Plant Responses to Water Deficit and Shade Stresses in Pigweed and Their Influence on Feeding and Oviposition by the Beet Armyworm (Lepidoptera: Noctuidae)

Abstract

Water deficit and shade stress in weed-infested crops could alter plant growth and biochemistry and feeding and oviposition by the beet armyworm, Spodoptera exigua Hubner. Palmer amaranth pigweed, Amaranthus palmeri S. Wats., was grown under 25% of full watering (water deficit), 30% of full light (shade), or combined stress. All treatments decreased plant height and weight. Shade and combined stresses decreased leaf counts and increased leaf water content. Water deficit stress increased leaf water potential, soluble protein and carbohydrate contents, peroxidase activities, and accumulations of 10 individual free amino acids (FAAs), summed essential FAAs, and total FAAs. Combined stress increased water potential, soluble carbohydrates, 12 individual FAAs, summed essential FAAs, and total FAAs. Shade stress decreased water potential, soluble carbohydrates, seven individual FAAs, and essential FAAs. Beet armyworm larvae consumed similar leaf areas on water deficit–stressed and nonstressed plants and larger areas on plants grown under shade or combined stress. Larval survival was reduced, and time to pupation was higher on shade-stressed leaves. Adult females deposited more eggs on shade and combined stress plants and fewer eggs on water deficit-stressed plants compared with controls. Beet armyworm feeding and oviposition responded to variation in water content. Stress-induced changes in nutrients were not tied to insect preference but could have negatively influenced survival under shade stress. The results have implications for the plant stress hypothesis and for the use of pigweeds for beet armyworm detection.