Milky Disease Bacterium as a Stressor to Increase Susceptibility of Scarabaeid Larvae to an Entomopathogenic Nematode

Mortality of Cyclocephala hirta by the entomopathogenic nematode Heterorhabditis bacteriophora was enhanced in grubs infected with the milky disease bacterium, Bacillus popilliae (Bp). In concentration-response assays with the nematode, LC50 values and 95% fiducial limits were 4.0 (<0.1, 14.6) infective juveniles per grub for Bp-infected insects and 47.7 (32.0, 64.7) for non-Bp-infected insects. The increased mortality of Bp-infected insects was partially attributed to rapid penetration of H. bacteriophora through the midgut wall. In in vitro assays with ligated midguts, the time required for H. bacteriophora to penetrate the midgut wall of Bp-infected grubs was 0.6 ± 0.1 hr, whereas the time required in non-Bp-infected grubs was 1.3 ± 0.1 hr. Total nematode progeny production per milligram grub tissue did not differ significantly between Bp-infected and non-Bp-infected grubs. However, the proportion of progeny remaining trapped in the non-Bp-infected cadavers at the end of the assay was greater than the proportion remaining in Bp-infected cadavers (0.52 ± 0.04 vs 0.13 ± 0.05, respectively). These data indicate that coexistence of these two pathogens within a host can occur and that B. popilliae could be used as a stressor on C. hirta to increase its susceptibility to H. bacteriophora.