The pine wood nematode (PWN), Bursaphelenchus xylophilus, is an invasive species causing high pine mortality in invaded areas outside its native range. European Union (EU) regulations stipulate a 500-m radius clear-cutting around infested trees to prevent the spread of PWN by the vector beetle Monochamus galloprovincialis, yet this measure is costly and not fully effective. We used an individual-based model describing the dispersal of the insect vector and its transmission of PWN to compare the cost-effectiveness of clear-cutting and selective cutting of only the PWN-infested trees for three different methods for surveillance: visual ground surveys, aerial surveys and insect trap networks. Both strategies eradicated PWN if intensive aerial surveillance was conducted at a time that all infested trees showed symptoms. Selective cutting reduced costs by up to a factor of 200 compared to clear-cutting, mainly due to the preservation of healthy trees. Additionally, selective cutting reduces environmental and societal costs. However, eradication is unlikely if symptom expression of infectious trees is incomplete at the time of survey. Policy implications. The results show that selective cutting applied together with modern remote sensing surveillance methods is an environmentally friendly alternative to the mandatory clear-cutting for PWN control in the EU.

In recent years, ectotherm species have largely been impacted by extreme climate events, essentially heatwaves. In Tunisia, the pine processionary moth (PPM), Thaumetopoea pityocampa , is a highly damaging pine defoliator, which typically lays eggs in summer. Its geographical range is expanding northwards in Europe while retracting from South Tunisia where summer temperatures can reach extremely high values. In this study, we aimed at exploring the effects of climate change on this species at its southern range edge. We investigated variations of fecundity and causes of egg mortality over time using historical and contemporary collections of egg masses from different Tunisian sites to seek relationships with regional climate change over three decades (1990-2019). Our results suggest negative effects of summer heat on egg survival, reflected in a decrease of hatching rate down to 0% in one site during a heatwave. Such a high hatching failure was found to result from both high egg sterility (our results did not allow distinguishing impeded mating success from failed egg maturation or early death of the embryo) and increased abortion of more developed embryos, but little effects of parasitism rate, thereby suggesting vulnerability to heat during embryonic development. We also observed decreasing female fecundity ( i.e ., number of eggs laid per female) in regions where data were available both in the 1990s and the 2010s, which was associated with a decrease in parasitism rate, while the climatic variability increased. This study investigated direct hatching failure in nature that may be related to the magnitude of warming in summer. Previous studies have confirmed the thermal sensitivity of early instars of the PPM to temperatures observed in the present work, including one population from South Tunisia. However, further work is required to evaluate the relative importance of warming summers among populations because the risk of heat stress depends on the phenology of sensitive instars, and populations from the warmest areas may not necessarily be the most vulnerable to climate change if they already evolved phenological heat avoidance. In addition to heat-induced mortality, the ultimate fitness of individuals that survive challenging heat stresses during early developmental stages should also be explored to determine potential carry-over effects on subsequent life stages.

Insect pests are one of the major threats to forests. Although invasive species cause more and more impacts, native species could also generate temporary very high damages. The population dynamics of insects relies on several factors, going from weather to stand conditions. Due to global change, insects could face conditions they have never encountered before, leading to unusual population outbreaks. Forest managers need to consider these possible emerging pests but predicting insect outbreaks is still very challenging. In this context, we have developed a mathematical model at the crossroad of statistical and mechanistic models to describe the likelihood of outbreaks for a set of 6 insect profiles: bark beetles, longhorn beetles, tortrix moth, other moths, aphids, and Hymenoptera. This model describes the probability of occurrence of an outbreak at a given time and at a given area, based on several conditions. It has been built and parametrized on the most documented orders of European forest pests. Parametrization for these species’ profiles can be used as a baseline to explore the risk of outbreaks for closely related pest species. We provide an illustration of the model application for the oak processionary moth, Thaumetopoea processionea , which reach epidemic levels in north-western Europe. This generic outbreak model is particularly performant to point out some years or areas as unlikely for an outbreak, and thus targets correctly factors that inhibit outbreaks. It is still at an exploratory level and should be further improved for an operational use in forest stand surveillance and management. ### Competing Interest Statement The authors have declared no competing interest.

Key message The invasive pine wood nematode is a major threat to pine forests worldwide, causing extensive tree mortality. Although scientific knowledge and control measures are continuously improving, important gaps remain. We argue that some key questions, notably related to early detection and pest management, need to be urgently tackled in countries at risk of invasion such as France.

Ectotherms are largely impacted by extreme climate events, essentially heatwaves. In Tunisia, the pine processionary moth (PPM), Thaumetopoea pityocampa , is a defoliator typically laying eggs in summer. Its geographical range is expanding northwards in Europe while retracting from southern Tunisia where summer temperatures can reach extreme heats. We explored the effects of climate change over the last 30 years on PPM at its southern range edge. We investigated variations of fecundity and causes of egg mortality over time using historical and contemporary collections of egg masses. Due to incomplete historical series, a clustering method was used to group climatically homologous sampling sites and perform allochronic analyses on clusters instead of individual localities. Our results suggest negative effects of summer heat on egg survival, down to 0% hatching rate in one site during a major heatwave. Such high hatching failures resulted partly from high egg sterility and aborted embryos, with little effects of parasitism rate, suggesting heat susceptibility during embryonic development, but our results do not allow distinguishing impeded mating success from failed egg maturation or early embryo death. Decreasing female fecundity was observed in regions where allochronic comparisons were possible, associated with a decrease in parasitism rate, while climatic variability increased. Previous studies have confirmed experimentally the thermal sensitivity of PPM early instars to temperatures observed in the present work, including one population from southern Tunisia. However, further work is required to evaluate the relative importance of warming summers among populations, since the risk of heat stress depends on the phenology of sensitive instars, and populations from the warmest areas may not necessarily be the most vulnerable to climate change if they already evolved phenological heat avoidance. In addition to heat‐induced mortality, the ultimate fitness of heat survivors should also be explored to determine potential carry‐over effects on subsequent life stages.

The African citrus psyllid, Trioza erytreae (Del Guercio) (Hemiptera, Triozidae), is native to tropical Africa and invasive species in North America and Europe. The main host plants are citrus, displaying a preference for lemon trees. This psyllid was recently detected in the northwest region of the Iberian Peninsula, both in Spain and Portugal. Here, we used a model combining a reaction-diffusion model to a stochastic long-distance dispersal model to simulate the invasion dynamics of T. erytreae in Portugal. The psyllid spread in Portugal was simulated between 2015 and 2021 for different combinations of model parameters: two fecundity levels; spread with and without stochastic long-distance dispersal; single or two introductions of T. erytreae ; and considering or not the urban and peri-urban citrus trees, besides citrus orchards, estimated using Google Street view imagery. The incorporation of long-distance human mediated dispersal significantly improved the F1-score in the model validation using the official reports as the observed data. Concomitantly, the dispersal rate of T. erytreae in Portugal was on average about 66 km/year, whereas removing long-distance dispersal events, the observed mean was 7.8 ± 0.3 km/year. The dispersal was mainly towards the south along the coastline, where human population is concentrated. The inclusion of the estimated citrus trees outside orchards areas significantly increased the F1-score in the model validation, revealing the importance these isolated host plants hold as stepping stones for the species current invasion and possibly for other species alike.

Understanding and predicting the spread of invading insects is a critical challenge in management programs that aim to minimize ecological and economic harm to native ecosystems. Although efforts to quantify spread rates have been well studied over the past several decades, opportunities to improve our ability to estimate rates of spread, and identify the factors, such as habitat suitability and climate, that influence spread, remain. We review emerging sources of data that can be used to delineate distributional boundaries through time and thus serve as a basis for quantifying spread rates. We then address advances in modeling methods that facilitate our understanding of factors that drive invasive insect spread. We conclude by highlighting some remaining challenges in understanding and predicting invasive insect spread, such as the role of climate change and biotic similarity between the native and introduced ranges, particularly as it applies to decision-making in management programs.

Climate change profoundly alters the phenology of insects, yet the mechanisms at play remain particularly elusive for univoltine species. Those species typically have to deal with contrasting thermal conditions across their development and life stages occurring at different seasons may have different thermal sensitivity. A modeling framework taking into account stage-specific thermal biology is lacking to predict the effect of climate change on the phenology of such species. Insect development rate scales non-linearly with temperature. This can be described with a thermal performance curve within each developmental stage, enabling higher accuracy near developmental thresholds than linear degree-day models. This approach, however, requires ample data to be correctly estimated. We developed a phenological model based on stage-specific performance curves to predict the phenology of a univoltine species undergoing uninterrupted larval development from summer to next spring, the pine processionary moth (Thaumetopoea pityocampa). This gregarious species is an important pine defoliator and is known to readily respond to climate change with a consistent and sustained range expansion/shift since the 1990s, as winter warming facilitates its survival in previously unsuitable areas. First, we determined the thermal performance curve of development rate for each stage from the egg to the fourth larval instar by monitoring molting in larval colonies exposed to fluctuating thermal treatments in controlled conditions. Second, we developed a phenology model to simulate the cumulated development rate across successive life stages, using observation data of adult flights and daily mean temperatures as input variables. A good fit was found between predictions and observations. Finally, the model was used to explore phenological consequences of hypothetical climate variations. With a simulated increase of temperature by 3°C, the model successfully predicted atypical ends of larval development before winter, which are being observed in nature in some regions or during years with autumnal heatwaves. With a simulated heatwave, carry-over effect on life stages development were predicted. On this winter-active species, we illustrate how variations in development rate caused by climate variations in early development can feedback into subsequent stages typically developing slowly in the cold season.

Temperature strongly drives physiological and ecological processes in ectotherms. While many species rely on behavioural thermoregulation to avoid thermal extremes, others build structures (nests) that confer a shelter against climate variability and extremes. However, the microclimate inside nests remains unknown for most insects. We investigated the thermal environment inside the nest of a temperate winter-developing insect species, the pine processionary moth (PPM), Thaumetopoea pityocampa. Gregarious larvae collectively build a silken nest at the beginning of the cold season. We tested the hypothesis that it provides a warmer microenvironment to larvae. First, we monitored temperature inside different types of nests varying in the number of larvae inside. Overall, nest temperature was positively correlated to global radiation and air temperature. At noon, when global radiation was maximal, nest temperature exceeded air temperature by up to 11.2-16.5 degrees C depending on nest type. In addition, thermal gradients of amplitude from 6.85 to 15.5 degrees C were observed within nests, the upper part being the warmest. Second, we developed a biophysical model to predict temperature inside PPM nests based on heat transfer equations and to explain this important temperature excess. A simple model version accurately predicted experimental measurements, confirming that nest temperature is driven mainly by radiation load. Finally, the model showed that nest temperature increases at the same rate as air temperature change. We conclude that some pest insects already live in warm microclimates by building their own sheltering nest. This effect should be considered when studying the impact of climate change on phenology and distribution.

To investigate enemy pressure across the range of the pine processionary moth (Thaumetopoea pityocampa, Den. & Schiff.) as it spreads northwards in Europe because of climate change, a survey of 206 egg batches from historical and newly colonized areas at both the westernmost and the easternmost ends of the front was carried out in 2016-2018. Out of them, 97 egg batches were collected from nine Locations in three regions in France and 109 from four Localities in two regions in Bulgaria, both within historical and newly colonized ranges of the pest. The average number of eggs per batch collected in Bulgaria (226.6 +/- 43.2) was higher than that in France (194.3 +/- 50.1). However, the hatching percentage was higher in French samples, varying from 69.8 to 95.7, vs 49.8 to 85.2 per cent in Bulgarian samples. Four primary parasitoids (Ooencyrtus pityocampae, Baryscapus servadeii, Anastatus bifasciatus, Trichogramma sp.) and a hyperparasitoid (Baryscapus transversalis, found only in two regions of the historical range in Bulgaria) were identified. The oligophagous species B. servadeii was present at all sites within the historical range of the pest. In newly colonized areas, the impact of primary parasitoids on the host was distinctly Low, suggesting that they Lag behind the range expansion of pine processionary moth. In France, the most abundant species in the T. pityocampa parasitoid complex were B. servadeii in Re Island and Orleans (97.3 and 87.4 per cent, respectively), and Trichogramma sp. (99.7 per cent) in a newly colonized Locality in Frehel. Ooencyrtus pityocampae prevailed in three of four Bulgarian Localities (72.1 per cent in Sandanski, 89.7 per cent in Maglizh and 65.7 per cent in Sladak kladenets), whereas B. servadeii was the most abundant in Gega (75.4 per cent). Mortality of B. servadeii and O. pityocampae caused by the hyperparasitoid B. transversalis amounted to 4.8-6.2 per cent. The impact of predators on the pine processionary moth in the egg stage was negligible at most sites, reaching 12.5 per cent in only one site (Maglizh).

Climate warming is inducing dramatic changes in species distribution. While many studies report the poleward range expansion of some species, some others report the range retraction and extinction risk of other species. Here we explore how climate warming affects the southern edge in North Africa of the pine processionary moth, Thaumetopoea pityocampa, which is a model insect currently expanding northwards and toward higher elevation in Europe. This Mediterranean forest insect was found in southern Tunisia until 2003. Field surveys were conducted to map the current southern edge of the species in Tunisia. Pheromone traps were installed on a north-south gradient, a translocation experiment of egg masses was conducted on this gradient, and local temperature change was analyzed. We thus proved that the pine processionary moth has disappeared from southern Tunisia, and that no more adult males were actually flying there. We also found a decrease of egg hatching and of the proportion of individuals able to reach larval stages along this gradient, while daily minimal and maximal temperatures globally increased. Furthermore, we showed that daily maximal and minimal temperatures as well as indices of extremely high temperatures have substantially increased during the study period (1980–2019). This study reveals the retraction of the pine processionary moth from southern Tunisia due to higher mortality rates that could be attributed to a significant local warming. The role of other factors (mainly the response of host trees and natural enemies to climate change) may amplify this direct effect and should be further explored.