Submitted to: Biological Control
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/12/2004
Publication Date: 3/15/2005
Citation: Tipping, P.W., Holko, C.A., Bean, R.A. 2005. Helicoverpa zea (lepidoptera: noctuidae) dynamics and parasitism in maryland soybeans . Biological Control. Interpretive Summary: The corn earworm is a major pest of soybeans in the Mid-Atlantic area, especially after the corn crop has begun to dry out. Soybeans with open canopies and pods just beginning to fill can suffer rapid and significant yield losses as the earworm larvae feed directly on them. Trapping adult moths in black-light traps has been used to monitor populations but has not been predictive for larval numbers in soybeans. We found that the earlier the peak of adult captures in traps, the greater the chance for outbreaks of earworms in soybeans. These peaks are influenced directly by the weather: dryer and warmer than normal weather in August explained most of the variation in larval densities over the three year period of this study. Also, larvae were found to be heavily parasitized by a suite of parasites that, in some years, quickly reduced the larval populations to very low levels. Parasitism generally doubled each week during the period in late August, early September when the moths switched to laying eggs in soybeans. Cooler, wetter conditions increased the incidence of fungal and viral infections in earworm larvae.
Technical Abstract: Larval populations of the corn earworm, Helicoverpa zea (Boddie), were surveyed in soybeans from 1995 to 1997 in order to catalogue larval parasites and their rates of parasitism and quantify the relationship between moth captures in black-light traps and larval densities. Parasitism was consistently high throughout the region averaging 80.3%, 82.3%, and 73.1% for all dates in 1995, 1996, and 1997, respectively, and appeared to suppress H. zea populations. The predominate parasite species was Microplitis croceipes (Cresson) with some parasitism by Cotesia marginiventris (Cresson), Meterous autographae Meus., and Winthemia rufopicta (Bigot). The date of the peak weekly capture of moths explained 99.7% of subsequent larval densities in soybeans, while the average weekly moth catches did not. The earlier moth peak in 1995 corresponded with higher populations of larvae while the later peaks in 1996 and 1997 were followed by very low, sub-economic larval populations. Departures from normal for precipitation and temperature during August explained 99.8% and 95.3%, respectively, of the variation in the date of peak moth capture.