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
Cabbage, Brassica oleracea var. capitata Linnaeus (var. Early Jersey) was grown in commercial potting mix (Sun-Gro Redi Earth) amended with 0 (control), 20% ,40% , or 60% food based vermicompost (Oregon Soil Corporation, Portland, OR). Cabbage was selected as the host plant because it belongs to the Brassicacea plant family, which has well documented chemical defense responses to herbivory. Additionally, this plant system has a wide number of herbivore species with different feeding habits associated with it.
All insect herbivore species selected for this study are known to be of economic importance for cabbage crop production. Helicoverpa zea and P. rapae are both foliar chewing insects whereas M. persicae and B. brassicae are phloem feeders. Cotesia marginiventris is a generalist lepidopteran parasitoid used to asses potential effects of the vermicompost resistance at the tri-trophic level.
Specific methodologies used for each insect species are as described in Little et al. 2011 and Little and Cardoza 2011.
Experiments carried out to evaluate the effect of vermicompost treatments on host preference of P.rapae and H. zea showed no feeding discrimination by the caterpillars. We also noticed that Pieris rapae fed more when given young leaves than when given older leaves; whereas, H. zea showed no feeding preference and fed indiscriminately of leaf age. Interestingly, P. rapae adults preferred the highest percentage vermicompost treatment over all other treatments for oviposition. Therefore, we conclude that no antixenosis resistance effects were found for either lepidopteran species.
Data collected from immature Lepidoptera no-choice performance bioassays, show strong antibiosis effects, leading to lower survival rates, against P. rapae but not H. zea. This could be due to the different feeding habits of the two pests: P. rapae preferred to feed on the young leaves where H. zea grazed indiscriminately. Therefore, we conducted a follow-up assay where we confined H. zea larvae to the top (young) plant canopy to measure effects on insect performance. However, no negative effects were observed on insect development time, pupal weight or survival. Thus, we conclude that the negative effects on P. rapae are due to antibiosis resistance and are unrelated to its preference for young plant tissue.
We also evaluated the preference and performance of two aphid species: the specialist B. brassicae and the generalist M. persicae on cabbage plants grown in soil amended with varying vermicompost levels. Plants were grown as described above. Results from this study show that vermicompost has no significant negative effects on the feeding preference of apterae of either species. To the contrary, apterae B. brassicae showed a significant preference for vermicompost treatments over control leaf discs. Contrastingly, alate M. persicae preferred alighting on control plants over vermicompost-grown plants, while B. brassicae showed no preference towards any of the treatments tested. Nonetheless, both aphid species deposited significantly more nymphs on control plants than on those grown in 20% vermicompost. Furthermore, plants grown in this treatment significantly suppressed mass accumulation, as well as, numbers of adults and nymphs of both aphid species compared to controls. These data clearly show that vermicompost soil amendments can significantly influence aphid preference and performance on plants. Moreover, these effects are not dose-dependent, but rather species and morph dependent.
Vermicompost treatments had no effect on attraction of the parasitic wasp C. marginiventris to cabbage plants infested with H. zea caterpillars. In fact, number of landings, time to discovery or total time spent on plant did not differ statistically for any of the treatments tested. Vermicompost treatments also had no effect on any of the wasp development parameters measured (i.e. percent host parasitism, time to pupation, time to emergence, and pupal weight).
Educational & Outreach Activities
Little, A. G., C. Arellano. G. Kennedy and Y. J. Cardoza. 2011. Bottom-up effects mediated by an organic soil amendment on the cabbage aphid pests Myzus persicae and Brevicoryne brassicae. Entomologia Experimentalis et Applicata, 139: 111–119.
Little, A. G and Y. J. Cardoza. Host plant effects on generalist and specialist lepidopterous cabbage pests modulated by organic soil amendment. Pedobiologia, 54: 353– 359.
Benefits of soil applications of earthworm-produced compost for sustainable agricultural pest suppression. Y. J. Cardoza. Center for Environmental Farming Systems (CEFS) Brown Bag Seminar Series, NC State University. March 19, 2009.
Mejoramiento del suelo con aplicaciones de compostas orgánicas para la reducción de plagas insectiles en la agricultura. Universidad Nacional de Agricultura (UNA, Catacamas, Olancho, Honduras). April 2, 2009.
Potential synergic benefits of vermicompost amendments for agricultural pest management. Y. J. Cardoza. 9th annual NCSU Vermiculture Conference, June 5-6, 2009, Durham, NC.
Bottom-up effects on plant insect interactions. Y. J. Cardoza. Virginia Tech University, Department of Entomology, February, 2011.
Influence of vermicompost on the preference and performance of cabbage aphid pests. A. G. Little & Y. J. Cardoza. North Carolina State University, Entomology graduate student symposium, 2010.
Work ocnducted as par of this project has been the subject of discussion in the vermicomposters.com Blog moderated by Allison Jack from Cornell University: http://vermicomposters.ning.com/forum/topics/new-research-on-insect?id=2094123%3ATopic%3A110905&page=2
The studies detailed herein clearly show that cabbage plants grown in soil enhanced through vermicompost amendment deployed resistance against the specialist P. rapae, but not the geenralist H. zea. Vermicompost also differentially affected performance by two aphids pests, M. persicae and B. brassicae and intra-species effects were morph-dependent. Furthermore, the resistance mechanisms were determined to be antibiosis, based on poor insect performance and significant mortality levels, for the caterpillars and a combination of antibiosis, leading to poor insect colonization, and antixenosis, leading to low offspring production, on the vermicompost-grown plants.
Therefore this research shows that vermicompost can be incorporated into pest management schemes for certain crops, such as cabbage. Furthermore for some pests (i.e., P. rapae) vermicompost-grown plants may be used as trap crops because females prefer to lay eggs on these plants, but their larvae perform significantly poorer when feeding on them.
Altogether these studies suggest soil amendment with vermicompost, alone or as part of an ecological management program, may reduce for synthetic pesticide application. This environmentally-friendly soil amendment has the potential to be easily adopted and incorporated into pest management programs. Furthermore, the resistance mediated by this soil amendment does not appear to have a negative effect on the tri-trophic interaction tested. However, this may change if evaluated with a different host-parasitoid system, since vermicompost did not induce resistance against H. zea in cabbage plants.
This grant provided partial project support for a MS student, Amos G. Little, in the Department of Entomology at NC State University, and resulted in 2 peer-reviewed publications and multiple professional presentations, as detailed below.
Areas needing additional study
There are many topics for future research stemming from this project, to name a few:
1) What are the factors in vermicompost responsible for the observed plant resistance to insects?
2) What are the plant defense mechanisms activated in different plants and that are effective against multiple attackers?
3) What chemical cues are involved in host discrimination for lepidopteran and aphid species? Our data show that P. rapae favored vermicompost-grown plants for oviposition, in spite of larval performance, whicle both alate morphs of the aphid species discriminated against plants grown in the amendend soil.
4) What is the lowest vermicompost concentration necessary to reap the benefits of plant defense activation?
5) Are natural enemies of pest species suppressed by vermicompost directly impacted by this resistance?