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 nucleopolyhedrovirus (AcMNPV) egt gene on speed of kill and virus productivity were compared in second and fourth instar Trichoplusia ni (Hübner) larvae. Time to death was significantly reduced in larvae infected with an egt deletion mutant compared to insects infected with the wild-type virus. Moreover, time to death was reduced by the same proportion (11%) in second and fourth instar larvae. Virus yield was also significantly lower in fourth instar larvae infected with the deletion mutant but no difference was apparent in second instar larvae. A comparison of cadaver weights showed that insects infected with the deletion mutant were lighter than those infected with the wild-type virus, suggesting that the decrease in virus yield resulted from a reduction in larval growth. An analysis of yield per unit body weight showed no evidence for differences in replication rate in the two viruses. To determine whether differences in larval growth rate were related to differences in feeding activity, frass production was monitored in fourth instar larvae. Larvae infected with the deletion mutant produced less frass than larvae infected with the wild-type virus. Whereas the average rate of feeding for fourth instars did not differ between virus treatments, the rate peaked and declined earlier for larvae infected with the deletion mutant than for those infected with wild-type virus, suggesting enhanced early feeding in the absence of egt expression.