The effects of Western corn rootworm (Diabrotica virgifera virgifera) infestation and refuge scenario on Bt toxin expression in transgenic corn

Corn rootworms are one of the most serious corn pests in the Americas.  Transgenic corn expressing Bacillus thuringiensis (Bt) toxin targeting rootworms has only been available since 2003, but resistance development by Diabrotica virgifera virgifera, the Western corn rootworm (WCR), has already been reported.  The mechanisms involved in WCR resistance to Bt corn are unknown, but it is likely a combination of insect and plant modulated factors.  We investigated WCR larval responses to Bt and non-Bt corn roots under scenarios mimicking both block/strip and seed mix refuges.  We also investigated Bt toxin expression in corn roots grown under both scenarios to determine if larval responses could be explained by differences in toxin expression along the root.  Not surprisingly, rootworms preferred non-Bt corn under no-choice scenarios, which may lead to sublethal toxin exposure.  However, differential toxin expression along the root length was not significant.  Interestingly, when pest pressure was present, Bt-expressing plants expressed more toxin when grown in tandem with other plants than when grown alone.  When pest pressure was not present, Bt-expressing plants expressed less toxin when grown in tandem with other plants than when grown alone.  Possible causes for this finding and its implications to resistance management are discussed.