largest swarm appeared in Zhangmu District (Nyalam County), and another in Jilong County at an altitude of about 1700 m. However, small groups of locusts have also appeared at altitudes above 4000 m (Figure 1, Table 1). How did they manage to climb to such places in the Himalayas? To answer this question, we mainly surveyed Nyalam County, from the lower elevations around 1700 m, up to 4200 m, following the G318 road for about 26 km along the Boqu River valley, passing through Zhangmu and Nyalam towns. This valley opens to the south at the China–Nepal border (Figure 2a) and gradually rises northward from 1700 to 3800 m.

<table><caption>**Table 1.** Monitoring sites and assessment of desert locust numbers along some valleys in South Tibet.</caption><thead><tr><th>Sites</th><th>Altitude (m)</th><th>Locust Density</th><th>Latitude</th><th>Longitude</th></tr></thead><tbody><tr><td>Saga (Jilong valley)</td><td>5400</td><td>Light</td><td>29°19'48"</td><td>85°13'56"</td></tr><tr><td>Zhongba (Zhongba County)</td><td>4800</td><td>Light</td><td>29°46'12"</td><td>84°01'53"</td></tr><tr><td>Jialong Lake</td><td>4750</td><td>None</td><td>28°03'54"</td><td>86°03'43"</td></tr><tr><td>Kuimai lake</td><td>4725</td><td>None</td><td>28°40'52"</td><td>85°56'27"</td></tr><tr><td>Langqiang Lake</td><td>4670</td><td>Very light</td><td>28°43'26"</td><td>85°53'31"</td></tr><tr><td>Puluo pasture</td><td>4200</td><td>Very light</td><td>28°10'39"</td><td>85°56'47"</td></tr><tr><td>Jimuxiong</td><td>3750</td><td>Very light</td><td>28°08'33"</td><td>85°58'17"</td></tr><tr><td>Pulan (Pulan valley)</td><td>3600</td><td>Light</td><td>30°16'05"</td><td>81°10'06"</td></tr><tr><td>Jilong (Jilong valley)</td><td>3530</td><td>Middle</td><td>28°45'01"</td><td>85°21'47"</td></tr><tr><td>4.1</td><td>3380</td><td>Light</td><td>28°06'27"</td><td>85°59'55"</td></tr><tr><td>Qu Town</td><td>3365</td><td>Middle</td><td>28°05'49"</td><td>85°59'46"</td></tr><tr><td>3.2</td><td>3350</td><td>Middle</td><td>28°05'59"</td><td>85°59'50"</td></tr><tr><td>3.1</td><td>3340</td><td>Middle</td><td>28°05'45"</td><td>85°59'46"</td></tr><tr><td>Chentang (Chentang valley)</td><td>3300</td><td>Light</td><td>27°51'39"</td><td>87°25'13"</td></tr><tr><td>Dingri (Gama valley)</td><td>3200</td><td>Light</td><td>28°34'00"</td><td>86°38'00"</td></tr><tr><td>Cuoxiamu</td><td>3150</td><td>Middle</td><td>28°04'37"</td><td>85°59'54"</td></tr><tr><td>2.2</td><td>2500</td><td>Heavy</td><td>27°59'37"</td><td>85°59'8"</td></tr><tr><td>2.1</td><td>2350</td><td>Heavy</td><td>27°59'34"</td><td>85°58'57"</td></tr><tr><td>1.3</td><td>2330</td><td>Very heavy</td><td>27°59'50"</td><td>85°58'47"</td></tr><tr><td>1.2</td><td>2320</td><td>Very heavy</td><td>27°59'50"</td><td>85°58'47"</td></tr><tr><td>1.1</td><td>1950</td><td>Very heavy</td><td>27°58'09"</td><td>85°57'54"</td></tr></tbody></table>

Estimated degree of desert locust density (number of locusts seen per site): Very heavy >10,000 (black), heavy >100 (red), middle 50–100 (brown), light 10–50 (green), very light <10 (light green). Unless otherwise indicated, all sites belong to the Boqu River valley in the Nyalam County.

The largest swarms were observed in Bang and Lixin villages (1700 m) in Zhangmu District, downstream of the Boqu River valley near its opening to Nepal (Figure 1 and Table 1, sites 1.1, 1.2, 1.3). The population size was estimated to be over 0.4 million adults. Locusts rested on both trees (mainly bamboo, but also willow, pine, and fir) and herbaceous vegetation at the base of valley cliffs (Figure 2a,h1–h3). On sunny or slightly cloudy days, locusts formed a large, cumuliform swarm. Crows and other birds were observed catching locusts in flight (Figure 2c). Some locusts formed into stratiform swarms migrating upward along the valley midway up the slope tops (Figure 2b). The size of each migrating group varied from several hundred to several thousand adults. During flight, many individuals fell into the river or landed on the road or mountain slopes.

Along the valley, at three sites between 2500 and 3200 m (Table 1, sites Cuoxiamu, 2.1, 2.2) locusts were observed forced to stop their flight by strong winds. Hundreds of individuals were found gathered on trees or in the grass. At four other sites located in the same valley at an altitude above 3300 m (Table 1, sites Qu town, 3.1, 3.2, 4.1), at a place where the river makes many bends, locusts were observed grouped on the ground (Figure 1). In Qu town (at 3365 m), a few locusts were found on the banks of the river as well as at the foot of the high cliffs, covered with lush vegetation, that overlook it (Figure 2d,f). Some were dead, others alive and basking on the rocks on sunny days (Figure 2g). The same situations were observed at the three other sites (Sections 3.1, 3.2 and 4.1).
The Boqu River valley ends near Nyalam, at about 3800 m elevation (Figure 1 and Table 1, Jimuxiong site). Several scattered locusts were observed flying in this area, with flight speed estimated at about 6 m/s. A few adults were also collected locally in potato fields. At Puluo pasture (4200 m), about 25 km northwest of Nyalam, 6 locusts were found in early July (Figure 1 and Table 1). On the shores of Langqiang (4700 m) and Kuimai (4725 m) lakes, about 30–40 km north of Nyalam, a few locusts in flight were observed as well as a few dead locusts in late July (Figure 2e, Table 1). It is interesting to note that while locusts were observed at Cuoxiamu, none were seen at Jialong Lake, although both sites are located at almost the same latitude (about 28°4′), but the former in the Boqu River valley at 3150 m, while the latter, at 4750 m, is outside this valley about 10 km to the east.

## 3.2. Population Dynamics of the Desert Locust in Tibet in 2020

### 3.2.1. Field Survey

Field surveys were conducted in areas at elevations from of 2300 m to 4750 m, from mid-July to early October 2020.

In Zhangmu, at 2300 m, from 13 to 27 July, the average density of locusts at surveyed sites gradually decreased from 3.1 locusts/m² to zero from 27 July. After this date, desert locusts were no longer observed in this area (Figure 3a). Twenty-five live females were collected on 25 July, and 38 and 177 dead females on 1 and 11 August. Females were dominant and the sex ratios on these dates were 60%, 68.4%, and 71.2%, respectively (Figure 3b, column graph). On August 1, all females were immature and on August 11, only 1.3% of the females were at maturity stage 1 (Figure 3b, pie chart).

<figure><img src="image_38.png" /><figcaption>**Figure 3.** Desert locust field monitoring in low and high altitudes in Nyalam County, Tibet, in July–August 2020. Lower altitudes: changes in locust average density as a function of temperature and rainfall (a), locust sex ratio and ovarian development (b). Higher altitudes: locust density at three sites (c), sex ratio (d), ovarian development (e). Filling degree of the digestive tract of locusts collected in the field (f).</figcaption></figure>

Weather conditions must have been unfavorable for the desert locusts. In Zhangmu, the temperature was relatively low and stable during the survey period, with the average ranging from 14.0 °C to 20.8 °C, and the lowest temperature being about 10 °C. In Qu City, in the middle of the Boqu River valley, the temperature was even lower, with an
average of 5 to 8 °C due to heavy rainfall, higher elevation, and strong wind. It rained almost every day and only three days had no rain. The highest rainfall was recorded on July 13 with 85.2 mm. On July 14, 16, and 17, more than 18 mm of rain fell, and on July 23 more than 40 mm. Since desert locusts prefer higher temperatures and drier weather, the low temperatures and sustained rainfall are expected to have been a major stress on these insects, resulting in high mortality and population loss in this area in late July.

At higher elevations, Jialong (4750 m), Langqiang (4670 m), and Kuimai (4725 m) lakes were monitored (Figure 1). No locusts were found near Jialong (11 and 22 August) and Kuimai lakes (22 August). Only at Langqiang lake were locusts observed on 22 August, at a low density of only 3 locusts/ha (Figure 3c). On the lake shore, some locusts were feeding or resting on shrubby plants such as acacia trees. A few dead locusts were also observed. On a sunny day, when the maximum temperature of the day was above 20 °C, a few locusts were observed flying 2–3 m above the ground in various directions at a speed of about 10 m/s. The distance flown in a single flight was about 20–50 m. The sex ratio was 70.6% and 62.5% for dead (n = 38) and live (n = 16) locusts, respectively (Figure 3d). Live females were mostly immature (55.6%), with the remainder in various stages of maturity (Figure 3e). No females had yet laid eggs. It appears that locusts lived longer in these drier, higher areas (above 3700 m) than in the lower (2300 m), very rainy areas.

Examination of the digestive system of locusts (n = 25) collected from Qu town area revealed that 72.0% of individuals were starved, including 53.3% of females and 100.0% of males (Figure 3f). Locust feces were liquid, probably due to the high humidity throughout the valley.

As of late July 2020, no swarms were observed in all previously infested areas and only dead locusts were found throughout the Boqu River valley.

### 3.2.2. Field Caging Experiments

At the same time, in Zhangmu and Nyalam, cage field studies were conducted. In Zhangmu (20 July to 4 August) locust life span averaged 26 days, ranging from 22 to 35 days. Probably under the influence of heavy rains, survival rate dropped sharply from 95% to 38% between July 22 and 25, and from 37% to 10% from July 26 to 28. On 4 August, no locusts were alive (Figure 4a). These figures are consistent with field observations made at the same time in this area (Figure 3a). This is despite the fact that locusts in cages were provided with sufficient food each day. In addition, 90% of the females were immature in the caged field studies, and no mating behavior was observed (Figure 4b), which is also very similar to the field study. These results confirm that low temperatures and high precipitation are critical limiting factors for locust survival in the southwestern Himalayan foothills.

In Nyalam (where the experiments were conducted from July 26 to October 18), the average lifespan of locusts was 70.9 days, with the longest being 110 days. The survival rate in the cages gradually decreased from 100% to 52% in 56 days from 26 July to 19 September. After the heavy rains on 23 and 24 September, survival continued to decline and then dropped rapidly after 10 October until the locusts disappeared completely on 18 October (Figure 4c). During the first 59 days, low precipitation occurred on 38 days (64.4%), and the mean daily temperature ranged from 8.7 °C to 18.8 °C. On September 23 and 24, heavier precipitation occurred (22.3 and 31.3 mm) followed by a sharp drop in temperature (daily average ranging from 10.5 °C and 1.5 °C between September 19 and October 18). Precipitation ceased after September 25 (Figure 4c).

Thus, locust mortality in the field cages in both areas was also clearly influenced by temperature and rainfall. Heavy precipitation seems a key factor, but low average temperatures over a long period and low minimum temperatures can also be critical.

All arriving locusts were pinkish and immature. Under the low temperatures prevailing from July to October, ovarian development of females was very slow. Twenty-six females could be dissected to examine the ovaries. On August 1, the females were immature. Early maturation stage 1 was observed on August 12, stage 2 on August 15, stage 3 on
September 10, and the only female (over 26) that laid eggs was observed on October 5. The egg-pod contained 78 eggs (Figure 4f), but none hatched after incubation in the laboratory (L:D = 12:12, T = 35 °C, RH = 30%).

<figure><img src="image_39.png" /><figcaption>**Figure 4.** Cage field studies in Zhangmu (2300 m) and Nyalam (3800 m). Desert locust survival and weather conditions in Zhangmu (a) and Nyalam (c) in 2020 (the % of survival is the mean of survival percentages per cage). Percentage of maturation of females in Zhangmu (b). In Nyalam: Number (d) and percentage (e) of maturation status of females; eggs in the ootheca (f); daily mating rate of locusts (g); mating (left) and pre-oviposition behavior (right) rates (h); mating duration (%) (i).</figcaption></figure>

In the cage study, adults mated many times during their lifetime, especially on sunny days and when the temperature was above 10 °C. However, no mating occurred on cloudy and rainy days. The period during which mating pairs were observed was very long, ranging from late August to early October, or 43 days. The vast majority (91%) of matings occurred from 25 August to 18 September, or 25 days (Figure 4g). During the day, matings could be observed from 09:00 to 18:00, but most (75.7%) occurred between 11:00 and 15:00 (Figure 4h). The duration of mating varied from a few minutes to several hours; 36% lasted 1–2 h; the longest being 19 h, probably due to the low evening temperatures that made the locusts not very active (Figure 4i). At the time of oviposition, the female makes regular movements of opening and closing the ovipositor valves and the abdomen elongates significantly. The whole oviposition process usually lasted a few minutes but
could sometimes last several tens of minutes. Overall, oviposition lasted about 2 h and eggs were deposited in sandy soil at a depth of about 12 cm. Pre-oviposition behavior (females using their abdomen to test whether the soil is suitable for oviposition) were observed from 11:00 to 18:00, but 75.3% occurred from 14:00 to 17:00 (Figure 4h). When the temperature reached 15 °C at noon in August and September, locusts flew into the cages but could still be hit easily.

3.2.3. Food Preference Tests

Most of the desert locusts (72%) collected on the Tibetan Plateau were starving, with an empty digestive tract (Figure 3f). In addition to the fact that low temperatures must have reduced feeding activity, we questioned whether or not local plants might be favorable and conducted food preference tests. A total of 34 local plant species (belonging to 19 families and 32 genera) were collected from Zhangmu area and presented to adult desert locusts (Figure 5). Only three plants were clearly consumed: *Lophatherum gracile* (LG), *Mentha canadensis* (MC), and *Fagopyrum esculentum* (FE) with, respectively, 2600, 1850, and 1400 mm² of leaf area consumed in 24 h. For the other plants, the consumed surface was lower or largely lower than 1200 mm² and ten plants were not or very little consumed.

<figure><img src="image_40.png" /><figcaption>**Figure 5.** Desert locusts' food preferences for some Tibetan plants. (a) Leaf area consumed by locusts (n = 10, female:male = 1:1; same color bars represent plants of the same family). (b) Leaf aspect of the 34 plant species tested: MC, *Mentha canadensis* (syn. *M. haplocalyx*); LG, *Lophatherum gracile*; FE, *Fagopyrum esculentum*; SM, *Salix matsudana*; TD, *Thladiantha dubia*; AA, *Ageratina adenophora* (syn. *Eupatorium adenophorum*); CO, *Calendula officinalis*; UF, *Urtica fissa*; PC, *Pogonatherum crinitum*; BM, *Bambusa multiplex*; PL, *Pueraria lobata*; NP, *Nicandra physalodes*; GPM, *Gynostemma pentaphyllum*; GPA, *Galinsoga parviflora*; IB, *Impatiens balsamina*; OC, *Oxalis corniculata*; PN, *Polygonum nepalense*; UP, *Ulmus pumila*; PF, *Perilla frutescens*; BP, *Bidens pilosa*; TB, *Thalictrum baicalense*; IS, *Impatiens sibilifer*; RH, *Rubus hirsutus*; RA, *Rumex acetosa*; HS, *Hylotelephium spectabile* (syn. *Sedum spectabile*); AAI, *Artemisia argyi*; RC, *Rosa chinensis*; CC, *Commelina communis*; HP, *Hypolepis punctata*; PR, *Polygonum runcinatum*; LL, *Ligustrum lucidum*; BS, *Boehmeria spicata*; AAS, *Acalypha australis*; EJ, *Eriobotrya japonica*.</figcaption></figure>
# 4. Discussion

## 4.1. Desert Locust Routes in Tibet in July 2020

The Himalayas, with their high altitudes and low temperatures, may seem an insurmountable natural barrier for locusts despite the great flight ability of these insects. However, some swarms have managed to penetrate it and our observations show that some isolated locusts have even been able to advance to very high areas. In fact, the valleys—thanks to the associated aerological systems—were the gateways of the locusts to the Tibetan highlands.

The highest concentrations of locusts in Tibet in 2020 were recorded along the Boqu River valley, with the highest densities observed south of the valley near the Nepalese border and at low elevations of only 1700 m (Figure 2a). Based on the observations, the flight path of the swarms mainly followed the river valley and gradually rose toward the Tibetan high plateau to an altitude of about 3200 m. Outside the valley floor, the density was much lower. Density decreased as one moved northward and elevation increased. A few isolated locusts were able to progress, still following the valley, to much higher areas between 3200 and 5400 m. They did not migrate in large swarms, but in small groups with several stops in the climbing route. These locusts do not seem to have been able to benefit from thermal lift that would have allowed them to rise to high altitudes, sometimes to the top of the convective layer at heights of 1 to 2 km [26]. The aerological conditions forced them to fly at low altitude, at a few tens of meters from the ground, while going up very gradually the bottom of the valley, until reaching too high altitudes where the thermal conditions must have ended up preventing them from flying. The observed flight speed along the Boqu River was only about 6 m/s, which is not much different from the average speeds of 7–18 m/s reported in the locust’s usual habitat range [27,28]; however, the flights were only of short duration. The progression of these locusts up the valley must have been aided by slope and valley breezes, blowing from the bottom up during the day, at the time of locust flight activity. It is known that transport by such anabatic winds can account for the frequent occurrence of grasshoppers in mountainous areas at altitudes well above their area of residence [29], as well as for the movement of desert locust swarms to and on mountain slopes [30] (see Appendix A).

This Boqu River valley is surely only one of the entry routes of the desert locust into Tibet in July 2020. At the same time, desert locusts are widely reported throughout Nepal, immediately to the south. This country was invaded in three successive waves, on 25–29 June, 12 July, and 16 July. The swarms then spread over much of Nepal in about 10 days [31,32] (Figure 1). This Himalayan region on the southern border of Tibet has four valleys (ChenTang, Gama, Zhangmu, and Jilong) leading into Nepal and of relatively low altitude. All these valleys could thus have been, like the Boqu River valley studied here, an access route to invade southern Tibet from the beginning of July.

## 4.2. High Altitude Climate as a Major Obstacle for Desert Locusts

Once in Tibet, the high-altitude climate must have been a major obstacle to entry, reproduction, and local survival for locust individuals. The desert locust's usual range, its recession area, covers about 19 million km² from western Africa to western India. These are generally low-lying areas (<1000 m) with a tropical desert or semi-desert climate. The average annual temperature is around 20–30 °C, the average monthly temperature from July to October is above 30 °C, and the annual rainfall is very low and below 250 mm [2]. Rainfall over 25 mm in two consecutive months is usually assumed to be enough for locust breeding and development [27]. In the highlands of Tibet, in areas where locusts have been observed, climatic conditions are quite different and a priori not very conducive to the survival of new arrivals. The average annual temperature is about 8 °C in southern Tibet. Zhangmu area, at relatively low elevations of 1700–3800 m, has a humid or semi-humid subtropical monsoonal plateau climate. Rainfall is abundant with an annual average of 2820 mm and a rainy season extending from June to October. The average daily temperature during this season is less than 20 °C or even 10 °C in the highest areas such as Qu City [33,34].
at elevations above 3800 m, such as Nyalam, have a plateau mountain climate where the highest daily temperature in August is mostly below 20 °C and the nighttime temperature is close to 0 °C [35–37].

The lowest temperature at which locusts can migrate and propagate over a long distance is 20 °C [27,38–40] and this limiting temperature is even higher (about 23 °C) if the sky is overcast, with insufficient sunlight rendering locusts unable to absorb sufficient heat energy [26,27]. Therefore, temperatures in Tibet make it nearly impossible for locusts to fly uninterrupted over long distances. Furthermore, above 4000 m, the oxygen content of the air is only about 14.6%, well below the 21.0% recorded in Africa at sea level [41–43]. Decreased oxygen availability (and decreased temperature) make life at such altitudes challenging. The physiological effects of environmental hypoxia may indeed contribute to limiting the ability of insects to persist at high altitudes [44]. This lack of oxygen at high altitudes must therefore have been another limiting factor in locust flight activity in Tibet. This reduced flight ability is probably responsible for the large number of drowned locusts found in small ponds and lakes in the Qu town area (3365 m) and at Langqiang Lake (4670 m), where individuals have been observed falling into the water, even on a sunny day, and although adults are accustomed to avoiding water when flying.

These extreme climatic conditions for desert locusts in the Tibetan highlands appear to have affected not only the flight ability of individuals but also their survival and reproductive ability. In a field cage, with sufficient food and no natural enemies, the average life span of desert locust adults was 26 days in Zhangmu (at 2300 m elevation) and 71 days in Nyalam (at 3800 m). Such life spans seem short even considering that these are immigrant individuals with only part of their imaginal life observed, with the average duration normally ranging from 2.5 to 5 months [27]. This longevity appears to have been significantly shortened even though many individuals remained immature, a state in which locusts can normally remain quiescent for several months until suitable conditions return.

Other evidence suggests that the locusts that entered Tibet were placed in unfavorable conditions.

First of all, they had a low food intake since 72% of the locusts collected in the field had an empty digestive tract. In the cages, the locusts also showed a reduced feeding activity. The majority of local plants tested appeared to have low palatability. Even though they represent a small proportion of the flora of Tibet, one third of these plants were not consumed or were consumed only slightly, which is surprising considering the very polyphagous nature of the desert locust. It is interesting to note that of the 34 plants tested, 15 belong to genera that are not represented in the usual range of this insect.

Secondly, sexual maturation was disturbed. In Zhangmu area, 99.4% of the females were immature. In the field cage only 10.0% of the females were observed in early maturation. In the Langqiang Lake area, 55.6% of the females were immature, the rest were in various stages of ovarian development, but none had laid eggs. In Nyalam, in a cage, only one female laid eggs while the other 35 tested died before laying eggs. In the end, most of the locusts died quickly in the immature state. For sexually matured females, the time from adult emergence to first matings was 43 days at 3800 m, whereas it may be as little as 15 days under optimal thermal conditions [27,45,46].

Finally, no eggs were observed except for one, containing 78 eggs, none of which hatched after incubation in the laboratory. At best, the embryonic development time was strongly slowed down by the low temperatures. Desert locust eggs require an air temperature of 20–35 °C and a soil temperature of 15.5–35 °C, low temperatures make egg survival impossible [27,47,48]. On the Tibetan plateau, the winter temperature is very low and the snow is deep. Even if locusts were successful in laying eggs, they would probably not withstand the low temperatures.

In conclusion, the combined influences of low temperatures, high precipitation, high altitude and low oxygen content, and unfavorable food supply weakened the immigrant locust populations, reduced their flight ability, disrupted sexual maturation, and made egg laying almost impossible. In addition, various natural enemies must have contributed
to rapidly limiting the populations, including various birds and entomopathogenic fungi actually observed. In the end, these locusts died quickly without any local offspring and without having been able to cross the Himalayan barrier.

**Author Contributions:** L.Z. and J.L. conceived the study, performed the field work, and collected data; J.L., M.L. and L.Z. wrote the first draft of the manuscript; all authors contributed substantially to data analysis, writing, review and editing. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was supported by grants from The Ministry of Agriculture and Rural Affairs of China (15206030 and 12210030), and the grant for innovation project from Shandong Academy of Agricultural Sciences (CXGC2021B12).

**Conflicts of Interest:** Authors declare no competing interests.

## Appendix A

Desert locust invasion pathways, survival, and reproduction on the Tibetan Plateau in China. The desert locust swarms cannot pass the high-altitude but enter through the lower parts of the Himalayan river valleys. A limited number of locusts climb along the river valleys, step by step. When the altitude exceeds 3000 m, only a few locusts can fly and some may fall suddenly in flight. They are not adapted to the low temperatures and high humidity of the plateau climate and eventually die abnormally and are unable to reproduce.

<figure><img src="image_41.png" /></figure>

## Highlights

1. Small desert locust swarms penetrated into southern Tibet in July 2020

2. Some locusts reach higher elevation areas, up to 5400 masl, climbing gradually by following the valleys

3. Locusts were severely stressed by low temperatures and high humidity: activities—feeding, flying and mating—were limited

4. Locusts did not survive long—max. 71 days—and did not produce any local offspring

<caption>**Figure A1.** Desert locust invasion pathways, survival, and reproduction on the Tibetan Plateau in China.</caption>

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Article

# Early Intervention against Desert Locusts: Current Proactive Approach and the Prospect of Sustainable Outbreak Prevention

Allan T. Showler <fnref n="1" />*, Mohammed Abdallahi Ould Babah Ebbe <fnref n="2" />, Michel Lecoq <fnref n="3" /> and Koutaro O. Maeno <fnref n="4" />

**Citation:** Showler, A.T.; Ould Babah Ebbe, M.A.; Lecoq, M.; Maeno, K.O. Early Intervention against Desert Locusts: Current Proactive Approach and the Prospect of Sustainable Outbreak Prevention. *Agronomy* **2021**, *11*, 312. https://doi.org/10.3390/agronomy11020312

Academic Editor: Imre J. Holb
Received: 8 January 2021
Accepted: 7 February 2021
Published: 10 February 2021

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

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

**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/).

**Abstract:** The desert locust, *Schistocerca gregaria* (Forskål) (Orthoptera: Acrididae), a major Old World pest, is associated with agricultural losses and undesirable societal effects. There are three broad approaches to its control: reaction, proaction, and outbreak prevention. Reaction protects crops from swarms but it is costly and disruptive. Proaction involves early intervention during outbreaks to avert further development to plague status; it is in current use because it is effective, relatively inexpensive, and it is the best available option for now. Outbreak prevention, largely unavailable since the 1970s, at least on a regional scale, will require highly sensitive surveillance to detect the onset of gregarization. Sufficiently early intervention can, hypothetically, extend desert locust recession indefinitely. While research on desert locust biology and behavior is, almost, no longer an urgent requirement to improve the efficacy of control, new priorities have arisen for developing outbreak prevention capability (and for enhancing proaction). Salient needs presently include long residual tactics for prophylactic (preventive) control in breeding areas, intervention thresholds, and improved, sustainable coordination among stakeholders at national, regional, and international levels. The most recent desert locust episode of 2020 provides an illustrative example of how prevention might have averted the entire upsurge, and how proaction in some countries contained the spread of swarms. The initial outbreak in Saudi Arabia escaped control due to unpreparedness, and impacts of armed conflict in Somalia and Yemen, which weakened surveillance and control, further contributed to the invasion of ≥22 countries, and the spraying of ≈4.9 million ha, by the end of 2020.

**Keywords:** control; plague; proaction; *Schistocerca gregaria*; surveillance; threshold; upsurge

## 1. Introduction

Basic scientific principles for controlling the desert locust, *Schistocerca gregaria* (Forskål) (Orthoptera: Acrididae), were established during the first half of the 20th century. Following the elucidation of phase theory (whereby the pest can change between solitary and gregarious morphologies and behaviors) by Uvarov [1] and the location of major desert locust breeding areas in which outbreaks tend to originate, the Fifth International Locust Conference, held in Brussels, 1938, promulgated the need for international coordination as well as the possibility of developing a preventative strategy against swarm formation [2]. Since that time, scientific and technical advances have been substantial, greatly improving approaches to surveillance and control [3,4]. Desert locust control strategy, however, has remained a topic of considerable debate. In recent years there has been extensive discussion of early intervention strategies [3–16]. The desert locust is a biologically and behaviorally complex insect [11,17] that, when gregarious and uncontrolled, is often fraught with adverse economic, environmental, and social consequences [3,8,10,11,16,18–21].

<footnote n="1">¹ USDA-ARS Knipling-Bushland U.S. Livestock Insects Research Laboratory, 2700 Fredericksburg Road, Kerrville, TX 78028, USA</footnote>

<footnote n="2">² Sahel Institute, Bamako, BP 1530, Mali; mohamedabdellahi.ebbe@cilss.int</footnote>

<footnote n="3">³ CIRAD, UMR CBGP, F-34398 Montpellier, France; mlecoq34@gmail.com</footnote>

<footnote n="4">⁴ Japan International Research Center for Agricultural Sciences, Livestock and Environment Division, Ohwashi 1-1, Tsukuba 305-8686, Japan; kmaeno@affrc.go.jp</footnote>

<footnote>* Correspondence: Allan.Showler@usda.gov</footnote>
## 2. Prevention Is Relative

Constructive discussion of desert locust control is often confounded by different perspectives on common terminology [11]. Regarding “prevention”, confusion arises from disharmonious views on what we intend to prevent. Do we permit the occurrence of upsurges before intervening to prevent plague development? Or is the ultimate goal to prevent outbreaks, thereby maintaining recession status indefinitely [9]? To prevent is to avert an event or condition, precluding its occurrence. Outbreak prevention is gregarization management; it entails intervention prior to, or during, phase transformation from solitary to gregarious [9,10,12,21]. Intervention during outbreaks, on the other hand, is aptly described as “proaction”, aimed at halting an outbreak’s expansion toward plague status [9]. There are, broadly, three desert locust control approaches: reaction, proaction, and outbreak prevention [4,8–11].

### 2.1. Reaction

The reactive approach involves intervention after economically and socially threatening desert locust upsurges and plagues are underway [11], a result of unpreparedness to take action earlier. Reaction primarily focuses on defensively protecting croplands from destruction instead of offensively intervening in the main breeding areas (commonly in remote regions) [11,17,19]. Major upsurges and plagues, unfortunately, are associated with serious economic losses and social upheaval [11,20].

Some observers from countries unafflicted by desert locusts have espoused allowing plagues to build and run their course without intervention; they propose that farmers who incur losses will later receive compensation through insurance policies [22], and that reactive campaigns are, on average, less expensive [13,14] than early intervention. Both views are cavalier, after Europe and United States took strong measures to decimate their own locust populations (e.g., Rocky Mountain locusts, *Melanoplus spretus* [Walsh]) [11,23], and because insurance is impractical where it is largely an unfamiliar concept and where subsistence agriculture predominates. Calculating costs associated with gregarious desert locust episodes is challenging [8,9,11,19,24] because it involves the monetary value of crops; the social implications of subsistence agriculture; negative impacts on pasturage, rangeland, and livestock; food aid; assistance for displaced people; and long-term impact on populations and education outcomes [20,21,25]. Economic assessments are available [17,18,26–31], but they are not necessarily complete. Agricultural losses were suggested as being insufficient to warrant investment in control [22], but that was founded on the cash value of selected crop systems without considering the array of other factors. The 2003–2005 upsurge, for example, was conservatively assessed to have inflicted an average crop loss of ≈30%, but that estimate failed to incorporate losses incurred by small Sahelian farms and to consider areas suffering from particularly severe food shortages [11,20,21]. The 2003–2005 upsurge further resulted in displacement of farmers and pastoralists to urban areas; conflicts between farmers, nomads, and pastoralists over limited resources; declines in market commodities that offset agricultural product prices; and food aid from international agencies (donors), and multiple cascades of indirect socio-economic impacts [20,29–34]. Belayneh [20] conservatively estimated crop losses of 10–20%, while loss of anticipated cereal harvest reached 80%, 90%, and 100% during 2004 in areas of Burkina Faso, Mali, and Mauritania, respectively [21]. Legume crop reductions were as much as 90% in the same three countries; further, ≈30% of pasture vegetation was destroyed in Mali and Burkina Faso, and fodder production was reduced by up to 80% in Mauritania, resulting in a total of ≈8.4 million affected sahelian farmers and pastoralists [21].

While it can be argued that reaction is, technically, preventive by aiming to avert crop losses under some circumstances, reaction is not regarded as being a preventive strategy. Reaction is instead a default response when desert locust activity becomes overwhelming and threatens agricultural production [9–12]. Examples of reactive control include the 1986–1989, 1992–1995, and the 2003–2005 campaigns [5,8,10,11,20]. Each of those campaigns was expensive in terms of international aid (US$ millions ≈300, ≈19, and
≈35, respectively) and involved 18–23 desert locust afflicted countries in Africa and Asia (Table 1) [5,8,10,11,20].

<table><caption>**Table 1.** Desert locust episode and control effort statistics 1986–2016 [11].</caption><thead><tr><th>Desert Locust Episodes</th><th>No. Countries</th><th>Ha Sprayed</th><th>Duration</th><th>Cost to Donors<sup>b</sup></th><th>Intervention Timing</th></tr></thead><tbody><tr><td>1986–1989</td><td>23</td><td>25 million</td><td>4 years</td><td>$310 million</td><td>Late</td></tr><tr><td>1992–1994</td><td>18</td><td>4 million</td><td>2 years</td><td>$18.8 million</td><td>Moderate</td></tr><tr><td>1997–1998</td><td>7</td><td>430,000</td><td>7 months</td><td>$30,000</td><td>Early</td></tr><tr><td>2003–2005</td><td>20</td><td>13 million</td><td>2.5 years</td><td>$35 million</td><td>Late</td></tr><tr><td>2007–2016<sup>a</sup></td><td>22</td><td>1.8 million</td><td>10 years</td><td>&lt;$1 million</td><td>Early</td></tr></tbody></table>

## 2.2. Proaction

Proaction is an intermediate response between reaction and outbreak prevention [9,12,26,35,36]. The progression of desert locust populations from recession to plague status is a continuum; outbreak prevention occurs at the initial stage, while later intervention during outbreaks is proactive. Although both terms refer to early interventions for averting plagues, the difference is in the timing. Conflation of outbreak prevention with proaction (i.e., [13,14]) obscures the important distinction between them [4,9,11]. Historical records show that the longer gregarious populations breed, the greater the risk of being overwhelmed by the locust situation and the greater the likelihood of reaching plague status [4,11,25,28,37].

Proactive intercession against desert locusts is initiated based on an often ad hoc blend of estimated gregarizing populations, local control capabilities, political pressure, and gestalt [9]; there is no empirically calculated intervention threshold. The default is killing all gregarious nymphal bands and swarms [4], representing zero-tolerance. The best available approach is proaction (Figure 1) and, following the 1986–1989 plague, it has had different degrees of success [4,8–11,20,25,38,39].

<figure><img src="image_43.png" /><figcaption>**Figure 1.** Bars represent years during which major desert locust activity (major upsurges and plagues) occurred (adapted and modified from 3), 1860–2020 (most recent upsurge extended past 2020); prevalent control approaches are indicated, prevention involved use of long-residual organochlorinated insecticides.</figcaption></figure>

Desert locust episodes from 1986 to the present (2020) are relatively well-documented. Post-1970s, campaigns discontinued use of highly persistent, broad spectrum organochlorinated insecticides (e.g., DDT, dieldrin) due to environmental concerns [19,40]. Long-residual insecticides, however, were useful (Figure 1) for applying in limited swathes, or

<footnote>a Ten-year period involved numerous outbreak episodes. b US$.</footnote>
bands, on vegetation in breeding areas that killed nymphs crawling into those areas by direct contact and ingestion of treated foliage [25]. Early control of desert locust outbreaks was obviously weakened by the banning of dieldrin [25,41–43]. Control since the 1970s has relied almost exclusively on relatively short-residual insecticides, including malathion, propoxur, and some pyrethroids [19]. Contemporary inability to control nymphal bands using long residual prophylactic methods means that spraying must be spatially and temporally accurate. Reliance on short residual insecticides has resulted in the need to take more individual control actions against nymphal bands, as well as highly mobile adult swarms. This has increased areas sprayed and numbers of afflicted countries, and intensified dependence on international aid [11]. In terms of insecticide treated areas, the 1997–1998 outbreak, involving proaction, was only 1.7% and 3.3% the magnitude of the 1986–1989 plague and the 2003–2005 upsurges, respectively [11]. During the ten-year period 2007–2016, when there were no plagues (Figure 1) but numerous outbreaks, proaction likely contributed to limiting the magnitude of sprayed area, across all ten years, to 7.2% and 13.8% of the 1986–1989 and 2003–2005 episodes, respectively (Table 2) [11]. International aid supporting proaction against the 1997–1998 outbreak was 0.01% and 0.3% of costs incurred during reactive 1986–1989 and 2003–2005 campaigns, respectively [11]. Accrued donor expenditures for 2007–2016 constituted only 0.1% and 2.9% of the 1986–1989 and 2003–2005 campaigns, respectively [11]. Furthermore, the proactive 1997–1998 campaign took just seven months, whereas the reactive campaigns persisted for 2.5–4 years [5,11,20].

<table><caption>**Table 2.** Numbers of countries infested, percentage of months in which proactive control occurred, area sprayed with insecticides, and number of times FAO Bulletin used “alarm” words to describe the desert situation, 2007–2016 (adapted from 11).</caption><thead><tr><th>Year</th><th>No. Countries Infested</th><th>% Months with Proactive Intervention</th><th>Hectares Sprayed</th><th>Number of Alarm Words <sup>a</sup></th></tr></thead><tbody><tr><td>2007</td><td>14</td><td>100</td><td>255,887</td><td>18</td></tr><tr><td>2008</td><td>9</td><td>75</td><td>64,744</td><td>0</td></tr><tr><td>2009</td><td>9</td><td>92</td><td>37,613</td><td>12</td></tr><tr><td>2010</td><td>10</td><td>75</td><td>48,163</td><td>3</td></tr><tr><td>2011</td><td>11</td><td>83</td><td>197,380</td><td>11</td></tr><tr><td>2012</td><td>9</td><td>92</td><td>215,660</td><td>26</td></tr><tr><td>2013</td><td>11</td><td>100</td><td>482,159</td><td>21</td></tr><tr><td>2014</td><td>13</td><td>100</td><td>373,231</td><td>11</td></tr><tr><td>2015</td><td>5</td><td>50</td><td>108,162</td><td>0</td></tr><tr><td>2016</td><td>11</td><td>92</td><td>54,916</td><td>15</td></tr></tbody></table>

Belayneh [20] concluded that lack of proactive intervention in 2003–2005 allowed desert locust populations to breed until attaining upsurge status; the dynamic was mainly curbed by unfavorable climate-driven conditions. Had proactive control been conducted sufficiently early to spare 10% of the 2003–2005 agricultural losses, US$ 226 million might have been conserved [20]. The cost of control during the 2003–2005 upsurge equaled projected costs of 170 years of proactive control [37].

Effective proaction hinges on the efficiency of outbreak suppression in breeding areas [19,25,37,44]. During the decade 2007–2016, outbreaks were usually controlled within five months [11]. Proactive interventions helped to hasten return to recession status before outbreaks could expand into upsurges (Figure 1) [11]. Although not every outbreak during the ten years might have become an upsurge, some of them produced swarms that moved beyond breeding areas and reproduced in other regions (Table 2) [45] before being controlled. The Food and Agriculture Organization (FAO) monthly bulletins warned, commonly in ominous terms (Table 2), that desert locust populations and habitat conditions favored imminent development toward upsurges [11,45]. Claims that ten years of proactive strategy is riskier than reactive campaigns [13] have not been supported by events [11].

<footnote><sup>a</sup> Alarm words: critical, extremely alarming, extremely/very serious, outbreak, potentially dangerous, potentially threatening, potentially very dangerous, serious, and threatening.</footnote>
## 2.3. Outbreak Prevention

Outbreak prevention refers to intervention that is sufficiently early to avert the onset, and initial development, of gregarization before and when nymphs first amass in small patches and adults gather in loose groups [9,10,12]. The approach will eliminate a sufficient proportion of the population to suppress pheromone cues associated with crowding (crowding elicits positive feedback for building fully gregarious populations) that trigger gregarization [4,46–54]. Sustainable interference with semiochemical gregarization triggers can, hypothetically, maintain recession status indefinitely [8,9,11,12]. Although proaction is an improvement over reaction, an earlier, preventive, approach offers an even more efficient control option [4,10,11,55]. While outbreak prevention is the logical ultimate goal of contemporary research efforts, it has not yet been implemented without using long-residual organochlorinated insecticides or through sufficiently early detection and intervention, as was accomplished in Mauritania in 2009, and again several years later [56]. On the other hand, preventive locust control strategies had achieved success against some swarming species, such as the red locust, *Nomadacris septemfasciata* (Serville), in southern Africa [57], and the South American locust, *Schistocerca cancellata* (Serville) [17,58]. Assuming that outbreak prevention becomes possible, it is likely that proactive interventions will continue intermittently until prevention is sustainable. A proactive contingency capability will likely remain prudent, even when outbreak prevention is sustainable, to counter unforeseen gregarization events.

# 3. Proaction: A Technological and Social Accomplishment

Surveillance locates desert locust populations for control. Both surveillance and control have benefitted from technological advances, as has communication, which is essential for gathering data from afflicted countries, disseminating desert locust forecasts from the FAO, and intra- and interregional coordination.

## 3.1. Surveillance

Proactive intervention relies upon well-targeted, accurate, and quickly communicated surveillance information, particularly in areas prone to producing gregarious populations. The geographic foci of surveillance efforts can change over time depending upon several factors, including weather and greenness, and it is accomplished by multiple means involving different levels of technological sophistication [5,8,11]. Surveillance must be sustainable in order to produce routine reports during times of calm as well as site-specific, detailed reports contributing to national, regional, and international data bases to better understand population dynamics [7,10,11,26,59]. In general, information gathered by technologically sophisticated methods requires relatively low-technology ground verification in order to detect and characterize actual desert locust activity.

Technological improvements to surveillance began during the mid-1980s, including use of remote sensing imagery (particularly to assess cover by green vegetation and soil moisture) and climatic and weather data collection methods, both of which help to anticipate desert locust activity [11,25,38,44,60–66]. These factors, combined with historical data and field-based reports, identify where conditions are conducive to outbreaks [8,10,11,15,59]. Suspect areas can be monitored for incidence of gregarizing populations [9,10,59] by scouts in terrestrial vehicles, and by obtaining reports from farmers, nomads, administrative authorities, militaries, and sometimes fire lookouts in the Atlas Mountains and rebel groups in contested territories [5,8,11,59]. Aerial surveillance (and control) is conducted, when possible, by the regional Desert Locust Control Organization for Eastern Africa (DCLO-EA), which has aircraft [67] (Saudi Arabia has its own aircraft). Field information is radioed to national ministries of agriculture that transmit reports by email and other electronic means to regional coordination centers and to the desert locust information service of FAO in Rome. Recently developed technology (elocust 3) [68] is used to transmit, in parallel, information to national locust units and to the Food and Agriculture Organization. The information is collated, synthesized, interpreted, and reported back to
the afflicted and donor countries accompanied by desert locust activity forecasts. The value of technological advances for enhancing surveillance, communication, and forecasting is corroborated by successful proaction from 2007 through 2016 [11].

## 3.2. *Control*

Following the shift away from long-residual organochlorinated insecticides during the 1980s [3,40], use of conventional, synthetic, relatively short residual insecticides sprayed directly onto nymphal bands and settled swarms has been the only available control tactic [10,25], excluding recent limited use of entomopathogens. Short residual insecticides are effective when desert locust populations are detected early and with sufficient accuracy to halt outbreaks. Currently used insecticides can also be applied as "barriers" against nymphal bands [66,69,70], but this is not particularly efficacious for long-term prophylactic control. Selective insecticides, including insect growth regulators [25], might also be efficacious as barrier treatments, but they are unlikely to be more persistent than conventional insecticides. Entomopathogens are commercially available, and others are being evaluated [45,66,71]. In 2020, entomopathogens were applied to at least 10,845 ha in Somalia against desert locusts [72,73]. Other alternative tactics have not been reported [10,25].

Additional technological advances include ultra-low volume (ULV) spray equipment, now in routine aerial and terrestrial use against desert locusts [44,66]. ULV formulations are oil-based, reducing droplet evaporation such that only 0.5–1.0 L of the insecticide is needed per hectare [44], decreasing environmental contamination and costs of transportation, handling, and storage [11]. In addition, global positioning systems (GPS) technology improves precision of insecticide applications, enhances navigation to targets that are often located in vast, remote wilderness [25,66], and reduces spurious environmental exposure to, and wastage of, insecticides. Research is underway to test the utility of unmanned aerial vehicles (UAV) or drones for desert locust detection and control.

# 4. Outbreak Prevention: Salient Requirements

The most important research goal now is arguably the development of a reasonably long-residual and environmentally benign control tactic that, whether chemical, biological, or other, is useful in a prophylactic context. Furthermore, the length of time constituting a reasonably long effective residual (days, weeks, months) should be established.

Because early intervention occurs within limited windows of opportunity, insecticide application timing is important even for reasonably long-residual tactics; hence, surveillance must be able to sustainably detect the onset of gregarization. It is not yet clear how this can be achieved, but it will rely, at least in part, on global information systems (GIS) imagery and weather information [4,11]. Outbreak prevention will require study of geo-historical desert locust survey and control to refine our knowledge of breeding and phase transformation in salient areas, such as has been accomplished in Mauritania and a few other countries [15,59,74–76]. Outbreak prevention will also entail use of empirical measurements that serve as triggers, or thresholds, for interventions [4,46]. It is likely that thresholds will have to be tailored for local conditions across time (e.g., seasonally) and in different areas [4]. An intervention threshold might be based on integrations of population density, food source abundance, and food source clumping which, in certain configurations, are associated with the initiation of gregarization [4,46]. In addition to using visual assessments of potentially gregarious desert locust activity as intervention threshold tools, it might be possible to develop a sensor for detecting semiochemicals associated with phase transformation as a cue to intervene.

Surveillance of population densities alone is inadequate for predicting gregarization because of differences between discrete desert locust habitats [4], which extend from Mauritania to India [11,17]. In western Africa, limited north-south movement of the intertropical convergence zone (ITCZ), which can be tracked, affects amounts of precipitation, and wind speed and direction, across the Sahel [15]. This provides a broad notion of where conditions are likely to favor desert locust population intensification, and, because swarms move
downwind, projections about areas at risk of invasion [15]. A study in Mauritania and Morocco determined that a displacement cycle for swarms, largely based on ITCZ position, has a seasonal association, but this has not been confirmed for solitarious populations [15]. Population increases during recessions were associated with areas that received sufficient rainfall, which changes spatially over time because the ITCZ moves [15]. Following rainfall, persistence of water beneath the soil surface in wadis and other low-lying areas support flushes of vegetational growth [15]. The foliage supplies enough food to increase desert locust populations that, when conditions dry and the vegetated areas recede, results in crowding, semiochemical release, and gregarization [11,49]. Anticipation of gregarization based upon suitable breeding conditions has helped to protect northwestern Morocco from swarms developing south of the Atlas Mountains (e.g., the Sahel) since 1988 [15], a dividend of proaction [11]. Improved surveillance by ground teams can be attributed to superior map-based orientation, the effort invested, and training [11,15]. Heightened ability of ground surveillance teams to find desert locusts; however, might produce skewed information unless scouts also report where desert locusts were not observed [15]. Failure to record nonpopulated locations can result in inaccurate estimates of area-wide desert locust activity (it can also bias historical desert locust activity data) [15].

Habitat differences involve variables that might affect the propensity of a desert locust population to initiate gregarization. Understanding how food source clumping, community and individual plant structure and phenology, nutritional value, and availability influence population density is important for assessing the risk of gregarization [4,46,52,75,77]. To streamline scouting, areas that need to be searched are reduced by use of timely forecasts [5,12], and although currently gathered information is sufficient to support proactive interventions, further refinements might better support outbreak prevention [4]. As an example, GIS technology has not yet been used to detect and map plant species that are particularly favorable to desert locust population increases [4] (GIS mapping of a selected low-growing plant species in Texas; however, has been accomplished to determine ixodid (hard tick) distribution [78]). Transect counts of desert locusts and important plant species might be useful for making risk assessments [4,46] and associated intervention determinations.

Other relatively recent reports indicate that there are other facets of spatio-temporal aggregation patterns inherent to desert locust ecology that might be relevant to surveillance efforts. A field study conducted in Mauritania [77] confirmed observations that marching nymphal desert locust bands passed some plants and eventually roosted on patchily distributed trees near dusk, forming aggregations [79]. Maeno and Babah [77] found that gregarious bands of nymphs condensed on, and around, the largest tree in the local plant community; adults mostly roosted on large trees and substantial bushes [80,81]. Surveillance might also benefit from findings that indicate variability desert locust defensive responses to predators and likely to approaching terrestrial surveillance personnel (on foot and in vehicles). Whereas adult desert locusts usually take flight when temperatures exceed the minimum threshold for flight (≈22 °C) [80,82], at cooler temperatures, locusts in low bushes tend to move into the middle of the bush upon which they are roosting in order to hide; individuals on taller plants (≥2 m) tend to remain stationary [80].

## 5. Impediments

Lecoq [25] and Doré et al. [29] emphasized that impediments to proactive and preventive strategies require thorough analysis and documentation. Although some authors (e.g., [13,14]) have suggested that the sole obstacle has been funding shortfalls, the scope of impediments is considerably wider and more complex. The array of challenges confronting many afflicted countries has been fundamental to the development of plagues for ≥40 years. Challenges that intertwine with and compound one another have been described [3–5,8,10,11,16,83]: unpreparedness [5,11,18,20,25,62,67], insecurity [11,83], remote, rugged terrain and poor infrastructure [8,11], environmental concerns [11,18,19,40,67], political hindrances [3,5,8,16,20,62], dogmas and assumptions [11], and research impediments [11]. The negative impacts of many challenges can be mitigated by advances in tech-
nology and appropriate funding. One notably intractable exception is insecurity [11,83,84],
which can cripple desert locust surveillance and control.

6. Changing Priorities

Major outbreak areas, and the ecological conditions and gregarization mechanism
that initiate outbreaks, are known, and an early warning system of surveillance and
reporting is in place [3,4,11,36]. Now that we have a substantially improved grasp of the
desert locust's complex ecology, the next steps should focus to an increasing extent on
the complexities of control systems governance [3,85]. The weakest link in desert locust
control is unpreparedness due to problems with organization, coordination, logistics at
international and national levels [3,4], and insecurity [83]. Unpreparedness is largely the
result of inactive recessions when the complex web of interactions that enable control seizes
up and donor funding declines [16,86]. Additionally, personnel turnover depletes trained
cadres and institutional memory fades in the afflicted countries [16]. Hence, when proactive
interventions hold desert locusts in the solitary phase for long periods, maintenance of
surveillance and control capabilities declines, accompanied by loss of focus, equipment,
insecticides, personnel, and expertise [16,86].

To muster coherent early responses to dynamic desert locust episodes, regional and
international cooperation must be well coordinated. Increasing preparedness will involve
strengthening national locust control units and making coordination among national, bilat-
eral, and multilateral stakeholders a sustainable priority [3]. At the urging of international
aid agencies, the FAO instituted the Emergency Prevention System (EMPRES) in 1996
to operate in parts of the desert locust recession areas [3]. EMPRES aimed to increase
the capabilities of afflicted countries, regional desert locust control organizations, and
the FAO [3,4,6,87]. The program has coordinated the development and application of
technologies, and training, that improved upon proactive control [3,11]. Because the issue
of financial sustainability of desert locust control systems is of critical importance, an
institutional framework providing for graduated mobilization of funding and sources,
depending on the severity of the desert locust episode, has been suggested [87,88]. An
initial effort to develop such a fund involves the Commission for Controlling the Desert
Locust in the Western Africa Region (CLCPRO), based in Algiers, in 2016 developed a
regional fund to proactively control desert locust invasions, endowed with ≈6.5 million
US dollars from member countries (Algeria, Burkina Faso, Chad, Libya, Mali, Mauritania,
Morocco, Niger, Senegal, and Tunisia) [89,90]. A risk management plan, possibly including
intervention thresholds, can be formulated prescribing coordinated stakeholder actions for
different levels of desert locust activity [4]. Coordination must be strong at all times in order
to maintain vigilance and early response capability during recessions [3,16]. Multi-agent
modeling has been suggested as a way of better understanding stakeholder interactions
and system responses [16].

7. Conclusions: New Upsurge

The most recent major desert locust upsurge became apparent during winter and
spring 2020. The episode originated when heavy rains fell during the latter half of 2018 in
the Rub al Khali, or Empty Quarter, of Saudi Arabia [90]. The resulting favorable breeding
conditions supported three undetected generations [90], and in December, swarms spread
elsewhere in Saudi Arabia, as well as to Yemen, Egypt, Eritrea, Sudan, and southwestern
Iran, where breeding occurred (Figure 2) [91–93]. Control operations were conducted
in Saudi Arabia, Iran, Eritrea, Egypt, and Sudan through spring 2019. Yemen, beset by
armed conflict, and Iran mustered weak responses and swarms from Iran invaded the
India-Pakistan border where three generations produced more swarms (Figure 2) [91].
Swarms from Yemen crossed the Red Sea to northern Somalia and Ethiopia (Figure 2),
where breeding produced additional swarms [90]. Survey reports were not obtained from
Somalia through spring 2019 [92–96], but it is likely that gregarious populations continued
to arrive from Yemen and reproduce. During summer 2019, desert locust activity and
control operations occurred in Ethiopia, Saudi Arabia, Iran, India, and Pakistan [96-98]. Surveillance remained weak in Somalia and Yemen, where gregarious populations were not controlled [97-99]. During fall and winter 2019-2020, desert locust activity resumed in Eritrea and Sudan, continued in Saudi Arabia, India, Pakistan, and, in December, Iran (Figure 2) [100-105]. Kenya was invaded (Figure 2) in December 2019, and infestations there intensified through February 2020, accompanied by control operations [104,105]. Furthermore, populations arriving from Saudi Arabia increased in Oman until, in the latter half of winter, intervention occurred [102-106]. Gregarious populations in Somalia and Yemen persisted while surveillance remained weak and control was limited [100-106]. During spring (March-June) 2020, infestations in Eritrea declined, while substantial gregarious populations and control operations continued in Ethiopia, Kenya, Oman, Saudi Arabia, Iran, Pakistan, and, by late spring, India [72,73,107]. Surveillance and control operations in Somalia and Yemen were spotty, but gregarious populations were likely to have been substantial [72,73,107]. During the episode, other countries were invaded and undertook control operations, particularly Iraq, Jordan, Kuwait and the United Arab Emirates, and infestations were comparatively light in Afghanistan, Bahrain, Djibouti, Libya, South Sudan, Tanzania, and Uganda (Table 3) [72,73,92-108]. During the first two years of the episode, June 2018 to June 2020, swarms were observed in ≥22 countries, and spray operations occurred in ≥19 countries, covering >2,792,840 ha (Table 3) [72,73,92-107].

<figure><img src="image_44.png" /><figcaption>**Figure 2.** Generalized movements of major gregarious desert locust swarms, beginning June 2018 in the Rub al Khali of Saudi Arabia, moving out from the Rub al Khali by December 2018 and beyond Saudi Arabia through June 2020 [72,73,90-106].</figcaption></figure>

We suggest that, had the initial outbreak in the Rub al Khali of Saudi Arabia been detected in summer 2018, swarms might not have reached Yemen, the Red Sea coast of Africa, and parts of Iran. While substantial control operations commenced December 2018 in Saudi Arabia, Egypt, Eritrea, and Sudan, the capabilities of Yemen and Somalia, both hobbled by insecurity, harbored and produced swarms that invaded other countries, and the initially weak response in Iran permitted swarms to move to Pakistan and India [91]. Because control was conducted in most of the other afflicted countries (Table 3), it is likely that once Saudi Arabia had begun to control its desert locusts, Somalia and Yemen continued to drive expansion and intensification. Effective surveillance and control in those two countries would have eliminated further spread in concert with the substantial control operations that occurred in Egypt, Eritrea, Ethiopia, and Sudan, and some other countries adjacent to them (e.g., Tanzania, Uganda). Similarly, effective early intervention in Iran would have spared Pakistan and India. The episode illustrates how a chain of worsening
desert locust population events resulted because of unpreparedness and insecurity in a few countries [109].

<table><caption>**Table 3.** Countries invaded by desert locust swarms and areas sprayed with insecticides, December 2018–June 2020 [72,91–106].</caption><thead><tr><th>Country</th><th>Area (ha) Sprayed</th></tr></thead><tbody><tr><td>Afghanistan</td><td>20</td></tr><tr><td>Bahrain</td><td>34</td></tr><tr><td>Djibouti</td><td>0</td></tr><tr><td>Egypt</td><td>19,802</td></tr><tr><td>Eritrea</td><td>93,586</td></tr><tr><td>Ethiopia</td><td>288,205</td></tr><tr><td>India</td><td>427,233</td></tr><tr><td>Iran</td><td>887,657</td></tr><tr><td>Iraq</td><td>2610</td></tr><tr><td>Jordan</td><td>2900</td></tr><tr><td>Kenya</td><td>102,342</td></tr><tr><td>Kuwait</td><td>15,653</td></tr><tr><td>Libya</td><td>70</td></tr><tr><td>Oman</td><td>13,164</td></tr><tr><td>Pakistan</td><td>453,892</td></tr><tr><td>Saudi Arabia</td><td>402,881</td></tr><tr><td>Somalia</td><td>26,845</td></tr><tr><td>South Sudan</td><td>0</td></tr><tr><td>Tanzania</td><td>0</td></tr><tr><td>Uganda</td><td>607</td></tr><tr><td>United Arab Emirates</td><td>5904</td></tr><tr><td>Yemen</td><td>49,435</td></tr><tr><td><strong>Total: 22</strong></td><td><strong>Total: 2,792,840</strong></td></tr></tbody></table>

The episode also demonstrates that proactive interventions, which occurred in many of the afflicted countries, impeded significant spread beyond their borders [72,73,92–107]. Once gregarious populations arrived in India and Pakistan, for example, relatively few swarms escaped those two countries, and the same is true for Egypt, Eritrea, Ethiopia, and Sudan. Saudi Arabia, the initially unprepared source of the upsurge, could not avert movement into Bahrain, Iraq, Kuwait, Oman, United Arab Emirates, Yemen, and likely parts of the Red Sea coast of Africa. A timely proactive approach (or, hypothetically, an outbreak prevention approach) in the Rub al Khali, a relatively limited area, would have pre-empted the possibility of an upsurge.

Based upon the advantages of early intervention, stakeholders must recognize the difference between proaction and outbreak prevention to distinguish intermediate from ultimate goals. While proaction has been successfully implemented starting in the 1990s (Figure 1) [4,9,11,12], a new approach to outbreak prevention that does not rely on environmentally persistent, broad-spectrum pesticides has not yet materialized. Advances that might contribute toward a viable outbreak prevention strategy, however, continue to unfold. Although surveillance and control were weak in Somalia, entomopathogens were applied there on at least two occasions [72,73], representing a step toward alternative, increasingly environmentally benign control, and possibly toward development of persistent formulations for prophylactic outbreak prevention. We also suggest that proaction and outbreak prevention require more smoothly operating national and regional organizations dedicated to early intervention, technical and funding mechanisms that provide sustainable support during recessions and gregarious episodes, heightened sustained vigilance, alleviation of impediments, and, with particular regard to armed conflict, some luck, even when mitigation measures are available [84].
**Author Contributions:** Conceptualization, A.T.S.; methodology, A.T.S., M.A.O.B.E., M.L. and K.O.M.; software, A.T.S. and M.L.; validation, A.T.S., M.A.O.B.E., M.L. and K.O.M.; formal analysis, A.T.S. and M.L.; investigation, A.T.S., M.A.O.B.E., M.L. and K.O.M.; resources, A.T.S. and M.L.; data curation, A.T.S. and M.L.; writing–original draft preparation, A.T.S.; writing–review and editing, A.T.S., M.A.O.B.E., M.L. and K.O.M.; visualization, A.T.S. and M.L.; and supervision, A.T.S. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Data sharing not applicable.

**Conflicts of Interest:** The authors declare no conflict of interest.

**Disclaimer:** 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 U.S. Department of Agriculture. USDA is an equal opportunity provider and employer.

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<figure><img src="image_45.png" /></figure>
Article

# Incidence and Ramifications of Armed Conflict in Countries with Major Desert Locust Breeding Areas

Allan T. Showler<sup>1,*</sup> and Michel Lecoq<sup>2 ID</sup>

**Abstract:** Despite many areas of progress in recent years, desert locust surveillance and control is impaired by many obstacles, the most intractable of which is insecurity. Insecurity involves rebellions, insurgencies, civil and international war, banditry, terrorism, and minefields. Obstruction of desert locust operations in breeding areas by ongoing armed conflict and landmines constitutes "direct" insecurity. "Indirect" insecurity, although less obvious, is arguably more broadly deleterious by debilitating government function and diverting funds, personnel, and equipment from desert locust management. Indirect "active" insecurity is armed conflict and civil unrest that is occurring at the same time as a desert locust episode, but not in the breeding areas. Indirect "inactive" insecurity refers to the after-effects of insecurity, including weak funding because of prior inattention to capacity maintenance during times of direct and indirect active insecurity, disabled or militarily-appropriated vehicles and other resources, destruction of infrastructure, and deployment of mines. We provide examples of direct and indirect insecurity across 35 years, from 1986 through May 2020, in 13 African and Asian countries (Chad, Eritrea, Ethiopia, India, Mali, Mauritania, Niger, Pakistan, Saudi Arabia, Somalia, Sudan, Western Sahara, and Yemen) with desert locust breeding areas to illustrate the complexity, pervasiveness, and chronic occurrence of insecurity. The upsurge of 2020 is used to show how direct insecurity still contributes to the genesis and expansion of desert locust episodes. Possible mitigation of direct insecurity effects on some desert locust operations is discussed.

**Keywords:** control; impediments; insecurity; plague; *Schistocerca gregaria*; surveillance; war

## 1. Introduction

The desert locust, *Schistocerca gregaria* (Forskål) (Orthoptera: Acrididae), is arguably the oldest and most agriculturally threatening migratory pest globally [1]. Since 1989, following a desert locust plague, control has evolved toward proactive intervention [2,3] during outbreaks to avert the development of upsurges and plagues with notable success [4,5]. Further, control is evolving past proactive control toward outbreak prevention with the aim of holding desert locust populations in their nongregarious recession stage for indefinite periods of time [4,5]. Despite notable successes and advances in the science of desert locust management and available technologies [4,5], major constraints remain, including insufficient funding, political issues, unpreparedness, impediments to research, and rugged and remote terrain with weak infrastructure [4]. Another key constraint, insecurity, involves rebellions, banditry, civil and international war, terrorism, and minefields (Figure 1) [4–8]. Large areas in the desert locust distribution [7,9] cannot be surveyed for desert locust activity due to chronic insecurity, which constitutes a serious risk for proactive and preventative strategies [4,5,10]. Insecurity has been a major challenge to surveillance and control by blocking access [4,8] to desert locust breeding areas in Africa and Asia [9] (Figure 2). Desert locust control is unique in that it is conducted across vast expanses and it requires coordination within and between afflicted countries. Surveillance should chiefly focus on the main breeding, or "outbreak areas" [11,12]. Major desert

**Citation:** Showler, A.T.; Lecoq, M. Incidence and Ramifications of Armed Conflict in Countries with Major Desert Locust Breeding Areas. *Agronomy* **2021**, *11*, 114. https://doi.org/10.3390/agronomy11010114

Received: 1 December 2020

Accepted: 5 January 2021

Published: 8 January 2021

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

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

**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/).

<footnote n="1">¹ Knipling-Bushland U.S. Livestock Insects Research Laboratory, USDA-ARS, 2700 Fredericksburg Road, Kerrville, TX 78028, USA</footnote>

<footnote n="2">² CIRAD, UMR CBGP, F-34398 Montpellier, France; mlecoq34@gmail.com</footnote>

<footnote>* Correspondence: allan.showler@ars.usda.gov; Tel.: +1-(830)-792-0341</footnote>
locust breeding areas are mainly located around massifs across the Sahara Desert and Mauritania, the Red Sea plains on the east and west coasts, parts of the southern Arabian Peninsula, and the India-Pakistan border [4,7,9], all of which are frequently beset with insecurity [8]. Occurrences and effects of insecurity are described herein from 1986 through May 2020 [4,6]. Desert locusts can invade and reproduce in countries beyond those harbor- ing key breeding areas [4–7,9], but we selected examples that have major breeding areas for illustrative purposes.

<figure><img src="image_47.png" /><figcaption>**Figure 1.** Landmines limit access to some areas frequented by desert locusts; photo credit Neil Rickards.</figcaption></figure>

<figure><img src="image_48.png" /><figcaption>**Figure 2.** Proximity and overlap of major desert locust breeding areas with areas advised against and formally advised against by the French Ministry of Europe and Foreign Affairs for safety concerns [13] and other foreign ministries and departments of other “western” countries; the dotted lines indicate the limits of the desert locust invasion area [12]; 1986 through October 2020.</figcaption></figure>
Insecurity can impede desert locust operations directly and indirectly. Direct insecurity obstructs ground and aerial surveillance and control operations in breeding areas by imperiling personnel and equipment. Further, swarms originating in non-contested areas can fly into conflicted zones where they might not be accessible for control. Unlike most other kinds of insect control campaigns, desert locust operations are, with relative frequency in some countries, impeded by direct insecurity.

Indirect insecurity, although less obvious than direct insecurity, is arguably more broadly deleterious by debilitating government function and diverting focus away from agricultural issues, and by diverting funding, personnel, and equipment (e.g., vehicles) from desert locust management. Indirect insecurity can be “active” and “inactive”. Indirect active insecurity is armed conflict and civil unrest that is occurring at the same time as the desert locust episode, but not in the breeding areas. Because of the apparent constancy and diversity of conflicts in many countries with desert locust breeding areas, indirect active insecurity is widespread, persistent, and dynamic. Indirect inactive insecurity refers to the after-effects of insecurity regardless of where it had occurred. Impacts include weak funding due to prior inattention to capacity maintenance during times of direct and indirect active insecurity, disabled or government-appropriated vehicles and other resources, destruction of infrastructure, and deployment of mines. Indirect inactive insecurity is, because of the frequency of indirect active insecurity, also persistent. Dormant hostilities, such as those between Israel and some of its neighboring countries, are examples of indirect inactive insecurity. Further, desert locust upsurges and plagues are linked to loss of feed production; displacements of human populations; conflicts between farmers, nomads, and pastoralists over limited resources [14], all of which are exacerbated and compounded by the chronic, sweeping, and pernicious effects of indirect (active and inactive) insecurity. While we cannot measure the influence of indirect insecurity on desert locust management, we can report the incidence of insecurity to provide a sense of scope and magnitude, as well as variability between afflicted countries.

Because early intervention and reactive control require concentrated use of resources during time-limited windows of opportunity, depletion of resources contributes toward non-sustainability, which underlies the primary obstacle to early intervention (proactive and preventive): Unpreparedness [4,6,10,15–17]. Insecurity is often accompanied by increased reliance on bilateral and multilateral international aid agencies for development, routine operations, and non-conflict-related disasters and emergencies, such as combatting desert locust upsurges and plagues. Ramifications of indirect insecurity erode national desert locust control capabilities particularly during recession periods [16,17]. In addition, chronic demand for external support to suppress a recurrent emergency has resulted in “donor fatigue”, a condition associated with dwindling funds, especially during recessions when there is no immediate emergency to mobilize and drive continued international aid agency support.

## 2. Insecurity in Countries with Major Desert Locust Breeding Areas

**Algeria.** *Direct Insecurity.* "Traditional" desert locust breeding areas are located in southern Algeria, particularly around the Ahaggar Mountains, and many swarms invaded north of the Atlas Mountains during the 1986–1989 plague and the 2003–2005 upsurge (Figure 3) [6,14]. Some areas in the 1980s and 1990s were insecure due to limited Tuareg unrest in the south and travel was restricted near Western Sahara. From 2007 through 2016, however, outbreaks were effectively controlled [4,18].
<figure><img src="image_49.png" /><figcaption>**Figure 3.** Locations and regions mentioned in the text in relation to desert locust activity and insecurity in Algeria. Primary breeding areas in red.</figcaption></figure>

*Indirect Insecurity*. Islamic extremism began after a general election was curtailed by the government in 1992 when Islamic fundamentalists were expected to win [19]. During the next 10 years, ≈150,000 people were killed and the government sank into political paralysis [20,21]. Attendant civil unrest included assassinations, massacres, and rioting, hindering travel to and within Algeria [8]. In 2001, a Berber uprising occurred in the Kabylie region (Figure 3) [22,23], and in 2007 an *al-Qaeda* faction carried out assassinations, kidnappings, and bombings; the army responded with anti-insurgency offensives [22–25]. Although a 19-year-long national emergency was lifted in 2011, public protests and terrorist attacks by *al-Qaeda* and other insurgent groups continued through 2016 [23]. The north was most insecure [22], but *al-Qaeda’s* southern branch accrued strength [26]; Libya’s civil war produced a weapons influx which helped arm *al-Qaeda* in Algeria [22,27], increasing ambushes and use of concealed explosives against security forces [28]. Indirect active and inactive insecurity obstructed governance, limited funding, and restricted movement, hindering anti-locust capabilities [8].

**Chad. Direct Insecurity.** Desert locust breeding largely occurs in the northwestern mountainous Tibesti and Ennedi regions (Figure 4) [29,30]. In 1973, Libya attempted to annex Chad’s Aouzou strip, part of the desert locust’s Tibesti (Figure 4) breeding area [29], but Chad retook it militarily in 1986 and travel there was prohibited until the early 1990s [31–34]. During the 1986–1989 campaign, control operations were often halted because of conflict with Libya [8]. Civil war, mostly in the Tibesti region between the government, and the Movement for Democracy and Justice and the National Resistance Army [35], obstructed travel to the region and impaired desert locust operations through 2016 [4,18].
<figure><img src="image_50.png" /><figcaption>**Figure 4.** Locations and regions mentioned in the text in relation to desert locust activity and insecurity in Chad. Primary breeding areas in red.</figcaption></figure>

*Indirect Insecurity.* The *Mouvement Patriotique du Salut* and other rebel groups replaced Chad's government in 1990 [27,36], and insurgency-based coup attempts became chronic [36,37]. During the 1990s, the border with the Central African Republic (Figure 4) was contested [38] between a group called the Federal Republic of Laouken and Chadian troops [8].

Border tension in eastern Chad deteriorated in 2005 when Chadian rebel groups used the region for bases (supported by Sudan), Sudanese militia groups also entered Chad and attacked civilians, instigating violence between eastern ethnic groups [36]. In April 2006, three rebel groupps, *Union des Forces pour la Démocratie et le Développement, Rassemblement des Forces pour le Changement*, and the *Front Uni pour le Changement*, started an unresolved offensive against the national army with casualties on both sides [36], prompting the government to declare a national emergency in 2007 [37]. In early 2008, the same rebel groups entered N’Djamena (Figure 4), but failed to topple the government [36]. A rebel advance in May 2009 elicited government airstrikes against their sanctuaries across the border in Sudan [37]. Sudan countered by launching an airstrike on a Chadian refugee camp and by pursuing Sudanese rebels that had taken refuge in Chad; Chad retaliated with July airstrikes against Sudan [37]. In 2010, Chad and Sudan deployed a 3000-soldier combined cross-border force to quell problematic opposition groups [37]. Other rebel organizations have since emerged and conflict continues. We suggest that indirect insecurity, active and inactive, have had a substantial deleterious impact on Chadian governmental functions, including locust surveillance and control.

**Eritrea. Direct Insecurity.** The Red Sea coastal plain of Eritrea, particularly north of Massawa (Figure 5), has, along with the contiguous coastal plains of Sudan, likely been the most prolific breeding area for desert locusts. The 1986–1989 plague and the 1992–1994 upsurge both originated on the Red Sea coastal plains in Eritrea and Sudan [6,7]. A war for independence waged by the Eritrean People’s Liberation Front (EPLF) against Ethiopia began in 1962 and continued for 30 years [39] (Figures 6 and 7), escalating to large tank battles. Fighting occurred everywhere, excluding the capital, Asmara (Figure 5) [39]. The war prevented desert locust control during the 1986–1989 plague [4,6,15], and during the
1992–1994 upsurge, minefields obstructed access to breeding areas [7]. Following indepen-
dence in 1993, minefields still made off-road travel in the lowlands so dangerous [40] that
some operations had to be conducted using aircraft [8]. The Sudan border was intermit-
tently conflicted after Eritrea’s independence [8] because of Eritrean support for Sudanese
rebels [41] and a suspected Sudanese-backed insurgency within Eritrea [8,42]. In 1999,
an Eritrean desert locust surveillance team was captured near the border, detained, and
tortured [8]. Despite continued hostilities, 1997–2020 desert locust control was reasonably
successful [4].

<figure><img src="image_51.png" /><figcaption>**Figure 5.** Locations and regions mentioned in the text in relation to desert locust activity and insecurity in Eritrea. Primary breeding areas in red.</figcaption></figure>

<figure><img src="image_52.png" /><figcaption>**Figure 6.** Eritrean People's Liberation Front (EPLF) soldiers during the war for independence from Ethiopia; photo credit Gwynne Roberts (copyright Gwynne Roberts).</figcaption></figure>
<figure><img src="image_53.png" /><figcaption>**Figure 7.** EPLF fighter, holding AK-47, who served 15 years on the front lines, with his wife who served on the front lines for 10 years; Eritrean women train and fight alongside men and they were reputed to be ferocious in battle; photo credit Allan Showler.</figcaption></figure>

*Indirect Insecurity*. In addition to the direct danger of landmines, the 1992–1994 upsurge started on the coastal plain [7] because of unpreparedness following the long war for independence [7,8]. In 1996, Eritrean forces displaced Yemeni troops on the Red Sea Hanish Islands (Figure 5), disrupting communication and travel between the two countries until resolution in 1998 [8,43]. Additionally in 1996, Islamic (possibly Sudanese) insurgents killed Eritreans and Belgians along the Abyssinian escarpment “Filfil” road connecting Keren to the coastal plain near Shelshela, a prolific desert locust breeding area (Figure 5) [8].

Ethiopia launched airstrikes against Asmara in 1998 over a border dispute and Eritrea responded with an airstrike against Mekele in northern Ethiopia (Figure 5) [8]. War ensued, involving tens of thousands of casualties until Ethiopia invaded part of southwestern Eritrea in June 2000. A United Nations peacekeeping force arrived in 2002 but disbanded in 2008 despite continuing tension [44] and the border remained inaccessible and tense through 2020, even after the two countries reached a peace agreement in 2018. Continuous military mobilization since independence has prevented Eritrea from reaching optimal capacity for desert locust control.

**Ethiopia.** *Direct Insecurity*. From 1986, parts of the Ogaden Desert (Figure 8), where desert locusts breed, were unsafe because of rebellions and banditry; in 1992, for example, a surveillance helicopter was shot down there, killing three [8]. The senior author, on low-flying reconnaissance near Jijiga (Figure 8), 1994, was targeted by small arms. Although
control operations have more recently been effective, dangers in some areas, particularly
the Ogaden, remain.

<figure><img src="image_54.png" /><figcaption>**Figure 8.** Locations and regions mentioned in the text in relation to desert locust activity and insecurity in Ethiopia. Primary breeding areas in red.</figcaption></figure>

*Indirect Insecurity*. Aside from the 1962–1992 war of Eritrean secession, rebellions and civil unrest besieged Ethiopia from 1986 through 2020. A civil war during the 1980s involved the Ethiopian People’s Revolutionary Democratic Front, which replaced the dictatorial Derg regime in 1991 [45]. The Sudan border was intermittently tense, and in 2000 Kenya reported that Ethiopian soldiers repeatedly crossed the border (Figure 8), killing Kenyan citizens and stealing cattle [8].

In 2004, ≈200 people died in ethnic clashes in Gambella Province (Figure 8), causing massive population displacement [46]. In 2005, disputed elections led to violent protests lasting months, in 2006 several bombs exploded in Addis Ababa, and Ethiopian troops were sent into Somalia for two years to combat Islamist militants [46]. In 2007, insurgents killed 74 workers at a Chinese oil facility in the southeast [46]. The Ogaden National Liberation Front (ONLF) captured several eastern towns in 2009 after heavy fighting [46], and clashes with the military continued into 2012, one raid killing 168 troops [46]. Further, European tourists were executed in the Afar region (Figure 8). In 2013, Ethiopia detained Somali *al-Shabaab* Islamist militants plotting to kidnap United Nations personnel [47], and, in 2015–2016, protests erupted in the northern city of Gondar (Figure 8) associated with the killing of >500 people and other human rights abuses by the government, prompting a state of emergency in late 2016 [46]. Occasionally, violent civil unrest in Ethiopia persisted through 2020. Although effects of indirect insecurity on Ethiopia’s ability to manage desert locusts are highly likely, it is difficult to assess the extent of impairment.

**India-Pakistan.** *Indirect Insecurity*. The Thar Desert (Figure 9) on the southern border of India and Pakistan comprise the easternmost key desert locust breeding area [9,29]. Direct insecurity in the breeding areas, 1986–2020, has not been reported. During the desert locust campaigns of 1986–1989 and 1992–1994, involving breeding in the border
deserts, control occurred before swarms escaped to other regions [6,7], but cooperation between India and Pakistan was problematic because of limited communications, travel restrictions to contested areas, and landmines [48]. Mutual hostility between the two countries escalated into a nuclear arms race [49,50]. From 1986 through 2020, conflict has mostly been focused on the Kashmir region (Figure 9). Terrorism and military skirmishes, including airstrikes, were conducted there, resulting in thousands of casualties [51–56]. India and Pakistan are likely among the countries (of the 13 presented) least impeded by indirect insecurity.

<figure><img src="image_55.png" /><figcaption>**Figure 9.** Locations and regions mentioned in the text in relation to desert locust activity and insecurity on the India-Pakistan border. Primary breeding areas in red.</figcaption></figure>

**Mali. Direct Insecurity.** The Adrar des Iforas region of northern Mali (Figure 10) is an important desert locust breeding area [29], inhabited by the Tuareg people [57]. In the early 1990s, the Tuareg turned from occasional banditry [58] to rebellion [59]. Five Malian troops protecting a desert locust surveillance team were killed by Tuareg rebels in 1994 [8] and the insurrection restricted travel around breeding areas [7,8]. Desert locust operations in northern Mali were impeded relatively often through 2016 [4,18]. If gregarious desert locusts were to become active in Mali in the near future, surveillance and control operations might experience direct insecurity-related impediments.
<figure><img src="image_56.png" /><figcaption>**Figure 10.** Locations and regions mentioned in the text in relation to desert locust activity and inScheme 2006. A Tuareg insurgency intensified when government army bases in Kidal, Menaka, and Tessalit (Figure 10) were attacked, but the conflict was resolved two months later [59,60]. In late 2011, another Tuareg rebellion began, strengthened by mercenaries from the Libyan civil war [59–61], and sporadic violent Tuareg unrest has continued through 2020.</figcaption></figure>

Five Europeans were kidnapped by *al-Qaeda* in eastern and northern Mali in 2006, and a German tourist was killed [61]. A massacre of Malian troops and civilians at Aguelhok (Figure 10) occurred in early 2012, three of four military units in northern Mali defected to Malian National Liberation Army (MNLA) rebels, United States military airdrops supplied besieged Malian troops in Tessalit (Figure 10) [61], and Senegal sent troops to assist the Mali government [60]. A coup in March 2012, however, overthrew the government, collapsing military command, and the MNLA and Islamist *Ansar al Din* seized the north [61]. By April, the MNLA declared part of Mali a separate state, but it retreated after being ousted from the town of Gao (Figure 10) by a faction of *al-Qaeda* [61]. In early 2013, France attacked insurgents in the north after government forces were defeated in Konna and Bamako (Figure 10) [61]. While the impacts of indirect insecurity on desert locust operations have not been assessed, they are likely to be relatively substantial.

**Mauritania.** *Direct Insecurity*. The Mauritanian desert is mostly favorable to desert locust breeding [29], which occurred during the 1986–1989, 1992–1994, and the 2003–2005 desert locust episodes [6,7,14]. Although landmines had at times obstructed desert locust operations, they have largely been cleared or marked and have not interfered with desert locust control operations [4,62]. Intervention against desert locusts from 2007 to 2020 were reasonably effective [18].

*Indirect Insecurity*. In 1989–1991, racial tensions triggered ethnic violence that resulted in hundreds of deaths in Mauritania and in Senegal [63–65]. After those incidents, emphasis on conflict prevention and ethnic reconciliation has occurred with a number of constitutional amendments affirming the multiethnic character of the state [66]. *Al-Qaeda* killed four French tourists in 2007, launched attacks in Nouakchott, and conducted ransom kidnappings beginning in 2008 [65]. Border tensions involved Western Sahara and its Polisario separatist guerrillas [65]. Indirect insecurity probably has had a low to moderate effect on desert locust control capability.
**Niger. Direct Insecurity.** A key desert locust breeding area encompasses the Air Mountains and the Tamesna region (Figure 11), which are inhabited by Tuareg. Tuareg insurrection hindered desert locust surveillance and control through 1994 [7]. In 1991, an encampment of scientists, including one of the authors (ML) researching desert locusts in the Tamesna was attacked by armed men, resulting in one casualty [67]. During 2007–2016, insecurity did not impede locust operations [4,18].

<figure><img src="image_57.png" /><figcaption>**Figure 11.** Locations and regions mentioned in the text in relation to desert locust activity and insecurity in Niger. Primary breeding areas in red.</figcaption></figure>

*Indirect Insecurity*. While violent incidents were associated with Tuareg bandits and rebels, by the late 1990s insecurity was less tumultuous than in Mali because government responses were more accommodating [59], and some Tuareg groups laid down arms [68]. It appears likely that, were gregarious desert locust populations to develop or arrive in 2020, insecurity would not pose significant obstacles.

**Saudi Arabia.** *Indirect Insecurity*. Desert locust breeding in Saudi Arabia mainly occurs along the Tihama, the *Rub al-Khali* (Empty Quarter), and the north-central desert [29]. No direct insecurity regarding desert locust control has been reported. Despite Saudi Arabian involvement in external wars (i.e., Iraq, Kuwait, Yemen) and limited internal civil unrest and Islamic terrorism confined to cities, indirect insecurity has had a negligible impact on desert locust control.

**Somalia.** *Direct Insecurity*. Desert locust breeding mostly occurs in northern parts of Somalia, and these areas have been particularly problematic in 2020 [5,69]. National locust control capabilities were eliminated in Somalia, from the 1980s through 2020, because of armed conflict [70]; hence, desert locust surveillance during the late 1990s was conducted by one United Nations volunteer, a Somali counterpart in Hargeisa (Figure 12), and occasional FAO (Food and Agriculture Organization of the United Nations)-funded aerial missions by the Desert Locust Control Organization for Eastern Africa (DLCO-EA), based in Addis Ababa, Ethiopia (Figure 8) [8]. During the relatively limited 1997–1998 desert locust episode [15], swarms developed in Somaliland (Figure 12) and flew into Ethiopia’s Ogaden region [4,8]. Direct insecurity has impeded desert locust operations in Somalia through 2020 [5], and Somalia is arguably the most problematic country with regard to insecurity in the desert locust’s distribution.
<figure><img src="image_58.png" /><figcaption>**Figure 12.** Locations and regions mentioned in the text in relation to desert locust activity and insecurity in Somalia. Primary breeding areas in red.</figcaption></figure>

*Indirect Insecurity*. After the Somali president was ousted and Mogadishu (Figure 12) was captured by rebel militias in 1991, thousands of civilians became casualties and Somalia lost its central government [71,72]. In 1992, ≈350,000 people died from disease, starvation, and civil war, prompting United States emergency aid airlifts [72–74]. In 1993, United States troops were deployed in Mogadishu to assist United Nations peacekeepers but heavy fighting resulted in 18 United States soldiers and >500 Somali fighters and civilians killed [72,75]. United States forces were withdrawn [74] and United Nations efforts to stabilize the government were aborted [75]. Puntaland and Jubaland (Figure 12) declared themselves as regional administrations in 1998 and factional fighting persisted after a transitional government was installed in 2000 [72,76]. A United Nations arms embargo on Somalia was in effect since the early 1990s, and in 2001–2002 Mogadishu was declared too dangerous for a United Nations office [72,77].

From 1996 through 1997, the *al-Qaeda*-linked Somali group, *al-Ittihad*, conducted attacks and bombings in Ethiopia. Starting in 1999, Ethiopia responded with incursions into southern Somalia to destroy *al-Ittihad* [78], capturing the regional capital of Garba Harre (Figure 12) [72]. In 2002, Ethiopian troops moved into Puntaland and, again, into southern Somalia [79–81]. Because the United States government suspected *al-Qaeda* activity in Somalia, military operations there increased [72]. An interim Somali government was formed in Kenya in 2003, and on moving to Somalia in 2005, internal divisions begat more violence [72]. Inter-militia fighting in Mogadishu during 2006 caused hundreds of civilian casualties and a militia-backed rival government, the Islamic Courts Union, seized most of southern Somalia and Mogadishu [72]. When Ethiopian troops re-invaded Somalia, Islamic government hard-liners declared *jihad* against Ethiopia, and subsequent fighting involved tanks and helicopters [72,82]. By 2007, the transitional government regained control, a United States airstrike killed the *al-Shabaab* insurgency's leader, and other airstrikes hit *al-Qaeda* targets in southern Somalia [72]. The government declared a state of emergency as fighting continued into 2008, including coordinated suicide bombings in semiautonomous regions [72].
By September 2011, >20 separate regional governments had developed across Somalia in addition to Puntaland and Somaliland (Figure 12), some warring against others [74]. Kenyan troops battled *al-Shabaab* in Somalia from late 2011 through 2016, and *al-Shabaab* claimed that it killed ≈100 African Union soldiers in El-Adda (Figure 12) [74]. In October 2016, Ethiopia pulled its 2000 troops (part of a 22,000-strong African Union force, Figure 13) from four towns in southern Somalia which *al-Shabaab* captured within hours [83]. Widespread insecurity in Somalia has continued through 2020.

<figure><img src="image_59.png" /><figcaption>**Figure 13.** African Union Mission in Somalia (AMISOM) soldiers in Jubaland, Somalia, to combat insurgent militias; photo credit Stuart Price.</figcaption></figure>

Over decades of conflict, thousands of landmines were scattered across parts of Somalia [84]. The Somalia–Ethiopia border and the regions of Galgadud, Bakool, Bay, Hiraan, and the Afgooye Corridor, and parts of Mogadishu (Figure 12) were the most heavily mined [84]. In addition to the instances of insecurity described above, regions of Somalia were also unsafe because of banditry and piracy. Indirect insecurity, like direct insecurity, continue to be major obstacles to desert locust control.

**Sudan.** *Direct Insecurity*. Sudan's Red Sea coastal plain, particularly the region from the Tokar Delta (Figure 14) to the Eritrean border, is a prolific desert locust breeding area [29]. Breeding also tends to occur in the western desert [29]. Civil war and insurgencies, especially around the Tokar Delta and Kassala [85] (Figure 14), impeded surveillance and control during the 1986–1989 plague. The Tokar Delta area was held by National Defense Alliance (Beja Congress) rebels in the 1990s and swarms developed there during the 1992–1994 upsurge and the 1997–1998 desert locust outbreak [4,15]. The northern coastal plain around Halaib (Figure 14) has been contested from 1986 to the present, by Egypt [86], rendering it off limits to locust scouts from both countries. Sudan is vulnerable to desert locust outbreaks becoming upsurges because direct insecurity persists in some breeding areas.
<figure><img src="image_60.png" /><figcaption>**Figure 14.** Locations and regions mentioned in the text in relation to desert locust activity and insecurity in North and South Sudan. Primary breeding areas in red.</figcaption></figure>

*Indirect Insecurity*. The government also fought other rebel armies, mostly in the south, following Sudan's 1956 independence [87,88] through 2020; the largest rebel force was the 40,000-strong Sudan People's Liberation Army (SPLA) [89]. During peak war years, 2003–2005, the government attacked villages using helicopters and fixed-wing Antonov bombers [90]. Ground forces followed, targeting the Fur, Massalit, and Zaghawa tribes especially in the Darfur provinces (Figure 14) [90]. The military was augmented by the *Janjaweed*, a militia comprised of former bandits, demobilized soldiers, young tribesmen, and unemployed youths [90]. The aim was ethnic cleansing accomplished through atrocities, killing hundreds of thousands [70,91]. Less than a year after Sudan split into the Republic of (North) Sudan and the Republic of South Sudan (Figure 14) in 2005, the two countries were fighting again [88], and limited conflict in the Darfur provinces erupted into another civil war. Further, in April 2012, war recommenced between North Sudan and South Sudan, with the SPLA crossing into the north and the North Sudan Armed Forces bombing southern villages [88]. In addition, North Sudan fought insurgencies in the South Kordofan, Blue Nile, and Abyei provinces (Figure 14) [88]. Conflict also occurred within South Sudan against the Juba-based government (Figure 14) [88]. In 2014 the president of North Sudan renamed the *Janjaweed* the Rapid Support Forces (RSF), comprised of ≈10,000 fighters, as his personal army to counter diminishing military loyalty [91]. The RSF continued inflicting atrocities, including aerial bombing of civilians in North and South Darfur, South Kordofan, and Blue Nile provinces (Figure 14) [91]. In 2016, in addition to its many other conflicts, the North Sudan government fought the Justice and Equality Movement, two different factions of the Sudan Liberation Movement, and the Liberation and Justice Movement [90].
Further, Sudan was cut off from international aid for harboring international terrorist cells, including *al-Qaeda* [8]; this was underscored by a 1997 United States missile strike on a “pharmaceutical factory” in Khartoum (Figure 14) suspected of manufacturing chemical weapons [8]. Despite the chronic and intense violence in Sudan that must certainly detract from optimal desert locust control capacity, and breeding areas compromised by direct insecurity, desert locust control operations in Sudan have been substantial and effective since 2003.

**Western Sahara. Direct Insecurity.** In 1988, a United States C-130 desert locust spray aircraft, flying from Senegal to Morocco, was downed by a shoulder-fired Polisario missile, killing the five-person crew [8]. Locust operations have been, at times, unfeasible in Western Sahara because of Polisario activity. During the 1986–1989, 1992–1994, and 2003–2005 control campaigns, swarms entering Western Sahara from Mauritania and Algeria could not be tracked until they crossed into Morocco or back into Algeria because of direct insecurity [4,6,7,18]. It is possible that direct insecurity will continue to be problematic for desert locust control for the foreseeable future.

*Indirect Insecurity*. Western Sahara (formerly Spanish Sahara), claimed by Morocco, has been vigorously contested by the Polisario Front since 1975 when Spain relinquished the territory [92,93]. The Polisario reported killing > 5000 Moroccan soldiers between 1982 and 1985 alone [53]. Many Polisario guerrillas took refuge in Algeria from Morocco's military [94], heightening tensions between Morocco and Algeria [8]. Additionally, the Polisario held 1360 Moroccan prisoners of war in Algeria as “bargaining chips”, some of whom had been imprisoned for >20 years [94,95]. A United Nations-monitored ceasefire began in 1991, but Western Sahara's status has remained undecided (pending referendum), interspersed by ceasefire violations [53]. Insecurity also persisted when violent extremist groups emerged [96]. In 2010, Moroccan forces stormed a protest camp in Western Sahara, triggering violent demonstrations in the capital, Elayoun. In late 2011, three European aid workers were kidnapped from Polisario-administered refugee camps in Tindouf, Algeria (Figure 2), by the Movement for Unity and Jihad in West Africa (a faction of *al-Qaeda*) [96]. Polisario-Morocco tensions continued through 2016 and, although peace talks have been underway since 2018, tensions remain high and part of the country is held by the Polisario. Uncleared landmines constituted an additional source of persistent insecurity [97]. The continual conditions of insecurity, akin to Somalia, have stymied the development of a functional government that has crop protection capabilities. It is likely that, in Western Sahara, desert locust incursions and breeding will have to be controlled by external interventions (e.g., by Morocco, Mauritania, or cross-border multinational strike forces), assuming infested areas are accessible [6–8].

**Yemen. Direct Insecurity.** The two main Yemeni desert locust breeding areas are in the Tihama and Hadramawt regions (Figure 15) [29]. Insecurity in Yemen has resulted in a variety of obstacles to desert locust operations since 1986 [4]. The division of Yemen, in addition to chronic tribal conflicts, was partially responsible for exacerbating the 1986–1989 desert locust plague [8]. Tribal and clan frictions also impaired surveillance and control to a lesser extent during the 1992–1994 upsurge. Border conflicts with Saudi Arabia have impaired communication, travel, and desert locust surveillance on the border [8].
<figure><img src="image_61.png" /></figure>

**Figure 15.** Locations and regions mentioned in the text in relation to desert locust activity and inScheme 1990. when traditionalist North Yemen and Marxist South Yemen (Figure 15) merged after years of skirmishing [98,99]. In mid-1994, a civil war erupted for several weeks, ending in defeat for separatist southern Yemen [99,100]. The war damaged infrastructure [100] and ministries were looted, including the ministry of agriculture, weakening Yemen's anti-locust capability [8]. North versus south enmity persisted [100,101] through 2020.

There are ≈60 million firearms in Yemen, nearly three per citizen [102]. Assault rifles, grenades, and light firearms were, into the 1990s, commonly sold from street stalls and at arms bazaars [102]. Long-standing animosities resulted in many inter-clan and anti-government clashes [98], and Yemeni clans often used foreign hostages to extort the government for funds and infrastructural improvements [103]. Tribal resistance [104–107], however, also intensified, involving surrounding and laying siege to Sana’a (Figure 15) in 2011 [108].

Chronic disputes over tribal homelands and national frontiers have afflicted Yemen since the 1930s [109]. As an example of border disputes, in 1998, Yemen alleged that Saudi Arabia violated its border 73 times over four weeks; transgressions included military deployments, shelling, and violations of air space and waters [8,110–113]. During 1999, a fundamentalist imam exhorted Muslims to kill all “infidels” in Yemen [8]. In 2000, *al-Qaeda* suicide bombers attacked the U.S.S. Cole in the Port of Aden (Figure 15) [8] and a French tanker [114]. The United States and Yemeni militaries struck at *al-Qaeda* by air and ground mainly in remote Marib and Hadramawt areas (Figure 15) [8,115]. In November 2002, a United States missile strike killed the leader of *al-Qaeda* in the Arabian Peninsula [100].

Mass protests in 2004 over myriad grievances led to arrests and military clashes with insurrectionists [100]. Emergent combatant groups in 2004 included the Houthis and the Believing Youth, and through 2016, six consecutive wars enveloped the Sa’ada, Hajja, Amran, and Al Jawf governorats (Figure 15), killing thousands [100,101].

The 2011 rise of Arab Spring resulted in President Saleh’s removal in 2012, fracturing the army and the government [100]. Associated protests pitted civilians against law enforcement and military personnel, and many people were consequently attacked by activists and gangs [100,116]. Houthis rebels captured Sana’a (Figure 15) in September 2014 and the insurgency became a civil war [116]. The Houthis also battled *al-Qaeda* and Sunni Salafist movements [117].

In 2009, *al-Qaeda* called for an Islamic caliphate in Yemen [116] and seized territory in outlying provinces [118]. In March 2015, a Saudi-led military coalition (10 countries) intervened to defend the government against the Houthis and *al-Qaeda* and its affiliates [116,117]. By October, the coalition uprooted fighters of Zaydi Shiite Ansar Allah (Partisans of God) from most of southern Yemen, and the initial air campaign escalated into a ground conflict committing thousands of troops [118]. From March to October, ≈5000 people were killed,
including 2355 civilians and >150 coalition soldiers [118]; a Saudi airstrike (Figure 16) in September, for example, killed 80–130 civilians [118], and the United States launched airstrikes against *al-Qaeda* leaders [118]. In late September, Islamic State of Iraq and Syria (ISIS) suicide bombers killed 30 at a Sana’a mosque and battled against Houthis [118]. The conflicts seriously damaged Yemen’s inadequate, already war-torn, infrastructure [117]. The conflicts have continued through 2020. Indirect insecurity has crippled Yemeni capacity and efforts to monitor and control desert locusts.

<figure><img src="image_62.png" /><figcaption>**Figure 16.** Yemeni child walks though rubble from Saudi/Emirati airstrike on Sana'a; photo credit Felton Davis/Flickr.</figcaption></figure>

**3. Insecurity and the Recent Desert Locust Upsurge**

The most recent desert locust upsurge originated in the remote *Rub al Khali*, or Empty Quarter, of Saudi Arabia during the late spring and early summer of 2018 [5,69,119,120]. The outbreak was not detected until late 2018 when swarms moved into Yemen, countries of the Horn of Africa, and Egypt [5,121]. While Egypt, Eritrea, Ethiopia, and Sudan mounted substantial and largely effective control operations [5,121–132], Somalia and Yemen, both crippled by direct and indirect [active and inactive] effects of insurrection, insurgency, international and civil war, lawless interclan fighting, terrorism, and, in the instance of Somalia, landmines, were unable to conduct effective surveillance and control. After Saudi Arabia began combatting gregarious desert locust populations [121–138], Somalia and Yemen produced many swarms; from Somalia they moved into Kenya, Tanzania, Uganda, and Ethiopia, and from Yemen they invaded Oman, Iran, Pakistan, and India [swarms continued to escape from Saudi Arabia into Yemen, Oman, Iraq, Jordan, and other neighboring countries [5,121–138]. In all, ≥22 countries in Africa and Asia were involved by May 2020, and ≈2.8 million hectares were sprayed with insecticides [5]. While it would have been best to proactively intervene against the initial outbreak in the *Rub al Khali*, possibly averting further intensification and dissemination of gregarious desert locust populations, efficacious control in Somalia and Yemen might also have had a pivotal impact on suppressing the upsurge [5]. Because of the effects of armed conflict, Somalia and Yemen became engines that continued to drive the upsurge further as Saudi Arabia, by May 2020, had treated ≈400,000 ha, mitigating its role, to an extent, as a source [5].
4. Insecurity and Possible Mitigation Measures

Inaccessibility of desert locust breeding areas due to insecurity is generally an in-
tractable challenge requiring political solutions rather than technological advances. Con-
flicts can be acute, long-term, fluid, and sporadic as they erupt, subside, spread, morph,
and multiply. Reliable forecasting in neighboring countries will likely enable preposi-
tioning of resources to control swarms when crossing borders from inaccessible areas. In
the past, multinational teams, such as the now-defunct Maghrebian Strike Force [MSF],
conducted cross-border operations when certain desert locust afflicted countries failed
to do so [7]. The MSF was formed by the *Commission de lutte contre le criquet pèlerin en
Afrique du Nord-Ouest* (CLCPANO) using Islamic Development Bank, FAO, and member
country (Algeria, Libya, Mauritania, Morocco, Tunisia) funds [139]. Active in the early
and mid-1990s, the MSF mostly operated in Mauritania and Mali [8]. The MSF, however,
was as vulnerable to insecurity as national units. DLCO-EA also does not operate in war
zones and most of its member countries (Djibouti, Eritrea, Ethiopia, Kenya, Somalia, South
Sudan, Sudan, Tanzania, Uganda) have been in arrears [8], and other regional locust control
organizations were nonfunctional or abolished [8]. A new mechanism, the Commission
for Controlling the Desert Locust in the Western Africa Region (CLCPRO), established in
2016, makes a regional fund available for proactively controlling desert locust invasions,
has been endowed with ≈6.5 million dollars from its member countries (Algeria, Burkina
Faso, Chad, Libya, Mali, Mauritania, Morocco, Niger, Senegal, and Tunisia) [5,140,141].

In situations where an outbreak developed in rebel-held territory adjoining a sym-
pathetic country, covert cross-border cooperation has occurred [4,8]. Tensions between
nations, however, might have increased if the collaboration was discovered, and further
complicated if involved resources originated from international aid agencies.

In relatively recent years, the FAO's EMPRES program (Emergency Prevention System) in West and North Africa has proposed some approaches for dealing with insecurity. In the event of some forms of insecurity in a given area, the country should make efforts to ensure the safety of surveillance and control activities when possible, and, in consultation with the other countries of the region, establish reinforced surveillance in breeding areas of neighboring countries as a safeguard against possible desert locust swarm movement from the insecure area [142]. In addition, satellite imagery information on breeding conditions in the inaccessible areas is almost immediately available, making it possible to assess the probability of outbreaks and the value of strengthening surveillance in accessible breeding areas of neighboring countries [142]. As a contingency against actively developing gregarious desert locust episodes in insecure areas, emergency control plans for the existing national and regional systems can enable rapid response at lower cost compared to times when control systems and plans were not in place [142]. It has been suggested that desert locust survey teams should be comprised of indigenous personnel, familiar with the remote breeding areas, and with the consent and protection of local populations [142]. While the problem of insecurity has long been known, the recent desert locust episode demonstrates that this crucial aspect of desert locust management has not yet been sufficiently addressed.

5. Conclusions

While insecurity is mostly envisaged as a direct obstruction to surveillance and con-
trol operations, we suggest that insecurity is considerably more complex and crippling
than direct obstruction. In 10 of the 13 countries (77%) we cited as examples, insecurity,
whether direct or indirect (Tables 1 and 2), occurred. Insecurity in some instances has been
chronic, and diverse, often involving multiple simultaneous conflicts. Indirect, active and
inactive, insecurity does not obviously impair surveillance and control, but the adverse
impacts of indirect insecurity, although unmeasured, are likely substantial, particularly in
countries with sever insecurity problems, such as, at present, Somalia, Yemen, Sudan, and
possibly Chad.
<table><caption>**Table 1.** Thirteen countries with major desert locust breeding areas, and where direct insecurity occurred during different time periods [4–8,14,15,121–138].</caption><thead><tr><th rowspan="2">Countries with Breeding Areas</th><th colspan="5">Direct Insecurity <sup>a</sup></th></tr><tr><th>1986–1989 <sup>b</sup></th><th>1992–1994 <sup>b</sup></th><th>2003–2005 <sup>b,c</sup></th><th>2007–2016 <sup>b</sup></th><th>2018–2020 <sup>b,d</sup></th></tr></thead><tbody><tr><td>Chad</td><td>√</td><td></td><td></td><td></td><td></td></tr><tr><td>Eritrea</td><td>√</td><td>√</td><td></td><td></td><td></td></tr><tr><td>Ethiopia</td><td></td><td>√</td><td></td><td></td><td></td></tr><tr><td>India, Pakistan</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Mali</td><td></td><td>√</td><td></td><td>√</td><td></td></tr><tr><td>Mauritania</td><td>√</td><td>√</td><td></td><td></td><td></td></tr><tr><td>Niger</td><td></td><td>√</td><td></td><td></td><td></td></tr><tr><td>Saudi Arabia</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Somalia</td><td>√</td><td>√</td><td></td><td></td><td>√</td></tr><tr><td>Sudan</td><td>√</td><td>√</td><td></td><td></td><td></td></tr><tr><td>Western Sahara</td><td>√</td><td></td><td></td><td></td><td></td></tr><tr><td>Yemen</td><td>√</td><td>√</td><td></td><td>√</td><td>√</td></tr></tbody></table>

<table><caption>**Table 2.** Thirteen countries with major desert locust breeding areas, and where indirect insecurity occurred during different time periods.</caption><thead><tr><th rowspan="2">Countries with Breeding Areas</th><th colspan="5">Indirect Insecurity <sup>a</sup></th></tr><tr><th>1986–1989 <sup>d</sup></th><th>1992–1994 <sup>d</sup></th><th>2003–2005 <sup>d</sup></th><th>2007–2016 <sup>d</sup></th><th>2018–2020 <sup>d</sup></th></tr></thead><tbody><tr><td>Chad</td><td>√</td><td>√</td><td>√</td><td>√</td><td>√</td></tr><tr><td>Eritrea</td><td>√</td><td>√</td><td>√</td><td></td><td></td></tr><tr><td>Ethiopia</td><td>√</td><td>√</td><td>√</td><td>√</td><td>√</td></tr><tr><td>India, Pakistan <sup>b</sup></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Mali</td><td>√</td><td>√</td><td>√</td><td>√</td><td>√</td></tr><tr><td>Mauritania</td><td>√</td><td>√</td><td></td><td></td><td></td></tr><tr><td>Niger</td><td>√</td><td>√</td><td></td><td></td><td></td></tr><tr><td>Saudi Arabia <sup>c</sup></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Somalia</td><td>√</td><td>√</td><td>√</td><td>√</td><td>√</td></tr><tr><td>Sudan</td><td>√</td><td>√</td><td>√</td><td>√</td><td>√</td></tr><tr><td>Western Sahara</td><td>√</td><td>√</td><td>√</td><td>√</td><td>√</td></tr><tr><td>Yemen</td><td>√</td><td>√</td><td>√</td><td>√</td><td>√</td></tr></tbody></table>

The challenge of insecurity on grand, varied, and complex scales presents a unique obstacle to desert locust control in breeding areas. Further, other desert locust afflicted countries are also beset by insecurity (e.g., Afghanistan, Iraq, Libya, Syria). While some desert locust scenarios might be managed within conflict zones through one creative means or another, operations will likely continue to be impeded and thwarted by insecurity.

<footnote><sup>a</sup> Includes areas made inaccessible because of land mines. <sup>b</sup> 1986–1989 was a plague, 1992–1994 and 2003–2005 were upsurges, 2007–2016 had no major desert locust episodes and many outbreaks, and 2018–2020 was an ongoing upsurge at the time of this writing. <sup>c</sup> No reports of direct insecurity, 2003–2005; in July 2005, one area in Sudan was not accessible to ground surveillance due to “access difficulties” [143].</footnote>

<footnote><sup>d</sup> This upsurge was ongoing at the time of this writing.</footnote>

<footnote><sup>a</sup> Both active and inactive indirect insecurity. <sup>b</sup> India and Pakistan desert locust control capacities were likely not impaired by the Kashmir conflict [8]. <sup>c</sup> Despite Saudi Arabia's involvement in its ongoing war with Yemen, desert locust control capacity was likely not impaired.</footnote>

<footnote><sup>d</sup> 1986–1989 was a plague, 1992–1994 and 2003–2005 were upsurges, 2007–2016 had no major desert locust episodes and many outbreaks, and 2018–2020 was an ongoing upsurge at the time of this writing.</footnote>
Impediments posed by insecurity must be better taken into account for desert locust risk management planning, and realistic mitigation measures must be carefully designed and implemented.

**Author Contributions:** A.T.S. conceived the article and drafted the manuscript with substantial input from M.L. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Conflicts of Interest:** The authors declare no conflict of interest.

**Disclaimer on Maps:** The designations employed and the presentation of material in the maps do not imply the expression of any opinion on the parts of the U.S. Department of Agriculture and *Agronomy* concerning the legal or constitutional status of any country, territory or sea area, or concerning the delimitation of frontiers.

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 U.S. Department of Agriculture. USDA is an equal opportunity provider and employer.

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Article

# Ups and Downs of the Italian Locust (*Calliptamus italicus* L.) Populations in the Siberian Steppes: On the Horns of Dilemmas

Michael G. Sergeev<sup>1,2 ID</sup>

**Citation:** Sergeev, M.G. Ups and Downs of the Italian Locust (*Calliptamus italicus* L.) Populations in the Siberian Steppes: On the Horns of Dilemmas. *Agronomy* **2021**, *11*, 746. https://doi.org/10.3390/agronomy11040746

Academic Editor: Salvatore Davino

Received: 2 April 2021

Accepted: 10 April 2021

Published: 12 April 2021

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<figure><img src="image_63.png" /></figure>

**Copyright:** © 2021 by the author. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/).

**Abstract:** The Italian locust is a common species and one of the most important pest in the steppes, semi-deserts, and deserts of Central and Southwestern Asia and also in the Mediterranean Region. The aim of this paper is to discuss some problems arising from studies of eco-geographical distribution and long-term dynamics of this species, especially in the southern part of West Siberia, and their consequences for management of pest species and rare forms. Peculiarities of the Italian locust populations' distribution and long-term dynamics are discussed for the north-eastern part of the species range (south-eastern West Siberian Plain, North-East and, partly, East Kazakhstan). Notable differences between local populations are revealed. Some issues arising from results of insecticide experimental treatments (mainly effectiveness of barrier applications with different compounds in variable environment) and ecological association between the Italian locust and rare Orthoptera are also discussed. Some suggestions as to requirements for improved monitoring, supervision, control, and forecasting of the Italian locust populations' distribution and dynamics are also provided.

**Keywords:** West Siberian Plain; outbreak; distribution; dynamics; population; population management; plant protection

## 1. Introduction

The Italian locust (*Calliptamus italicus* (Linnaeus, 1758)) is a common and widely distributed species and the most important acridid pest in the steppes and semi-deserts of Eurasia, from Ukraine to the southern part of West Siberia, East Kazakhstan, and North-West China. Many of its outbreaks were in these areas, in mountains and oases of Central Asia, and also in the Mediterranean region. In the steppes, during the 20th century, its outbreaks occurred in all decades, but the situation became especially dangerous in the end of the last century, when in 2000 more than 16 million ha were infested and more than 10 million ha were treated by different acridicides from organophosphates to insect growth regulators [1,2].

The Italian locust is an intermediate form between typical gregarious and solitarious acridid species [1,3,4]. During outbreaks, the dense bands and swarms are common; however, their dispersal rates are usually limited. Both hopper bands and swarms can migrate over comparatively small distances, usually up to several hundred meters (rarely up to several kilometers) for bands and up to 100–200 km (rarely up to 750–800 km) for swarms [5–7]. Morphologically, the gregarious form of adults may be distinguished only by relatively long wings [3,8]. Gregarious nymphs of the Italian locust are usually characterized by dominance of dark colors (brown, grey, and black) [5,9].

Ecological peculiarities and distribution patterns of *C. italicus* are described in many publications for some parts of its range. An analysis of many publications was given by B.P. Uvarov [3]. He emphasized an insufficient level of our knowledge on the Italian locust ecology for periods between outbreaks. During recent decades, results of many studies concerning this species were published as well; however, nearly all of them are limited by

<footnote n="1">¹ Department of General Biology and Ecology, Novosibirsk State University, 2 Pirogova Street, 630090 Novosibirsk, Russia; mgs@fen.nsu.ru</footnote>

<footnote n="2">² Laboratory of Invertebrate Ecology, Institute of Systematics and Ecology of Animals, Siberian Branch, Russian Academy of Sciences, 11 Frunze Street, 630091 Novosibirsk, Russia</footnote>
outbreak periods [1,4,10,11]. Recently, the main part of all data was summarized for the territory of the former USSR [9].

The Italian locust is a typical univoltine form with overwintering eggs. Its spatial population distribution during periods between outbreaks shows that, in the northern part of its range, the species occurs in very dry habitats, in the central one, it prefers relatively dry and diverse habitats of the steppe and semi-desert life zones, while in the southern part, its colonies are usually localized in meadow habitats of river valleys or in mountains [4,12]. The species prefers highly heterogeneous semi-arid landscapes (especially with dominance of sagebrushes—*Artemisia* spp.) in the dry steppes and the semi-deserts. During outbreaks, the Italian locust can penetrate and colonize different types of fields cultivated and abandoned, and also other types of transformed habitats, e.g., roadsides, lawns, overgrazed sandy plots, etc. However, the Italian locust usually prefers to consume dicotyledon plants, e.g., *Artemisia* Linnaeus, *Lactuca tatarica* (Linnaeus) C.A.Mey, *Kochia* Roth, *Medicago* Linnaeus, *Glyzyrhiza* Linnaeus, etc. [3,9]. This means the species can seriously damage some cash crop fields, for instance, sunflowers, and, at least on the first stage of infestation, may eat and destroy different weeds across wheat fields.

The aim of this paper is to discuss some problems arising from studies of long-term dynamics of the Italian locust populations, especially in the southern part of West Siberia, and their consequences for population management of both possible pest and rare orthopteran species.

## 2. Materials and Methods

### 2.1. Study Territory

Original data were collected from 1979 until 2019 in the south-eastern part of West Siberian Plain and in the adjacent north-eastern and eastern parts of the Kazakh Uplands (Saryarqa). This region borders the Ishim River (left tributary of the Irtysh) to the west and south-west, the southern edge of the Kazakh Uplands to the south, the Altay Mts. to the south-east, and the Ob River to the east. Its northern boundary is approximately defined by the 56th parallel north. Average temperatures are relatively low (mean temperatures of the warmest month vary from 17 to 22 °C, the same for the coldest month—from −17 to −20 °C), and annual precipitation amounts vary from 280 to 520 mm [13]. Originally, this territory was covered with grasslands (from meadows to semi-deserts) and forests (mainly birch and pine) [13–15]. The main part of the plains was transformed in agricultural lands (fields and pastures), while the mountain steppes and semi-deserts are used for livestock grazing. Besides, there are some flood-plains with meadows and forest patches, sandy plots, solonchaks, and swamps. The Kazakh Uplands consist of small low ranges, numerous hills and small mountains, where some elements of altitudinal zonation may be observed.

### 2.2. Observation Sites

Nine fixed sites were selected to study long-term dynamics of the Italian locust populations in the central part of the so-called Kulunda steppe (between the Irtysh River and the central part of the Altaj Region). Almost all plots were covered by more or less typical steppe zonal vegetation. In some cases, vegetation cover was damaged by moderate grazing or haymaking. These sites were distributed from the northern steppes to the southern ones:

1. SE Aleksandrovskij settlement (Novosibirskaya Oblast (Novosibirsk Region), 53.67° N 78.25° E, northern steppe, studied in 2000–2008, 2015, 2018, 2019; in 2003 the local control, i.e., untreated, plot was moved about 100 m southwards, because the main part of the area was plowed) (Figure 1A).

2. S Burla settlement (Altai Krai (Altaj Region), 53.23° N 78.43° E, old abandoned field, 2000–2008).

3. SW Yarovoe town (Altai Krai (Altaj Region), 52.85° N 78.57° E, dry steppe (actually very old crested wheatgrass field), 1979, 1992, 1999–2008, 2015).
4. E Platovka settlement (Altai Krai (Altaj Region), 52.40° N 79.05° E, dry steppe, 1999–2008, 2015).

5. SE Severka settlement (Altai Krai (Altaj Region), 52.12° N 79.32° E, typical steppe, 1999–2008, 2015).

6. S Pokrovka settlement (Altai Krai (Altaj Region), 52.00° N 79.37° E, typical steppe, 1999–2008, 2015).

7. NW Mikhaylovskoye settlement (Altai Krai (Altaj Region), 51.82° N 79.62° E, typical steppe, 1999–2008, 2015; in 2003, we counted locusts on the adjacent steppe plot, 51.83° N 79.58° E, because we could not visit the stationary plot due to some technical problems).

8. W Bor-Forpost settlement (Altai Krai (Altaj Region), 51.87° N 80.03° E, typical steppe (actually very old crested wheatgrass field), 2000–2009, 2015).

9. E Ust-Volchikha settlement (Altai Krai (Altaj Region), 51.93° N 80.28° E, dry steppe, 1999–2009, 2015, 2018) (Figure 1B).

<figure><img src="image_64.png" /><figcaption>**Figure 1.** Typical habitats of the Italian locust in the Kulunda steppe: (A)—northern steppe (near Aleksandrovskij settlement); (B)—dry steppe (near Ust-Volchikha settlement).</figcaption></figure>

As a rule, locust abundance/density was counted in the first half and middle of July, while adults prevailed (see the Section 2.4). In this study, I analyzed data mainly for 2000–2008 when all nine sites were explored.

## 2.3. *Experimental Area*

In 2000, during the Italian locust outbreak, the dynamics of its local population was studied on the experimental area near Alexandrovskij settlement (see Section 2.2). This area included some plots with steppe vegetation, some agricultural fields, several small birch forests, a shallow depression with halophytes vegetation, meadows and willow bushes, and forest belts [16]. The steppe vegetation was quite diverse and heterogeneous (Figure 2). Several plant species, namely, *Phleum phleoides* (Linnaeus) H. Karst., *Agrostis capillaris* Linnaeus, *Koeleria macrantha* (Ledeb.) Schult., *Potentilla argentea* Linnaeus, *Galium verum* Linnaeus, *Veronica spuria* Linnaeus, *Artemisia glauca* Pall. ex Willd., *A. gr. frigida*, were common dominants. The local steppe habitats were used for very moderate livestock grazing and haying. The general distribution of *C. italicus* was studied over this area in June and July. These studies were continued in 2001 and 2002. Later we checked this population of the Italian locust once a summer, commonly in July.
<figure><img src="image_65.png" /><figcaption>**Figure 2.** Typical heterogeneous steppe habitat of the Italian locust with vegetation mosaic (grey—short sagebrushes, green—grasses or forbs) in the Kulunda steppe (near Aleksandrovskij settlement): red dot—exact position of an adult (31 July 2019).</figcaption></figure>

In 2000 and 2001, some experimental treatments were organized. The steppe parts were split into several experimental and control plots. In 2000, experiments were conducted with a water-based formulation of fipronil (phenylpyrazole insecticide). The acridicide was applied to 50% of the main experimental plot, by alternating, 15-m-wide swaths. The dose rate in the “barriers” was 4 g of a.i. per ha (i.e., 2 g of a.i. per protected ha). In 2001 the main experiments included different types of treatments with a suspension formulation of teflubenzuron (insect grow regulator—IGR). Four experimental treatments were conducted: (1) blanket, (2) barrier (alternating swaths of 15 m wide), (3) alternating treated (15 m) and untreated (30 m) swaths, and (4) alternating treated (15 m) and untreated (45 m) swaths. The dose rate in the “barriers” was 30 g of a.i./ha, while the rate in the “blanket” area was 22.5 g of a.i./ha. In 2003, some parts of the experimental area (north-eastern and south-western) were plowed to cultivate perennial grasses (mainly *Bromus inermis* Leyss.) However, after several years, the steppe vegetation has been recovered almost completely.

## 2.4. Field Studies

General distribution patterns of the Italian locust over the south-eastern part of West Siberian Plain and the adjacent north-eastern and eastern parts of the Kazakh Uplands were revealed on the basis of quantitative and qualitative samples collected in natural and transformed ecosystems, usually in the middle of summer when adults were dominant.

Field sampling was organized along gradient (usually transzonal), regional and local transects [4,17]. Each transzonal transect crossed the set of life zones, e.g., from the southern taiga to the northern deserts, and consisted of the set of regional and local transects. Each regional transect laid out a geographic region (e.g., the Kulunda steppe), commonly along river valleys, and was composed of local transects. Each local transect crossed a river valley or a lake basin from a local lower flood-plain to a watershed plain (plakor or flat interfluve) or mountain slopes. The length of local transects was from hundreds of meters to several kilometers. Grass ecosystems and similar transformed habitats were studied along local transects. As a rule, each part (habitat) of the local transect (lower and upper
flood-plains, lower and upper terraces, watershed plain, in some cases, their variants) was explored separately. Samples captured during a fixed period of time were made in every habitat studied [4,17,18]. Using this method, insects were captured with a standard net (40 cm diameter) over a period of 10–30 min. Results for each habitat were extrapolated to an hour. In some cases, a simple sweep netting was used too. Many habitats were also investigated to find species with very low abundance. This approach allowed us to obtain repeatable and comparable results over a number of years and habitats and independently of collectors (Figure 3).

<figure><img src="image_66.png" /><figcaption>**Figure 3.** Population distribution of the Italian locust along the local transect crossing the Ayagöz River Valley (semi-desert, eastern part of the Kazakh Uplands, 47.92° N 80.25° E). Collectors: 1975—I. Stebaev, E. Moiseeva; 1976—A. Lopatkin; 1986 and 1991—M. Sergeev).</figcaption></figure>

Some old data, mainly from the expeditions of Novosibirsk State University (1972–1977), were also used. We employed the Glonass/GPS receivers to determine geographical coordinates. We also used Google Earth Pro (©Google, 2020) to get the same parameters for habitats explored before 2000. The main part of studied specimens is in the collections of Novosibirsk State University and the Institute of Systematics and Ecology of Animals (Novosibirsk).

Peculiarities of long-term and seasonal dynamics and results of experimental treatments were revealed on the basis of the Italian locust density estimations. In each habitat studied, locusts were counted on arbitrarily placed plots 0.25 × 0.25 m² (in some cases—0.5 × 0.5 m² or 1 × 1 m²) [4,16,19]. We determined densities on 25 plots, while locust densities were high, and on 50–200 plots (sometimes more), when densities were low. After that, the average density was estimated for every habitat studied.

## 2.5. *Data Analysis*

The general distribution patterns of the Italian locust along transects and habitats were analyzed by simple ratio scaling to avoid some problems with variations of abundance estimations [4,20] (see also Figure 3). Three scales were used: from very low up to 19 ind. per hour; from 20 to 99 ind. per hour; more than 99 ind. per hour.

Data on the long-term and seasonal dynamics and results of experimental treatments are commonly non-parametric. The long-term dynamics patterns of the Italian locust populations on the model plots (see Section 2.2) were compared by the Spearman rank order correlation analysis. Significance of difference between years for each model plot was estimated by the Kruskal–Wallis ANOVA and median tests. The Mann–Whitney U test was also calculated for every consecutive pair of years on each plot. The same tests were used for the analysis of seasonal dynamics peculiarities and results of treatments (see Section 2.3). Effectiveness of treatments was estimated by the modified Abbott's formula considering population dynamics on both test and control plots [21]. Data analyses were mainly conducted using PAST 4.02 [22] and Statistica 10 (© StatSoft. Inc., Tulsa, OK, USA; now—© TIBCO Software Inc., Palo Alto, CA, USA).

The tegmen length/posterior femur length ratios (E/F) were also counted for populations studied [8,19], especially for outbreak seasons. According to K.A. Vasil'ev [8], the typical solitarious specimens have relatively short tegmina (E/F for females < 1.418 and