Differential susceptibility of pine weevil, Hylobius abietis (Coleoptera: Curculionidae), larvae and pupae to entomopathogenic nematodes and death of adults infected as pupae

The large pine weevil Hylobius abietis is a serious pest of reforestation in northern Europe. Development takes place in the stumps of felled conifer trees and emerging adults feed on and kill newly planted trees. Application of entomopathogenic nematodes around tree stumps has been shown to reduce the emergence of adult weevils. In order to target application at the most susceptible stage, the susceptibility of larvae and pupae to Heterorhabditis downesi and Steinernema carpocapsae was compared in a close-contact assay on filter paper. An average of 95.8 % of larvae were killed by H. downesi and 82.1 % by S. carpocapsae while only 16.3 and 15.0 % of pupae were killed by these two species, respectively. However, many of the H. abietis that were exposed as pupae died after metamorphosis to callow adult, with mortality of pupae and callow adults combined reaching 62.5 % for H. downesi and 69.9 % for S. carpocapsae. For both nematode species significantly more insects died as larvae than as either pupae or pupae/callow adults. When pupae were exposed to infective juveniles (IJs) for 2 days and were then washed while still pupae to remove surface IJs, adults were later found to be infected indicating that IJs can infect pupae, survive metamorphosis and subsequently kill adults