Resistance against Pissodes strobi (Coleoptera : Curculionidae) in severedleaders and in a water-soluble bark extract of Picea sitchensis (Pinaceae): evidence for a post-ingestive mode of action

The levels of ovarian growth and transcription of the vitellogenin gene wer e compared in Pissodes strobi Peck (Coleoptera: Curculionidae) feeding on s evered leaders from susceptible and resistant Picea sirchensis (Bong) Carr (Pinaceae) and in weevils given a dose of an aqueous extract from the bark of such leaders. A force-feeding method was developed to deliver the extrac ts into the alimentary canal of the weevils. Weevils given one dose of the aqueous extract from resistant leaders, followed by feeding on sections of laterals from susceptible trees, exhibited a 60% inhibition of oocyte growt h relative to insects given the extract from susceptible leaders. In simila r experiments, transcription of the vitellogenin gene of the weevils was in hibited by about 48%. Transcription of the vitellogenin gene was reduced by about 60% in weevils feeding on the severed leaders from the resistant clo ne relative to those feeding on severed leaders from susceptible trees. Res ults indicate that these effects of resistance do not require an intact tre e. Experiments using extracts show that the observed effects result from a post-ingestive effect of the extract.