Laboratory and field evaluation of an entomopathogenic nematode genetically selected for improved host-finding

A strain of Steinernema carpocapsae selectively bred in the laboratory for improved host-finding of scarab larvae did not provide enhanced field efficacy of Japanese beetle larvae, Popillia japonica. Selected (S20) and wildtype (All) strains reduced larval populations by 25-30%, compared with 70% for the chemical insecticide. Similar results were obtained in laboratory exposures conducted in soil-filled pots against the scarab Maladera matrida. The selected S20 strain was, nevertheless, superior to the All and HP88 strains at locating larvae. S20 nematodes, which are known to have enhanced chemosensitivity to carbon dioxide, were observed to aggregate heavily at or near the spiracles. Scarab spiracles are covered with sieve plates and, therefore, are not a portal of nematode entry. Despite selection for greater response to host cues, S20 persisted in nictating, standing on their tails, even when a host was present. The unfavorable field results usually obtained with S. carpocapsae against scarab larvae have been attributed to low powers of host search. Our results indicate that even when larvae are located S. carpocapsae is poorly adapted to cause infection. Genetic improvement cannot he expected to overcome poor adaptation. Future selection efforts with nematodes should begin with species adapted for the target host.