The comparative susceptibility of the diamondback moth Plutella xylostellaand some other major lepidopteran pests of Brassica crops to a range of baculoviruses

The susceptibility of larvae of the Diamondback moth (DBM), Plutella xylost ella to infection by three buculoviruses was evaluated in the laboratory us ing a microdroplet feeding assay. The viruses tested were a granulovirus (G V), originally isolated in Taiwan from P. xylostella larvae (PxGV-Taiwan); the nucleopolyhedrovirus (NPV) from Galleria mellonella (GmNPV), and the NP V from Autographa californica (AcNPV). Neonate P. xylostella larvae were su sceptible to infection by all thr ee viruses. In an extensive ser ies of bi oassays carried out over a 21-month period LD(50)s for neonate DBM larvae r anged from 1.0-8.9 viral occlusion bodies (OB) for PxGV-Taiwan, and 9.5-30. 2 OB for GmNPV and AcNPV. LT(50)s for the three viruses ranged from 3.8-6.0 days at 27 degrees C, with GmNPV having a significantly shorter LT50 than the other two viruses. Second and third instar larvae of P. xylostella were significantly less susceptible to infection by PxGV-Taiwan (LD(50)s rangin g from 18-57 OB/larva) than were neonate larvae. GmNPV also initiated infec tion in sever al other lepidopterous pest species that colonize brassica cr ops. In particular neonate Crocidolomia binotalis larvae proved highly susc eptible to GmNPV with mean LD(50)s ranging from 2.1 to 9.3 OB/larva ann a m ean LT50 of 4.8 clays at a nose of 8.08 OB. Heliothis virescens neonate lar vae were also highly susceptible to GmNPV (LD50, 7.1 OB), but Mamestra bras sicae larvae were less so (LD50, 80-270 OB). The results of the bioassays s uggest that PxGV-Taiwan is highly infective and could be developed as a sel ective microbial pesticide for DBM. While GmNPV has a higher LD50 in DBM la rvae, its wider host range may be of considerable value in situations where DBM occurs on cruciferous crops together with a complex of other lepidopte rous pests.