Epsilon Open Archive

Abstract

It is generally believed that diversity leads to stability in ecological systems. One consequence would be that insect outbreaks (i.e. drastic increases in insect density) should be more frequent in managed systems, such as forest plantations and crop fields, than in natural and more diverse systems. The leaf beetle Phratora vulgatissima is a major insect pest in plantations of the willow Salix viminalis (‘energy forests’) and outbreaks are frequently observed. In this thesis, food-web interactions and population fluctuations of leaf beetles are compared between managed (Salix viminalis) and natural (S. cinerea) willow stands. In a five-year study, we found no difference in temporal variability of leaf beetle populations between managed and natural willow stands. However, drastic increases in leaf beetle density (‘outbreaks’) tended to be more frequent in managed stands. The two willow species studied responded differently to leaf beetle attack. The natural willow (S. cinerea) responded to grazing by adult beetles by producing new leaves with an increased density of trichomes (leaf hairs). Larvae were shown deterred from feeding on the induced leaves. This type of plant response may reduce the overall damage done by leaf beetles. However, no induced defence in response to adult grazing could be detected for the willow used in plantations, S. viminalis. Among the main predators attacking egg aggregations laid by leaf beetles, one type (mirid bugs) often stay and consume most eggs before moving on to search for other prey, a behaviour characterised as ‘find and stay’. Another type (anthocorid bugs) have more of a ‘run and eat’ behaviour; they visit many aggregations but consume fewer eggs within aggregations than mirids. In a simulation model, populations of leaf beetles were less likely to establish and to increase in abundance when attacked by ‘find and stay’ predators than when attacked by ‘run and eat’ predators. In the field, the leaf beetle P. vulgatissima occurred at low densities, and the predation on eggs was high, in natural willow stands where ‘find and stay’ mirids were abundant. The results suggest that a high abundance of heteropteran predators may prevent outbreaks of the leaf beetle P. vulgatissima.