Increased field and greenhouse efficacy against scarab grubs with a combination of an entomopathogenic nematode and Bacillus thuringiensis

In previous laboratory studies, entomopathogenic nematodes and Bacillus thu ringiensis subspecies japonensis (Btj) caused additive or synergistic morta lity in third-instar Cyclocephala (Coleoptera: Scarabaeidae) grubs when the nematodes were applied at least 7 days after Btj. This type of mortality w as observed for C. hirta, a species that is not very susceptible to Btj, an d for C. pasadenae, a species that has intermediate susceptibility to Btj. In the present study, this observation was confirmed for third-instar Anoma la orientalis, a species that is highly susceptible to Btj. In greenhouse a nd field studies, additive or synergistic interactions between nematodes an d Btj were observed for all three scarab species. The interactions were var iable, and to achieve acceptable grub control, high concentrations of Btj h ad to be applied. A subsequent greenhouse experiment showed that young thir d-instar C. pasadenae were more susceptible to Btj than older third instars . In addition the interaction between nematodes and Btj was also more syner gistic in the younger than in the older third instars. In a field test agai nst a population of C. hirta consisting of late second and early third inst ars, combinations of nematodes and Btj at economic application rates provid ed acceptable control levels whether applied simultaneously or with a 4-day delay between Btj and nematode application. Our observations suggest that curative control of white grubs is possible with combinations of entomopath ogenic nematodes and Btj if the applications are done when populations cons ist of second and early third instars. (C) 1999 Academic Press.