Out breaking herbivore escapes parasitoid by attaining only a small body size

Abstract

The reason why parasitoids are often unable to control insect herbivores during outbreak years has remained unclear to modern ecologists. Here I show that the blue willow beetle (Phratora vulgatissima) can escape parasitoids (Perilitus brevicollis) by reaching only a small body size during the increased competition which coincides with outbreaks. Different phases of outbreaks were simulated by allowing leaf-beetle larvae to develop at different densities on willow. The denser the herbivore populations, the smaller body sizes they attained. In the field too, herbivore body size decreased when the level of defoliation increased. The parasitoids were clearly limited by herbivore body size, exhibiting reduced survival in parallel with decreasing herbivore size, with none of them surviving in beetles from larvae reared at the highest density. The results demonstrate how bottom-up control of the herbivore becomes more important as outbreaks intensify, while top-down control gradually becomes less important and collapses during the outbreak peak. Because the mechanism limiting P. brevicollis parasitoids involves herbivore body size, which is reduced at high densities, I anticipate that these parasitoids are unable to terminate herbivore outbreaks.