Host plant effects on generalist and specialist lepidopterous cabbage pests modulated by organic soil amendment

An organic product of earthworm digestion upon organic matter (vermicompost) has been shown to increase plant growth, production, and resistance to a variety of pests. However, the mechanisms of resistance effective against different insect species still need to be fully characterized. Furthermore, host plant resistance to herbivory has been shown to affect natural enemies yet, this has not been investigated for vermicompost-mediated resistance. For these reasons, we tested the effects of plants grown in various vermicompost concentrations on the preference (antixenosis) and performance (antibiosis) of a generalist (Helicoverpa zea Boddie) and specialist (Pieris rapae L.) lepidopterous cabbage pests. We also tested the generalist parasitoid, Cotesia marginiventris Cresson responses to H. zea feeding on vermicompost grown plants. Insect preference was evaluated in leaf disc larval and whole plant oviposition choice assays. After 24 h of feeding, H. zea larvae showed no aversion for any vermicompost treatment or leaf age, and P. rapae larvae consumed more of younger leaves indiscriminate of vermicompost treatments. There was also no antixenosis effects against P. rapae adults, which oviposited significantly more on plants grown on 60% vermicompost. Potential detrimental effects on performance was also evaluated in no-choice larval development assays. No significant treatment effects were found for H. zea development; however, vermicompost treatments had a significant negative effect on P. rapae survival. Vermicompost treatments did not have an effect on attraction or development of the parasitoid C. marginiventris. Results from this study suggest that the mechanism of resistance mediated by vermicompost is antibiosis, which results in lower performance and death of the specialist, but not the generalist insects tested in this study. Furthermore, this resistance appears to be caused by factors other than nutrient levels in plant tissue.