Characterizing the enhanced susceptibility of milky disease-infected scarabaeid grubs to entomopathogenic nematodes

Grubs of the scarabaeid Cyclocephala hirta infected with the milky disease bacterium, Bacillus popilliae (Bp), were more susceptible to the entomopathogenic nematodes Heterorhabditis bacteriophora and Steinernema glaseri than healthy grubs. Increased susceptibility was due to greater ease of nematode penetration through the midguts of Bp-infected insects and occurred after > 10 days exposure to B. popilliae spores. By that time, the bacterium was proliferating rapidly in the insect hemolymph and possibly stressed the insect by physiological starvation. Attraction of the nematodes to the insects and cuticular penetration by the nematodes were ruled out as possible reasons for the enhanced susceptibility of Bp-infected grubs. In greenhouse pot trials, S. glaseri killed more Bp-infected grubs than healthy grubs, whereas no differences were noted with H. bacteriophora. Thus, to achieve the best grub control in the presence of B. popilliae, application of S. glaseri is recommended. The compatibility of B. popilliae and entomopathogenic nematodes in the field is ensured by the existence of a diseased insect class with a low level of susceptibility to the nematode, the presence of B. popilliae in grubs not interfering with nematode progeny production, and the ability of B. popilliae spores to survive invading nematodes and their associated bacteria.