Effects of Cyanogenic Plants on Fitness in Two Host Strains of the Fall Armyworm (Spodoptera frugiperda)

Spodoptera frugiperda (J. E. Smith) consists of two genetic subgroups (host strains). The corn strain prefers crop plants such as corn, sorghum, and cotton, while the rice strain is found in small grasses such as Cynodon spp. and rice. Little is known about the physiological factors that drive this host preference. Here, we report a feeding study with natural host plants and an artificial diet containing cyanide. We found that corn, two Cynodon spp. bermudagrass C. dactylon (L.) Persoon, ‘NuMex Sahara’, and stargrass C. nlemfuensis var. nlemfuensis Vanderyst, ‘Florona’), and a hybrid between bermudagrass and stargrass, ‘Tifton 85’, exhibited differences in the cyanogenic potential (HCNp) and the cyanogenic capacity (HCNc). Corn plants released low levels of hydrogen cyanide, while stargrass had greater HCNp/HCNc than bermudagrass and ‘Tifton 85’. Feeding studies showed that corn strain larvae experienced higher mortality than the rice strain when fed stargrass or artificial diet supplemented with cyanide. Also, corn strain larvae excreted higher levels of cyanogenic compounds than the rice strain when fed Cynodon spp. These differences in excretion suggest potential disparities in cyanide metabolism between the two strains. We hypothesize that differences in the susceptibility to cyanide levels in various host plants could play a role in driving strain divergence and what appears to be the incipient speciation of this moth.