Results obtained from this study show that a soil organic amendendment (earthworm compost=vermicompost) mediates antibiosis-based resistance against specialist, but not generalist, caterpillars. Soil vermicompost amendment at concentrations > 20% does not affect insect feeding preference, but reduces performance of the specialist Pieris rapae larvae on cabbage. However, no measurable effects were dectected against Helicoverpa zea (generalist) larvae. Vermicompost also differentially affected performance by two aphids pests, Myzus persicae (generalist) and Brevicoryne brassicae (specialist) and intra-species effects were morph-dependent. Data obtained with all insect species tested indicate that vermicompost-mediated resistance effects are not dose-dependent, as lower concentrations resulted in greater resistance effects against insects than higher concentrations.
Plants grown in organically managed soils, fertilized with manure and compost, have been shown to be less favorable hosts for phytophagous insects than plants grown in conventionally managed soils, receiving synthetic fertilizers. Vermicomposts are produced through the digestive action of earthworms upon organic waste and are a rich source of microbial diversity and available plant nutrients. More importantly for the purpose of this study, vermicompost has been shown to reduce damage by, and populations of, pests such as, plant parasitic nematodes and a variety of arthropods including jassids, aphids, spider mites, mealy bugs, and caterpillars. However, the mechanisms responsible for resistance against this wide variety of insects have not been fully investigated. Furthermore, host plant resistance to herbivory has been shown to affect natural enemies yet, this has not been investigated for vermicompost-mediated resistance.
To determine if the mechanisms responsible for herbivore resistance in vermicompost-grown plants were antixenosis and/or antibiosis, we tested the effects of plants grown in various vermicompost concentrations on the preference and performance of generalist (H. zea and M. persicae) and specialist (P. rapae and B. brassicae) cabbage pests. Potential tri-trophic effects due to vermicompost resistance was evaluated using the generalist braconid parasitoid wasp Cotesia marginiventris. Specifically we aimed to:
1) Evaluate treatment effects on insect feeding preference (choice tests)
2) Test effects in insect performance (no-choice tests)
3) Assess potential impacts of induced resistance on parasitoid searching and performance