Studies on the application of a nuclear polyhedrosis virus to control populations of the Egyptian cottonworm, Spodoptera littoralis

Abstract

Variations in the resistance to nuclear polyhedrosis virus (NPV) were found in three populations of Spodoptera littoralis. The LD 50 for the most resistant population was 1.07 × 10 4 PIB/5th instar larva as compared to 8.4 × 10 2 and 5.8 × 10 2 PIB/larva in the other two populations. The effect of NPV persisted in larvae which survived and pupated. Some of the pupae died, and those which survived produced normally shaped adults. While fecundity was sharply reduced in the less resistant populations, the effect on the most tolerant population was less pronounced. A 3-year-old inoculum, stored unprotected from daylight and without cooling, was much less effective even against the most sensitive larval population as compared to a relatively fresh and refrigerated batch. Larvae in their 6th instar proved to be approximately 10-fold more resistant to the NPV than 5th instar ones, while the difference in weight was only about twice. These variations in resistance to NPV are also discussed from the point of view of applying S. littoralis NPV in pest control schemes.