Pine Processionary Research Articles

Effects of climate and density-dependent factors on population dynamics of the pine processionary moth in the Southern Alps

Forest pest populations can fluctuate dramatically in relation to climate and density-dependent factors. Although the distributional range of the pine processionary moth Thaumetopoea pityocampa (Lepidoptera Notodontidae) appears to be expanding northward and upslope with climate warming, the relative importance of climate and endogenous, density-dependent factors has not been clearly documented. We analyzed the population dynamics of the moth using long-term data from two provinces in the Southern Alps (Trento: 1990–2009, Bolzano/Bozen: 1975–2011) to evaluate the relative importance of climate and density-dependent factors as regional drivers. Both summer temperatures and rainfall significantly affected population growth rate, with different outcomes depending on the local conditions. Although previous studies indicated that low winter temperatures have negative effects on insect performance, our analyses did not show any negative effect on the population dynamics. A negative density dependent feedback with a 1-year lag emerged as the most important factor driving the population dynamics in both regions. Potential mechanisms explaining the observed negative density feedback include deterioration of host quality, increased mortality caused by pathogens, and increase of prolonged diapause as an adaptive mechanism to escape adverse conditions.

Contrasting vulnerability and resilience to drought-induced decline of densely planted vs. natural rear-edge Pinus nigra forests

The southernmost European natural and planted pine forests are among the most vulnerable areas to warming-induced drought decline. Both drought stress and management factors (e.g., stand origin or reduced thinning) may induce decline by reducing the water available to trees but their relative importances have not been properly assessed. The role of stand origin – densely planted vs. naturally regenerated stands – as a decline driver can be assessed by comparing the growth and vigor responses to drought of similar natural vs. planted stands. Here, we compare these responses in natural and planted Black pine (Pinus nigra) stands located in southern Spain. We analyze how environmental factors – climatic (temperature and precipitation anomalies) and site conditions – and biotic factors – stand structure (age, tree size, density) and defoliation by the pine processionary moth – drive radial growth and crown condition at stand and tree levels. We also assess the climatic trends in the study area over the last 60years. We use dendrochronology, linear mixed-effects models of basal area increment and structural equation models to determine how natural and planted stands respond to drought and current competition intensity. We observed that a temperature rise and a decrease in precipitation during the growing period led to increasing drought stress during the late 20th century. Trees from planted stands experienced stronger growth reductions and displayed more severe crown defoliation after severe droughts than those from natural stands. High stand density negatively drove growth and enhanced crown dieback, particularly in planted stands. Also pine processionary moth defoliation was more severe in the growth of natural than in planted stands but affected tree crown condition similarly in both stand types. In response to drought, sharp growth reduction and widespread defoliation of planted Mediterranean pine stands indicate that they are more vulnerable and less resilient to drought stress than natural stands. To mitigate forest decline of planted stands in xeric areas such as the Mediterranean Basin, less dense and more diverse stands should be created through selective thinning or by selecting species or provenances that are more drought tolerant.

Foliar terpene chemistry of Pinus pinaster and P. radiata responds differently to Methyl Jasmonate and feeding by larvae of the pine processionary moth

Jasmonic acid and Methyl Jasmonate (MeJA) are important signaling molecules for plant defensive responses upon mechanical wounding, herbivory damage, or fungal inoculation. There is an increased interest in the use of exogenous chemical elicitors such as jasmonic acids to induce natural host defense mechanisms as an alternative to synthetic chemical pesticides. However few studies have compared the response caused by application of elicitors vs. real herbivores and few studies have analyzed the impact on particular pest species of plant responses induced by exogenous application of MeJA. We compared the defensive response of two pine species, one native to our study area (Pinus pinaster) and one introduced (Pinus radiata), after exogenous application of MeJA and after feeding of a regionally important indigenous pest (pine processionary moth, PPM). We analyzed variation in foliar terpenes, nitrogen and water. Additionally we assessed the consequences of plant responses to MeJA for the growth and mortality of PPM larvae. Because the defensive reaction may be influenced by resource availability, we also evaluated effects of site quality on plant responsiveness to MeJA and insect feeding. The exogenous application of MeJa had different effects in the two pine species. In P. radiata, there were no conspicuous increases from MeJA in any measured terpene, and on good sites, there were general decreases in overall concentrations of the three terpene classes. In P. pinaster, MeJA by itself had no detectable effect on overall concentrations of any class of terpenes, but there was a 6-fold increase in the concentration of one monoterpene, linalool. Feeding by PPM induced decreases in terpenes, especially monoterpenes, in both species. Foliar nitrogen contents were markedly higher in P. radiata than P. pinaster – especially on good sites. Foliar water content was similar between pine species and, in both species, was reduced relative to control trees in treatments that included PPM feeding. There were no differences in larval mortality between trees that were and were not exposed to exogenous MeJA. In summary, the exogenous application of MeJA had effects on the terpene chemistry of both pine species, but the effects were less consistent across species and compounds than expected under some models of MeJA biochemistry. It seems likely that operational manipulations of MeJA to manage pests in pine forestry will require further understanding of differences among pine species in their responses to MeJA.

Assessing Species Distribution Using Google Street View: A Pilot Study with the Pine Processionary Moth

Mapping species spatial distribution using spatial inference and prediction requires a lot of data. Occurrence data are generally not easily available from the literature and are very time-consuming to collect in the field. For that reason, we designed a survey to explore to which extent large-scale databases such as Google maps and Google street view could be used to derive valid occurrence data. We worked with the Pine Processionary Moth (PPM) Thaumetopoea pityocampa because the larvae of that moth build silk nests that are easily visible. The presence of the species at one location can therefore be inferred from visual records derived from the panoramic views available from Google street view. We designed a standardized procedure allowing evaluating the presence of the PPM on a sampling grid covering the landscape under study. The outputs were compared to field data. We investigated two landscapes using grids of different extent and mesh size. Data derived from Google street view were highly similar to field data in the large-scale analysis based on a square grid with a mesh of 16 km (96% of matching records). Using a 2 km mesh size led to a strong divergence between field and Google-derived data (46% of matching records). We conclude that Google database might provide useful occurrence data for mapping the distribution of species which presence can be visually evaluated such as the PPM. However, the accuracy of the output strongly depends on the spatial scales considered and on the sampling grid used. Other factors such as the coverage of Google street view network with regards to sampling grid size and the spatial distribution of host trees with regards to road network may also be determinant.

Matching methods to produce maps for pest risk analysis to resources

Decision support systems (DSSs) for pest risk mapping are invaluable for guiding pest risk analysts seeking to add maps to pest risk analyses (PRAs). Maps can help identify the area of potential establishment, the area at highest risk and the endangered area for alien plant pests. However, the production of detailed pest risk maps may require considerable time and resources and it is important to match the methods employed to the priority, time and detail required. In this paper, we apply PRATIQUE DSSs to Phytophthora austrocedrae, a pathogen of the Cupressaceae, Thaumetopoea pityocampa, the pine processionary moth, Drosophila suzukii, spotted wing Drosophila, and Thaumatotibia leucotreta, the false codling moth. We demonstrate that complex pest risk maps are not always a high priority and suggest that simple methods may be used to determine the geographic variation in relative risks posed by invasive alien species within an area of concern.

Solar radiation directly affects larval performance of a forest insect

Solar radiation can affect the performance of insect herbivores directly by increasing body temperature, or indirectly through alteration of either host plant quality or natural enemy activity.To test for the direct effect of solar radiation on larval performance, youngPinus sylvestristrees growing on the island ofGotland (Sweden) were assigned to one of four shading treatments for the whole duration of the first larval instar of the northern pine processionary mothThaumetopoea pinivora.There was a strong, linear relationship between shading and the temperature of the first‐instar colonies ofT. pinivora, resulting in higher growth of the larvae exposed to full sunlight, but there were no effects on developmental rate or larval mortality. Putative negative effects ofUVradiation on the larvae are not consistent with higher growth in full sunlight, but it is possible thatUVeffects might have modulated the response.Thaumetopoea pinivorahas a strong preference for light and open pine stands, i.e. habitats with frequent intense incoming solar radiation. The data in the present study suggest that the opportunity for young larvae to bask in the sun during cold spring weather is an important determinant of the spatial distribution ofT. pinivora.

Are heat waves susceptible to mitigate the expansion of a species progressing with global warming?

A number of organisms, especially insects, are extending their range in response of the increasing trend of warmer temperatures. However, the effects of more frequent climatic anomalies on these species are not clearly known. The pine processionary moth, Thaumetopoea pityocampa, is a forest pest that is currently extending its geographical distribution in Europe in response to climate warming. However, its population density largely decreased in its northern expansion range (near Paris, France) the year following the 2003 heat wave. In this study, we tested whether the 2003 heat wave could have killed a large part of egg masses. First, the local heat wave intensity was determined. Then, an outdoor experiment was conducted to measure the deviation between the temperatures recorded by weather stations and those observed within sun-exposed egg masses. A second experiment was conducted under laboratory conditions to simulate heat wave conditions (with night/day temperatures of 20/32°C and 20/40°C compared to the control treatment 13/20°C) and measure the potential effects of this heat wave on egg masses. No effects were noticed on egg development. Then, larvae hatched from these egg masses were reared under mild conditions until the third instar and no delayed effects on the development of larvae were found. Instead of eggs, the 2003 heat wave had probably affected directly or indirectly the young larvae that were already hatched when it occurred. Our results suggest that the effects of extreme climatic anomalies occurring over narrow time windows are difficult to determine because they strongly depend on the life stage of the species exposed to these anomalies. However, these effects could potentially reduce or enhance the average warming effects. As extreme weather conditions are predicted to become more frequent in the future, it is necessary to disentangle the effects of the warming trend from the effects of climatic anomalies when predicting the response of a species to climate change.

Needle terpene concentrations and emissions of two coexisting subspecies of Scots pine attacked by the pine processionary moth (Thaumetopoea pityocampa)

Mediterranean pine forests are often attacked by caterpillars of Thaumetopoea pityocampa (Lep., Thau- metopoidae), one of the most important defoliators in the Mediterranean region causing large economic losses and ecological effects. The needle terpene concentrations and emissions may play a key role in the defense of pines. We studied two subspecies of Pinus sylvestris, nevadensis (an endemic and relict subspecies) and iberica, with different levels of caterpillar attack in Sierra Nevada mountains (Spain). GC-MS analyses showed large total concentra- tions of terpenes (6 to 39 mg g -1 of dry weight) in the needles of both subspecies under field conditions. Con- centrations were 25 % higher in ''Non-Attacked Trees'' (NATs) of the iberica than in the nevadensis subspecies. The branches of NATs had terpene concentrations 20 % higher than those of ''Attacked Branches of attacked trees'' (ABs). Within attacked trees, the ''Non-Attacked Bran- ches'' (NABs) also had terpene concentrations 20 % higher than those of ABs. Mainly a-pinene and germacrene D had higher concentrations in NATs and NABs than in ABs. Some terpenes had higher concentrations in NABs than in NATs, indicating possible systemic reactions. In subsp. nevadensis, the percentage of monoterpenes relative to total terpenes was higher in ABs than in other attack states. The rates of emission in nevadensis (standardized to 30 C) were ca. three times higher in ABs than in NABs and NATs. These results suggest that the lower terpene con- centrations and high percentages of monoterpenes in ABs were produced by a combination of emission losses and terpene induction in response to herbivorous attack.

Inducibility of chemical defences by two chewing insect herbivores in pine trees is specific to targeted plant tissue, particular herbivore and defensive trait

There is increasing evidence that plants can react to biotic aggressions with highly specific responses. However, few studies have attempted to jointly investigate whether the induction of plant defences is specific to a targeted plant tissue, plant species, herbivore identity, and defensive trait. Here we studied those factors contributing to the specificity of induced defensive responses in two economically important pine species against two chewing insect pest herbivores. Juvenile trees of Pinus pinaster and P. radiata were exposed to herbivory by two major pest threats, the large pine weevil Hylobius abietis (a bark-feeder) and the pine processionary caterpillar Thaumetopoea pityocampa (a folivore). We quantified in two tissues (stem and needles) the constitutive (control plants) and herbivore-induced concentrations of total polyphenolics, volatile and non-volatile resin, as well as the profile of mono- and sesquiterpenes. Stem chewing by the pine weevil increased concentrations of non-volatile resin, volatile monoterpenes, and (marginally) polyphenolics in stem tissues. Weevil feeding also increased the concentration of non-volatile resin and decreased polyphenolics in the needle tissues. Folivory by the caterpillar had no major effects on needle defensive chemistry, but a strong increase in the concentration of polyphenolics in the stem. Interestingly, we found similar patterns for all these above-reported effects in both pine species. These results offer convincing evidence that induced defences are highly specific and may vary depending on the targeted plant tissue, the insect herbivore causing the damage and the considered defensive compound.

Estimation of population allele frequencies from next‐generation sequencing data: pool‐versus individual‐based genotyping

Molecular markers produced by next-generation sequencing (NGS) technologies are revolutionizing genetic research. However, the costs of analysing large numbers of individual genomes remain prohibitive for most population genetics studies. Here, we present results based on mathematical derivations showing that, under many realistic experimental designs, NGS of DNA pools from diploid individuals allows to estimate the allele frequencies at single nucleotide polymorphisms (SNPs) with at least the same accuracy as individual-based analyses, for considerably lower library construction and sequencing efforts. These findings remain true when taking into account the possibility of substantially unequal contributions of each individual to the final pool of sequence reads. We propose the intuitive notion of effective pool size to account for unequal pooling and derive a Bayesian hierarchical model to estimate this parameter directly from the data. We provide a user-friendly application assessing the accuracy of allele frequency estimation from both pool- and individual-based NGS population data under various sampling, sequencing depth and experimental error designs. We illustrate our findings with theoretical examples and real data sets corresponding to SNP loci obtained using restriction site-associated DNA (RAD) sequencing in pool- and individual-based experiments carried out on the same population of the pine processionary moth (Thaumetopoea pityocampa). NGS of DNA pools might not be optimal for all types of studies but provides a cost-effective approach for estimating allele frequencies for very large numbers of SNPs. It thus allows comparison of genome-wide patterns of genetic variation for large numbers of individuals in multiple populations.

Stepping‐stone expansion and habitat loss explain a peculiar genetic structure and distribution of a forest insect

It is challenging to unravel the history of organisms with highly scattered populations. Such species may have fragmented distributions because extant populations are remnants of a previously more continuous range, or because the species has narrow habitat requirements in combination with good dispersal capacity (naturally or vector borne). The northern pine processionary moth Thaumetopoea pinivora has a scattered distribution with fragmented populations in two separate regions, northern and south-western Europe. The aims of this study were to explore the glacial and postglacial history of T.pinivora, and add to the understanding of its current distribution and level of contemporary gene flow. We surveyed published records of its occurrence and analysed individuals from a representative subset of populations across the range. A 633bp long fragment of the mtDNA COI gene was sequenced and nine polymorphic microsatellite loci were genotyped. Only nine nucleotide sites were polymorphic in the COI gene and 90% of the individuals from across its whole range shared the same haplotype. The microsatellite diversity gradually declined towards the north, and unique alleles were found in only three of the northern and three of southern sites. Genetic structuring did not indicate complete isolation among regions, but an increase of genetic isolation by geographic distance. Approximate Bayesian model choice suggested recent divergence during the postglacial period, but glacial refugia remain unidentified. The progressive reduction of suitable habitats is suggested to explain the genetic structure of the populations and we suggest that T.pinivora is a cold-tolerant relict species, with situation-dependent dispersal.

Metapopulation modelling and area-wide pest management strategies evaluation. An application to the Pine processionary moth

Forecasting pest population abundance is a time and resource consuming task, and in particular for area-wide pest management is complicated by demographic and environmental stochasticity. These factors make difficult the development of quantitative tools to design and evaluate different management strategies performances by taking into account various form of variability and uncertainty. Pest management could benefit from methods supporting decision making based on models ease of development under scarce data and high uncertainty. Host plants for many agricultural and forest pests are often patchily distributed, therefore population dynamics can be suitably described in terms of metapopulations. Despite the fact that metapopulation models were originally proposed for pests, they remain a widely used tool in conservation biology but receive little attention in large scale pest management.The aim of this paper is to propose a framework allowing the ranking of the efficacy of area-wide pest control strategies, taking into account population spatial distribution in discrete patches. The Kullback–Leibler divergence, well known in Information Theory, Probability and Statistics, is used to measure how far the state of the metapopulation as predicted by a spatially explicit metapopulation model is from a suitable reference state.The method is applied to compare the efficacy of different types of predefined control strategies of the Pine processionary moth (Traumatocampa pityocampa (Den. and Schiff)). The analysis of a dataset on metapopulation dynamics of this moth from a fragmented Mediterranean pine forest allows to derive some rules of thumb for the rational allocation of control effort, in terms of spatial and temporal distribution of the interventions.

Potential spread of the pine processionary moth in France: preliminary results from a simulation model and future challenges

Some forest insect pests are currently extending their range as a consequence of climate warming. However, in most cases, the evidence is mainly based on correlations and the underlying mechanism is not clearly known. One of the most severe pests of pine forests in Europe, the pine processionary moth, Thaumetopoea pityocampa, is currently expanding its distribution as a result of climate warming and does not occupy entirely its potential habitat. A model describing its spread was developed to simulate its potential range in France under various climate change scenarios. The spread model was divided into several sub-models to describe the growth, survival and dispersal of the species. The model was validated on the observed change of species distribution, its sensitivity was tested, and spread scenarios were simulated for the future. The model shows that climate warming initiated the species range expansion in France since the early 1990s. The spread is now limited by dispersal capability, but human-mediated dispersal could accelerate the range expansion. Species range expansion is an indicator of climate change. However, time lags can appear due to limited dispersal capabilities, and human-mediated dispersal could create satellite colonies and artificially accelerate the spread.

Winter bird numerical responses to a key defoliator in mountain pine forests

The challenging question of predator–prey interactions contributing or not to limit insect outbreaks in forest ecosystems is still ongoing. Winter numerical responses of generalist insectivorous birds to overwintering larvae of a key pine defoliator, the pine processionary moth, has received little attention to date. Here, we tested the effects of forest structure and composition at stand and landscape scales on overwintering moth density and winter bird communities, including generalist moth predators. We assessed moth nest density and bird communities during two consecutive winters on 25 linear transects along an elevation gradient in mountain pine forests of the southern French Prealps. We found that winter severity significantly decreased both moth density and total bird abundance and richness. Pine processionary moth density increased with south-facing aspects and forest fragmentation, especially edge density, at the landscape scale. Winter bird communities were influenced more by forest fragmentation at the landscape scale than stand-scale variables, except moth density. Total bird abundance increased significantly with moth density only in the colder winter. Great tit Parus major abundance also increased significantly with moth density, but irrespective of winter severity. Winter bird numerical responses were mediated primarily by landscape composition, and secondarily by forest stand structure and composition. We suggest that landscape-scale conservation of bird functional diversity in both breeding and wintering seasons would increase opportunities for biological control of pest insects in mountain forests.

Timing and intensity of bush cricket predation on egg batches of pine processionary moth: no evidence of population control

Abstract Bush crickets (Orthoptera, Tettigoniidae) are known predators of egg batches of the pine processionary moth (Thaumetopoea pityocampa Denis and Schiffermüller, Lepidoptera: Notodontidae), a severe pest in Mediterranean pine woodlands. Bush crickets have been proposed as biological control agents of T. pityocampa populations, although their characteristics as generalist predators suggest that this would be possible only during the low‐density phases of the processionary population cycle. We combined observational and experimental approaches aiming to determine the role played by the bush cricket Steropleurus andalusius as a regulator of T. pityocampa. The synchrony of the phenology of T. pityocampa egg batches and the activity of bush crickets was poor, with the activity of bush crickets being longer than the period during which egg batches were available. The predation rate of bush crickets reached 23%, varied in time and space, and was independent of T. pityocampa egg‐batch density. There was no relationship between the egg‐batch densities and bush cricket abundance, and no response to experimental variations in egg‐batch density. Analysis of microhabitat selection suggests that T. pityocampa egg‐batch predation is a by‐product of the substrate selected by bush crickets for mating, and does not correspond with active searching of egg batches as food. These findings do not rule out the possibilty of bush crickets being natural regulators of T. pityocampa populations, although they impose limits with respect to their future use in biological control.

Host and phenology shifts in the evolution of the social moth genus Thaumetopoea.

The genus Thaumetopoea contains the processionary moths, a group of lepidopteran associated with forest trees, well known for the social behaviour of the larvae and for carrying urticating setae. The taxonomy of the genus is partly unresolved and a phylogenetic approach is lacking. The goal of this work is to produce a phylogeny for Thaumetopoea and to identify the main traits driving the evolution of this group. Eighteen mitochondrial and three nuclear genes were fully/partly sequenced. Markers were aligned and analysed singularly or in various combinations. Phylogenetic analyses were performed according to maximum likelihood and Bayesian inference methods. Trees obtained from largest data sets provided identical topologies that received strong statistical support. Three main clades were identified within Thaumetopoea and were further supported by several signatures located in the mitochondrial tRNAs and intergenic spacers. The reference topology was used to investigate the evolution of life history traits related to biogeography, host plant, ecology, and morphology. A multigenic approach allowed to produce a robust phylogenetic analysis of the genus Thaumetopoea, with the identification of three major clades linked to different ecological and life history traits. The first clade is associated with Angiosperm host plants and has a fast spring development of larvae on young foliage. The other clades have originated by one event of host plant shift to Gymnosperm Pinaceae, which implied a longer larval developmental time due to the lower nutritional quality of leaves. These clades showed different adaptations to such a constraint, the first with a switch of larval feeding to cold season (winter pine processionary moths), and the second with a retraction to high altitude and latitude and a development cycle extended over two years (summer pine processionary moths). Recent global warming is affecting all species and seems able to further shape the evolution of the group.

Linking Climate Suitability, Spread Rates and Host-Impact When Estimating the Potential Costs of Invasive Pests

Biosecurity agencies need robust bioeconomic tools to help inform policy and allocate scarce management resources. They need to estimate the potential for each invasive alien species (IAS) to create negative impacts, so that relative and absolute comparisons can be made. Using pine processionary moth (Thaumetopoea pityocampa sensu lato) as an example, these needs were met by combining species niche modelling, dispersal modelling, host impact and economic modelling. Within its native range (the Mediterranean Basin and adjacent areas), T. pityocampa causes significant defoliation of pines and serious urticating injuries to humans. Such severe impacts overseas have fuelled concerns about its potential impacts, should it be introduced to New Zealand. A stochastic bioeconomic model was used to estimate the impact of PPM invasion in terms of pine production value lost due to a hypothetical invasion of New Zealand by T. pityocampa. The bioeconomic model combines a semi-mechanistic niche model to develop a climate-related damage function, a climate-related forest growth model, and a stochastic spread model to estimate the present value (PV) of an invasion. Simulated invasions indicate that Thaumetopoea pityocampa could reduce New Zealand’s merchantable and total pine stem volume production by 30%, reducing forest production by between NZ$1,550 M to NZ$2,560 M if left untreated. Where T. pityocampa is controlled using aerial application of an insecticide, projected losses in PV were reduced, but still significant (NZ$30 M to NZ$2,210 M). The PV estimates were more sensitive to the efficacy of the spray program than the potential rate of spread of the moth. Our novel bioeconomic method provides a refined means of estimating potential impacts of invasive alien species, taking into account climatic effects on asset values, the potential for pest impacts, and pest spread rates.

Corneal Inflammation From Pine Processionary Caterpillar Hairs

Hairs from the caterpillar of the pine processionary moth, Thaumetopoea pityocampa, located in the pine forests of southern Europe and elsewhere, can become embedded in the cornea and conjunctiva. Disorders produced by the urticating hairs can be classified into a broad spectrum of severity from mild keratoconjunctivitis to anaphylactic shock. This report presents 3 cases that illustrate the range of corneal responses to embedded hairs of the processionary moth caterpillar. Case series and review of the literature. A 51-year-old man (case 1), a 46-year-old woman (case 2), and a 67-year-old man (case 3) presented different manifestations as a result of contact with pine processionary caterpillar hairs. Case 1 had acute keratitis with decreased vision. He was treated with a corneal depot steroid 4 times daily for 3 months. After 4 months, he was asymptomatic. Case 2 had acute keratitis without vision symptoms. She was released without treatment, and 1 month later was asymptomatic. Case 3 experienced dry gritty sensations, probably because of blepharitis, but also presented asymptomatic caterpillar hairs embedded in the corneal stroma. He was instructed regarding lid hygiene for his blepharitis, and after 3 months the embedded caterpillar hairs had disappeared. The most common ocular presentation of embedded hairs from the pine processionary moth caterpillar is keratitis. The urticating hairs can cause corneal infiltrates that disappear progressively over time. Steroid eye drops accelerate the resorption of these infiltrates.

Pine growth response to processionary moth defoliation across a 40-year chronosequence

Estimating the impact of pest insects on forest productivity requires a better understanding of host tree responses. While many studies have focused on juvenile trees, studying the impacts of defoliation on trees of increasing age helps to better characterize underlying mechanisms regulating growth responses to defoliation. During winter 2009–2010 a large outbreak of pine processionary moth occurred in Southwestern France. We established a field experiment to examine the effects of pine processionary moth defoliation varying from 25% to 100% on the growth of Pinus pinaster in stands ranging from 3 to 40years old. Our results showed that pine processionary moth defoliation resulted in significant loss of radial growth for at least two years following defoliation. Stem growth loss in the first and second years was proportional to defoliation intensity and ranged from 32% to 93% in year 1, and from 17% to 68% in year 2. Stem growth was most reduced in older trees. Carbohydrates and nitrogen contents in needles and stem sapwood were also affected by defoliation. Our results suggest that defoliation affects stem growth through nitrogen and carbon resource limitation and that stem growth would be a lower priority sink for resources than other physiological processes in pine trees.

Toxicity of native Bacillus thuringiensis isolates on the larval stages of pine processionary moth Thaumetopoea wilkinsoni at different temperatures

Pine processionary moth, Thaumetopoea wilkinsoni Tams, is an important defoliating lepidopteran pest of pine trees. The aim of this study was to determine the required spore-crystal concentration of local Bacillus thuringiensis (Bt) isolates, optimal ambient temperature, and larval stage to control this troublesome forest pest. The susceptibility of T. wilkinsoni larvae decreased with older stage and lower temperatures. The optimum temperature was found to be 15 °C or higher for the control of early larval stages. At the highest spore-crystal concentration (500 µg g^{-1}), the most effective isolate (SY49.1) caused 83% mortality for the second-stage larvae at 5 °C. However, an approximately 4-fold decrease in mortalities was observed in late-stage larvae for all isolates examined at this temperature. Nevertheless, other Bt isolates, excluding SY27.3, caused nearly complete mortality at 25 °C for early-stage larvae. Considering the distribution of seasonal temperature, Bt products should be applied at the highest ambient temperature to earlier stages for efficient control. We propose that local Bt isolates SY27.1, SY49.1, and SY62.1 could be used to develop environmentally safe bioinsecticides to control this important pest species. These results indicate that larval stage and environmental temperature should be taken into consideration for efficient control of T. wilkinsoni using the spore-crystal mixture of Bacillus thuringiensis isolates.