Suppression of diamondback moth (Lepidoptera: Plutellidae) with an entomopathogenic nematode (Rhabditida: Steinernematidae) and Bacillus thuringiensis Berliner.

We tested the efficacy of the All strain of Steinernema carpocapsae (Weiser) against larvae of the diamondback moth, Plutella xylostella (L.). In laboratory bioassays we found that (1) commercially formulated nematodes produced in vitro were as effective as nematodes produced in vivo, (2) resistance of P. xylostella to Bacillus thuringiensis Berliner subsp. kurstaki did not confer cross-resistance to nematodes, (3) mortality caused by nematodes was higher for early than late 3rd-instar P. xylostella larvae, and (4) no interaction occurred when B. thuringiensis and nematodes were combined against a susceptible strain of P. xylostella, but an antagonistic interaction occurred between the 2 pathogens against a strain of P. xylostella resistant to B. thuringiensis. In field trials conducted on 2 watercress [Rorippa Nasturtium-aquaticum (L.) Hayek] farms in Hawaii, nematodes provided 41% control, B. thuringiensis subsp. aizawai gave 44% control, and the combined treatment (B. thuringiensis plus nematodes both at half rate) resulted in 58% control. Using nemodes to control diamondback moth can theoretically reduce resistance development in diamondback moth populations to B. thuringiensis products, but repeated applications of nematodes will probably be ineffective in attaining control (suggested in simulation model). The results of this study demonstrate that nematodes may be a useful component of integrated pest management programs if efficacy can be increased, especially for populations of P. xylostella that are resistant to B. thuringiensis.