Holes in cabbage leaves are a telltale sign of cabbage worm feeding. Hand remove adults. Look for eggs on the undersides of the leaves.
Signs of Cabbage Worms
From Marie Iannotti, About.com
Cabbage Worm Damamge
Photo: © Marie Iannotti (2008) licensed to About.com, Inc.©2009 About.com, a part of The New York Times Company.
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