Habitat loss and alien tree invasion reduce defoliation intensity of an eruptive forest pest

Abstract

In forest ecosystems, bush-cricket species often occur at low density for very long periods until they suddenly outbreak with important economic and ecological consequences. In the last decades, isolated but severe outbreaks were reported across Europe for several species belonging to the genus Barbitistes. The aim of the study was to evaluate the role of local (i.e. stand composition) and landscape factors (forest area and composition in the landscape) on B. vicetinus populations during two outbreak years. We sampled frass quantity and defoliation intensity in 2014 and 2015 in 200 forest stands across north-east Italy where the forest landscapes have been fragmented and native forests have been heavily invaded by Robinia pseudoacacia, a North-American alien tree species. We found a strong negative effect of decreasing forest area on the frass quantity at relatively small spatial scale (250–500 m). The negative effect of forest habitat loss was likely related to the reduced ability to find resources across the landscape and possibly to higher pest mortality in the non-forest matrix due to multiple impacts such as agriculture management and road kill. The presence of R. pseudoacacia, a non-host alien tree species, at both the local and landscape scale further reduced the frass quantity indicating lower population density. We found a negative density-dependence in the population dynamics during the two outbreak years indicating that greater population density probably imposed a fitness cost to the offspring due to competition for food. Our results can help predicting spatial patterns of forest defoliation according to landscape structure and can improve the spread risk assessment and the design of potential control or containment strategies of an important forest pest. Any management intervention to contain this eruptive pest should consider the pervasive effects of landscape beside local processes.