Combination of two entomopathogenic. Nematode species for suppression of an insect pest

Laboratory and greenhouse studies were conducted to determine whether combining 2 entomopathogenic nematode species would result in synergistic, additive, or antagonistic effect against End-stage larvae of the western spotted cucumber beetle, Diabrotica undecimpunctata undecimpunctata Mannerheim. After an initial screening of 12 nematode species/strains, the most efficacious Steinernema and Heterorhabditis and 2 Steinernema species that were intermediate in effectiveness were selected for further evaluations. In laboratory soil bioassays, the lethal concentration resulting in 50% mortality (LC(50)) for the most virulent nematode, Steinernema carpocapsae (Weiser) All strain, was 14.5 infective juveniles followed by 18.4 infective juveniles for Heterorhabditis bacteriophora Poinar NC1 strain, 48.1 infective juveniles for S. riobravis Cabanillas, Poinar and Raulston Texas strain, and 98.6 IJs for Steinernema sp. Hanrim strain. Combining 2 of the 4 nematode species at the LC(25) values always resulted in an additive effect, regardless of the effectiveness of the individual nematode species. In the greenhouse, application of S. carpocapsae, S. riobravis, Steinernema sp., or H. bacteriophora was made alone or in paired combinations at the rates of 1 x 10(8) and 2.5 x 10(8) infective juveniles/ha against western spotted cucumber beetle larvae. At both rates, the most efficacious single nematode species against the larvae was S. carpocapsae. The combination of 2 nematode species showed variable results. Its efficacy ranged between as high as the better of the 2 combined species alone and as low as the less effective of the 2 species alone. In addition, the combined nematode treatments were always less effective than the most efficacious single nematode species, S. carpocapsae. We conclude that no advantage was gained by combining 2 entomopathogenic nematode species against western spotted cucumber beetle larvae.