Induced host plant resistance effects on insecticide efficacy

Soybean looper, Chrysodeixis includens (Walker), is a defoliating pest of southern U.S. soybean.  Soybeans are its primary host, however, soybean looper (SBL) can utilize other agronomic crops; cotton, sweetpotato, okra, cowpea, and alfalfa and can also grow on weeds; morningglory, lambsquarter, dock, and pigweed.  SBL is highly resistant to most insecticides and few products exist for control.  An alternative strategy for suppressing soybean pest populations is through induced host plant resistance.  Jasmonic acid (JA) was used to induce resistance in cotton, sweetpotato, okra, cowpea, soybean, and common bean to SBL.  Neonate larvae were fed leaf cores from treated and untreated plants.  Mortality and defoliation were recorded daily and larval weights were taken on the seventh day to assess induction effects.  To assess impacts of induction on insecticide efficacy, larvae were then transferred to a meridic diet with or without methoxyfenozide and daily mortality was recorded until pupation.  JA applications had no effect on larval mortality.  However, larval weights were significantly reduced on JA treated okra, soybean, and sweetpotato, indicating activation of plant resistance.  SBL fed induced cotton leaves and then placed on methoxyfenozide incorporated diet took significantly longer to reach 50 and 100% mortality as compared to SBL feed control cotton leaves.  The opposite occurred with okra.  Preliminary data indicates that a single application of JA can induce host plant resistance to SBL in some hosts.