Submitted to: Environmental Entomology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: May 9, 2005
Publication Date: August 1, 2005
Citation: Moran, P.J., Showler, A.T. 2005. PLANT RESPONSES TO WATER DEFICIT AND SHADE STRESSES IN PIGWEED AND THEIR INFLUENCE ON FEEDING AND OVIPOSITION BY THE BEET ARMYWORM (LEPIDOPTERA: NOCTUIDAE). Environmental Entomology. 34(4):929-937. Interpretive Summary: Water stress and shading in weed-infested crops could alter feeding and egg laying by the beet armyworm. Palmer amaranth pigweed was grown under 25% of full watering (water stress), 30% of full light (shade) or combined stress. All treatments decreased plant height and fresh weight. Beet armyworm consumed similar leaf areas on water deficit-stressed and nonstressed plants and higher amounts on plants grown under shade and combined stress. Adult females deposited more eggs on shade- and combined-stressed plants and fewer eggs on water deficit-stressed plants, compared to controls. Larval survival was reduced on shaded plants. Beet armyworm feeding and oviposition were sensitive to water content and related stimuli. Pigweed may have value as a test pest in assessing crop damage potential by the beet armyworm.
Technical Abstract: Many insect pests like to feed on weeds, which act like a magnet to attract pests, who then move onto new crop plants. Both weeds and crops have to deal with stress when they are short on something they need to grow, like water and light. Pigweeds, also known as carelessweeds, are major problems for cotton and melon growers in south Texas. The beet armyworm is a moth larva that feeds on the leaves of many important crops. Does drought or shading influence the ability of beet armyworm to feed and reproduce on pigweeds? We grew Palmer amaranth, a species of pigweed, under normal greenhouse conditions. Separate plants were grown with 25% of normal watering (Drought), 30% of normal lighting (shade), or reduced water and light (combined). Plants grown under these 3 treatments were shorter and lighter. Drought caused leaf wilting, while shade actually increased the water content of the leaves, while decreasing the number of leaves. Drought, or combined treatment, increased lots of things that insects like, including carbohydrates and free amino acids, the building blocks of proteins. Shade treatment decreased these nutrients. Beet armyworms cared more about water content than nutrients. Larvae ate more tissue from plants treated with shade alone or combined drought and shade than from other plants. High water content probably motivated the larvae to feed. The same was true for adult female moths, who laid more eggs on shade and combined treatment plants, and fewer eggs on drought plants. Reduced nutrients did hurt the larvae in the long run - survival was lower for larvae raised on leaves from shaded plants. The results show that searching only crop plants for beet armyworm may not be enough. Shaded pigweeds may provide an early warning sign of beet armyworm infestation.