Evaluation of Steinernema riobravis, S. 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 earworm, Helicoverpa zea (Boddie) prepupae and pupae in corn plots at the Lower Rio Grande Valley of Texas. The most effective S. riobravis concentration 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 receiving S. carpoeapsae. The ineffectiveness of S. carpocapsae was attributed to high (>38 C) soil temperatures. Parasitism was higher when S. riobravis was applied at 200,000 IJ/m(2) through furrow irrigation (97%) 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.

Evaluation of the Spatial Pattern of Steinernema riobravis in Corn Plots.

The vertical and horizontal spatial patterns of a naturally occurring population of the entomopathogenic nematode Steinernema riobravis (Rhabditida: Steinernematidae) were investigated in corn field soil by laboratory and field bioassays. This nematode appears to be endemic to the Lower Rio Grande Valley of Texas, where it was found parasitizing prepupae and pupae of both corn earworm, Helicoverpa zea, and fall armyworm, Spodoptera frugiperda (Lepidoptera: Noctuidae). Corn earworm prepupa was the bioassay host used to detect S. riobravis from soil in previously harvested corn plots. Steinernema riobravis occurred at soil depths of 5-30 cm. The maximum nematode density was in the upper 20 cm of soil, and the lowest density occurred at soil depth of 25-30 cm. The field and laboratory bioassays performed on the top 20 era of soil resulted in S. riobravis-infected corn earworm of 49 and 34%, respectively, whereas at 25-30 cm soil depths 11 and 4.5% of the H. zea were infected, respectively. The horizontal spatial pattern of this nematode was patchy or aggregated. Our study provides new information on the spatial pattern of S. riobravis in its natural habitat and indicates the need to augment its natural biocontrol efficacy.