Impacts of Virus Infected Soybean and Cowpea on Soybean Looper and Fall Armyworm Larval Weight Gain

Plant disease vectors profit from feeding on virus infected plants by developing more rapidly, with increased longevity and fecundity.  Virus infected plants have improved host plant nutritional quality, with increased levels of amino acids, carbohydrates and nitrogen.  Thus, there is the potential for non-vector herbivores to benefit from virus infected plants.  Soybean looper, Chrysodeixis includens (Walker) and fall armyworm, Spodoptera frugiperda (Smith), feeding behavior on soybean, Glycine max (L.) and cowpea, Vigna unguiculata (L.) infected with Cucumber mosaic virus (Cucumovirus, Bromoviridae) and Sunn-hemp mosaic virus (Tobamovirus, Virgaviridae) was evaluated under laboratory conditions.  Plant leaves showing virus symptoms three weeks post inoculation were fed to soybean looper and fall armyworm in order to study the effect of virus infection on non-virus vector herbivore growth.  Area consumed was measured daily and larval weights were taken after 7 days.  There were no statistically significant differences in weight gain and leaf area consumed among soybean looper larvae fed virus infected and non-infected leaves, regardless of host or virus.  However, for fall armyworm, both viruses benefited fall armyworm on cowpea, increasing weight gain.  This suggests that under field conditions, soybean looper would not benefit from virus infected hosts while fall armyworm would benefit.