Dataset documenting prevalence and counts of pine processionary moth tents on local host trees in 3 regions of France with different climatic environments.

The pine processionary moth Thaumetopoea pityocampa is a defoliating lepidopter that develops during winter. The larvae are gregarious and bear urticating setae that are harmful to humans and vertebrates. They shelter in conspicuous silk tents that are easy to detect. We here present a dataset comprising tree characterization and tent counts from 3 agglomerations in France located in regions with different climatic environments. The studied trees belong to various conifer species that are potential hosts for the caterpillars. In each site, we defined clusters as one target tree and its 10-62 nearest neighbors, and surveyed each tree within the clusters by informing: tree species, coordinates, size, number of tents. We characterized a total of 3690 trees, including 2009 trees in Orléans (grouped in 68 clusters), 359 trees in La Baule (18 clusters) and 1322 trees in Montpellier (52 clusters). We provide the raw data characterizing each individual tree, graphs showing the prevalence and mean number of tents for the tree species included in the survey, and maps allowing to locate each tree. This dataset brings information about host preference of the pine processionary moth and will be useful as a baseline to study spatio-temporal variability of host-insect relationships. It can also be informative for decision-makers and managers of urban greenings to avoid trees that are likely to be heavily infested for plantation in proximity to vulnerable people.

Diapause Regulation in Newly Invaded Environments: Termination Timing Allows Matching Novel Climatic Constraints in the Box Tree Moth, Cydalima perspectalis (Lepidoptera: Crambidae).

The association between indirect environmental cues that modulate insect diapause and the actual stressors is by no means granted when a species encounters new environments. The box tree moth, Cydalima perspectalis, is an Asian pest whose rapid invasion in Europe causes considerable economic and ecological impacts. Larvae enter a winter diapause induced by the photoperiod in both native and invaded ranges, but factors that trigger the return to an active phase are still unknown. Yet, identifying them is crucial to understand how diapause end synchronizes with the end of the winter stress encountered in Europe. To test whether activity resumption is regulated by thermal and/or photoperiodic thresholds, or additive effects between these factors often involved in diapause termination, diapausing caterpillars from an invaded area were exposed to crossed treatments at the laboratory. The evolution of diapause rate was monitored over time and compared to that of nearby field sites invaded. A strong positive effect of increasing temperature was found on the rate and dynamics of diapause termination, whereas no compelling effect of photoperiod appeared. Resuming development directly when main stressors fade, not in response to indirect photoperiodic cues that could be mismatched outside native areas, likely contributes to the good match observed between diapause and the new climates that this pest encountered in the invaded range.

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.