The killing speed of egt-inactivated Bombyx mori nucleopolyhedrovirus depends on the developmental stage of B. mori larvae.

Several lines of evidence have shown that the deletion of the ecdysteroid UDP-glucosyltransferase gene (egt) from the nucleopolyhedrovirus (NPV) genome increases the killing speed of host lepidopteran larvae. However, it has not been investigated in detail whether the effects of egt deletion depend on the larval stages of the host insect. In this study, we performed bioassays using 10 continuous larval stages of the 4th- or 5th-instar Bombyx mori larvae and B. mori NPV egt mutants. The fast-killing phenotype was observed in the egt mutants only when the infection process progressed through larval-larval transition. All day-2 4th-instar larvae infected with the egt mutants entered the molting stage and died much earlier than wild-type-infected larvae. Bodies of egt mutant-infected larvae were filled with excessive fluid immediately after head capsule slippage, owing presumably to the degeneration of Malpighian tubules. Fourth- or 5th-instar larvae infected with the egt mutants at early stages of each instar died similarly to those infected with the wild-type virus. Under infection in the middle stages of the 5th-instar, the survival time of egt mutant-infected larvae was significantly longer than that of the wild-type virus-infected larvae. These results clearly show that the effects of egt deletion on killing speed of NPV are largely dependent on the developmental stage of the host larvae infected by the virus.