Behavioral resistance of the western corn rootworm (Diabrotica virgifera virgifera) to Bt corn

Diabrotica virgifera virgifera (Coleoptera: Chrysomelidae), the Western corn rootworm (WCR), causes serious economic damage in the larval stage feeding on the roots of corn plants. Transgenic plants, genetically modified to express Bacillus thuringiensis (Bt) toxins that are effective against rootworms are now widely used to manage these pests. In the few years that rootworm-active Bt corn has been available, no resistance development has been reported. However, the variability of toxin expression in corn roots and WCR’s proven ability to develop resistance to other forms of control are calls for caution. WCR has been shown to develop resistance to Cry3Bb1 Bt toxin expressed in corn in as little as 3 generations under both greenhouse and laboratory conditions. The exact mechanisms leading to this resistance remain to be ascertained. Behavioral avoidance leading to exposure to low Bt doses in plants or target tissues is a possible route towards resistance. Seed-mix, or refuge in a bag, planting methods were designed to reduce resistance risk for rootworms. However, they may in fact increase resistance development by allowing early susceptible instar WCR feeding on susceptible corn roots until development allows them to tolerate feeding on Bt corn.

For these reasons we are 1) investigating WCR larval feeding preference for different portions of roots in Bt and non-Bt corn isolines, under no-choice (mimicking block refuge planting) and choice (mimicking refuge in a bag planting) scenarios, and 2) determining if preference is correlated with differential Bt toxin expression across root tissues.