Additive and Synergistic Interaction between Entomopathogenic Nematodes andBacillus thuringiensisfor Scarab Grub Control

We studied the interactions betweenBacillus thuringiensissubspeciesjaponensisBuibui strain (Btj) and entomopathogenic nematodes on the white grubs,Cyclocephala hirtaandC. pasadenae.Field-collected third instar grubs were kept individually in microcosms filled with soil and fed grass seeds. Grubs were exposed to various concentrations ofBtjand/or entomopathogenic nematodes and grub mortality was assessed at weekly intervals. Nematodes were added at 0 to 14 days after application ofBtj.Throughout most of our experiments, combinations ofBtjand nematodes caused additive grub mortalities or greater than additive mortalities, indicating synergism. To achieve additive or synergistic effects, grubs had to be exposed toBtjfor at least 7 days before the addition of nematodes. We observed this interaction betweenBtjandHeterorhabditis bacteriophoraorSteinernema glaseri,but not with the most pathogenic nematode,S. kushidai.The additive or synergistic interaction betweenBtjandS. glaseriorH. bacteriophoramay offer a powerful and reliable tool for scarab grub control.