EVALUATION OF STEINERNEMA-RIOBRAVIS, STEINERNEMA-CARPOCAPSAE, AND IRRIGATION TIMING FOR THE CONTROL OF CORN-EARWORM, HELICOVERPA-ZEA

Two entomopathogenic nematodes, Steinernema riobravis and Steinernema carpocapsae, were compared for their ability to parasitize corn earwor m, Helicoverpa zea (Boddie) prepupae and pupae in corn plots at the Lo wer Rio Grande Valley of Texas. The most effective S. riobravis concen tration was 200,000 infective juveniles (IJ)m(2) (95% parasitism), as compared with 100,000 IJ/m(2) (81%), 50,000 IJ/m(2) (50%), 25,000 IJ/m (2) (31%), and the control (13%), No parasitism occurred in plots rece iving S. carpocapsae. The ineffectiveness of S. carpocapsae was attrib uted to high (> 38 C) soil temperatures. Parasitism was higher when S. riobravis was applied at 200,000 IJ/m(2) through furrow irrigation (9 7%) or post-irrigation (95%) than when nematodes were sprayed onto the soil before irrigation (82%). Parasitism of corn earworm prepupae by S. riobravis persisted up to 36 days after application and was higher in the treated plots (80%) than the natural parasitism of the control plots (14%). These results show that at high field soil temperatures S , riobravis is more effective against corn earworm than S. carpocapsae .