# Sentinel prey settings differ in their ability to distinguish organic from conventional apple orchards and in their sensitivity to weather conditions during field exposure

Claire Lavigne, Bertrand Gauffre, Cécile Thomas, Jean-Charles Bouvier, Xavier Said, Filipa Knapen, Jérôme Olivares, Alexis Cavazzini, Pierre Franck

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Claire Lavigne, Bertrand Gauffre, Cécile Thomas, Jean-Charles Bouvier, Xavier Said, et al.. Sentinel prey settings differ in their ability to distinguish organic from conventional apple orchards and in their sensitivity to weather conditions during field exposure. Biological Control, 2025, 210, pp.105875. 10.1016/j.biocontrol.2025.105875. hal-05265404

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# Sentinel prey settings differ in their ability to distinguish organic from conventional apple orchards and in their sensitivity to weather conditions during field exposure

Claire Lavigne*, Bertrand Gauffre, Cécile Thomas, Jean-Charles Bouvier, Xavier Said, Filipa Knapen, Jérôme Olivares, Alexis Cavazzini, Pierre Franck

INRAE, UR 1115, Plantes et Systèmes de culture Horticoles, Avignon 84000, France

## HIGHLIGHTS

* Monitoring of the predation of four sentinel prey for nine years in orchards.

* Sentinel prey were more informative when exposed in the canopy than on the ground.

* Correlated predation of aphids and *Ephestia kuehniella* eggs in the tree canopy.

* Higher predation of aphids and lepidoptera eggs but not seeds in organic orchards.

* *Ephestia* eggs are good candidate sentinel prey for large-scale studies.

## GRAPHICAL ABSTRACT

<figure><img src="image_2.png" /></figure>

## ARTICLE INFO

**Keywords:**

Aphid

Crop management

*Ephestia*

Indicator

Predation

Rain

Temperature

## ABSTRACT

Determining factors that affect pest predation by natural enemies is essential for designing cropping systems that promote conservation biological control. Predation of sentinel prey has become an increasingly popular measure of predator activity for comparing fields with differing characteristics of interest. However, many different sentinel prey exist, and such studies rarely examine how the specific prey used influences their conclusions, especially in perennial crops. We compared the ability of four sentinel prey, placed either in the tree canopy (eggs of two Lepidoptera, *Cydia pomonella* and *Ephestia kuehniella*, and aphids, *Acyrthosiphon pisum*) or on the ground (aphids, *A. pisum*, and seeds of *Viola arvensis*), to differentiate between organic and conventional apple orchards. Furthermore, we assessed whether their predation depended on weather conditions during field exposure. We monitored predation of these sentinel prey during two field sessions (April and May) over nine years in 14 apple orchards. Predation of aphids exposed for 24 h on the ground was always high. Predation of all other prey, except seeds, was higher in organic than in conventional orchards, particularly during the April field session. Additionally, the predation of seeds, and to a lesser extent of aphids, was influenced by weather conditions during exposure. *Ephestia* eggs emerged as a suitable choice for comparing predation by generalist predators among orchards, as they were easy to handle and their predation rates were positively correlated with

<footnote>* Corresponding author at: INRAE, PSH, 128 route de l'aérodrome, 84914 Avignon cedex 9, France.
E-mail address: claire.lavigne@inrae.fr (C. Lavigne).</footnote>
those of aphids exposed in the tree canopy. Codling moth eggs displayed a somewhat different predation pattern and could be a valuable complement.

# 1. Introduction

Many predatory species feed in agroecosystems, including birds, bats, small mammals, and arthropods. Predation is a key component of pest regulation, and it is estimated that resident predators on average reduce pests by 73% when comparing crops with and without predators (Boldorini et al., 2024). Conservation biological control, i.e. supporting populations of natural enemies present in the agroecosystem and promoting their effectiveness (Begg et al., 2017), is a key lever to reduce the use of insecticides that have detrimental effects on human health, biodiversity and the environment (e.g. Nicholson et al., 2024; Stehle and Schulz, 2015). Understanding which factors affect pest predation by natural enemies is thus necessary to design cropping systems that promote conservation biological control. The quantification of the predation achieved by predators on pest species is difficult because predation leaves elusive traces. Numerous pests are not easily tracked in commercial fields, predation acts can be brief, and some predators are nocturnal and small in size. A number of approaches have been employed to estimate predation in agroecosystems. Among these, the most common are observations of predation events by field observers or via camera traps, morphological or molecular identification of prey remnants in faeces, gut contents, or regurgitates of predators, enclosure/exclusion experiments, and sentinel prey (Daluwatta Galappaththige, 2024).

Sentinel prey are increasingly used in research projects (Fig. A1). The sentinel prey method involves exposing prey in the field and assessing the number of intact prey after a limited time interval. Additionally, predatory species may be identified using cameras (Seimandi-Corda et al., 2025; Gardarin et al., 2023; Tresson et al., 2022; Zou et al., 2017). Sentinel prey provide a standardized measure of the activity of a subset of the predators and can be used to compare the predation potential among plots differing in some characteristic of interest. They are not used to estimate predation on a specific pest since the setting is most generally artificial and the pest behaviour is modified (Ferrante et al., 2024; Zou et al., 2017). Sentinel prey offer many advantages: they are generally easy to produce and display, and allow standardized protocols on large numbers of sites. Sentinel prey mimic pests of interest to a greater or lesser extent. Prey can be live, dead, frozen, or fake (e.g., Nagy et al., 2020; Lövei and Ferrante, 2017). Several studies, for example, have used aphids stuck on cards (Boetzl et al., 2020; Rusch et al., 2015), lepidoptera, hemiptera, or diptera egg masses (e.g., Daelemans et al., 2023; Marliac et al., 2015; Monteiro et al., 2013), dead or live larvae or pupae of diptera or lepidoptera (e.g., Mathews et al., 2004). Researchers have also used plasticine caterpillars (e.g., García et al., 2021; Zvereva and Kozlov, 2023) and fake eggs (Gillis et al., 2012). Among sentinel prey that are used to estimate arthropod predation, the eggs of the Mediterranean flour moth *Ephesia kuehniella* (Zeller, 1879) are cost-effective as they are easily purchased fresh (possibly sterilized to prevent hatching) or frozen. Furthermore, developments are also underway to automatically estimate their predation rates using image recognition tools (Mouratidis et al., 2023).

In agricultural settings, sentinel prey have been used to compare predation between plots differing in management, presence of agro-ecological infrastructures, or landscape context (e.g., in perennial crops: Daelemans et al., 2023; Monteiro et al., 2013; Rusch et al., 2015). Although sentinel prey are increasingly used, only a few studies have determined how the choice of the prey can affect the study results by comparing outcomes obtained with different sentinel prey placed within the same plot. A comparative study on the predation of Helicoverpa armigera (Hübner, 1805) egg masses (live or dead) and larvae (dead or artificial) placed in a wheat field and a meadow revealed that prey status

(live or not) played a role in distinguishing between the two habitats for larvae, but not for eggs (Nagy et al., 2020). Similarly, a study tested the consistency of the effects of semi-natural habitats on different sentinel prey across a landscape gradient. These included five sentinel prey exposed on the ground, including *E. kuehniella* eggs, two seed species, larvae of two Diptera species, and one prey, glued aphids, exposed on the wheat crop. Predation of all prey was sensitive enough to detect the influence of semi-natural habitats at both local and landscape scales, but the response of predation rates varied among prey (McHugh et al., 2020). Lastly a three-year multisite study including both arable and orchard crops reported different response patterns of four sentinel prey to local pesticide use and landscape complexity with both *E. kuehniella* egg and aphid predation showing a positive response to landscape complexity when local pesticide use was low, while seed predation showed no significant response to local pesticide use (Ricci et al., 2019). Finally, in a slightly different vein, live and dead sentinel larvae of the beetle species *Zophobas morio* (Fabricius, 1776) exposed in rapeseed plots showed the same latitudinal gradient of predation, different from that indicated by fake clay larvae (Rodriguez-Campbell et al., 2024).

Perennial crops such as vineyards and orchards are likely to host diverse communities of pest enemies thanks to their multistrata structure and their stability. Several studies thus investigated conservation biological control in orchard crops, using sentinel prey both on the ground and in the tree canopy to compare the predation potential between landscapes or farming systems. Studies focusing on arthropod natural enemies have used a variety of sentinel prey, but most have used only one prey, generally designed to mimic a sensitive stage of the target pest (e.g., egg masses: Daelemans et al., 2023; Marliac et al., 2015; Monteiro et al., 2013; lepidoptera larvae: Mathews et al., 2004; aphids: Boetzl et al., 2020; Rusch et al., 2015; diptera larvae: Blaise et al., 2021). Since studies in annual crops indicate that the use of different sentinel prey could lead to different recommendations in terms of crop or landscape management, comparing the response of different prey to different management options appeared also necessary in perennial crops that moreover are multistrata habitats and thus harbour a large diversity of predators with different ecological requirements (Simon et al., 2010).

Sentinel prey can be considered as indicators of a predation potential and their choice should be guided by the typical qualities expected from indicators. These qualities depend on the context. At least a good ecological indicator should be relevant (i.e., vary with the indicated process), reliable (i.e., consistent in measurement), easy to sample and measure, and cost-effective. To guide the choice of sentinel prey, particularly in orchards, we present results of predation on four different sentinel prey exposed either in the canopy or on the ground twice a year in 14 commercial apple orchards over 9 years. The codling moth (*Cydia pomonella*, Linnaeus, 1758) and aphids, particularly the rosy apple aphid (*Dysaphis plantaginea*, Passerini, 1860) and the woolly apple aphid (*Eriosoma lanigerum*, Hausmann, 1802), are the most serious insect pests in apple orchards in southern France. We thus used three prey mimicking the predation-sensitive stages of these two pests and one prey related to seed predation. To help the choice of a sentinel prey, in addition to ease of experimentation, we investigated 1) which prey had a variable predation rate among orchards, 2) whether the predation rates of the different prey covaried and in particular if covariation among predation rates of prey exposed in the tree canopy was larger than with prey exposed on the ground, assuming that they would share the same generalist predators, 3) which prey had a predation rate that was not significantly sensitive to weather conditions during field exposure. Furthermore, from an ecological viewpoint, we hypothesized 4) that predation would increase from early to late spring, since the abundance of predatory arthropods in orchards of the study area was shown to
increase from early to late spring (Dib et al., 2010), and 5) that predation would be higher in organic than conventional orchards, since natural enemies are expected to be more abundant in organic orchards (Porcel et al., 2018), particularly in late spring after pesticide treatments accumulate in conventional orchards.

## 2. Materials and methods

### 2.1. Study sites

The study was carried out within a long-term monitoring site in the Lower Durance Valley (centre: WGS84 53° 50′ 10′ N; 4° 56′ 40′ E), in south-eastern France. This is a flat agricultural plain characterized by a dense network of ditches and hedgerows and by diverse farming systems. Fruit orchards are the dominant crop. The study was carried out in the same 14 commercial apple orchards from 2014 to 2022. These orchards had an average area of (mean ± se) 1.3 ± 0.4 ha, a plantation density of approximately 2000 trees per ha, a grassy ground cover between rows, and were bordered by wind-break hedgerows. The minimum distance between orchards was on average 1.4 km (range [0.85,3.2]).

Due to the conversion of four orchards to organic management during the course of the experiment (one in 2016, 2017, 2019, and 2020), the number of organic orchards increased from four in 2014 to eight in 2022. All other orchards were conventional and managed according to Integrated Pest Management (IPM) practices. The codling moth (*Cydia pomonella*) and the rosy apple aphid (*Dysaphis plantaginea*) were the two main insect pests in the study orchards. Regarding pest management, conventional orchards differed from organic orchards by the use of chemical herbicides rather than mechanical weeding of tree rows, and by the type of insecticides and fungicides used to manage pests and diseases. Conventional orchards were typically sprayed with six chemical insecticide treatments and 11 fungicides (including sulfur and copper) yearly from 2014 to 2022. Organic orchards were typically sprayed each year with two non-specific insecticides (including spinosyns and mineral or neem oils), seven treatments based on the CPGV virus, a virus specific to the codling moth, and five fungicides (only sulfur and copper). Tree rows of most organic orchards were also covered by nets as a protection against the codling moth.

Spring pest abundance was estimated each year based on plant symptoms observed on 32 to 50 trees per orchard. Using the presence-absence of leaf rolling symptoms due to the rosy apple aphids (2 shoots per tree) and of fruit damage by the codling moth (20 fruits per tree), we found that the abundance of these two pests appeared higher in organic than conventional orchards: throughout the experiment, fruit damage by the codling moth and shoot damage by aphids were on average (mean ± sd) 1.4 ± 3.9 % and 5.2 ± 8.6 % in organic orchards and 0.4 ± 1.2 % and 2.7 ± 6.2 % in conventional orchards.

### 2.2. Sentinel prey

Five types of predation cards were prepared each with one type of sentinel prey: Three 2.5 cm x 5 cm sandpaper cards with either eggs of the codling moth, eggs of *E. kuehniella* (thereafter named *Ephestia* for simplicity), or aphids, and two 5 cm x 5 cm sandpaper cards with either aphids or seeds. Sandpaper (here: LEMAN grain 120) is a robust standard support for sentinel prey that allows predatory insects to walk across the surface (e.g., Boetzl et al., 2020; Ricci et al., 2019; Seimandi-Corda et al., 2024). Cards were prepared approximately one month before field exposition. Regarding aphid sentinel prey, three adult unwinged aphids of the species *Acyrtosiphon pisum* (Harris, 1778) were recovered from a local rearing and glued alive on the centre of each predation card (glue: UHU® Twist&Glue solvent-free). For codling moth sentinel prey, egg laying data sheets were collected from our lab rearing. Pieces of approximately 1 cm x 1 cm containing 8 to 12 eggs were cut out and glued on the sandpaper cards (glue: SADER® All-purpose solvent-free).

*Ephestia* eggs were sprayed over glue dots of approximately 0.3 cm diameter (glue: SADER® All-purpose solvent-free). Ten seeds of *Viola arvensis* (Murray, 1770) were also glued directly on the cards (glue: SADER® WOOD PRO D3). Cards were frozen at -20 °C after preparation, once the glue was dry, except for seed cards.

Cards were set in the orchards during two field sessions each year at the end of April and the end of May, except for April 2020 due to the COVID-19 pandemic. Cards were set on 10 trees of the central row in the orchard. These trees were at least 15 m from the end of the row and were separated by 10 m. Cards with codling moth and *Ephestia* eggs, as well as the 2.5 x 5 cm aphid cards, were stapled to the underside of a leaf, at a height of about 1.50 m into the tree foliage. The three cards were attached to different branches, at least 50 cm apart. The 5 cm x 5 cm cards with seeds or aphids were nailed to the ground along the tree row, approximately 50 cm on each side of the trunk. Aphid cards were left for 24 h, and the other cards were left for four days following preliminary experiments showing that these durations resulted in intermediate levels of predation. Cards were set in all orchards the same day and, as much as possible, during a period without rain. Once cards were brought to the lab, they were examined for the number of remnant prey under a binocular magnifier. Prey that were removed during exposure were considered as predated, although other causes of prey removal cannot be excluded (e.g., for seeds, rain or loss of adhesive power of the glue, Westerman et al., 2003). Each card was characterized by a binary variable corresponding to the presence or absence of predation. As a comparison, for a supplementary analysis, predation rates were also calculated as the ratio of predated to exposed prey at the orchard level (Appendix A2).

### 2.3. Weather data

We characterized the weather during field exposure by the average of hourly temperatures (*Temp*), the sum of hourly rainfall (*Rain*), and wind speed (*Wind*, measured as the equivalent of distance travelled by the wind over the exposure period). Data were collected from the local INRAE weather station (INRAE CLIMATIK platform (https://agroclim.inrae.fr/climatik/), in French, managed by the AgroClim laboratory of Avignon, France). Distances between this weather station and monitored orchards were less than 15 km. Even though prey exposure occurred at similar periods of the year and we tried to avoid rainy days, conditions during exposure varied. For example, for prey exposed for four days, mean temperatures ranged from 11.8 °C to 18.8 °C for the April session and from 16.5 °C to 22.8 °C for the May session. It rained during aphid exposure (one day only in 2019, 23.5 mm) for the April session and never for the May session; rainfall ranged from 0 to 46.5 mm for the April session and from 0 to 6.5 mm for the May session for prey exposed for four days.

Spearman correlations among weather variables during field exposure were not significant, but correlations were strongly negative (r < -0.55) between wind speed and temperature at the first session for the 1-day exposure and at both sessions for the 4-day exposure. Wind speed during exposure had thus to be removed from statistical analyses due to excessive collinearity (variance inflation factors > 3). Correlation values can be found in Fig. A2.

### 2.4. Statistical analyses

To assess if predation rates covaried between prey types for each session and year, we calculated pairwise Spearman correlations among predation rates. To summarize these correlations, we then calculated the mean and standard error of the correlation values for each session over the years, and the number of years for which these correlations were significant.

To assess if different predation cards had different responses to crop management, we analysed variation in the occurrence of predation with generalized linear mixed models (glmm) with the R package lme4 (Bates
et al., 2015). Predation of aphids exposed on the ground was not analyzed due to a too low variation among orchards (see results). The occurrence of predation was modelled as a binomial variable with a logit link function. Orchard identity and year were included as random factors in all models. First, to assess if all types of predation cards responded similarly to crop management, we performed a model that included the type of predation card, the crop management (organic vs IPM), and their interaction, as well as the field session as fixed variables. Since the interaction was significant in this first model (see results), we then performed a model for each type of predation card independently, including crop management and session as fixed effects. Second, to assess if the crop management effect would be better detected at one or the other of the two field sessions, we performed models for each type of predation card and each field session, including only crop management as fixed effects. Lastly, to assess the sensitivity of predation rates to conditions during field exposure, we performed models including weather conditions during field exposure (i.e., Temp, Rain) and crop management as fixed effects, for each predation card type and session. Weather variables were scaled to allow comparison of effect sizes. Variance inflation factors were < 3 for all models, indicating low collinearity (Zuur et al., 2010). All model fits were assessed via residual analyses with the R package DHARMa. Following residual analysis, an observation-level random factor was added in the case of overdispersion (Harrison, 2014). Post-hoc Tukey tests were performed using the R package multcomp (Hothorn et al., 2008). All statistical analyses were performed with R 4.3.3 in the RStudio environment (R core team, 2024).

# 3. Results

## 3.1. Variation of predation among prey

Except for aphid cards exposed on the ground, predation rates were highly variable among orchards with a range [0,1] (Fig. A3). Predation rates also varied among prey (Table 1, Fig. A4). The most predated prey on average were aphids on cards laid on the ground with an average predation rate of (mean ± sd) 94.1 % ± 13.1 %, while prey in the tree canopy were least predated (codling moth eggs: 42.3 % ± 32.0 %, *Ephestia* eggs 38.6 % ± 29.6 %, aphids 45.3 ± 26.6 %). Seed predation was intermediate (55.2 % ± 32.3 %). Overall, predation rates were on average larger (estimate ± se: 0.23 ± 0.11) at the second field session (Table 1, Fig. A4), a trend confirmed in statistical models by prey for *Ephestia* eggs only (Table A1, Fig. A5).

## 3.2. Correlations among prey

Within-year pairwise correlations between predation rates of different prey mainly showed a pattern for prey located within the canopy. This pattern was most clear at the second session, where *Ephestia* egg predation and aphid predation were positively correlated for seven years, and codling moth egg predation was significantly positively correlated for three years with *Ephestia* egg predation and two years with aphid predation. The pattern was similar but weaker at the first session. The correlation between the predation of the two prey that were set on the ground was only significant once. It was positive.

<caption>**Table 1**</caption>

Analysis of the predation rates of four sentinel prey as a function of prey, crop management, field session, and the Prey x Management interaction. P-values < 0.05 are in bold.

<table><thead><tr><th></th><th>Df</th><th>Chisq</th><th>P</th></tr></thead><tbody><tr><td>Prey</td><td>3</td><td>45.64</td><td><strong>6.7 10<sup>-10</sup></strong></td></tr><tr><td>Management</td><td>1</td><td>17.04</td><td><strong>3.6 10<sup>-5</sup></strong></td></tr><tr><td>Session</td><td>1</td><td>4.75</td><td><strong>2.9 10<sup>-2</sup></strong></td></tr><tr><td>Prey x Management</td><td>3</td><td>11.43</td><td><strong>9.6 10<sup>-3</sup></strong></td></tr></tbody></table>

Predation rates of prey set on the ground and those set on the canopy were only significantly correlated once (Table 2, details in Table A2).

## 3.3. Crop management

Predation rates were significantly larger in organic than conventional orchards when all prey were considered together, but the crop management effect depended on prey type (significant prey x management interaction) (Table 1, Fig. A4). When prey were analysed independently, a significantly higher predation was observed in organic versus conventional orchards for all prey except for seeds (Table A1, Fig. 1). Models by field session further indicate that the effect of crop management was more frequently detected at the first session, being significant for all prey except seeds, than at the second session, when it was only significant for the codling moth and *Ephestia* eggs (Table A3, Figs. 1, A5).

## 3.4. Weather conditions during field exposure

Seed predation was the most sensitive to exposure conditions, with increased predation under colder (session 1) and wetter (both sessions) conditions. Aphid predation was little sensitive to weather conditions during exposure; only aphid predation in the tree canopy increased with temperature at session 2. Lastly, *Ephestia* and codling moth egg predation were not sensitive to conditions during exposure (Table A4, Figs. 2-3).

# 4. Discussion

A key criterion for sentinel prey is their ability to exhibit predation rates that vary among orchards with different levels of predator activity. We detected variation in predation rates for all prey types except aphids exposed on the ground, which were highly predated despite the short 24-hour exposure period. This high predation rate in comparison to that observed in arable crops (e.g., Boetzl et al., 2020) could be attributed to the stability of orchards, their grassy ground cover, and their irrigation, which created favourable conditions for a diversity of arthropods and slugs. Cameras placed above aphid sentinel prey exposed on the ground in arable crops indeed demonstrated that they could be predated by many generalist species, including slugs and scavengers (Gardarin et al., 2023). Predation rates on other prey, except seeds, furthermore, responded in the expected direction of higher predation in organic orchards (Monteiro et al., 2013; Porcel et al., 2018), thus confirming their ecological relevance. Indeed, several studies have shown that organic orchards have a greater diversity and abundance of predatory arthropods, which are less affected by pesticides and attracted by the large quantity of food (Hambäck et al., 2021; Odorizzi Santos et al., 2018; Porcel et al., 2018; Albert et al., 2017; Lefebvre et al., 2017). These higher abundance and diversity are expected to have a positive effect on sentinel prey predation (Snyder et al., 2019). However, despite being significant, the magnitude of the management effect in the present study was limited. Predation rates varied considerably among orchards for a given crop management, and further analysis of our data showed that the effect of management was rarely significant on a yearly basis (not shown). Predation rates on sentinel prey may, in fact, underestimate the difference in predators' activity between organic and conventional orchards. Many generalist predators are known to switch to the most abundant prey (Symondson et al., 2002). In the study orchards, codling moths and aphids were, on average, more abundant in organic orchards than in conventional ones, and organic orchards likely hosted more alternative prey, which may have deterred predators from sentinel prey.

The ability to detect differences between organic and conventional orchards depended on the timing of field exposure for some prey. Predation was significantly higher under organic management for the three insect prey at the first session. However, management was only significant for *Ephestia* and codling moth eggs at the second session. The
<caption>**Table 2**</caption>

Average of the yearly correlation *ρ* among predation rates of different sentinel prey measured either at the end of April (session 1) or end of May (session 2). When correlations were positive and significant (*p* < 0.05) more than two years out of nine, the colour indicates the number of years for which the correlation was significant (dark green: 7/9; medium green: 3/9; light green: 2/9). Values are underlined when the correlation was only significant for one year (full line: one significant positive correlation; dotted line: one significant negative correlation). Details on yearly correlation are provided in Table A2.

<table><thead><tr><th></th><th></th><th>Ephestia eggs</th><th>Seeds</th><th>Aphids-ground</th><th>Aphids-canopy</th></tr></thead><tbody><tr><td rowspan="4">Session 1</td><td>Seeds</td><td>&lt; 0.001</td><td></td><td></td><td></td></tr><tr><td>Aphids-ground</td><td>0.114</td><td><u>0.152</u></td><td></td><td></td></tr><tr><td>Aphids-canopy</td><td><u>0.256</u></td><td>-0.106</td><td>0.122</td><td></td></tr><tr><td>Codling moth eggs</td><td><u>0.124</u></td><td>0.056</td><td>0.200</td><td><u>0.083</u></td></tr><tr><td rowspan="4">Session 2</td><td>Seeds</td><td>0.010</td><td></td><td></td><td></td></tr><tr><td>Aphids-ground</td><td>0.159</td><td>0.188</td><td></td><td></td></tr><tr><td>Aphids-canopy</td><td><u>0.436</u></td><td>-0.101</td><td><u>0.208</u></td><td></td></tr><tr><td>Codling moth eggs</td><td><u>0.298</u></td><td>-0.109</td><td>0.103</td><td><u>0.308</u></td></tr></tbody></table>

<figure><img src="image_3.png" /><figcaption>**Fig. 1.** Predation rates in organic and conventional orchards at the first (April) and second (May) field session. For each session, different letters above the boxplots indicate statistically significant differences between conventional and organic orchards (Table A3). Tests were not performed for aphids exposed on the ground due to a lack of variation.</figcaption></figure>

reduced importance of management in late May was unexpected, given that insecticide treatments accumulate from April onwards, which could have increased the difference in predation between organic and conventional orchards from the first to the second field session. It may result from the arrival of additional predators in May in both organic and conventional orchards, which aligns with the increased predation rates

observed during the second session. Generalist species such as ladybugs and earwigs, for example, were shown to be mostly present in the aphid colonies at the end of May in the study region (Dib et al., 2010). Consequently, late predation may be less indicative of local management conditions and more sensitive to the quality of the seminatural elements surrounding the orchard. From a practical perspective, these
<figure><img src="image_4.png" /><figcaption>**Fig. 2.** Relationship between predation rates and average temperature during exposure at the first (April) and second (May) field sessions. Each data point is a yearly average, error bars are standard errors over orchards. When relationships are significant, the line represents the model estimate and the shaded area represents its standard error.</figcaption></figure>

results suggest that the ability to detect differences in local ecological functioning is less sensitive to the date of field exposure for *Ephestia* and codling moth eggs, which is an interesting feature when field work is constrained. These results also suggest that aphid predation may be more sensitive to the landscape environment in late May. A larger number of orchards would be required to investigate this question.

Exposing different sentinel prey is most useful when the prey are not redundant, i.e., when they are predated by different subsets of the predator community. Predation rates at the field level are expected to correlate positively among sentinel prey that share a pool of predators, or if their predators exhibit a similar response to environmental variables. We found that predation rates covaried little among prey in the early spring, with occasional negative correlations. In contrast, in late spring, they frequently covaried positively between *Ephestia* eggs and aphids exposed in the canopy, and, to a lesser extent, between these two prey and codling moth eggs. This outcome was surprising given the biological similarity between *Ephestia* eggs and codling moth eggs. *Ephestia* eggs and aphids may share a group of generalist predators in late spring, whereas codling moth eggs may be preyed upon by a somewhat different predator assemblage, notably in early spring. A similar response of *Ephestia* egg and aphid sentinel prey was also observed in a multi-crop setting, with predation of these prey increasing with landscape complexity in extensive fields (Ricci et al., 2019). In orchards, several types of natural enemies are known to feed on aphids, including earwigs, spiders, predatory bugs, and ladybirds (Dib et al., 2010). The majority of these natural enemies could also consume *Ephestia* eggs, a food source commonly utilised in insect rearing. To our knowledge, no study has yet investigated predators of sentinel aphids or Lepidoptera eggs in the tree canopy. Ladybirds have been observed preying on sentinel aphids glued on predation cards in arable crops (Greenop et al., 2019; Grez et al., 2024). Ants were also considered as predators of sentinel lepidoptera eggs in tropical vegetables (Frizzo

et al., 2020) and of sentinel diptera larvae on the ground and trunks of Mediterranean vineyards (Blaise et al., 2021).

To overcome the risk of poorly estimating predation by not considering the dependence of predation events on a single card, we transformed predation rates per card into presence-absence data. Dependence among predation events may have two origins. First a single predator may consume multiple prey on a single card, whereas it would not if prey were mobile or were scattered in the orchard (as is the case for codling moths eggs (Thiéry et al., 1995)). Second, some predators may learn their way to a food source (Pfeffer and Wolf, 2020). Lövei and Ferrante (2017) thus recommend exposing the prey for no longer than 24 h to ensure that predation events are independent. However, in the studied orchards, preliminary experiments had shown that predation rates of prey exposed for 24 h were too low, except for aphids. From a practical point of view, assessing the presence-absence of predation was also both faster and less prone to error than counting predated prey. Nevertheless, to ensure that this approach did not cause us to overlook any interesting results, we also analysed raw predation rates, i.e. the total number of prey predated divided by the total number of prey exposed per orchard at each session (e.g., Boetzl et al. 2020; Ricci et al. 2019). These complementary analyses revealed a stronger effect of the field session, while some effects became non-significant (e.g., the effects of weather conditions at session 1). However, most results were robust, and the primary conclusions concerning the effects of crop management and the differences and correlations among prey remained unchanged (Appendix 2).

To avoid the confounding effects of uncontrolled environmental factors, the sentinel prey should be exposed simultaneously in all fields. For practical reasons, this may prove difficult, and it may be necessary to stagger the exposure of the prey. However, if exposure dates are too far apart, weather conditions may change and plants and insects may be at different phenological stages. Considering a single session with
<figure><img src="image_5.png" /><figcaption>**Fig. 3.** Relationship between predation rates and total rainfall during exposure at the first (April) and second (May) field sessions. Each data point is a yearly average, error bars are standard errors over orchards. When relationships are significant, the line represents the model estimate and the shaded area represents its standard error.</figcaption></figure>

comparable phenological conditions, our results indicate that predation rates were little affected by weather conditions during exposure, except for seeds, although these varied to some extent from one year to the next. It should be noted, however, that aphid exposure was always carried out in the absence of rain, meaning that it was not possible to test its effect. Only predation on aphids in the canopy increased with temperature at the second session, which also corresponded to lower winds, favourable weather conditions for arthropod activity. The strong dependence of seed predation on wet weather conditions during exposure is a significant limitation of this prey type and is consistent with predation by snails or slugs (Gardarin et al., 2023). Conversely, the low sensitivity of insect prey to weather conditions within the range observed in our study is advantageous when field exposures cannot be fully synchronised for practical reasons. Predation rates, however, differed between the April and May sessions when considering all prey pooled, and particularly for *Ephestia* eggs. It therefore seems necessary to group all exposures over a short period.

In conclusion, our study confirms the relevance of sentinel prey in ecological studies that aim to compare the activity of predators at a large number of sites in a standardized and replicable setting that allows, for example, landscape-scale analyses or comparisons among studies. It also highlights the importance of selecting appropriate sentinel prey. Prey set on the ground proved little informative in contrast to prey located in the tree canopy. *Ephestia kuehniella* eggs appeared as a good option due to their cost-effectiveness, ease of handling, and the positive correlation between their predation rates and those of aphid prey. Codling moth eggs exhibited a slightly different pattern and, although more difficult to obtain, could be a good complement in apple orchards since they are close to the susceptible stage of a problematic pest. From a conservation biological control perspective, the enhanced ability of sentinel prey to detect management effects early in spring is of interest, as early predation of pests, before they become too numerous, is the most effective

(Symondson et al., 2002).

Caution should, nevertheless, be taken when interpreting the results of sentinel prey experiments. Sentinel prey represent only a loose proxy for pest predation, and they may be predated by species that would not predate live, mobile, or scattered prey (Zou et al., 2017). Scavengers were for example observed consuming glued aphids exposed on the ground (Gardarin et al. 2023). Furthermore, some aphids, such as *D. plantaginea* in apple orchards, are reared by ants that defend them against predators (Dib et al., 2010; Albert et al., 2017). When possible, it is therefore necessary to complement sentinel prey with observations of predators and pests (e.g., Boetzl et al., 2020; Charalabidis et al., 2025; Zou et al., 2017; Zvereva and Kozlov, 2023). Numerous studies have estimated pest enemy abundance or diversity and have attempted to relate them to sentinel prey predation (e.g., Badenhausser et al., 2020; Boetzl et al., 2020; Rusch et al., 2015). Methodological advancements have created new opportunities to more directly estimate pest-predator interactions. The deployment of light and robust video cameras could, for example, both indicate whether species most active in pest control were also active on sentinel prey, and point to the most active predators (Gardarin et al., 2023; Tresson et al., 2022; Zou et al., 2017). Genetic analysis of the gut content of predatory arthropods and the faeces of birds or bats has allowed the identity of predators to be determined under different crop managements (e.g., in apple orchards, García et al., 2024; Lefebvre et al. 2017), and the frequency of positive predators has been interpreted as a measure of their role in biological control (e.g., Szendrei et al. 2010). Passive acoustic monitoring of bats was also used to determine the frequency of predation events (Britton and Jones, 1999). Relating sentinel prey monitoring with such understanding of pest-predator interactions is instrumental in promoting conservation biological control.
## CRediT authorship contribution statement

**Claire Lavigne:** Writing – review & editing, Writing – original draft, Investigation, Funding acquisition, Formal analysis, Data curation, Conceptualization. **Bertrand Gauffre:** Writing – review & editing, Investigation, Data curation. **Cécile Thomas:** Writing – review & editing, Investigation, Data curation. **Jean-Charles Bouvier:** Writing – review & editing, Investigation, Data curation. **Xavier Said:** Investigation, Data curation. **Filipa Knapen:** Investigation, Data curation. **Jérôme Olivares:** Investigation. **Alexis Cavazzini:** Investigation, Data curation. **Pierre Franck:** Writing – review & editing, Investigation, Funding acquisition, Data curation, Conceptualization.

## Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

## Acknowledgements

The authors wish to thank the farmers for providing access to their orchards, students and Pauline L'hote for help with field work, and Sandrine Maugin and Elodie Lecerf for providing codling moth eggs. This work was partially funded by the STABIREG project funded by the French Office for Biodiversity (OFB). Field data were acquired within the SEBIOPAG-PHYTO project funded by the Ecophyto Plan of the French Ministry of Agriculture and Food and the French Foundation for Research on Biodiversity (FRB), the SEBIOPAG-STAR project funded by the French Office for Biodiversity (OFB) and the FRAMEwork project (European Union's Horizon H2020 research and innovation program under grant agreement No 862731).

## Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.biocontrol.2025.105875.

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