Studies on the entomopathogenicity and bacterial associates of the nematode Oscheius carolinensis

Abstract

The entomopathogenic potential of Oscheius carolinensis (Nematoda: Rhabditidae), a recently described nematode species, was tested on various developmental stages of five insect species. The nematode penetrated, killed, and reproduced in all of the insect species tested; however, some insect developmental stages proved more susceptible to infection than others. O. carolinensis was consistently associated with four bacterial species, one of which, Serratia marcescens, appears to be carried on the cuticle of the nematodes and through its association provides the worms with entomopathogenic potential. In manual inoculation assays, S. marcescens killed fourth-instar Helicoverpa zea larvae within 24h at concentrations as low as 330μg/mL (3.96×103CFU). Another species, Enterococcus mundtii, inflicted levels of mortality of 33.3% only after mechanical wounding (pin-pricking) of H. zea and at much higher concentrations. A third species, Achromobacter xylosoxidans, did not cause mortality even at the highest dose tested (500mg/mL or 6×106CFU), with or without wounding. The fourth species, Providencia rettgeri, predominant in ground-up surface sterilized nematodes, caused low levels of mortality (25%) when manually applied to pin-picked larvae larvae. Nematodes did not performed well on insect hosts after surface sterilization; nonetheless, it is possible that P. rettgeri in combination with S. marcescens is responsible for the facultative insect colonization ability of O. carolinensis. Based on results from this study it would appear that O. carolinensis association with its bacteria allows the nematode a dual life: style as an entomopathogen when associated with S. macescens, and to an extent with P. rettgeri, and as a free-living entity when associated with all others. Further studies of are needed to fully characterize the association between O. carolinensis and S. marcescens, to determine the exact role of the bacterial associates in the nematode’s ecology and to assess this nematode’s applicability for agricultural insect pest management.