Vertical distribution, persistence, and activity of entomopathogenic nematodes (Nematoda: Heterorhabditidae and Steinernematidae) in alfalfa snout beetle-(Coleoptera: Curculionidae) infested fields

The vertical movement, persistence, and activity of four isolates of entomopathogenic nematodes, Heterorhabditis bacteriophora Poinar (Oswego), Heterorhabditis bacteriophora Poinar (NC), Steinernema carpocapsae (Weiser) (NY001), and an undescribed Steinernema species (NY008-2E), were evaluated for 24 mo at field locations in northern New York. Nematodes were released into three alfalfa fields naturally infested with alfalfa snout beetle, Otiorhynchus ligustici (L.). Each field differed in soil type and soil textural composition: salt loam, sandy loam, and loamy sand. Nematodes were recovered from soil using trap insects, Galleria mellonella larvae, and their vertical distribution was monitored at 5-cm intervals to depths of 20 cm for Steinernema species and 35 cm for Heterorhabditis species. All nematodes persisted (no significant reduction in percentage of infection of G. mellonella) for 6 mo after the initial application in all soil tapes. However, by the end of the second growing season (17 mo after application), all nematodes showed significant reductions in infection rates of G. mellonella except H. bacteriophora (Oswego) which showed high levels of infection for 24 mo. Nematode vertical movement was affected by soil type and varied by isolate. S. carpocapsae (NY001) and Steinernema sp. (NY008-2E) remained primarily in soil depths 15 cm, whereas both heterorhabditids dispersed to soil depths of 35 cm. Vertical movement of H. bacteriophora (Oswego) was greatest in loamy sand and vertical movement of Steinernema sp. (NY008-2E) was greatest in sandy loam. Percentage of infection of G. mellonella by H. bacteriophora (Oswego) and S. carpocapsae (NY001) was significantly correlated with rising soil temperatures in early spring