Effects of Elicitor Induced Host Plant Resistance on Lepidopteran Insecticide Efficacy

Abstract

Soybean looper (SBL), Chrysodeixis includens (Walker), is an important defoliating Lepidopteran pest of southern U.S. soybean and utilizes other agronomic crops and weeds as hosts. With increasing resistance to insecticides, alternative control strategies such as induced host plant resistance were evaluated against SBL. Jasmonic acid (JA) is an elicitor of host plant resistance, and was selected to determine its fit in an IPM plan for SBL. JA was applied to the top of meridic SBL diet and fed to SBL; no effects were found. JA applied as an exogenous elicitor to cotton, sweet potato, okra, cowpea, and soybean did result in differences. Less leaf area was consumed on all JA treated hosts aside from sweet potato, where SBL larvae consumed 10% less leaf area from control plants. Larval weight was reduced on all JA hosts except cowpea. To assess impacts of JA induction on insecticide efficacy, larvae were fed induced or uninduced host plant tissue for seven days and then transferred to diet incorporated with or without methoxyfenozide. The number of days to adult emergence was longer on JA treated cotton (1.8) and soybean (0.9), while shorter on sweet potato (1.1). However, JA treatment to host plants did not affect methoxyfenozide efficacy. Another pesticide that may induce plants is the herbicide, glyphosate. Glyphosate was applied to glyphosate resistant soybeans in the field, and in the greenhouse to glyphosate resistant cotton, Palmer amaranth, and soybeans to test induction effects on SBL survival, weight gain, and defoliation. Life table studies revealed non-induced Palmer amaranth could be an alternative host for SBL. However, consumption was half the leaf area and pupal weights were lower than larvae placed on soybean. vii A glyphosate diet overlay bioassay revealed SBL neonates had lower weights after seven days than those that fed on control diet and SBL 3rd instars were not affected. On foliage from glyphosate treated host plants after 7 days, SBL third instar weights were similar across treatments. On cotton, SBL consumed more leaf area on glyphosate treated leaves than on non-treated leaves but on soybean, consumed less leaf area of glyphosate treated vs non-treated leaves.