Natural control of Helicoverpa zea (Boddie) in Trans-Pecos cotton

Abstract

During the summers of 1996 and 1997 Helicoverpa zea aphics. (Boddie) life stages were monitored in insedicide-free cotton, Gossypium hirsutum L., in the Trans-pecos physiogeographic region of Texas. zea in each of the two vowing seasons. Partial ecological life tables were construed for H Total mortality (egg to 5th instar) approached 97% in both 1996 and 1997. The egg stage experienced the greatest amount of age-specific (qx) mortality in both years (82.5% in 1996 and 86.0% in 1997). Egg mortality was attributed to Trichogramma spp., Encarsia sp. ltr. artery Xercet), Orius tristicolor (White) and unexplained mortality. Unexplained mortality represented the largest portion of egg mortality (67.5% in 1996 and 65.4% in 1997). Third instars exhibited the highest age-specific mortality of the larval stages sampled in 1996 (67.2%) and 1997 (51.8%). No 6th instars were round in either year and 100% of the life stages sampled consisted of H. zea. Life stages monitored in 1996 and 1997 conformed to a Type III survivorship curve. Enzyme linked immunosorbent assay (ELISA) of field-collected 0. tristicolor revealed that adult and nymphal 0. tristicolor fed on H. zea eggs. No pattern was evident between the number of bookworm eggs consumed by 0. tristicolor in relation to the seasonal abundance of H. zea eggs. The consistency of the H. zea survivorship data from our study suggests that natural control of bookworm may be plausible in Trans-Pecos cotton without insecticidal intervention. Future studies should attempt to extrapolate this survivorship data on a larger scale to test whether bookworm populations are kept below economic thresholds without insecticidal intervention on an area-wide basis.