Impact of climate change on insect pests of trees

In The Netherlands, insect pests on trees and shrubs are being monitored continuously since 1946. During these years, almost all insect pest populations showed marked changes, which may be the result of climate change, arrival of new pests, changes in forest management, shifts in forest composition etc. In an earlier study, we analyzed the number of observations for all pest species in the database on deciduous trees. The results showed that since 1985, pest insects hibernating in the egg stage, numerically exceed insects hibernating as larva, pupa or adult. During the last 2-3 decades, the winters in The Netherlands have become relatively warm and more humid. In literature, it is stated that mild winter temperatures can reduce winter survival of adult, larval and pupal stages more than of the eggs, presumably because the first stages are more vulnerable for entomopathogenic nematodes and fungal activity. This phenomenon may be the cause of our observed increase of egg hibernators (Moraal et al. 2004). In a later study, we have analyzed trends in 61 years of population development of the 91 most abundant species in our database, in such a way that frequently observed species did not bias the results. Of the observed species, only a minority occurred regularly over the entire observation period of 61 years. The remaining species showed population fluctuations that varied from single short-term outbreaks to long-lasting increases or decreases. On coniferous trees, most insect species showed decreasing numbers, while increasing numbers were found most on deciduous trees. In the increasing trend-group of Lepidoptera, more egg hibernating species were observed compared with the decreasing trend-group (Moraal & Jagers op Akkerhuis, in prep). Future climate change models for our region, predict increasing temperatures, drought periods, and heat waves during the growing season. The European literature on pest outbreaks that followed after the exceptional drought of 2003, give us some indications of the impacts of extreme climatic conditions. Primary pest insects, mostly leaf-consuming larvae, are not dependant on the vitality status of the host trees. Secondary pests, mostly barkboring species, are dependant on weakened trees e.g. by drought. In literature, some generalized predictions were made, based on current pest distributions and the severity of insect outbreaks in individual regions after the summer drought of 2003. The predictions are that tree mortality due to secondary pest insects may become more important in the future, because dry summers will reduce the resistance of trees. A combination of global trade and a changing climate makes it possible for new invasive species to establish in the EU and The Netherlands. In the absence of specific natural enemies, these species may cause tree mortality on a large scale (Moraal, in press). There are many interactions and it is extremely difficult to predict the impact of climate change on insect pests in the future, but we may expect an increase of certain primary pests as well as secondary pests and invasive species. References Moraal, L.G. Expected impact of climate change on insect pests of forest trees in The Netherlands. Wageningen, Alterra-report 1761. In press. Moraal, L.G., Jagers op Akkerhuis, G.A.J.M., Siepel, H., Schelhaas M.J. & Martakis G.F.P., 2004. Verschuivingen van insectenplagen bij bomen sinds 1946 in relatie met klimaatverandering. Met aandacht voor de effecten van stikstofdepositie, vochtstress, bossamenstelling en bosbeheer. Wageningen, Alterrarapport 856. 52 pp. Moraal, L.G. & Jagers op Akkerhuis, G.A.J.M. Population changes of insect pests on trees and shrubs in The Netherlands since 1946 in relation with climate change, forest management and site factors. In prep.