Physiological basis for increased AgipMNPV infection following feeding of Agrotis ipsilon larvae on Herculex® I corn

Larvae of the black cutworm, Agrotis ipsilon Hufnagel, were more susceptible to infection by Agrotis ipsilon multiple nucleopolyhedrovirus (AgipMNPV, Baculoviridae) after feeding on Herculex^® I (Dow AgroSciences LLC), a transgenic corn hybrid expressing the Bacillus thuringiensis (Bt) toxin Cry1Fa2 compared to larvae fed on isoline corn. We investigated the physiological basis for increased susceptibility to virus infection following exposure to Herculex^® I with emphasis on factors important for both Bt Cry toxin action and baculovirus infection, namely midgut pH, gut protease activity and peritrophic membrane structure. There were no significant treatment differences in midgut pH in larvae fed Herculex^® I or isoline diet. Membrane-associated aminopeptidase activity and soluble chymotrypsin-like proteinase activity were significantly lower in Herculex^® I -fed larvae, compared to isoline-fed larvae, although the number and relative molecular masses of soluble chymotrypsin-like proteinases did not differ between the two treatments. In vitro analysis indicated that baculovirus polyhedra are not susceptible to degradation by chymotrypsin. From SEM analysis of the peritrophic matrices of Herculex^® I -fed larvae and isoline-fed larvae we concluded that Herculex^® I did not damage the peritrophic matrix, which could facilitate subsequent baculovirus infection. Additional research is required to further delineate the physiological basis for enhanced baculovirus infection following exposure to sublethal doses of Bt toxins.