Larvae of the black cutworm, Agrotis ipsilon Hufnagel, were more susceptible to
infection by A. ipsilon multiple nucleopolyhedrovirus (AgipMNPV: Baculoviridae) after feeding on Herculex I, a transgenic corn hybrid expressing the Bacillus thuringiensis (Bt)-derived 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 by analyzing the midgut pH, gut
protease activity and peritrophic matrix structure which are important factors for both
Bt toxin action and baculovirus
infection. No significant treatment differences were found in the pH of anterior midgut, central midgut or posterior midgut in larvae fed Herculex I or
isoline diets. Analysis of soluble and membrane-associated gut
proteinase activities from larvae fed Herculex I or
isoline diets indicated that membrane-associated
aminopeptidase activity and soluble
chymotrypsin-like
proteinase activity were significantly lower in Herculex I -fed larvae compared to
isoline-fed larvae. The number and relative molecular masses of soluble
chymotrypsin-like
proteinases did not differ. Baculoviruses were not susceptible to in vitro degradation by bovine
chymotrypsin, suggesting that
chymotrypsin degradation of baculovirus occlusion-derived virus did not result in reduced
infection of larvae fed on
isoline diet. Scanning electron micrographs of the peritrophic matrices of Herculex I -fed larvae and
isoline-fed larvae indicated that Herculex I did not result in damage to the peritrophic matrix that 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.