The Comparative Susceptibility of the Diamondback Moth Plutella xylostella and Some Other Major Lepidopteran Pests of Brassica Crops to a Range of Baculoviruses

The susceptibility of larvae of the Diamondback moth (DBM), Plutella xylostella to infection by three baculoviruses was evaluated in the laboratory using a microdroplet feeding assay. The viruses tested were a granulovirus (GV), originally isolated in Taiwan from P. xylostella larvae (Px GV-Taiwan); the nucleopolyhedrovirus (NPV) from Galleria mellonella (Gm NPV), and the NPV from Autographa californica (Ac NPV). Neonate P. xylostella larvae were susceptible to infection by all three viruses. In an extensive series of bioassays carried out over a 21-month period, LD 50S for neonate DBM larvae ranged from 1.0-8.9 viral occlusion bodies (OB) for Px GV-Taiwan, and 9.5-30.2 OB for Gm NPV and Ac NPV. LT 50S for the three viruses ranged from 3.8-6.0 days at 27 C, with Gm NPV having a significantly shorter LT 50 than the other two viruses. Second and third instar larvae of P. xylostella were significantly less susceptible to infection by Px GV-Taiwan (LD 50 s ranging from 18-57 OB/larva) than were neonate larvae. Gm NPV also initiated infection in several other lepidopterous pest species that colonize brassica crops. In particular, neonate Crocidolomia binotalis larvae proved highly susceptible to Gm NPV, with mean LD 50 s ranging from 2.1 to 9.3 OB/larva and a mean LT 50 of 4.8 days at a dose of 8.08 OB. Heliothis virescens neonate larvae were also highly susceptible to Gm NPV (LD 50 , 7.1 OB), but Mamestra brassicae larvae were less so (LD 50 , 80-270 OB). The results of the bioassays suggest that Px GV-Taiwan is highly infective and could be developed as a selective microbial pesticide for DBM. While Gm NPV has a higher LD 50 in DBM larvae, its wider host range may be of considerable value in situations where DBM occurs on cruciferous crops together with a complex of other lepidopterous pests.