The specific "worms" you are most likely referring to are known as
cabbage loopers, so-called because caterpillars tend to crawl with a distinctive
"looping" motion. The "worms" (this stage is more correctly termed as the larva
or caterpillar stage) grow to be about 2 inches long and are light green in
color. The caterpillars have three pairs of "true" legs behind the head in
addition to three pairs of thick or fleshy "false legs" located near the back
segments of the body. The adult stage is a moth which is a mottled grayish-brown
with a 1.5-inch wingspan.
In addition to cabbage and broccoli, the caterpillar stage of cabbage loopers
also feed on leaves of a wide variety of other plants including beets,
carnations, kale, lettuce, nasturtiums, parsley, peas, potatoes, soybeans,
spinach, and tomatoes. Be aware that caterpillars have chewing mouthparts.
Injured leaves typically appear tattered, with irregularly shaped holes removed
between major leaf veins.
Satisfactory control can be obtained using a product containing Bacillus
thuringiensis, commonly referred to as Bt. This is a biological
type insecticide which gives excellent control of a wide variety of
caterpillars. It is sold under a variety of trade names, such as Dipel, BioWorm
Killer, and Thurgicide.
For this material to be effective, it must be eaten by the caterpillar. Please
note that the caterpillars will stop eating very soon after ingesting this
product, but death will occur about 2-to-3 days afterwards. This is a relatively
safe and very effective insecticide and can be used for control of most types of
caterpillars on most commonly grown garden vegetables.
Cabbage loopers can also be controlled with other insecticides such as liquid
Sevin, pyrethrin or rotenone. Through coverage of the plant is important. If
only a few worms are found, they can be handpicked and removed from the site.
Check your garden regularly for best control. Remember, whenever using any
insecticide or other pesticide, always be sure to read and follow directions
provided on the pesticide label.