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PostPosted: Mon May 07, 2012 5:19 pm 
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North Carolina State University researchers have dug deeper into the mechanisms behind insect resistance of plants grown in soil amended with vermicompost, and the preliminary results suggest this may provide an ecological friendly means of managing pests in vegetable crops.

Yasmin Cardoza, an entomologist and soil arthropod ecologist, along with graduate student Amos Little, found that different insect species responded differently to host plant resistance of brassica crops. They also determined that it takes as little as a 20 percent concentration of vermicompost mix in the soil to boost the resistance of host plants against caterpillar and aphid pests.

However, the level of resistance may depend on a variety of factors, including the type of pest involved, the origin of the vermicompost, what was fed to the worms, and plant defense mechanisms that have yet to be identified. The work was funded by a Southern Sustainable Agriculture Research and Education (SARE) Graduate Student Grant.

“Vermicompost research papers produced at Ohio State University show that plants grown in soil mixed with different proportions of vermicompost sustained lower plant damage from insects than those where vermicompost was not applied,” said Cardoza. “I was interested in learning what the mechanisms are that drive resistance in the plant. Are the insects not feeding on the plant because it simply looks, feels or tastes like a normal plant (called antixenosis), or do they feed on the plant, but succumb to toxic effects that lead to slower development or even death (known as antibiosis)?”

Insect species, like the corn ear worm (top) and cabbage worm, exhibit varying levels of susceptibility to plants growing in vermicompost-amended soils. Photo courtesy of Yasmin Cardoza.

Cardoza and Little studied two species of caterpillar larvae, Pieris rapae, which feed on a narrow host range (specialist), and Helicoverpea zea, which feed on multiple host species (generalist). In addition, the researchers looked at two aphid species, Myzus persicae, which is a generalist, and Brevicoryne brassicae, which is a specialist. The brassica plant of choice was cabbage, which has well-documented chemical defense responses to pests and has a wide number of pests with different feeding habits associated with it.

In the experiment, the researchers introduced the insects to plants grown in varying levels of vermicompost and plants grown in plain soil, with no vermicompost, to evaluated which ones the insects preferred to feed on.

“With the caterpillars, we hypothesized that the generalist would be more impacted by host plant resistance than the specialist simply because the specialist-cabbage relationship has evolved in close association with one another and the insect has mechanisms to cope with the common defense compounds in the host plant,” said Cardoza. “However, the results turned out to be just the opposite. The specialist caterpillar species was more impacted by the host plant resistance than the generalist, which surprised us.”

The specific plant host resistance was antibiosis, said Cardoza, meaning that the insects recognized the cabbage as a suitable host, but didn’t perform well or died after feeding on the plant.

“There is a chemical in the plant that has a toxic effect on the insect. We don’t yet know what antibiosis properties exist in cabbage that would affect specialists more than generalists,” said Cardoza.

One theory may be related to the organic compounds known as glucosinolates, which are favored by specialist insects and not by generalists, said Cardoza. “Or there could be a novel compound, which is activated in the plants grown in soil mixed with vermicompost that is different from the normal defense compounds.”

Cardoza said that one potential factor involved in modulating the resistance could be the microbial community in the vermicompost that can form symbiotic relationships with the plant roots.

With the aphids, the researchers found that the vermicompost mixed with soil resulted in plant host defenses that were both antibiosis and antixenosis, which were effective against different insect forms, wingless and winged, respectively.

“Winged aphids landed less and produced fewer nymphs on plants grown in soil mixed with vermicompost, which is good news because the vermicompost mix can help reduce colonization by the stage of insect that disperses and is most likely to affect a larger area of crops,” said Cardoza.

The overall impacts of the results are still in their preliminary stages, said Cardoza, as the data raised more questions than found answers.

“How long can one use the vermicompost mix to get the most benefits? What quality of vermicompost is needed? How much vermicompost do you really need? These are just some of the questions that require further study,” said Cardoza. “It’s not as simple as mixing vermicompost in your soil and applying it to your crops.”

The results, however, show that soil amended with vermicompost, as part of an ecological management program, may reduce the need for synthetic pesticide applications. In addition, vermicompost-grown plants may be used as trap crops because while females prefer to lay eggs on the plants, the larvae perform poorly when feeding on them.

To learn more about the SARE-funded project, “Evaluating Vermicompost-Mediated Host Plant Resistance as a Sustainable Alternative to Manage Agricultural Insect Pests", visit the national SARE projects database and search by project number GS09-078.

Published by the Southern Region of the Sustainable Agriculture Research and Education (SARE) program. Funded by the USDA National Institute of Food and Agriculture (NIFA), Southern SARE operates under cooperative agreements with the University of Georgia, Fort Valley State University, and the Kerr Center for Sustainable Agriculture to offer competitive grants to advance sustainable agriculture in America's Southern region.

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