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 thuringiensis subspecies japonensis (Btj) caused additive or synergistic mortality in third-instar Cyclopephala (Coleoptera:Scarabaeidae) grubs when the nematodes were applied at least 7 days after Btj. This type of mortality was observed for C. hirta, a species that is not very susceptible to Btj, and for C. pasadenae, a species that has intermediate susceptibility to BTj. In the present study, this observation was confirmed for third-instar Anomala orientalis, a species that is highly susceptible to Btj. In greenhouse and field studies, additive or synergistic interactions between nematodes and Btj were observed for all three scarab species. The interactions were variable, and to achieve acceptable grub control, high concentrations of Btj had to be applied. A subsequent greenhouse experiment showed that young third instar C. pasadenae were more susceptible to Btj than older third instars. In addition the interaction between nematodes and Btj was also more synergistic in the younger than in the older third instars. In a field test against a population of C. hirta consisting of late second and early third instars, combinations of nematodes and Btj at economic application rates provided 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 entomopathogenic nematodes and Btj if the applications are done when populations consist of second and early third instars.