Climate change is a multi-faceted phenomenon, including elevated CO2, warmer temperatures, more severe droughts and more frequent storms. All these components can affect forest pests directly, or indirectly through interactions with host trees and natural enemies. Most of the responses of forest insect herbivores to climate change are expected to be positive, with shorter generation time, higher fecundity and survival, leading to increased range expansion and outbreaks. Forest insect pest can also benefit from synergistic effects of several climate change pressures, such as hotter droughts or warmer storms. However, lesser known negative effects are also likely, such as lethal effects of heat waves or thermal shocks, less palatable host tissues or more abundant parasitoids and predators. The complex interplay between abiotic stressors, host trees, insect herbivores and their natural enemies makes it very difficult to predict overall consequences of climate change on forest health. This calls for the development of process-based models to simulate pest population dynamics under climate change scenarios.

Responses of forest insect pests to climate change: not so simple.

It is difficult to find a genus of Lepidoptera showing the high variability of life history traits observed in Thaumetopoea. There are typical summer feeding close to winter feeding species, and in one special case a recent switch has been detected even within one species, the pine processionary moth, indicating that the natural history traits are constantly evolving at a fast rate. There are species adapted to cold conditions of high mountains and high latitude close to truly Mediterranean and sub-desert region species. All species have gregarious behaviour as larva and are protected against vertebrate predators by urticating setae.

Natural History of the Processionary Moths (Thaumetopoea spp.): New Insights in Relation to Climate Change

Apparent feeding damage by insects on plants is often slight. Thus, the influences of insect herbivores on plant populations are likely minor. The role of insects on host-plant populations can be elucidated via several methods: stage-structured life tables of plant populations manipulated by herbivore exclusion and seed-addition experiments, tests of the enemy release hypothesis, studies of the effects of accidentally and intentionally introduced insect herbivores, and observations of the impacts of insect species that show outbreak population dynamics. These approaches demonstrate that some, but not all, insect herbivores influence plant population densities. At times, insect-feeding damage kills plants, but more often, it reduces plant size, growth, and seed production. Plant populations for which seed germination is site limited will not respond at the population level to reduced seed production. Insect herbivores can influence rare plant species and need to be considered in conservation programs. Alterations due to climate change in the distributions of insect herbivores indicate the possibility of new influences on host plants. Long-term studies are required to show if density-related insect behavior stabilizes plant populations or if environmental variation drives most temporal fluctuations in plant densities. Finally, insects can influence plant populations and communities through changing the diversity of nonhost species, modifying nutrient fluxes, and rejuvenating over mature forests.

Impacts of Insect Herbivores on Plant Populations

Various species of forest trees are commonly used for ornamental purposes and are therefore frequently found in nonforest ecosystems. They constitute an important component of the so-called trees outside forests (TOF). Not much is known, however, about the drivers of TOF spatial distribution either in urbanized or in agricultural landscapes since they are generally absent from forest inventories. The present study focused on the spatial distribution of TOF across agricultural landscapes and their potential role in the dispersal of a forest pest insect, the pine processionary moth, Thaumetopoea pityocampa (PPM). All the TOF belonging to the genera Pinus, Cedrus and Pseudotsuga were considered as potential hosts and inventoried within a 22 × 22 km study window. We fitted a nonstationary Poisson process to the empirical data and used the distance to the nearest building as a covariate. Both empirical and simulated data indicated that TOF associated to human artifacts/urbanized areas constituted the main source of landscape connectivity for the PPM in the open fields under study. Because they do not account for TOF, forest inventories dramatically underestimate landscape connectivity and provide an erroneous picture of the PPM habitat distribution. 
We conclude that TOF, especially the ornamental component, must be taken into account when it comes to understanding forest insect landscape dynamics or genetics. The omnipresence of TOF also suggests a potentially huge role in pest dispersal and invasive species expansion.

/pdf/trees-outside-forests-in-agricultural-landscapes-spatial-44n2418waa.pdf

Trees outside forests in agricultural landscapes: spatial distribution and impact on habitat connectivity for forest organisms

Predicting shifts in the distribution and abundance of pest organisms relies on an accurate forecasting of their response to climate change. The oak processionary moth (OPM) Thaumetopoea processionea causes serious damages to oak trees in forest, urban and other landscapes as well as severe allergic reactions to humans and animals. In the 1990’s and 2000’s, the OPM extended its range from mainland Europe and the Middle East into northern Europe. In 2005, it was also accidentally introduced in the United Kingdom. Moreover, the intensity and the frequency of OPM outbreaks are thought to have recently increased in several countries of Europe including Belgium, the Netherlands, Germany and Austria. In the present study, we aimed at forecasting the potential distribution of the OPM in Europe under current and future climate conditions. We thoroughly compiled available records of established populations all across Europe and fitted MaxEnt and BIOCLIM models to infer bioclimatic requirements for this species. Both models showed good predictive performance under current climate conditions. In particular, the surroundings of London where the OPM recently got established were predicted as highly climatically suitable. Models also predicted that many parts of northern Europe where the OPM currently does not occur (e.g. central UK, Wales, Ireland, southern Scotland, Denmark, southern part of the Scandinavian Peninsula, etc.) might become climatically suitable by 2050. Our predictions warrant the need for proper communication and management planning around the risks associated with the potential expansion of the OPM in Europe.

Current and future distribution of the invasive oak processionary moth

Pine processionary moth, Thaumetopea pityocampa, is a model insect indicator of global warming, the northwards and upwards range expansion of this Mediterranean species being directly associated with the recent warming up. The knowledge about the drivers of moth expansion is synthesized. A first standardized mapping of the northern expansion edge, from Western Europe to Turkey, is presented, then detailed for 20 countries of Europe, Asia Minor and North Africa, including future trends. Additional data about the responses of the other Thaumetopoea species are given. Finally, the chapter points out the importance of the man-mediated introductions in the expansion process.

Climate Warming and Past and Present Distribution of the Processionary Moths (Thaumetopoea spp.) in Europe, Asia Minor and North Africa

The forest cockchafer Melolontha hippocastani is a serious forest pest whose infestation levels have been increasing in South-West Germany since the mid-1980 s. During the past 25 years several control measures have been taken to manage its populations. In 2007 and 2008, 122 and 73 hectares of forest stands comprising of deciduous trees were sprayed with NeemAzal®-T/S (1% azadirachtin A) by helicopter, respectively. Here we summarize the results of the accompanying research with the aim to evaluate the efficacy of NeemAzal®-T/S in the control of M. hippocastani populations under field conditions. As expected from azadirachtin’s mode of action, no immediate mortality occurred but maturation feeding and oogenesis was interrupted. It appeared that precise timing of the application is a crucial issue regarding the level of its effect on the population density. In 2007, the first application was performed in good synchrony with the climax of female emergence from the soil, resulting in a remarkable decline of the population density in the next generation of beetles in 2011. In 2008, our data suggest that maturation feeding and oogenesis had already been completed when the agent was sprayed. Hence, no significant decline of the population density could be registered in 2012. Practicability of the use of NeemAzal®-T/S against M. hippocastani in forests is discussed.

Aerial Spraying of Neemazal®-T/S Against the Forest Cockchafer (Melolontha hippocastani, Col.: Scarabaeidae) in South-West Germany: The Effects of Two Field Trials Performed in 2007 and 2008 on Local Populations

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Climate Warming and Past and Present Distribution of the Processionary Moths (Thaumetopoea spp.) in Europe, Asia Minor and North Africa

Aerial Spraying of Neemazal®-T/S Against the Forest Cockchafer (Melolontha hippocastani, Col.: Scarabaeidae) in South-West Germany: The Effects of Two Field Trials Performed in 2007 and 2008 on Local Populations