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Wormy cabbage: blame the victim.

Wormy cabbage: Blame the victim

Farmer Bill's field remains largely free of pests and is carpeted with healthy heads of cabbage. Farmer Bob's cabbage plot, just down the road, shows signs of serious chomping by the diamonback caterpillar, Plutella xylostella -- the world's number-one pest of plants in the cabbage family. Clearly, the caterpillars deserve the blame for Bob's poor yield.

Not so, say Jamin Eisenbach and Laurel R. Fox of the University of California, Santa Cruz. They have found that P. xylostella has a penchant for plants already doing poorly -- even though greenhouse studies indicate these caterpillars survive better, grow plumper and ultimately lay more eggs when raised on healthier plants. The caterpillar's seemingly poor judgment makes sense, however, given the researchers' finding that its major predator, the parasitoid wasp Diadegma insulare, seeks its victims primarily on healthy plants. Thus the diamondback caterpillar, in an effort to browse peacefully in enemy-free space, has developed a taste for undernourished plants, the biologists conclude.

Their experiments suggest that by raising nitrogen levels as little as 3 to 5 percent in cabbage-family crops (such as collard, broccoli and cauliflower), farmers can make their plants less attractive to the pesky larvae while increasing the number of caterpillar-killing wasps. Unfortunately, Eisenbach adds, better-nourished plants can also attract more aphids, another major crop pest. The researchers hope to identify the wasp-attracting chemicals in the well-nourished leaves so that plant breeders might develop crop strains rich in these compounds.
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Title Annotation:what causes caterpillars to attack cabbage
Author:Weiss, Rick
Publication:Science News
Date:Aug 11, 1990
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