Larval mortality and injury patterns of the African stalk borer, Busseola fusca (Fuller) on various plant parts of Bt-transgenic maize

Abstract

During the 1998/99 season considerable stem damage was caused by B. fusca in commercial plantings of Bt-maize, without leaf feeding damage during the vegetative stages of plant development. This indicated that larvae may survive on some less toxic plant part subsequent to the vegetative stages. In this study a laboratory bioassay was conducted, using forced feeding of larvae on various parts of Bt-maize. In a field trial larval survival and plant damage were recorded following artificial infestation of Bt- plants one week before, and one and three weeks after tasseling. It appears that there is a window of approximately 14 days immediately preceding tasseling, in which relatively late infestations may result in increased larval survival, due to the availability of silks as a primary food source to neonate larvae. The Bt-protein concentration in silks appears to be low enough to allow survival of some larvae until completion of the first two instars, after which the ear tips and husk leaves serve as important feeding sites. The upper stem appears to be less toxic than the lower stem, providing a site for early penetration of stems by-young larvae, culminating in the eventual successful penetration of the lower stem at later stages of plant development. Increased larval survival resulted in a significant increase in the incidence of ear damage, which appears to be most pronounced within the first 14 days after infestation. Observed plant damage did not result in significant yield losses, but does not exclude the possibility that currently used Bt-hybrids may suffer economically important yield losses at the high levels of natural infestation often experienced with late planting.