Age-related effects of the Autographa californica multiple nucleopolyhedrovirus egt gene in the cabbage looper (Trichoplusia ni)

The effects of deleting the Autographa californica multiple nucleopolyhedro virus (AcMNPV) egt gene on speed of kill and virus productivity were compar ed in second and fourth instar Trichoplusia ni (Hubner) larvae. Time to dea th was significantly reduced in larvae infected with an egt deletion mutant compared to insects infected with the wild-type virus. Moreover, time to d eath was reduced by the same proportion (11%) in second and fourth instar l arvae. Virus yield was also significantly lower in fourth instar larvae inf ected with the deletion mutant but no difference was apparent in second ins tar larvae. A comparison of cadaver weights showed that insects infected wi th the deletion mutant were lighter than those infected with the wildtype v irus, suggesting that the decrease in virus yield resulted from a reduction in larval growth. An analysis of yield per unit body weight showed no evid ence for differences in replication rate in the two viruses. To determine w hether differences in larval growth rate were related to differences in fee ding activity, frass production was monitored in fourth instar larvae. Larv ae infected with the deletion mutant produced less frass than larvae infect ed with the wildtype virus. Whereas the average rate of feeding for fourth instars did not differ between virus treatments, the rate peaked and declin ed earlier for larvae infected with the deletion mutant than for those infe cted with wild-type virus, suggesting enhanced early feeding in the absence of egt expression. (C) 2000 Academic Press.