Damage

• The three moths that produce cabbage worms from their hatched eggs begin to deposit the eggs on plants in early to mid-spring, when temperatures are cool but progressively warming. If the cabbages are small seedlings, the worms can fully and quickly defoliate them, killing them as they sever the stem's vascular tissues. On larger plants, they elude notice by crawling down in the leaf bases or rosette in the plant's core and chew at the foliage's base. Although unsightly, the leaf holes aren't dangerous, but boring into the main plant stem can prevent further crop development.
  
  

Pesticide Control

• An effective but potent pesticide control for cabbage worms is the chemical ingredient carbaryl. However, products containing carbaryl readily kill many other bugs in the vegetable patch, including honeybees. The least toxic and best pesticide to use to curb cabbage worms is Bt -- Bacillus thuringiensis. Bt is a natural bacteria that targets insects and is more appropriately called a microbial insecticide. It does not harm people, pets or beneficial insects that maneuver through the vegetable garden after treatment.
  
  

How It Works

• Bt is sprayed on the foliage to leave a dry bacteria residue. Once the cabbage worms begin to eat the coated leaves, the bacteria enter their gut. The bacteria act to paralyze the digestive tract, causing the worm to slowly starve to death. Exposure to sunlight rays, rainfall and irrigation spray diminishes the effectiveness of Bt. Reapplication every couple of days or once a week is necessary to ensure plants have a bacteria coating to continually kill new hatching, hungry larvae.
  
  

Safety

• The minimal toxicity, ease of application and reliable effectiveness of Bt make it the best pesticide to use on cabbage worms. While handpicking and destroying the worms are the most environmentally sound and safe procedures, they're not always practical. Follow product label directions on both liquid and powder forms. While not dangerous, Bt must dry thoroughly on plants before pets and people walk among or handle them. As a safety precaution, don't get wet Bt residue in your eyes, mouth or open wounds, as it can cause infections. Consult the label for recommended times to wait after application before harvesting any leaves.