Developmental resistance in fourth instar Trichoplusia ni orally inoculated with Autographa californica M nuclear polyhedrosis virus.

Larvae of lepidopteran insects commonly become increasingly resistant to baculovirus infections as they age. The mechanism responsible for this development resistance is not known, but the phenomenon does not occur if the viral inoculum is administered intrahemocoelically instead of orally, which is the natural route of infection. This observation indicates that the factors mediating developmental resistance are operative during infection of the primary target tissue, the larval midgut, and not during subsequent systemic infection. To learn more about the mechanism of developmental resistance, we orally inoculated four cohorts of fourth instar Trichoplusia ni larvae with a recombinant of Autographa californica M nuclear polyhedrosis virus expressing a reporter gene. While these cohorts differed only by a few hours in age, we found increasing resistance to infection in successively older cohorts. By assessing the presence and location of infected cells at intervals during the first 48 hr after inoculation, we identified two key factors relevant to the resistance pattern among the developmental cohorts. These factors were: (i) an age-dependent rate of establishing and/or sloughing infected midgut cells and (ii) the ability of fourth instar T. ni to completely clear infection of the midgut epithelium by ecdysis to the fifth instar.