# Functional role of hedgerows and landscape composition in controlling a forest insect pest

Nattan Plat, Yohan Charbonnier, Irene Garcia-Celada, Mathieu Sannier, Olivier Bonnard, Jean-Baptiste Rivoal, Thomas Ribot, Thibaud Coupart, Inge van Halder, Hervé Jactel

► To cite this version:

Nattan Plat, Yohan Charbonnier, Irene Garcia-Celada, Mathieu Sannier, Olivier Bonnard, et al. Functional role of hedgerows and landscape composition in controlling a forest insect pest. Landscape Ecology, 2025, 40 (8), pp.152. 10.1007/s10980-025-02170-5. hal-05502517

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# Functional role of hedgerows and landscape composition in controlling a forest insect pest

Nattan Plat<sup>D</sup> · Yohan Charbonnier · Irene Garcia-Celada · Mathieu Sannier · Olivier Bonnard · Jean-Baptiste Rivoal · Thomas Ribot<sup>D</sup> · Thibaud Coupart<sup>D</sup> · Inge van Halder<sup>D</sup> · Hervé Jactel<sup>D</sup>

Received: 15 March 2025 / Accepted: 4 July 2025 / Published online: 19 July 2025
© The Author(s) 2025

## Abstract

*Context* Mixed forests exhibit greater resistance to pests compared to monospecific forests. However, stand-level diversification is challenging for managers. An alternative approach is to enhance tree diversity at the landscape scale.

*Objectives* Building on the agricultural landscapes with hedgerows, we hypothesized that in a pine plantation landscape broadleaved hedgerows along pine stands locally increase tree diversity, while broadleaved stands increase landscape level diversity. According to the associational resistance concept, both methods should reduce pest damage.

*Methods* We tested these hypotheses in a pine plantation landscape (dominated by *Pinus pinaster*), by counting nests of the pine processionary moth (*Thaumetopoea pityocampa*, hereafter “PPM”) along 36 pine edges adjacent or not to broadleaved hedgerows, in landscapes with low or high broadleaved cover. PPM bird and bat predators activities were assessed using acoustic devices.

*Results* At the local level, PPM nests were 42% less abundant in pine edges adjacent to taller hedgerows. This reduction was attributed to the ability of hedgerow to disrupt PPM host tree location and to the predation by great tit (*Parus major*), which was more active in pine edges adjacent to hedgerows. At the landscape level, higher broadleaved cover resulted in 72% reduction of PPM infestation. Although broadleaved cover did not significantly affect the activity

**Supplementary Information** The online version contains supplementary material available at https://doi.org/10.1007/s10980-025-02170-5.

N. Plat (✉) · O. Bonnard · J.-B. Rivoal · T. Ribot ·

T. Coupart · I. van Halder · H. Jactel

Univ. Bordeaux, INRAE, BIOGECO, 33610 Cestas,

France

e-mail: nattan.plat@inrae.fr

O. Bonnard

e-mail: olivier.bonnard@inrae.fr

J.-B. Rivoal

e-mail: jean-baptiste.rivoal@inrae.fr

T. Ribot

e-mail: thomas.ribot98@gmail.com

T. Coupart

e-mail: tcoupart99@gmail.com

I. van Halder

e-mail: inge.van-halder@inrae.fr

H. Jactel

e-mail: herve.jactel@inrae.fr

Y. Charbonnier · I. Garcia-Celada · M. Sannier

Ligue Pour La Protection Des Oiseaux, Bourrou, France

e-mail: yohan.charbonnier@lpo.fr

I. Garcia-Celada

e-mail: Irene.Garcia-Celada@lpo.fr

M. Sannier

e-mail: mathieu.sannier@lpo.fr
of PPM predators along pine edges, PPM nest abundance decreased with increasing species richness of bird predators consistently with the natural enemies hypothesis.

*Conclusions* Planting broadleaved hedgerows, combined with the conservation of broadleaved remnants, represents an effective management strategy for mitigating PPM infestation in pine plantation landscapes.

**Keywords** Pest control · Natural enemies · Landscape heterogeneity · *Pinus pinaster* · Pine processionary moth · *Thaumetopoea pityocampa*

## Introduction

In the context of climate change, the increasing frequency and intensity of natural disturbances are causing growing damage to forest ecosystems (Patacca et al. 2023; Forzieri et al. 2024). Over the past two decades, biotic disturbances, particularly insect outbreaks, have intensified in interaction with abiotic hazards such as wildfires, wind throws and droughts. As a result, insect outbreaks have become the first cause of tree biomass loss in Europe (Hlásny et al. 2021; Forzieri et al. 2024). This trend is especially pronounced in monospecific forests like conifer plantations, which have been shown to be more vulnerable to disturbances than mixed forests (Jactel et al. 2017, 2021; Sebald et al. 2021). Increasing tree species diversity at both stand and landscape levels has been proposed as a strategy to enhance forest resistance to disturbances (Brockerhoff et al. 2017; Marini et al. 2022). Regarding insect pest damage, a recent global meta-analysis demonstrated that tree species growing in pure stands suffer greater damage than the same species in mixed-species stands (Jactel et al. 2021). This phenomenon, known as "associational resistance" is well documented and can be explained by two key mechanisms. First, in mixed stands, the likelihood of an insect locating its host tree is reduced due to several factors: a lower density of host trees (i.e. the resource concentration hypothesis; Root 1973), the disruptive effect of cues emitted by non-host trees (i.e. the host apparency hypothesis; Castagneyrol, et al., 2014b), and the preferential attraction to non-host trees (i.e. the decoy hypothesis; Ruttan & Lortie 2015). Second, increased tree diversity can positively influence the richness and abundance

of natural enemies like predator and parasitoid species (i.e. the natural enemies hypothesis; Root 1973; Stemmelen et al. 2022), thereby enhancing pest control (Vázquez-González et al. 2024).

Although mixed forest stands generally provide higher levels of ecosystem services compared to pure stands (Brockerhoff et al. 2017), their adoption remains limited due to higher management costs and more difficult timber production, which slow down their implementation by stakeholders (Jactel et al. 2009; Sebald et al. 2021). An alternative and innovative approach to increasing local tree species diversity is by planting linear woody structures at the edges of monospecific stands. This method has been extensively studied in agricultural "bocage" landscapes, where crops are surrounded by perennial hedgerows that provide multiple ecosystem services, including biodiversity conservation and pest control (Montgomery et al. 2020; Boinot et al. 2023). In homogeneous agricultural landscapes, hedgerows offer habitats for biodiversity, including natural enemies of crop pests, such as bats (Coulthard et al. 2016; Hiller et al. 2025), birds (de Zwaan et al. 2024), spiders (Kratschmer et al. 2024), parasitoid wasps and lady beetles (Morandin et al. 2014), which can lead to an increase in predation pressure (Badenhausser et al. 2020; Ferrante et al. 2024). In conifer forest ecosystems, the role of broadleaved hedgerows in reducing insect pest infestations has been investigated in only a few studies on the pine processionary moth (*T. pityocampa*, hereafter "PPM") (Dulaurent et al. 2012; Castagneyrol, et al. 2014a; Plat et al. 2025). These studies identified mechanisms similar to those observed in associational resistance, including "the host apparency hypothesis" and "the natural enemies hypothesis", to explain how broadleaved hedgerows can mitigate PPM infestations in adjacent pine stands.

Increasing tree species diversity at the landscape scale can also influence insect pest dynamics through mechanisms similar as those at the stand scale (Marini et al. 2022). High concentration of host tree species within forest landscapes has been shown to benefit for insect pests (Senf et al. 2017; McNie 2023). Furthermore, non-host tree stands in the landscape can protect adjacent host stands by emitting chemical cues or acting as visual barriers, thereby slowing the spread of insect pests (Rigot et al. 2014; Nunes et al. 2021; Lantschner & Corley 2023). Some studies have hypothesized that part of the reduction in
insect pest damage in more diverse forest landscapes may be attributed to increased activity of natural enemies (Campbell et al. 2008; Marini et al. 2022). This relationship has been extensively studied in agricultural landscapes, with global reviews demonstrating that cover of native habitat, proximity of crops to native habitat, and landscape heterogeneity enhance the abundance and species richness of natural enemies (e.g. birds and arthropods), thereby improving pest control in crops (Boesing et al. 2017; Dainese et al. 2019). In forest landscapes, however, this question has only been explored indirectly. Studies have reported positive effects of broadleaved remnants in coniferous dominated forest landscapes on bat activity (Charbonnier et al. 2016) and bird abundance (Betts et al. 2010). To our knowledge, only one study, working on the interaction between the spruce budworm and its parasitoid wasps, has reported a direct effect of tree diversity at the landscape level on pest control by natural enemies (Cappuccino et al. 1998).

To investigate how tree diversity can mitigate insect pest damage at both the local and landscape level, we focused on a monospecific forest a, the “Landes de Gascogne Forest”, and its main defoliator, the pine processionary moth. This forest is predominantly composed of pure maritime pine (P. pinaster) plantations, with broadleaved remnants embedded within the pine-dominated matrix (Barbaro & van Halder 2009). PPM is the main native defoliator of pine trees in southern Europe, with periodic outbreaks that can significantly impact tree growth (Jacquet et al. 2013). The adult female moths lay their eggs in summer in the crown of pine trees (Uemura et al. 2021). From late summer to late winter, caterpillars go through five larval instars, feeding on pine needles. PPM larvae spend the day clustered together in silky nests, making their presence easily recognizable. By the end of winter, the caterpillars leave infested trees and move in procession toward the soil, where they burrow to enter the pupal stage, remaining underground until early summer (Roques 2015).

Increasing tree diversity at the stand level has proven to be an effective method for reducing PPM damage (Damien et al. 2016; Castagneyrol et al. 2020; Poeydebat et al. 2021). In pure pine plantations, PPM infestations are predominantly concentrated at edges, with far fewer attacks in the interior (Samalens & Rossi 2011). Dulaurent et al. (2012) counted PPM nests along five transects parallel to the edge of the

plots, ranging from the edge of the plantation (0 m) to 100 m inside, at intervals of 25 m. They found that 70% of caterpillar nests were located on the first line of trees (the pine edge), although this area represented only 20% of the trees sampled. This marked difference has been attributed to the preference of females for laying their eggs on trees whose dark silhouette stands out against a light background (Démolin 1969), or on the first host trees encountered after emerging from the ground in open areas (Régolini et al. 2014), as well as greater exposure to sunlight at the edges of plots, which favors the survival of larvae in nests (Uemura et al. 2021). Therefore, the presence of broadleaved hedgerows adjacent to pine stand edges had been shown to be a relevant diversification method for reducing PPM infestation in monospecific stands (Dulaurent et al. 2012; Plat et al. 2025). Non-host trees in hedgerows may serve as visual or chemical barriers, preventing female PPM moths from locating their pine hosts, particularly when hedgerows exceed the height of the adjacent pines. Hedgerows may also provide suitable habitat for natural enemies that may contribute to pest control in adjacent pine stands. This hypothesis is supported by evidence showing increased number of dummy caterpillars (models made of plasticine) attacked by birds along pine edges adjacent to hedgerows compared with absence of hedgerows (Plat et al. 2025). Nevertheless, this study focused solely on the great tit as a predator, highlighting the need for further research on the combined effects of multiple natural enemies. Hedgerows can also increase the mortality of PPM pupae by creating non optimal soil humidity and temperature conditions for their development (Dulaurent, et al. 2011a).

At the landscape scale, several variables have also been shown to influence PPM infestations in pine-dominated forests. One study found a negative correlation between riparian forest cover in the surrounding landscape and PPM infestation in pine edges, likely due to a combination of host tree reduction and increased natural enemies activity (Samalens & Rossi 2011). Conversely, grasslands in the surrounding landscape have been associated with higher PPM infestation, as these low-vegetation areas provide optimal light conditions for soil warming, thereby enhancing pupal survival (Azcárate et al. 2023).

Several natural enemies of PPM have been identified in the study region, including birds (Barbaro &
Battisti 2011), bats (Charbonnier et al. 2014; Garin et al. 2019), and parasitoid hymenoptera (Dulaurent et al., 2011b). Most of these natural enemies can benefit from the presence of broadleaved trees at both local and landscape scales. Broadleaved trees provide essential nesting cavities for several bird species, such as the Eurasian hoopoe (Upupa epops) (Barbaro et al. 2008), the great tit (P. major) (Mänd et al. 2009), the blue tit (Cyanistes caeruleus) (Kirwan, Gosler, et al., 2024a), the crested tit (Lophophanes cristatus) (Gosler & Clement 2020), and roost sites for various bat species (Charbonnier et al. 2016). This is particularly important in pine plantation landscapes, where nesting cavities are scarce (Acloque et al. 2023). Additionally, the long-tailed tit (Aegithalos caudatus) can find nesting sites in the broadleaved shrub layer (Harrap 2020). Beyond nesting opportunities, broadleaved trees also offer complementary feeding resources (e.g. greater abundance of lepidopteran larvae; Kennedy & Southwood 1984) that are particularly beneficial for tit species during their breeding season (Billerman et al. 2022). Furthermore, the presence of broadleaved trees can support insectivorous

passerines, which are parasitized by the common
cuckoo (Cuculus canorus), a highly specialized
predator of PPM caterpillars (Barbaro & Battisti
2011).

In this study, working in a landscape dominated
by monospecific pine plantations, we examined
whether broadleaved hedgerows (hereafter “hedge-
rows”) at the local level and broadleaved stands at
the landscape level could play a functional role in
reducing PPM abundance. While previous studies in
the same region have focused on the role of broad-
leaved hedgerows or stands in influencing the activ-
ity of individual PPM natural enemy species, using
specific experimental designs (Charbonnier et al.
2016; Plat et al. 2025), our study aims to consider
the entire community of PPM natural enemies.

More particularly, we examined the following
hypotheses (Fig. 1):

H1: The number of PPM nests is lower at edges of pine stands adjacent to broadleaved hedgerows than in the absence of hedgerows, particularly when the hedgerow is taller than adjacent pine trees. Additionally, the number

**Fig. 1** Hypothetical Framework. Black boxes represent variables, while arrows indicate relationships between variables. Broadleaved trees and stands are represented in orange while pine stands are represented in green (differences in the intensity of the green color indicate differences in the age of pine stand)

<figure><img src="image_2.png" /></figure>
of PPM nests is lower when the pine edge is in a landscape with a high cover of broadleaved stands compared to a low cover.

H2: The activity of vertebrate natural enemies of PPM is higher in pine edges adjacent to hedgerows than in the absence of hedgerows. Additionally, the activity of natural enemies is higher when the pine edge is in a landscape with a high cover of broadleaved stands compared to low cover.

H3: Vertebrate natural enemies of PPM, benefiting from the improved habitat conditions provided by broadleaved stands, can contribute to reducing the number of PPM nests.

# Methods

## Study region

Located in the southwest of France, the “Landes de Gascogne Forest” is a 1.16 million-hectare region, predominantly composed of pure stands of maritime pine (P. pinaster; 66% of area), thereby constituting the largest planted forest in Europe (Barbaro et al. 2005). These even-aged plantations are intensively managed, with harvesting occurring every 30–40 years, resulting in open areas following clearcutting. Other forest types (13%) in the region are not managed (or low intensity managed) and include riparian forests dominated by *Alnus glutinosa* and *Quercus robur*, broadleaved stands primarily composed of native oak species (mainly *Q. robur*, *Q. pyrenaica* and *Q.suber*). The remaining landcover is divided among agricultural land (15%), primarily dedicated to corn production, urban zones (4%), and inland water bodies (2%) (IGN, 2013; Mora et al. 2012). Within this pine plantation-dominated landscape, linear broadleaved hedgerows—similar to those found in agricultural bocage landscapes (Montgomery et al. 2020)—are located at the edges of maritime pine plantations and near roads, forest tracks, or ditches. These hedgerows have been preserved for their function as property boundaries, for their cultural and recreational value, particularly for hunting, mushroom picking, firewood collection, and their aesthetic value (Plat et al. 2025). Based on field observations, we defined hedgerows for this study as linear formations of broadleaved trees measuring at least

100 m in length, 8 m in height and one or two trees in width, with continuous tree crowns and dominated by the most abundant native broadleaved species (*Q. robur* and *Q. pyrenaica*).

## Experimental design

We conducted our study in an area of 50,000 ha (center coordinates: X: -0.776865; Y: 44.560623), characterized by a land cover representative of the “Landes de Gascogne Forest” (i.e. pure pine plantation 72%, other forest types 13%, agricultural land 10%, urban zones 4% and inland water bodies 1%) (Fig. 2). Within this area, we used infrared colour orthophotographs (IGN, 20 cm pixel resolution, https://geoservices.ign.fr/documentation/donnees/ortho/bdortho) at a 1:2500 scale in GIS to identify 1) broadleaved hedgerows and 2) broadleaved and mixed forest patches. Then, we selected 36 study sites to assess the effects of hedgerows and broadleaved stands cover on (1) PPM infestation and (2) multitaxonomic biodiversity, including vertebrate natural enemies of PPM. The study sites consisted of edges of maritime pine plantations selected on three criteria: the presence or absence of a hedgerow along the pine edge, the direct connectivity of the hedgerow with a broadleaved stand, and the cover of broadleaved stands in the surrounding landscape.

Pine stand edges were 100–325 m long, at least 5 m high, and adjacent to either (1) a hedgerow (n=24) or (2) a forest road, representing the control (absence of a hedgerow; n=12). In both cases, another pine stand was located on the opposite side of the hedgerow or forest road, with an average distance of 26 m between the two pine stands (with a standard deviation of 12 m). As connectivity of hedgerows to broadleaved stands could affect species dispersal, 12 of the 24 hedgerows were connected at one end to a broadleaved stand. Study sites without hedgerows and half of those with hedgerows were classified as non-connected, as we ensured they were at a minimum distance of 100 m from broadleaved stands and 50 m from other hedgerows. This distance was considered relevant for evaluating the effect of connectivity on multitaxonomic diversity, including groups with low dispersal capacity like spiders or ground beetles. However it can be considered relatively short when applied to taxa with high dispersal capacity,
<figure><img src="image_3.png" /><figcaption>**Fig. 2** Map of the study area (50 000 ha). Circles and triangles show the location of study sites and the corresponding modalities. Bottom left map shows the location of the "Landes de</figcaption></figure>

Gascogne Forest” in southwest Europe. The background map comes from Google Map (Google Maps 2025)

such as bats and birds (including predator species of PPM; Fahrig 2013; Horner-Devine et al. 2003).

Percentage cover of broadleaved stands in the surrounding landscape (hereafter “broadleaved cover”) was evaluated within a 500 m buffer around the pine edge of each study site. Broadleaved stands included riparian forests or patches dominated by native oaks (*Q. robur* and *Q. pyrenaica*), sometimes mixed with maritime pine. The scale at which landscape composition has an effect on species presence or diversity is influenced by the movement range of species within each taxonomic group, making it difficult to determine the most relevant buffer size when considering multiple species or taxonomic groups. However, in our study, broadleaved cover in the 200 m, 500 m, and 1000 m buffer areas were highly correlated (P=0.002, R²=0.22; P<0.001, R²=0.75). Of these, the 500 m buffer zone was chosen as it corresponds to

the dispersal capacity of most vertebrate predators of PPM (Nicholls & Racey 2006; Slagsvold et al. 2013). For each modality of hedgerow (presence/absence) and connectivity (connected, non-connected), study sites were equally selected between low]0-;6%] (n=18) or high [14–36%] (n=18) broadleaved cover in the surrounding landscape. Sites from each category were evenly distributed across the study area (Fig. 2).

The average distance between study sites was 1.7 km (ranging from 0.7 to 3.9 km). Additionally, because the height difference between the hedgerow and the adjacent pine stand (i.e. hedgerow height minus pine stand height) can influence PPM behaviour (Dulaurent et al. 2012), we categorized hedgerows into two groups based on their height: tall hedgerows, with height differences ranging from +5 m to – 1 m (n=11), and small hedgerows,
with height differences ranging from – 2 m to – 10 m (n=13). We checked that tall and small hedgerows were evenly distributed among the study modalities and randomly distributed in the landscape. We then created a new variable (hereafter referred to as "edge type") with three categories: tall hedgerow, small hedgerow, and no hedgerow.

## Pine processionary nests monitoring

PPM winter nests were monitored in February 2023 and 2024 as a proxy for PPM infestation level. Given that the number of PPM nests is consistently higher at the edges of pine stands than inside stands (Sama-lens & Rossi 2011; Dulaurent et al. 2012; Régolini et al. 2014), we considered that infestation levels observed at the pine stand edges are representative of the overall stand infestation. Therefore observations were conducted on 30 pines from the first row of trees along the edge. Nests were counted on sunny days by two trained observers using binoculars, with obser-vations made from two opposite directions to ensure accuracy (Dulaurent et al. 2012).

## Bird surveys

Bird monitoring was conducted using passive acous-tic recorders, combined with expert identification of bird species calls and songs (Schillé et al. 2024). In April 2023, Song Meter Mini Bat recorders equipped with acoustic microphones (Wildlife Acoustics) were installed at each site. The recorders were placed at the edge of the 36 pine stands, on a pine tree in the first row, at a height of 2 m. The recorders were active for five consecutive days, operating between sunrise and sunset, and recorded one minute every three minutes. To create a sub-sample of audio files for bird iden-tification, we selected two days with no precipitation and wind speeds below 30 km/h. For each day we selected a sub-sample of five one-minute audio files recorded between 8:30 and 8:45 AM (corresponding to 1.5 h after sunrise). This process yielded a total of 360 1-min files (5 files×2 days×36 sites), corre-sponding to six hours of recordings. The audio files were analysed by expert ornithologists (YC, MS and IGC), who identified all bird species present in each file. For each study site, we used the number of files where a species was detected as a measure of bird species vocal activity (ranging from 0 to 10; Schillé

et al. 2024). We focused on bird species described as predators of the PPM in the study area (Barbaro & Battisti 2011). Six species were present in our dataset: the common cuckoo, the great tit, the Eurasian hoopoe, the crested tit, the Eurasian blue tit, and the long-tailed tit.

The four tit species maintain the same breeding territories across years. Their home range is small in spring (1-2 ha for the great tit; Bouvier et al. 2022) and is larger in winter when they forage in flocks (3.9 ha; Saitou 1979). Common cuckoo and Eurasian hoopoe are migratory species nesting in the area; they usually return to the same nesting location each year (Yavin 1987; Slagsvold et al. 2013; Theillout 2015; Billerman et al. 2022). Although these bird species prey on PPM at different seasons, for technical rea-sons we chose to monitor their activity only once, in spring, which is known to be the most appropriate season for bird monitoring. Therefore, we considered that the benefits of hedgerows and broadleaved cover for birds in spring, when they need suitable habitats and resources for nesting and feeding the chicks, could lead to an increase in predation pressure on the PPM throughout the year (as it has been shown for the great tit in the same region; Plat et al. 2025). To assess the overall impact of the PPM bird preda-tor community, we used the species richness of PPM bird predators per site (ranging from 0 to 6 species), as summing up or averaging bird vocal activity across different species is not appropriate due to their differ-ences in acoustic traits (Funosas et al. 2024).

## Bat surveys

Bat ultrasounds were recorded using the same devices and setup as for birds, but recordings were conducted in July 2023 to coincide with the peak emergence of PPM moths (Démolin 1969) and main hunting activity of bats during the nursing season (Barataud et al. 2020). Recorders were active for five consecu-tive nights, operating from sunset to sunrise and sav-ing a 5-s file each time ultrasounds were detected. Bat activity was analysed based on two nights with-out rain and wind speeds below 30 km/h (Barataud et al. 2020). To ensure the recordings captured bats potentially preying on PPM, we focused on the period between one and four hours after sunset as it was reported as the most active for PPM moths (Charbon-nier et al. 2014), resulting in a dataset of 74,697 files
for analysis. Identifying bats to the species level from ultrasound sequences is challenging due to the complexity of sonotypes, which may represent multiple species, and requires specialized expertise. Previous studies conducted in similar contexts within the study region indicated that bats of the genus *Pipistrellus* accounted for 63 to 92% of bat activity depending on the season (Charbonnier et al. 2014, 2016). Consequently, we opted to use bat activity as our primary metric instead of species richness. Using Kaleidoscope software (v. 5.6.8), bat activity was visually identified by four observers (IVH, JBR, TC, and NP) as presence of three or more pulse calls within a 5-s file (Jung et al. 2012). The mean number of files with bat pulses across the two nights was then calculated and used as a measure of bat activity. The number of feeding buzz, which correspond to active attempts to capture prey, were also counted in the complete dataset of 74,697 files (Schnitzler & Kalko 2001).

## Data analysis

As connectivity of hedgerows to broadleaved stands had no statistically significant effect on PPM infestation, bird or bat activity (Table S1), we excluded this variable from further analysis.

We used the following modelling approach to test our three hypotheses (Fig. 1):

H1: To evaluate the effect of edge type and broadleaved cover on the number of PPM nests, we used Generalized Linear Mixed-Effect Models (GLMMs) with a Poisson distribution, which is appropriate for count data. Since PPM nests were counted in two consecutive years at the same study sites, we included year as a fixed effect in the model and the study site ID as a random effect. With only two levels, year is better treated as a fixed effect, as random effects with few levels are poorly estimated and can skew the results (Harrison et al. 2018).

H2: The effect of edge type and broadleaved cover on the activity of PPM bird predators was assessed using Generalized Linear Models (GLMs) with a binomial distribution. Bird activity (ranging from 0 to 10) was used as the response variable. Among the six bird species studied, four species (long-tailed tit, blue tit, crested tit, and Eurasian hoopoe) were

not abundant, being absent from more than 50% of the study sites. While a Zero-Inflated model would have been appropriate to model the effects of variables on their activity, this approach requires at least 10 replicates per modality, whereas our study included only 6 replicates per modality. To ensure the robustness of the analysis, we therefore restricted the model to species present on at least 50% of the study sites (i.e. great tit and common cuckoo; Kouakou 2022).

The effect of edge type and broadleaved cover on the species richness of PPM bird predators was evaluated using a GLM with a binomial distribution, as species richness ranged from 0 to 6. For bats, a linear model with log-transformed bat activity values was used to reduce the influence of outliers and meet assumptions of normality and homoscedasticity of model residuals. A GLM with a quasi-Poisson distribution was used for feeding buzz to account for the overdispersion of the data.

H3: To evaluate the relative contributions of edge type, broadleaved cover and natural enemies to the number of PPM nests, structural equation modelling (SEM) would have been the ideal approach as it allows differentiation between direct and indirect effects. However, due to the limited number of replicates, we could not carry out SEM and therefore opted to construct separate models based on H1 with the sequential addition of activity or richness variables of PPM natural enemies (H3a-e). Because great tit activity was the only variable positively influenced by hedgerow presence, we built an additional model (H3f) replacing the edge type variable by great tit activity variable to evaluate whether the effect of hedgerow on PPM nests was mediated by the associated activity of great tit.

The interaction between edge type and broadleaved cover was tested for H1 but was removed in the final model as it was not significant. Interactions were not included in other model as we sought to ensure the robustness of the results given the limited number of replicates. Model assumptions of residual normality and homoscedasticity were assessed graphically. All statistical analyses were
conducted in R (R Core Team, 2016, version 4.4.0) using the lmerTest package.

## Results

### H1: Effect of hedgerows and broadleaved cover on PPM infestation

Based on observation of 36 pine edges with two time replicates (February 2023 and 2024), the number of PPM nests per edge (located on 30 pines of the first tree row) ranged from 0 to 163 with a mean of 19.1. The three factors studied (edge type, broadleaved cover and year) significantly contributed to the model and explained 40% of the variation in the number of PPM nests (Table 1; H1). The number of PPM nests was lower on pine stand edges adjacent to hedgerows, with 14.7±7.4 (mean±standard error) PPM nests in pine edges adjacent to tall hedgerows, 17.0±2.8 PPM nests along small hedgerows and 25.2±6.6 PPM

nests in absence of a hedgerow. Although this difference was significant only for tall hedgerows compared to the absence of hedgerows, it represents a reduction of 33% (small hedgerows) and 42% (tall hedgerows) in the number of PPM nests compared to the absence of hedgerows (P=0.965 and P=0.012 respectively, Table 1, H1, Fig. 3A). The broadleaved cover in the landscape surrounding pine edges showed a strong and significant negative effect on the number of PPM nests, with 29.7±6.0 nests in landscapes with a low broadleaved cover and 8.4±1.4 nests in landscapes with a high broadleaved cover, which represents a reduction of 72% (P<0.001, Fig. 3A). Finally, the number of PPM nests significantly increased by 75% between February 2023 and 2024 (P<0.001).

### H2: Effect of edge type and broadleaved cover on PPM natural enemies' activity and richness

Among PPM bird predators recorded on the 36 sites, the common cuckoo was the most present (29 sites)

<table><caption>**Table 1** Summary of GLM and GLMM testing the effect of the type of pine edge and the broadleaved cover in the landscape on the number of PPM nests per pine edge (H1) and on bird and bat natural enemies of PPM (H2a-e)</caption><thead><tr><th>H</th><th>Response variable</th><th>Predictors</th><th>×2</th><th>Df</th><th>Coeff. ± SE</th><th>P</th><th>R2m (R2c)</th><th>n</th></tr></thead><tbody><tr><td rowspan="4">H1</td><td rowspan="4">Number of PPM nests per pine edge</td><td>Small hedgerow</td><td>7.08</td><td>2</td><td>-0.01±0.39</td><td>0.965</td><td>0.40 (0.97)</td><td rowspan="4">72</td></tr><tr><td><strong>Tall hedgerow</strong></td><td>7.08</td><td>2</td><td>-1.04±0.41</td><td><strong>0.012</strong></td><td></td></tr><tr><td><strong>High broadleaved cover</strong></td><td>12.8</td><td>1</td><td>-1.16±0.33</td><td><strong>&lt;0.001</strong></td><td></td></tr><tr><td><strong>Year</strong></td><td>100</td><td>1</td><td>0.56±0.06</td><td><strong>&lt;0.001</strong></td><td></td></tr><tr><td rowspan="2">H2a</td><td rowspan="2">Great tit activity</td><td>Small hedgerows</td><td>7.24</td><td>2</td><td>1.05±0.50</td><td><strong>0.030</strong></td><td>0.22</td><td rowspan="2">36</td></tr><tr><td>Tall hedgerows</td><td>7.24</td><td>2</td><td>1.16±0.50</td><td><strong>0.020</strong></td><td></td></tr><tr><td rowspan="3">H2b</td><td rowspan="3">Common cuckoo activity</td><td>High broadleaved cover</td><td>7.48</td><td>1</td><td>-0.16±0.34</td><td>0.625</td><td></td><td rowspan="3">36</td></tr><tr><td>Small hedgerows</td><td>20.8</td><td>2</td><td>-0.90±0.29</td><td><strong>0.002</strong></td><td>0.41</td></tr><tr><td>Tall hedgerows</td><td>20.8</td><td>2</td><td>0.29±0.27</td><td>0.277</td><td></td></tr><tr><td rowspan="3">H2c</td><td rowspan="3">Species richness of PPM bird predators</td><td>High broadleaved cover</td><td>22.1</td><td>1</td><td>-0.26±0.23</td><td>0.245</td><td></td><td rowspan="3">36</td></tr><tr><td>Small hedgerows</td><td>1.17</td><td>2</td><td>0.01±0.35</td><td>0.969</td><td>0.13</td></tr><tr><td>Tall hedgerows</td><td>1.17</td><td>2</td><td>0.31±0.35</td><td>0.382</td><td></td></tr><tr><td rowspan="3">H2d</td><td rowspan="3">Bat activity</td><td>High broadleaved cover</td><td>3.84</td><td>1</td><td>-0.46±0.28</td><td>0.103</td><td></td><td rowspan="3">36</td></tr><tr><td>Small hedgerows</td><td>7.20</td><td>2</td><td>-1.02±0.45</td><td><strong>0.029</strong></td><td>0.08</td></tr><tr><td>Tall hedgerows</td><td>7.20</td><td>2</td><td>-0.82±0.47</td><td>0.088</td><td></td></tr><tr><td rowspan="3">H2e</td><td rowspan="3">Bat feeding buzzes</td><td>High broadleaved cover</td><td>7.84</td><td>1</td><td>-0.27±0.37</td><td>0.478</td><td></td><td rowspan="3">36</td></tr><tr><td>Small hedgerows</td><td>32.5</td><td>2</td><td>-1.28±0.59</td><td><strong>0.038</strong></td><td>0.41</td></tr><tr><td>Tall hedgerows</td><td>32.5</td><td>2</td><td>-0.58±0.47</td><td>0.890</td><td></td></tr><tr><td></td><td></td><td>High broadleaved cover</td><td>33.0</td><td>1</td><td>-0.04±0.39</td><td>0.916</td><td></td><td></td></tr></tbody></table>

For each model, pine edges without hedgerows and low broadleaved cover in the landscape are the reference modality compared respectively to pine edges adjacent to small or tall hedgerows and high broadleaved cover in the landscape. The effect of the year was included in model H1 to account for the temporal repetition of PPM nest counts. Significant variables are indicated in bold. R2m and R2c correspond to the variance explained by fixed effects and fixed+random effects respectively. Values of n indicate sample size
<figure><img src="image_4.png" /><figcaption>**Fig. 3** : Boxplots showing the effect of edge type (absence of hedgerows or presence of small or tall hedgerows) and broadleaved cover on (A) number of PPM nests on the pine edge (mean values of winter 2023 and 2024) and (B) great tit vocal activity. Boxes indicate the lower and upper quartiles. Thick horizontal lines represent the median, white dots indicate the</figcaption></figure>

and active with a mean activity of 3.69±0.52 per site, followed by the great tit (18 sites) with a mean activity of 1.08±0.26 per site and crested tit (14 sites) with a mean activity of 0.72±0.21. Blue tit (8 sites), long-tailed tit (6 sites) and Eurasian hoopoe (5 sites) were less present and less active with a mean activity of 0.44±0.16, 0.31±0.12 and 0.33±0.17, respectively.

Great tit was significantly more active in hedge-
rows, with a mean activity of 1.30±0.3 and 1.45±0.7
for tall and small hedgerows respectively, compared
to 0.5±0.2 in absence of hedgerows (P=0.03 and
P=0.02 respectively, Table 1, H2a, Fig. 3B). The
common cuckoo was similarly active in pine edges
adjacent to tall hedgerows and in absence of hedge-
rows but less active in pine edges adjacent to small
hedgerows (P=0.277, P=0.002, H2b, Fig.S1A).
Broadleaved cover had no significant effect on great
tit or common cuckoo. The species richness of PPM
bird predators varied from 0 to 5 with a mean of 2.2
species per site. Neither edge type nor broadleaved

mean value. The sample size was n=36 for each figure. Black
dots show extreme values. Different letters show significant
differences between edge types according to the model (A) H1
and (B) H2a (Table 1). Broadleaved cover had A) a significant
effect on number of PPM nests (P<0.001, Table 1) and B) no
significant effect on great tit activity (P=0.625, Table 1)

cover affected the species richness of PPM bird pred-
ators (Table 1, H2c, Fig.S1B).

Based on 3 h recording during two nights, we identified 12,996 bat passes with a mean of 180 per site per night and high variation between sites from 5 to 1273. Bat activity was higher along pine edges with no adjacent hedgerows (216±37), than at pine edges adjacent to a small hedgerow (179±37) or a tall hedgerow (142±44), although the difference was only marginally significant for tall hedgerows (P=0.029 and P=0.088 respectively, Table 1, H2d, Fig.S1C). Broadleaved cover in the landscape had no significant effect on bat activity. Both variables did not explain an important part of the variation in bat activity (R²=0.08). We also identified 130 feeding buzzes with a mean of 3.6 per site including 13 sites out of 36 without any feeding buzzes. While bat activity and number of feeding buzzes were positively correlated (P=0.002; R=0.45), and broadleaved cover had no effect on the number of feeding buzzes, we observed a slight difference for the relationship
<caption>**Table 2** Comparison between GLMM models testing the effect of the type of pine edge, the broadleaved cover in the landscape (H1) and the effect of great tit activity (H3a, H3a')</caption>

or the species richness of PPM bird predators (H3b) on the number of PPM nests per pine edge

<table><thead><tr><th>H</th><th>Response variables</th><th>Predictors</th><th>×2</th><th>Df</th><th>Coeff.±SE</th><th>P</th><th>R2m (R2c)</th><th>AIC</th></tr></thead><tbody><tr><td rowspan="4">H1</td><td rowspan="4">Number of PPM nests per pine edge</td><td>Small hedgerow</td><td>7.08</td><td>2</td><td>-0.01±0.39</td><td>0.965</td><td>0.40 (0.97)</td><td>590.2</td></tr><tr><td><strong>Tall hedgerow</strong></td><td>7.08</td><td>2</td><td>-1.04±0.41</td><td><strong>0.012</strong></td><td></td><td></td></tr><tr><td><strong>High broadleaved cover</strong></td><td>12.8</td><td>1</td><td>-1.16±0.33</td><td><strong>&lt;0.001</strong></td><td></td><td></td></tr><tr><td><strong>Year</strong></td><td>100</td><td>1</td><td>0.56±0.06</td><td><strong>&lt;0.001</strong></td><td></td><td></td></tr><tr><td rowspan="4">H3a</td><td rowspan="4">Number of PPM nests per pine edge</td><td>Small hedgerow</td><td>7.10</td><td>2</td><td>-0.09±0.42</td><td>0.821</td><td>0.40 (0.97)</td><td>592.0</td></tr><tr><td><strong>Tall hedgerow</strong></td><td>7.10</td><td>2</td><td>-1.10±0.43</td><td><strong>0.011</strong></td><td></td><td></td></tr><tr><td><strong>High broadleaved cover</strong></td><td>12.8</td><td>1</td><td>-1.18±0.33</td><td><strong>&lt;0.001</strong></td><td></td><td></td></tr><tr><td><strong>Year</strong></td><td>100</td><td>1</td><td>0.56±0.06</td><td><strong>&lt;0.001</strong></td><td></td><td></td></tr><tr><td rowspan="4">H3b</td><td rowspan="4">Number of PPM nests per pine edge</td><td>Bat activity</td><td>0.23</td><td>1</td><td>-0.07±0.15</td><td>0.632</td><td></td><td></td></tr><tr><td>Small hedgerow</td><td>7.19</td><td>2</td><td>-0.16±0.40</td><td>0.695</td><td>0.42 (0.97)</td><td>591.2</td></tr><tr><td><strong>Tall hedgerow</strong></td><td>7.19</td><td>2</td><td>-1.05±0.41</td><td><strong>0.010</strong></td><td></td><td></td></tr><tr><td><strong>High broadleaved cover</strong></td><td>13.0</td><td>1</td><td>-1.17±0.32</td><td><strong>&lt;0.001</strong></td><td></td><td></td></tr><tr><td rowspan="4">H3c</td><td rowspan="4">Number of PPM nests per pine edge</td><td><strong>Year</strong></td><td>100</td><td>1</td><td>0.56±0.06</td><td><strong>&lt;0.001</strong></td><td></td><td></td></tr><tr><td>Bat feeding buzzes</td><td>1.03</td><td>1</td><td>-0.03±0.04</td><td>0.310</td><td></td><td></td></tr><tr><td>Small hedgerow</td><td>8.38</td><td>2</td><td>-0.02±0.35</td><td>0.965</td><td>0.50 (0.97)</td><td>586.2</td></tr><tr><td><strong>Tall hedgerow</strong></td><td>8.38</td><td>2</td><td>-0.89±0.38</td><td><strong>0.018</strong></td><td></td><td></td></tr><tr><td rowspan="4">H3d</td><td rowspan="4">Number of PPM nests per pine edge</td><td><strong>High broadleaved cover</strong></td><td>15.6</td><td>1</td><td>-1.40±0.30</td><td><strong>&lt;0.001</strong></td><td></td><td></td></tr><tr><td><strong>Year</strong></td><td>100</td><td>1</td><td>0.56±0.06</td><td><strong>&lt;0.001</strong></td><td></td><td></td></tr><tr><td>Species richness of PPM bird predators</td><td>6.75</td><td>1</td><td>-0.38±0.15</td><td><strong>0.009</strong></td><td></td><td></td></tr><tr><td>Small hedgerow</td><td>7.16</td><td>2</td><td>-0.07±0.40</td><td>0.840</td><td>0.41 (0.97)</td><td>591.9</td></tr><tr><td rowspan="4">H3e</td><td rowspan="4">Number of PPM nests per pine edge</td><td><strong>Tall hedgerow</strong></td><td>7.16</td><td>2</td><td>-1.02±0.41</td><td><strong>0.013</strong></td><td></td><td></td></tr><tr><td><strong>High broadleaved cover</strong></td><td>13.0</td><td>1</td><td>-1.19±0.33</td><td><strong>&lt;0.001</strong></td><td></td><td></td></tr><tr><td><strong>Year</strong></td><td>100</td><td>1</td><td>0.56±0.06</td><td><strong>&lt;0.001</strong></td><td></td><td></td></tr><tr><td>Common cuckoo activity</td><td>0.34</td><td>1</td><td>-0.03±0.06</td><td>0.561</td><td></td><td></td></tr><tr><td rowspan="4">H3f</td><td rowspan="4">Number of PPM nests per pine edge</td><td>Small hedgerow</td><td>7.15</td><td>2</td><td>0.03±0.39</td><td>0.965</td><td>0.41 (0.97)</td><td>592.0</td></tr><tr><td><strong>Tall hedgerow</strong></td><td>7.15</td><td>2</td><td>-0.98±0.41</td><td><strong>0.012</strong></td><td></td><td></td></tr><tr><td><strong>High broadleaved cover</strong></td><td>12.9</td><td>1</td><td>-1.17±0.33</td><td><strong>&lt;0.001</strong></td><td></td><td></td></tr><tr><td><strong>Year</strong></td><td>100</td><td>1</td><td>0.56±0.06</td><td><strong>&lt;0.001</strong></td><td></td><td></td></tr><tr><td rowspan="4">H3g</td><td rowspan="4">Number of PPM nests per pine edge</td><td>Great tit activity</td><td>0.27</td><td>1</td><td>-0.05±0.11</td><td>0.604</td><td></td><td></td></tr><tr><td><strong>High broadleaved cover</strong></td><td>9.85</td><td>1</td><td>-1.12±0.35</td><td><strong>&lt;0.001</strong></td><td>0.27 (0.97)</td><td>595.0</td></tr><tr><td><strong>Year</strong></td><td>100</td><td>1</td><td>0.56±0.06</td><td><strong>&lt;0.001</strong></td><td></td><td></td></tr><tr><td>Great tit activity</td><td>0.79</td><td>1</td><td>-0.10±0.11</td><td>0.375</td><td></td><td></td></tr></tbody></table>

For each model, pine edges adjacent to no hedgerows and low broadleaved cover in the landscape are the reference modality compared respectively with pine edges adjacent to small or tall hedgerows and high broadleaved cover in the landscape. The effect of the year was included to account for the temporal repetition. Significant variables are indicated in bold. R2m and R2c correspond to the variance explained by fixed effect and fixed+random effects respectively. AIC Akaike Information Criterion; lower values indicate better model fit. Sample size was n=72 for all models

between feeding buzzes and edge types. The number of feeding buzzes was higher in pine edges without hedgerows (5.0±1.4), similar in pine edges adjacent to tall hedgerows (4.7±1.8) and lower in pine edges adjacent to small hedgerows (1.38±0.5) (Table 1, H2e, Fig.S1D).

H3: Contribution of PPM natural enemies to the reduction in number of PPM nests

Neither bat activity nor feeding buzzes had a significant effect on the number of PPM nests. However, models that included these variables (H3a and
H3b) exhibited AIC values similar to the baseline model (H1) (difference between AIC < 2). Incorporating feeding buzzes increased the proportion of explained variance slightly, from 0.40 to 0.42 (Table 2). The species richness of PPM bird predators significantly and negatively affected the number of PPM nests in pine edges (P = 0.009; Table 2; H3c; Fig. 4). The model that incorporated the species richness of PPM bird predators obtained the lowest AIC, and this variable increased the proportion of explained variance from 0.40 in H1 to 0.50 in H3c. Neither common cuckoo nor great tit activity showed a significant additional effect on the number of PPM nests when these variables were added to the baseline model (H1) (Table 2; H3d, H3e). Although the corresponding models had similar AIC values (590.2 for H1, 591.9 with common cuckoo activity, and 592.0 with great tit activity), including these variables only marginally increased the explained variance (from 0.40 to 0.41). When the edge type was removed from the model including great tit activity (H3f), great tit activity remained non-significant, and the overall explained variance dropped from 0.41 to 0.27.

<figure><img src="image_5.png" /><figcaption>**Fig. 4** Effect of species richness of PPM bird predators on the number of PPM nests in pine edge (P = 0.009; Table 2; H3c). Mean values of PPM nests were used between winter 2023 and 2024, data were square root transformed to reduce the impact of extreme values on the graphical representation. Dots show the transformed data distribution. Grey area indicate standard deviation</figcaption></figure>

## Discussion

By monitoring PPM infestation in a pine plantation landscape, our study demonstrated that broadleaved trees reduce the abundance of PPM nests at both the local and landscape level. At the local level, PPM nests were 42% less abundant in pine edges adjacent to a taller broadleaved hedgerow, while at the landscape level, higher broadleaved cover was associated with 72% lower PPM nests abundance in pine edges. This reduction could be partly explained by the role of natural enemies. The number of PPM nests in pine edges significantly decreased with increasing species richness of PPM bird predators, and, locally, with increasing activity of the great tit in pine edges adjacent to hedgerows.

### Effect of broadleaved hedgerows on PPM infestation and vertebrate natural enemies

Our results showed that the presence of a hedgerow reduced the number of PPM nests in the adjacent pine edge, particularly when the hedgerow was taller than the adjacent pine stand, corresponding to a pattern of “associational resistance”. Several mechanisms can be responsible for this associational resistance. Although hedgerows contributed to increasing tree diversity at the local level, the abundance of host trees in the adjacent pine stand remained high. Therefore, the mechanism of reduced host concentration was probably not involved in the reduction of PPM nests. On the contrary, this reduction is most likely explained in part by the reduced “host apparentcy” mechanism, in line with the results of Dulaurent et al. (2012) who showed that a hedgerow taller than the adjacent pines may act as a visual or chemical barrier for PPM female moths when locating their host trees for egg-laying (Jactel et al. 2011). Although our measurements were limited to the edges of pine stands, results from Dulaurent et al. (2012) suggest that the barrier effect of hedgerows may also extend up to 25 m into the plantation interior. Beyond this distance, infestation levels were near zero, even in the absence of hedgerows. PPM vertebrate natural enemies sheltered by the hedgerow could have also contributed to the reduction in PPM nests. By evaluating their activity along the same pine edges, we found that the great tit was more active in pine edges adjacent to hedgerows compared to pine edges without
hedgerows. Great tit activity was recorded in April, during the breeding season, when broadleaved trees in hedgerows provide more food resources (mainly lepidopteran larvae) than pine plantations (Billerman et al. 2022). Since the number of microhabitats is generally higher in broadleaved trees than in conifers and increases with tree age (Acloque et al. 2023), great tit may have also benefitted from a greater availability of nesting cavities in broadleaved trees compared to pine plantations (Billerman et al. 2022). Similarly, in late summer when PPM egg masses are present, great tits were found to be three times more abundant in pine edges adjacent to hedgerows than in pine edges without hedgerows (Castagneyrol et al. 2014a). Likewise, while working in the same region but with a different experimental design, Plat et al. (2025) showed that in winter, when great tits prey on mature PPM caterpillars, there was a higher predation rate by birds in pine edges adjacent to hedgerows, and that the predation rate by birds was positively correlated with great tit vocal activity. Broadleaved hedgerows may thus benefit great tits throughout the year by providing nesting sites and food resources, leading to higher great tit densities and increased predation on PPM caterpillars in winter when other food sources are scarce (Kirwan et al., 2024b). This predation pressure may occur both along the edges and within the interior of pine stands, as great tits are capable of flying several hundred meters in search of food. However, when we tried to disentangle the effect of great tit activity from the presence of hedgerows on PPM nests we observed that great tits contributed only marginally to the reduction in PPM nests. Although a previous study found that great tit vocal activity was correlated with bird predation in winter (Plat et al. 2025), vocal activity in spring did not accurately reflect predation pressure during winter. In winter, great tits can vocalize while foraging in flocks, whereas in spring, vocalization is more closely linked to territorial behavior (Kirwan et al., 2024b). Additionally, statistical limitations—such as the use of a successive linear model instead of structural equation modeling (SEM)—may have hindered our ability to disentangle the confounding effects of hedgerows and great tit activity on the reduction of PPM nests.

The activity or species richness of other PPM bird predators were not higher in pine edges adjacent to hedgerows compared to the absence of hedgerows. In forest landscapes, bird biodiversity is generally

higher in mixed broadleaved-conifer stands compared to monospecific stands (Ampoorter et al. 2020) and is typically greater in deciduous forests than in coniferous forests (Willson & Comet 1996). The use of narrow hedgerows to enhance forest diversity at a local scale is likely less effective in providing habitat for birds than larger mixed stand could do. Another possible explanation for the lack of significant effect of hedgerows on bird richness is that some of the bird species studied may have found complementary resources in pine plantations, which prevented us from detecting significantly higher vocal activity in hedgerows. For example, Eurasian hoopoes and crested tits use nesting cavities provided by broadleaved trees while foraging in pine plantations for the food resources they offer (Barbaro et al. 2008; Gosler & Clement 2020). Contrary to our hypothesis, bats were more active in pine edges without hedgerows. In agricultural landscapes, hedgerows can serve as flyways for bats foraging in open areas (Coulthard et al. 2016). By contrast, in forested landscapes, bats tend to use forest edges as flyways to avoid the denser structure of forest stand interiors (Morris et al. 2010). Thus, the presence of a dense hedgerow could reduce the open space available between pine stands and thereby the possibility for bats to disperse easily. Moreover, we sampled bat activity during the PPM flying period, which may have led to a positive numerical response of bats in pine edges with high PPM infestation (Charbonnier et al. 2014). This idea is further supported by the fact that feeding buzzes, corresponding to prey capture attempts, were also more frequent in pine edges in the absence of hedgerows.

### Effect of broadleaved forest cover in the landscape on PPM infestation and vertebrate natural enemies

We found that the number of PPM nests in pine edges significantly decreased with increasing broadleaved cover in the pine plantation landscape. There was a reduction of more than 70% of PPM nest abundance in pine edges in forest landscapes with 14–30% of broadleaved cover compared to landscapes with less than 6% broadleaved cover. This result is consistent with a study in Canada, which showed a 20% higher growth of spruce trees during a *C. fumiferana* outbreak when spruce stands were surrounded by a high broadleaved cover (> 50%) compared to those with
low broadleaved cover (<50%) in the surrounding landscape (Campbell et al. 2008). In the same study area as our research, a previous study reported a negative relationship between PPM nest density and the percentage of riparian broadleaved forests in the landscape (Samalens & Rossi 2011).

While reduced host concentration is a mechanism often proposed to explain the limitation of insect pest infestations in heterogeneous landscapes (Marini et al. 2022), we believe that in our study area, pine plantations largely dominate the landscape (still over 70% of land cover) and therefore provide unlimited food resources. Broadleaved stands, especially riparian forests, differ from pine plantations in composition, structure, stratification, and chemical cues, potentially acting as barriers that could disrupt host tree location by PPM moths during their dispersal flight (Marini et al. 2022). However this reduced “host apparenty” effect at the landscape level is unlikely to be responsible for such an important reduction in the number of PPM nests, given the low cover of non-host tree stands (ranging from 14 to 30%) in the landscape.

Instead, increased natural enemy activity in landscapes with a higher broadleaved cover is the most probable mechanism driving the reduction in PPM infestation. A global review in agricultural landscapes reported a positive relationship between pest suppression provided by bird species, and native habitat cover, landscape heterogeneity, or proximity to native habitats (Boesing et al. 2017). Similarly, Tortosa et al. (2023) found that landscape heterogeneity positively influenced bat species richness and activity, with greater diversity and abundance observed in landscapes combining crops, vineyards and forests compared to crop-dominated landscapes. When considering forest stand diversity at the landscape scale, two studies reported significant effects of broadleaved cover on bat activity and bird abundance. In a conifer-dominated landscape in Oregon, the abundance of 12 bird species was positively correlated with broadleaved cover within a 2 km buffer around survey points (Betts et al. 2010). In the same study area as our research, Charbonnier et al. (2016) found a positive effect of broadleaved cover within a 500 m radius on bat activity in pine stands.

Our results showed that species richness of PPM bird predators contributed to the reduction of PPM nests. While this finding should be interpreted with caution, as species richness was assessed in spring, it

suggests that multiple bird species, rather than a single species, may play complementary roles in reducing PPM infestation across different stages of its life cycle (from eggs to moths; Barbaro & Battisti 2011). However, none of the PPM natural enemies observed were significantly more active in pine edges surrounded by landscape with high broadleaved cover. The discrepancies between previous studies and our results could be explained by a relatively low cover of deciduous forest in our study. A global review indicated that the optimal threshold for conserving forest-dwelling species in agricultural landscapes is around 40% native forest cover (Arroyo-Rodríguez et al. 2020). In our study, high broadleaved cover ranged from only 14% to 30% due to the dominance of pine stands in the landscape, which may have been insufficient to observe significant effects on natural enemies. Additionally, the pine plantation matrix of our study sites may be also fairly suitable for natural enemies (Brockerhoff et al. 2008), providing complementary resources (Barbaro et al. 2008).

Methodological biases could also explain the lack of effect of broadleaved cover on bird natural enemies. Firstly, we assessed the activity of birds at the local level (in pine edges) rather than across the landscape as a whole, whereas birds often exploit a mosaic of habitats, using different areas for nesting and feeding, and easily moving between them throughout the day (Barbaro et al. 2008). Secondly, in our study, birds were sampled only in spring during their nesting period (Kirwan, 2024b). A more appropriate approach would have been to sample their activity during the period when they prey on PPM, which would have required recording bird vocal activity throughout the year at different phases of the PPM cycle. Thirdly, the recorded bird vocal activity may not have accurately reflected the actual abundance of birds at the study sites, as the same individual may have been recorded multiple times. In addition, species such as the common cuckoo or the Eurasian hoo-poe, whose calls are louder than those of tit species, may have been detected further away from the study sites.

For bats, the lack of a relationship between their activity and broadleaved cover in the studied landscapes could also be attributed to the ecology of the species. Previous research identified *Pipistrellus* as the most active bat genus in our study area (63% of total bat activity in the same season in 2012;
Charbonnier et al. 2014). In agricultural landscapes, Hiller et al. (2025) showed that this bat genus does not respond to the amount of natural habitat, its proximity, or hedgerow length within a 50–500 m radius around recording devices, which may explain the lack of observed effects in our study. Additionally, *Pipistrellus* bats may roost in old buildings or under bridges, particularly in regions with limited natural roosting sites, potentially masking the effect of broadleaved cover if a roost site was nearby (Bashta 2022). Finally, as a previous study highlighted a seasonal response of bats to broadleaved cover (Charbonnier et al. 2016), further work should monitor bat activity throughout the year to better assess the benefits of broadleaved stands in providing complementary habitat and food resources, at each stage of development of individuals and populations.

## Conclusion

Our study is one of the first to demonstrate that, in a coniferous plantation landscape, increasing compositional heterogeneity (Fahrig 2013), through the presence of broadleaved stands and hedgerows, can reduce insect pest infestation. Furthermore we showed the additive effect of the two types of broadleaved elements on pest regulation, showing that associational resistance can be achieved by spatially segregating tree diversity and not only with intimate species mixing at the stand level.

While reduced host tree concentration has been proposed as a primary mechanism for associational resistance (Jactel et al. 2021; Marini et al. 2022), it is unlikely to have played a role in our study due to the large cover of host trees at both local and landscape levels. Instead, reduced host apparenty was a more relevant driving factor of PPM regulation, especially at the local level. At the local level, hedgerows may have prevented host tree location by female moths (Dulaurent et al. 2012), while at the landscape level, riparian forests may have acted as barriers to moth dispersal (Samalens & Rossi 2011). However, the most important mechanism explaining the reduction in PPM infestation was probably the role of broadleaved woodlands as a favorable habitat for its natural enemies. Our results show that great tits were more active in spring along pine edges adjacent to hedgerows, while a previous

study has shown that great tits predate PPM caterpillars in winter at pine edges adjacent to hedgerows (Plat et al. 2025). The common cuckoo, which preys on PPM caterpillars, may have also benefited from broadleaved trees where passerine nests are abundant. The Eurasian hoopoe preys on PPM pupae buried in open areas along pine stand edges while nesting in broadleaved hedgerows or stands (Barbaro et al. 2008). In summer, bats can predate flying PPM moths and find complementary foraging resources in broadleaved stands during spring (Charbonnier et al. 2016). Last, several tit species, which can feed on PPM egg masses in late summer, are more abundant in pine edges adjacent to hedgerows than in absence of hedgerow (Castagneyrol et al. 2014a).

Our results indicate the functional importance of increasing the compositional heterogeneity of the forest mosaic to prevent pest damage in plantation landscapes. As the establishment of new broadleaved stands across the pine plantation landscape can be difficult due to private property constraints and governance issues, forest planning should focus on preserving existing broadleaved remnants and planting new broadleaved hedgerows. These hedgerows can reinforce the biological control of forest pests by their natural enemies, providing the latter with both dispersal corridors and favorable habitats. More generally, broadleaved hedgerows and stands can also support local biodiversity (Montgomery et al. 2020), thus contributing to the multifunctionality of pine plantation landscapes.

**Acknowledgements** We thank Tom Barlier, Yannick Mellerin, Aurelien Kohler for their help in collecting the samples.

**Author contribution** HJ, IVH and NP designed the study, analysed the data, and wrote the first draft. JBR, OB, TR and TC collected the data. JBR, TC, IVH and NP processed and analysed the bat acoustic data. YC, MS and IGC processed and analysed the bird acoustic data. All authors contributed substantially to subsequent improvements of the manuscript.

**Funding** This work was supported by the project Horizon Europe eco2adapt (Grant Agreement No. 101059498) and the project Horizon 2020 Research and Innovation Programme SUPERB (Grant Agreement No. 101036849).

**Data availability** For the moment, there is an embargo on data and codes, which will be lifted after open acceptance.

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

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