Tuesday, October 19, 2010

Who's Eating My Vegetables?

Now that we are in the "green" season - the time to grow leafy vegetables and fall/winter staples such as broccoli and cauliflower - more and more questions are coming in, asking: who's doing this?
The answer, most probably? This critter, the cabbageworm.

The cabbageworm started out as an egg, laid on the underside of the leaf.

Who laid that egg...and many, many more?  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 leaves...to 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 cabbageworm 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. But it's not just cabbage. It's all the cole crops, including broccoli and cauliflower.

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.

A similar pest that does the same kind of damage is the cabbage looper. It, too, is a greenish caterpillar, but with a distinctive looping movement in which they arch the middle portion of their body to bring the hind legs forward to meet the front legs.  

According to the University of Florida, the cabbage looper feeds on an even wider variety of popular fall and winter garden vegetables. "As the common name implies, it feeds readily on crucifers, and has been reported damaging broccoli, cabbage, cauliflower, Chinese cabbage, collards, kale, mustard, radish, rutabaga, turnip, and watercress. Other vegetable crops injured include beet, cantaloupe, celery, cucumber, lima bean, lettuce, parsnip, pea, pepper, potato, snap bean, spinach, squash, sweet potato, tomato, and watermelon. Additional hosts are flower crops such as chrysanthemum, hollyhock, snapdragon, and sweetpea, and field crops such as cotton and tobacco."

Tachinid Fly
The best control for either of these pests? 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.

Another active "green eater" this time of year: snails or slugs. Their damage, however, is more ragged looking, and they are much more voracious, decimating young plants overnight. Look for their slime trails to help I.D. this garden glutton. Control with a product that contains iron phosphate.

Row covers, placed over newly planted cole crops or seedlings, can also offer protection until the plants are large enough to overcome any damage caused by these caterpillars. Generally, you don't want more than 50% of any plant damaged by the cabbageworm or cabbage looper; too much damage and heads of cabbage, broccoli and cauliflower may not form.

Don Shor of Redwood Barn Nursery in Davis offers another organic cure: "Cut off and discard the infested leaves of green leafy vegetables, but keep the crown of the plant intact. The plant should grow back."

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.


  1. Good information. Although. Many of the NON-pesticides will NOT work.