><figcaption>**Fig. 2** Partial dependency plots for the explanatory variables included in the final boosted regression tree models based on analyses of the twelve most influential variables. The y-axis represents the logit scale</figcaption></figure>

used for the indicated variable, and hash marks at the top of each plot indicate deciles of the indicated variable

based on scientific evidence. Environmental/biological and management aspects of the 12 factors our analysis identified as most influential are discussed below.

## Rainfall (F1)

Annual rainfall ranged from 465 to 1102 mm among the various farms during the survey. Producers with farms receiving more than 850 mm of rain were more likely to treat than producers with farms in areas receiving less rainfall, that is 6.7 versus 5.89 times respectively. Climatic factors such as temperature, humidity, and rainfall are abiotic factors that regulate the development and survival of the off-host stages of *Rhipicephalus* ticks parasitizing cattle (De Barros et al. 2017; Cruz et al. 2020; Wang et al. 2017). Temperature affects the length of non-parasitic life stages and thus the number of tick generations per annum (Desquesnes and Vignon 1987; De Barros et al. 2017; Wang et al. 2016), whereas rainfall correlates stronger with larval density (Jones and Kitron 2000). In areas with annual

rainfall below 500 to 760 mm, cattle ticks are absent or rare (Yeoman et al. 1967; Wilkinson 1970; de Vos 1979). Desiccation of tick eggs due to low rainfall results in larval mortality and accounts for a large proportion of the variation in the number of off-host larvae (Sutherst and Bourne 2006). During the driest period in areas receiving 635 mm rainfall annually, Harley (1966) noted that many cattle ticks failed to reproduce. In semiarid conditions, with annual rainfall from 220 to 819 mm, Barros et al. (2017) observed that, while almost 100% of the engorged females started oviposition, most often no larvae hatched from these eggs and, when larvae did hatch, the mean hatching percentage was most often < 50%. The number of egg masses that yielded larvae, and the mean hatching percentage of the egg masses that yielded larvae, both were positively correlated with rainfall.

## Duration of monitoring efforts (M11)

Producers that monitored for more than 2 years during the study period treated less (5.94 times per year) than those
who monitored for less than 2 years (6.75 times per year). When producers monitored for more than 2 years, the number of acaricide treatments decreased to 5.32 times per year. Duration of monitoring efforts indicates the level of interest in understanding tick population dynamics on the farm. Even if an initial farm inspection leads to correction of some pasture and herd management practices, e.g., replacing inefficient fences, efficacy of some recommendations can only be evaluated after several months of monitoring. Stock-yard extensions and corridors are typically locations where cattle can be infested. They are infested each time they are visited by tick-infested cattle for acaricide treatment, and depending upon amount of grass cover and season, they may represent ideal tick refuges and become sources of reinfestation each time cattle use them. It can be a long and potentially expensive operation for producers to correct all the high-risk tick practices initially identified. Continuous monitoring facilitates identification of problems and prioritization of management efforts and investments to enhance farm biosecurity (Gerry and Murillo, 2019).

## Percentage of tick-resistant cattle in herd (F5)

Producers with herds composed of more than 70% tick-resistant cattle treated less (5.91 times per year) than those who had less than 70% tick-resistant cattle (6.81 times per year). The Caledonian herd was initially composed mainly of European *Bos taurus* breeds (Limousine and Charolais) until the end of the 1990s. Due to the emergence of acaricide resistance, Brahman bulls were introduced in the early 2000s, followed by other tick-resistant breeds, such as Senepol (2007), Droughtmaster (2010), and Belmont Red (2013) (Hüe 2019). Most Caledonian farms now are breeding crossbreeds, mixed from European *Bos taurus* and tick-resistant breeds. The genetic transition to tick-resistant herds is not always completed for different reasons. First, it takes at least 7 years to change from a tick-susceptible herd to a tick-resistant herd. But this genetic transition comes also with negative effects and producers are reluctant to completely change their herd. Indeed, Zebu breeds are known for their relatively low fertility rates compared to fertility in *Bos taurus* breeds (Frisch et al. 1987). European *Bos taurus* cattle are valued for their meat conformation and maternal instincts. For all of these reasons, Caledonian producers still keep some Limousine or Charolais in their herd. The aim is to find an equilibrium between the overall tick resistance of the herd and herd productivity. Tick resistance of different breeds is well documented but, to our knowledge, there is no information about the ratio required to allow reduction in number of tick treatments when breeders are mixing susceptible and resistant breeds. Thus, although reaching 70% tick-resistant cattle may take time, it seems the minimum objective if producers want to decrease the number of treatments.

## Amitraz resistance (F6)

Producers with herds in which the LD⁹⁹ of amitraz was lower than 0.33 treated more (7.16 times per year) than those with herds in which the LD⁹⁹ was higher than 0.33 (5.62 times per year). This result seems counter intuitive as it is further suspected that producers treat more frequently when acaricide resistance appears and products are less efficient. However, this may be explained by the fact that producers faced with resistance have to change their strategy and not rely only on acaricide treatments for tick control. We usually observed a genetic transition from susceptible to resistant cattle in farms where acaricide resistance occurred early. Thus, producers in these farms with herds having a high level of acaricide resistance ended treating less than those in farms with herds infested with ticks susceptible to chemical acaricide treatments.

## Ticks observed during the month before LLA treatment (M8)

Producers who observed ticks during the month before the long-lasting acaricide treatment treated more (8.58 times per year) than those who did not observe ticks (8.13 times per year). This result confirms the conclusion of Hüe et al. (2020), who reported the period between LAA treatment and the following tick treatment was 11.2 ± 5.3 weeks if ticks were observed the day of the acaricide application and 18.5 ± 11.7 weeks if ticks were not observed. In New Caledonia, the period between dropping-off of engorged ticks and hatching of eggs ranges from 6 to 10 weeks depending on season (Desquesnes and Vignon 1987). Since the efficacy of LAA treatment is less than 6 weeks, if engorged ticks have fallen into the paddock just before LLA treatment, the acaricide may not be active when larvae hatch in the paddock (Hüe et al. 2020). Therefore, animals will need to be treated more often. This highlights an interesting point regarding the preferential use of long-lasting acaricides to decrease tick populations in paddocks rather than to treat animals against ticks. When ticks are observed on cattle, it is advisable to treat animals with an efficient short-lasting acaricide and delay use of the long-lasting acaricide to enhance the role of cattle in collecting and killing larvae in pastures.

## Good internal fences (F3)

Producers having good internal fences in their farm treated less (5.92 times per year) than those who did not (7.91 times per year). Pasture management to control ticks requires efficient internal fences to allow herd rotations. Thus far, herd rotations to control ticks have been studied from two perspectives: pasture spelling and comparison between rotational and continuous grazing. Pasture spelling is based

<figure><img src="image_51.png" /></figure>
on natural mortality of off-host tick larvae over time (Harley and Wilkinson 1964; Wharton et al. 1969; Wilkinson 1957, 1964). Off-host larvae could survive up to 4.5 months (Wharton et al. 1969). Depending on the season, under Caledonian climatic conditions, off-host larvae could still be present in a pasture 3 to 5 months after engorged ticks are observed on animals (Desquesnes and Vignon 1987). However, pasture spelling of this duration is not compatible with good forage management in New Caledonia, where a grazing rotation cycle of 25 to 35 days provides appropriate utilization of the local tropical forage (Correa and Santos 2003; Salas et al. 1994).

Different grazing systems often have been studied using
mathematical models (Teague et al. 2015; Wang et al. 2020b),
and some models highlight the interest in rotational versus
continuous grazing to decrease tick infestations (Corson
et al., 2003; Hernandez et al. 2000; Teel et al. 1997; Teel
et al. 1998). However, Nicaretta et al. (2020), in a field study
conducted in Brazil, observed that cattle in a rotational graz-
ing system required more tick treatments than those in a con-
tinuous grazing system. In that study, cattle were rotated to a
new paddock every day and each paddock was rested (without
animals) for 20 days. On the farms surveyed in our study,
rotational grazing was conducted within the global pasture
management scheme described by Hüe (2019). This scheme
includes a strategy to prevent infested cattle from dropping
ticks in multiple pastures, and to prevent unintentional move-
ment of cattle into tick-infested pastures. Thus, although
replacing old fences may be particularly expensive, our study
underscores the value of a such an investment.

Number of parcels available to herd (F4)

Producers with less than 7 paddocks per herd treated more (6.93 times per year) than those with more than 7 paddocks per herd (5.06 times per year). This result is consistent with the conclusions of Hernández-A et al. (2000) who observed that the density of questing larvae encountered by cattle decreased as the number of pastures increased. Correlation between a decrease in the number of tick treatments with increasing numbers of parcels per herd can be explained in the context of our comments related to rotational grazing and pasture vacation in the previous section. Increasing the number of paddocks into which cattle can be rotated facilitates avoidance of infested paddocks and also permits more lengthy pasture vacation periods. This would increase off-host larval mortality. Given a sufficient number of paddocks, a particularly interesting possibility during the hot and rainy season when forages are abundant is to remove an infested paddock from the rotation program and use it to produce hay for the dry season. When many paddocks are infested, a LAA treatment can be planned to decrease tick

populations with treated cattle. But, as presented above, it
is important to treat with LAA when ticks are not observed
during the month before the treatment. For this purpose,
having more than 7 paddocks helps to select tick-free pas-
tures where cattle can stay before a LAA treatment. If new
fences are required to increase the number of parcels avail-
able to the herd, as mentioned in the previous section, this
may be particularly expensive, but our study underscores the
value of a such an investment.

Acaricide treatment as soon as breeder sees a tick
(M1)

Producers treating as soon as they see ticks on cattle treated more (7.24 times per year) than producers who tolerated some ticks on animals before treating (5.81 times per year). The initial strategy of tick eradication in New Caledonia using treatments every 2 weeks or monthly was implemented during decades but never succeeded and led to the development of acaricide resistance. The strategy changed from systematic monthly treatments to treatment “at sight” (when ticks are observed) at the end of the 1990s, when deltamethrin began to fail (Hüe 2019). The aim is now to control tick population to allow beef cattle production. This change was made to delay the development of acaricide resistance. Today, some producers are treating as soon as they see ticks, whereas others can tolerate a few ticks before treating. It mainly depends on whether or not they have had some cattle die due to tick infestations. The strategy to treat “at sight” does not necessarily lead to eradication (Wang et al. 2019). Producers using this strategy usually continue to treat frequently. Likewise, the option to treat only if tick infestations are moderate or severe does not necessarily lead to development of high tick populations in paddocks, and producers who have used this strategy for several years are not treating more today.

Parcels dedicated to each herd when several herds
on farm (M3)

Producers with several herds and different herds using com-
mon paddocks treated more (7.20 times per year) than pro-
ducers with different paddocks dedicated to each herd (5.98
times per year). This was mainly due to cross-infestations
between/among herds. When animals have engorged ticks,
they infest the paddock where they are, and if another herd
shares the same paddock, the tick population will infest this
second herd. This can create a "domino effect" as each herd
regularly contaminates another, resulting in the need to
increase the number of tick treatments. It was also observed
that, when producers use dedicated parcels for each herd,
cattle do not need to use common corridors or cut across par-
cels when being moved to the dipping area, which can result

<figure><img src="image_52.png" /></figure>
in rapid decreases in tick infestations in the different herds. Having a parcel dedicated to each herd may require new fences. Once again, as mentioned in previous sections, this may be particularly expensive, but our study underscores the value of such an investment.

## Use of "strategic" treatments (M12)

Producers who used strategic treatments treated less (6.17 times per year) than producers who did not (6.87 times per year). Planning strategic treatments is a way to interrupt the tick life cycle and avoid reinfestations of paddocks. When the herd is introduced into a paddock where tick larvae are suspected to be present, based on the monitored history of the paddock, the recommendation is to closely inspect the cattle 2 to 2.5 weeks after their introduction. If ticks are found, a treatment can be applied before the ticks complete their cycle and reinfest the paddock.

## Cows with calves during dry period (October–December) (N1)

Producers who let calves suckle cows during the dry period treated more (6.80 times per year) than producers who did not (5.10 times per year). Depending on reproduction management, some producers leave bulls with the cows all year, while other producers leave bulls with the cows only during a few months, such that the calving period coincides with the period when forages are abundant. This second option requires a separate paddock for the bulls during part of the year, which is not always possible on small farms. Moreover, income is restricted to the few months when calves are ready to be slaughtered. When bulls are with the cows all year, births occur regularly and calves can be on pasture with suckling cows during the dry season. The presence of calves impacts tick infestations on two levels. First, lactating cows are known to be more susceptible to tick infestations (Siefert 1971; Sutherst et al. 1988). Second, calves graze fodders that are insufficient in quantity and quality during the dry season. As shown by Sutherst et al. (1983), animals with a poor nutritional diet may have up to 5 times more ticks than cattle correctly fed. This should encourage producers who use a seasonal breeding strategy to decrease the stocking rate during the dry season to preserve cattle resistance to ticks.

## Frequency of herd observation per week (F7)

Producers who observed their herd less than twice per week treated more (7.88 times per year) than producers who observed their herd more than twice per week (6.14 times per year). Many Caledonian cattle producers keep another

job besides their livestock activity to remain economically viable. They do not always live on the farm and spend only the weekends working their cattle herds. In such cases, if tick infestation occurs, cattle are not treated until the weekend. During such delays, paddocks can be highly and regularly infested. This distinction between producers who are able to observe their cattle frequently and those that can tend their herds during weekends only highlights the need to adopt the PCM method to control *R. australis* populations sustainably in New Caledonia.

## Conclusion

This study demonstrated the ability of PCM to control cattle tick infestations in New Caledonia while reducing acaricide use. However, it is recognized that if followed, PCM recommendations to producers regarding modifications of farm characteristics, herd nutrition, as well as the details of pasture and herd management per se require more time and effort than traditional management approaches. The implementation of some of these recommendations also requires substantial financial investments, particularly those involving fencing modifications. This requires further investigations to assess the return on investment, which will provide the financial evidence for producers to make informed agribusiness decisions (Hurtado and Giraldo-Ríos 2019; Wang et al. 2019). Our study highlights the need for partnerships based on trust between technical advisors and producers to maximize the practice of the PCM method to advance integrated *R. australis* management.

**Acknowledgements** The authors would like to thank the technicians of the CANC/GDSA for their help in data collection. Mention of trade names or commercial products in this publication is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the authors. USDA is an equal opportunity provider and employer.

**Author contribution** Thomas Hüe: conceptualization, methodology, writing (original draft), supervision. Anna Berger: investigation and resources. Hsiao-Hsuan Wang: methodology, software, formal analysis, and writing (original draft). William E. Grant: methodology, validation, and writing (review and editing). Pete D. Teel: validation and writing (review and editing). Adalberto A. Pérez de León: validation and writing (review and editing).

**Funding** The research of T. Hüe was funded by the 2017–2021 Development Contract with contributions from the French Government, the New Caledonian Government, and the South, North, and Loyalty Islands Provinces. H.-H. Wang was partially supported by USDA-ARS and USDA-APHIS projects 58–3094-8-010 and 58–3094-9-016. The research of A.A. Pérez de León was funded through USDA-ARS appropriated project number: 3094–32000-042-00-D.

<figure><img src="image_53.png" /></figure>
**Data availability** Not applicable.

**Code availability** Not applicable.

**Declarations**

**Ethics approval** Not applicable.

**Consent to participate** Not applicable.

**Consent for publication** Not applicable.

**Competing interests** The authors declare no competing interests.

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# Optimizing long-acting acaricide use for integrated tick management of Rhipicephalus australis-infesting cattle in New Caledonia

Thomas Hüe<fnref n="1" />, Chloé Fontfreyde<fnref n="2" />, Hsiao-Hsuan Wang<fnref n="3" />, William E. Grant<fnref n="3" />, Pete D. Teel<fnref n="4" />, Adalberto A. Pérez de León<fnref n="5" />

Received: 3 March 2021 / Accepted: 18 June 2021
© The Author(s), under exclusive licence to Springer Nature B.V. 2021

## Abstract

The tick *Rhipicephalus australis*, formerly known as *Rhipicephalus microplus*, is the most economically important ectoparasite of livestock in New Caledonia, affecting cattle health and production. Decades of control attempts based on the application of chemical acaricides have exerted a strong selective pressure on *R. australis* populations, some of which have evolved resistance to these treatments. Research to develop integrated tick control programs is now focused on decreasing applications of chemicals. This study reports the implementation of a method of pasture and herd management involving minimal strategic use of long-acting acaricides, here defined as those having substantial efficacy against larvae for several weeks. Diverse parameters concerning the utilization of long-acting acaricides in association with pasture and herd management on 21 New Caledonian farms over a 5-year period were analyzed to optimize their strategic use. Longer larval acaricidal effect was achieved with a commercial pour-on formulation of fluazuron than with a commercial injectable (subcutaneous) formulation containing 3.15% ivermectin. Pasture and herd management allowed an increase in the delay between a long-lasting acaricide application and the subsequent acaricide treatment from 11.0 weeks to 17.7 weeks. However, if ticks were detected and reported by producers on the day of a long-acting acaricide application, the delay to the following treatment was reduced from 18.5 weeks to 11.2 weeks. The impact of a long-acting acaricide treatment on larval populations in pastures was greatest with a stocking rate of 5 animals per hectare grazing during 1 week. These results provide science-based evidence to cattle producers for adaptive integrated tick management in order to delay the development of acaricide resistance.

**Keywords** Cattle tick · *Rhipicephalus australis* · Long-acting acaricides · Integrated tick management · New Caledonia

## Introduction

Ticks are economically important ectoparasites of cattle that affect their health and production directly through their parasitic lifestyle and indirectly as vectors of

pathogens causing animal diseases, several of which are zoonotic and of veterinary public health relevance (Pérez de León et al. 2020). In New Caledonia, the cattle tick *Rhipicephalus australis*, previously known as *Rhipicephalus microplus* (Estrada-Peña et al. 2012), causes major health

<figure><img src="image_54.png" /></figure>

Thomas Hüe
hue@iac.nc

Chloé Fontfreyde
cfontfreyde@canc.nc

Hsiao-Hsuan Wang
hsuan006@tamu.edu

William E. Grant
wegrant@tamu.edu

Pete D. Teel
pteel@tamu.edu

Adalberto A. Pérez de León
beto.perezdeleon@usda.gov

Published online: 01 July 2021

<footnote n="1">¹ Laboratoire de Parasitologie Animale, Institut Agronomique Néo-Calédonien (IAC), Paita, New Caledonia</footnote>

<footnote n="2">² Groupement de Défense Sanitaire Animal, Chambre D'Agriculture de Nouvelle-Calédonie, Bourail, New Caledonia</footnote>

<footnote n="3">³ Ecological Systems Laboratory, Department of Ecology and Conservation Biology, Texas A&M University, College Station, TX, USA</footnote>

<footnote n="4">⁴ Department of Entomology, Texas A&M AgriLife Research, College Station, TX, USA</footnote>

<footnote n="5">⁵ Agricultural Research Service, USDA, San Joaquin Valley Agricultural Sciences Center, 9611 S. Riverbend Av., Parlier, CA 93648, USA</footnote>
problems among cattle herds (Hüe et al. 2017). Since its
introduction to New Caledonia from Australia during the
second world war, synthetic acaricides have been provided
free of charge to cattle producers by the Caledonian Gov-
ernment to treat cattle infested with R. australis (Bianchi
et al. 2003). However, the intense application of these
types of veterinary drugs to treat infested cattle caused
selected populations of R. australis to become resistant to
them (Beugnet and Chardonnet 1995). Thus, the choice of
acaricides provided by the Government is determined by
the detection of widespread resistance to a chemical class
of veterinary drugs.

Tick management was initially supervised by veterinary
official services, but since 2010, it has been supervised by a
producer association: the Groupement de Défense Sanitaire
Animal (GDSA). Acaricides in a single chemical class were
generally used during a decade before resistance appeared,
after which acaricides from a different chemical class have
been used. Amitraz is an acaricide belonging to the forma-
midine chemical class that has been used by all Caledonian
breeders since 2003. Antiparasitic drugs of short-action con-
taining the macrocyclic lactones ivermectin and moxidectin,
which are used to treat cattle infested with ticks because of
their endectocidal activity (Rodríguez-Vivas et al. 2014),
are also used once a year when cattle breeders want to treat
against internal parasites. A survey conducted in 2014 doc-
umented that 25% of R. australis strains were resistant to
amitraz (Petermann et al. 2016). However, this acaricide is
still used, even in farms where acaricide resistance is known,
and some of these farms were included in the present study.

The tick control program proposed by the GDSA is based
on adaptive integrated management of herds and pastures
to reduce tick infestations, and includes the use of long-
acting acaricides (LAA). Herein, LAA are defined as veteri-
nary drug products with a chemical acaricide as the active
ingredient(s) having lasting efficacy against cattle ticks,
including R. australis larvae, sustained for several weeks
(Davey et al. 2011; Nava et al. 2015). In New Caledonia,
the rational use of LAA has not yet been fully adapted to
enhance integrated tick management and allow use of desir-
able beef-producing European cattle breeds. Thus, resistance
to the widely used acaricides is forcing New Caledonian
cattle producers to change herd genetics away from more
tick susceptible European breeds (Petermann et al. 2016).
The genetic transition of cattle herds in New Caledonia is
accomplished by introducing bulls from breeds with known
tick-resistant traits (Hüe 2019). Within a herd, the complete
transition to a tick-resistant cattle herd occurs over a 5- to
7-year period. LAA are mainly used to help breeders con-
trol tick populations during this herd transition period. In
contrast to non-residual acaricides, LAA can be particularly
useful when treated cattle are grazing in tick-infested pad-
docks and collect and kill tick larvae.

Two LAA currently used in New Caledonia are (1) a
commercial injectable (subcutaneous) formulation con-
taining 3.15% ivermectin, which has been in use since
2005 (note that short-action ivermectin is used at 200 µg/
kg of body weight and the pharmacology of LAA with
ivermectin is 630 µg/kg of body weight), and (2) a com-
mercial pour-on formulation of fluazuron, which has been
used since 2008. These LAA are applied only by GDSA
technicians as part of official management plans. These
products are not accessible over the counter for produc-
ers. Research on the strategic application of LAA in New
Caledonia is needed because their improper use can select
for resistant populations of Rhipicephalus spp. cattle ticks,
as has been documented elsewhere (Rodríguez-Vivas
et al. 2018; Reck et al. 2014). Thus, the integrated use of
LAA with other approaches like pasture management can
enhance biosecurity, for example, against R. australis, by
mitigating the risk of moving cattle infested with acari-
cide-resistant (amitraz-resistant) ticks within and between
farms (Miraballes et al. 2019).

This study reports the analysis of field observations reg-
istered by cattle producers during and after treatments with
LAA. Efficacy data were considered according to the cir-
cumstances under which the LAA were applied. Although
resistance to macrocyclic lactone products used for their
endectocidal activity has not been reported in New Caledo-
nia (Petermann et al. 2016), the information reported herein
provides an empirical basis for reduced and more effective
use of these antiparasitics, which could delay the develop-
ment of resistance not only to ticks but to gastrointestinal
nematodes (Alegría-López et al. 2015). Resistance to fluazu-
ron among R. australis populations remains to be assessed
in New Caledonia.

**Materials and methods**

Study area

New Caledonia is a French island in the South Pacific Ocean with an oceanic tropical climate (Petermann et al. 2016). The cattle population consists of ≈80,000 animals, which are raised under extensive conditions by ≈650 producers (Hüe 2019). Due to poor soil quality, the average stocking rate is ≈0.5 head per hectare. The hot and wet season is usually from late January to April, followed by a cold season from May to August, with possible rain. The dry season is from September to December. Producers monitored in this study are located on the West Coast, where annual rainfall ranges from 600 to 1200 mm. Daily mean temperatures are 27–28 °C during the hot season and 20–22 °C during the cold season.
## Tick and pasture management

The method to control *R. australis* through herd and pasture management was developed by the Agronomic Institute of New Caledonia (IAC) in 2015 as described by Hüe and Fontfreyde (2019). This approach was transferred from IAC to the technicians of GDSA in 2017. At the end of 2019, 33 producers were monitored by the staff of this service. This method requires maintaining a calendar of herd movements among paddocks, as well as detection of on-host ticks and tick treatments within each paddock. Considering the duration of off-host tick development, this allows reliable estimations of when and where tick larvae were present in paddocks. It was thus possible to predict which paddocks would be infested by tick larvae. With this prediction, a LAA treatment was planned to maximize the collection and killing of larvae by treated cattle. The ivermectin (IVM) injectable product (IVOMEC GOLD®, Boehringer Ingelheim Animal Health) and the commercial fluazuron (FLZ) pour-on formulation (Acatak®, Elanco) were used per label instructions.

The policy in New Caledonia is to use LAA as infrequently as possible, and never more than two times per year. When LAA were first used in New Caledonia, their lethal activity on tick larvae was expected to be 6 weeks (Citroni et al. 1999; Ferlat 2004). Once the LAA treatment was completed, the planning of rotations was established by a technician in consultation with the producer to rotate treated cattle among the infested paddocks during the expected period of LAA efficacy. Initially, advice for post-LAA treatment rotations was not always given or was given orally and unevenly followed. However, since 2018, this advice has been transmitted in writing by technicians directly to the producer. After the subsequent tick treatment, technicians collected information about actual herd rotations to determine whether the advised rotation plan had been followed. From January 2015 to December 2019, 85 LAA treatments were recorded on 39 herds present in 21 farms (Table 1). These 85 treatments included the 29 for which rotation plan was written and the 56 for which rotation plan was not specified or specified orally. The choice of the product applied depended on several factors. For example, the long withholding period of the FLZ-LAA on calves was considered when lactating cows were treated. Therefore, this product was avoided if producers planned to slaughter calves in the next 4 months. Furthermore, alternating use between FLZ-LAA and IVM-LAA was considered to delay the development of resistance. If rain was forecasted, injection of IVM-LAA was preferred rather than a pour-on application of FLZ-LAA.

## Indicators of treatment efficacy

The first indicator was the minimal delay between the LAA treatment and the following tick treatment with amitraz.

<table><caption>**Table 1** Characteristics of the 21 New Caledonian farms and 39 herds on which 85 LAA treatments were recorded from January 2015 to December 2019</caption><thead><tr><th></th><th>No. herds per farm<sup>a</sup></th><th>Surface area per herd (ha)<sup>b</sup></th><th>No. parcels per herd<sup>c</sup></th><th>Stocking rate<sup>d</sup></th></tr></thead><tbody><tr><td>Mean</td><td>2.1</td><td>186.8</td><td>5.5</td><td>0.7</td></tr><tr><td>Maximum</td><td>8</td><td>769.0</td><td>13</td><td>2.4</td></tr><tr><td>Minimum</td><td>1</td><td>8.5</td><td>2</td><td>0.2</td></tr><tr><td>Median</td><td>1.0</td><td>111.0</td><td>5.0</td><td>0.6</td></tr></tbody></table>

Breeders were free to decide when they wanted to apply this following treatment. There was no threshold of infestation recommended to treat, and treatments were usually done when adult ticks were detected. The average delay between the LAA treatment and the following tick treatment was then analyzed considering several factors that can influence the reinfestation of animals. These factors included which product was used, whether technical advice was followed by producers, and whether producers observed adult ticks the day of the LAA treatment.

The last indicator was related to the length of grazing time needed to collect a maximum number of larvae in a plot, which depended on the stocking rate. This approach aimed to know how long a herd treated with a LAA had to stay in a paddock to decrease larval infestation low enough to avoid reinfestation of animals when cattle were moved back into the paddock. Assessment of this indicator required that a paddock was pastured during the period of activity of the LAA, and pastured once more by the herd just after the end of the expected efficacy of the treatment. If cattle were treated 2.5 to 3 weeks after their second stay in the pastured paddock, the stocking rate and/or the length of stay was considered insufficient to collect enough larvae during the first stay. If animals were not treated, the stocking rate and the length of stay in the paddock under LAA activity were considered sufficient.

## Statistical analysis

Because the data were not normally distributed and the samples were small, a nonparametric test, Mann-Whitney *U* test, was used to compare (1) the minimal periods between a LAA treatment and the following tick treatments with FLZ-LAA and IVM-LAA; (2) the effect of adaptive herd rotation on the period between LAA application and the following tick treatment; (3) the effect of presence of ticks the day of LAA treatment on the period between LAA application and

<footnote><sup>a</sup>Herd represents a group of cattle that rotate through pastures together; two herds do not rotate in the same pastures</footnote>

<footnote><sup>b</sup>Surface area (hectares) used per herd</footnote>

<footnote><sup>c</sup>Number of fenced pastures through which cattle can rotate</footnote>

<footnote><sup>d</sup>Number of animal per hectare</footnote>
the following tick treatment; and (4) the effect of stocking
rate on length of stay required to collect larval ticks under
LAA. The stocking rate was calculated as the ratio between
the total number of animals and the surface area of the pad-
dock. To compare the different situations, the length of stay
per paddock under LAA activity was divided by seven to
obtain a "week equivalent." A stocking rate equivalent of a
week of pasture was calculated with the following formula:
stocking rate per week = (number of cattle/surface in ha)/
(length of stay in days/7). This value was then aggregated
into 4 classes: >3, >4, >5, >6 head/ha/week. Depending on
whether or not a treatment was necessary for a herd stay-
ing in a certain paddock, the estimation of pasture infesta-
tion for cattle to be exposed to larvae described above was
considered as efficient or not. Percentage of efficiency and
inefficiency for each class was then calculated. All tests were
performed under the α = 0.05 significance threshold.

Results

Minimal period between LAA treatment
and the following tick treatment

Among the total 85 treatments, 38 treatments with FLZ-LAA and 47 treatments with IVM-LAA were registered. Minimal delays between a treatment with FLZ-LAA and IVM-LAA and the following tick treatment were 6 weeks and 7 weeks, respectively (Fig. 1). Since the development of *R. australis* on cattle from larva to adult requires ~3 weeks, it was assumed that cattle were infested by larvae 3 weeks earlier. Thus, the minimal periods of larval efficacy between a treatment with FLZ-LAA and IVM-LAA and the following larval infestation were estimated at 3 weeks and 4 weeks, respectively. However, except for one treatment observed within 6 weeks after an FLZ-LAA application, 11% and 8%

of the following tick treatments occurred 8 and 9 weeks,
respectively, after LAA application. After an injection of
IVM-LAA, 11%, 9%, and 19% of the following tick treat-
ments occurred within 7 weeks, 8 weeks, and 9 weeks,
respectively. An observation of particular biological rel-
evance was that within 9 weeks after initial LAA treatment,
follow-up treatment was required for 21% of the cases after
FLZ-LAA pour-on application and for 38% of the cases
after prior subcutaneous injection of IVM-LAA. However,
the periods between treatment and the following tick treat-
ment within these 9 weeks were not significantly different
between the LAA tested (Mann-Whitney *U* test, *U* = 55,
*z*-score = 0.4539, *P* = 0.3264).

Effect of adaptive herd rotation on period
between LAA application and the following tick
treatment

Beginning in 2018, written technical advice with recom-
mended pasture rotations was provided to producers and
compared afterward with the effective rotations for 29
LAA treatments. Advice was followed for 18 (62.1%) of
these treatments. Some producers did not follow the advice
because it did not correspond to their expectations or
because the forage resources in the pastures did not allow
them to rotate the animals. Or, conversely, some producers
did not want to remove animals from a parcel with unutilized
forage resources just to follow the recommendations. The
average period between the LAA application and the follow-
ing tick treatment was 11.0±5.3 weeks when advice was not
followed and 17.7±12.6 when advice was followed (Fig. 2).
Following recommendations for the rotations after a LAA
application significantly increased the period before the
following tick treatment (Mann–Whitney *U* test, *U* = 51.5,
*z*-score = 2.1125, *P* = 0.0174). This technical advice was
critical to extend the average period until the next treatment

<figure><img src="image_55.png" /><figcaption>**Fig. 1** Distribution of minimal period between initial and the following tick treatment with commercial long-acting acaricides consisting of an ivermec-tin injectable formulation (IVM-LAA) or a pour-on fluazuron formulation (FLZ-LAA)</figcaption></figure>