Midgut-based resistance of Heliothis virescens to baculovirus infection mediated by phytochemicals in cotton

The decrease in susceptibility to polyhedrosis disease when Heliothis vires cens larvae feed on cotton is profound, limiting the utility of baculovirus es for controlling noctuids on this important crop. We observed that the mo rtalities of H. virescens larvae challenged with a reporter-gene construct of Autographa californica M nucleopolyhedrovirus (AcMNPV-hsp70/lacZ) and fe d either lettuce or artificial diet were approximate to 2.5-fold higher tha n that of cotton-fed insects. This decrease in susceptibility on cotton was observed following oral but not intrahemocoelic inoculation of virus, and it was negatively correlated with levels of foliar peroxidase. The rates of development of both infected and uninfected larvae also were correlated ne gatively with levels of foliar peroxidase, and hence, were significantly lo wer for insects fed cotton. When Calcofluor White M2R, an optical brightene r reported to enhance the retention of AcMNPV-infected midgut cells, was in cluded in inoculum administered orally to larvae, mortality levels were equ ivalent regardless of diet. These results suggest that sloughing of infecte d midgut cells occurred at a higher rate in insects that fed on cotton comp ared to the other two diets, and that midgut cell sloughing is the mechanis m whereby susceptibility to mortal infection by AcMNPV-hsp70/lacZ is decrea sed on cotton. This conclusion is consistent with previous reports that ing estion of cotton can generate reactive oxygen species within the midgut lum en that may damage midgut epithelial cells. As far as we know, this is the first study to link resistance intrinsic to the physiology of the insect (e .g., developmental resistance) and resistance conferred by host plant chemi stry to a single mechanism, i.e., midgut cell sloughing. (C) 2000 Elsevier Science Ltd. All rights reserved.