How does a butterfly benefit peppergrass?
By Chelsie Vandaveer
October 30, 2002
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Virginia peppergrass (Lepidium virginicum Linnaeus) is native to North America. Like other members of the Brassicaceae (mustard family), peppergrass produces pungent-tasting glucosinolates (mustard oil glycosides) to deter herbivory.
The European cabbage butterfly (Pieris rapae Linnaeus) was first noticed in Quebec, Canada around 1860. Since then, the butterfly has spread over all but the hottest areas of North America.
The cabbage butterfly is a specialist; its larvae feed only on plants containing glucosinolates. The caterpillars are considered an agricultural nuisance.
Although cabbage butterfly caterpillars eat peppergrass, this damage may work in the plant's favor. The damage induces a response, a change in physical or chemical defenses. Anurag Agrawal with the University of Toronto experimented with peppergrass and cabbage butterflies.
Agrawal allowed the caterpillars to feed on some plants while other plants were simply trimmed with a scissors. Those the caterpillars chewed on responded by growing sixty-four percent more trichomes (plant hairs) on new leaves. The damage did not affect the amount of glucosinolates, but the plant responded by making a wider variety of these chemicals. Trimmed plants did not grow extra trichomes or change the makeup of the chemicals.
These changes did not deter the caterpillars' feeding, but did reduce the numbers of aphids which injure the plants by sucking the sap. Agrawal noted, "These same chemicals serve as feeding stimulants for specialized herbivores, and may also induce egg laying by adults." (Anurag A. Agrawal, "Benefits and Costs of Induced Plant Defense for Lepidium virginicum", Ecology, 81(7), 2000)
After a female cabbage butterfly lays her eggs, she scent marks the plant. Other females will not lay their eggs on that plant. The plant is protected from total destruction by too many caterpillars.
The cabbage butterfly has become common worldwide. North American Insects and Spiders - Cirrus Digital Imaging has excellent photographs of this pest. To view the photographs, click on the link:
http://www.cirrusimage.com/butterfly_cwhite.htm
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