Wednesday, July 7, 2010

Oh, what a pretty butterfly! Hey...wait a minute...

A morning stroll through the vegetable garden may be highlighted by the sight of several pretty white butterflies, flitting back and forth among your leafy green crops. 

Wait a minute, wipe that smile off your face...that's no innocuous flying creature! The small, white to creamy yellow butterfly with the black dots or splotches on the wing is the cabbageworm butterfly. This garden pest is busily hunting down locations on the undersides of plant lay its eggs. A few days later, out pops the cabbageworm, a garden pest of leafy crops that will begin chewing irregular holes in the leaves, while pooping below.

According to the UC Davis Integrated Pest Management website, the cabbage worm larvae (caterpillars) are green and very hairy, with an almost velvetlike appearance. Older larvae may be up to an inch long and often have one faint yellow-orange stripe down their backs and broken stripes along the sides. Compared to other caterpillars, cabbageworms move slowly and are sluggish but they feed voraciously on both the outer and inner leaves, often feeding along the midrib, at the base of the wrapper leaves, or boring into the heads of cabbage. 

After 2 to 3 weeks of feeding, larvae pupate attached by a few strands of silk to stems or other nearby objects; pupae are green with faint yellow lines down the back and sides; there is no spun cocoon. 

The adult cabbage butterfly is white with one to four black spots on the wings; they are often seen fluttering around the fields. They have a wingspan of about an inch and a half.

The whitish, rocket-shaped eggs are laid singly on the undersides of leaves. The cabbageworm is active throughout the year in California.

In the fall and spring, when you see those butterflies flitting about, check the undersides of the leaves of your cole crops (cabbage, broccoli, cauliflower, mustard). In the summer, look for this pest on the leaves of any leftover greens that may be on their last legs: chard, spinach, lettuce.

Tachinid Fly
The best control? Handpick the larvae and the eggs. Avoid harsh pesticides as they will do more damage to the "garden good guys", the beneficial insects, who are also going after the cabbageworm: trichogramma wasps and tachinid flies.

If you must use a spray pesticide, choose an organically acceptable biological insecticide, such as Bt (Bacillus thuringiensis. After ingesting the Bt, the cabbageworm larvae cease eating and will die within a few days. Bt works as a stomach poison on leaf and bud-feeding caterpillars.  Larvae of desirable butterflies will not be injured if their food sources are not sprayed. Few desirable butterflies feed on vegetables.

Another naturally derived insecticide that is effective on chewing insects, including caterpillars, is spinosad. When using any pesticide, be sure to read and follow all label directions.

A good reference book with great pictures for battling garden pests safely yet effectively is the UC Agricultural and Natural Resources book, Pests of the Garden and Small Farm. I highly recommend it.