Erratically flying white butterflies that look more like moths and favor members of the cabbage family are insidious garden pests. If that weren't bad enough, chemicals found in their favorite foods make them distasteful to birds.
Cabbageworms start out as tiny yellow, green, or white, rocket-shaped eggs that are laid singly on the undersides of leaves. If you have a hand lens, you can see distinct ridges on the eggs. Whether you can see that well or not, do your plants a favor and extermigate the eggs whenever you see them.
Cabbageworm butterflies tend to be white or off-white with one to four black spots on the wings. Larvae are green and fuzzy and can grow to one inch long. Larvae often have a faint yellow stripe on the back and broken yellow lines along the sides. Pupae are a mottled brownish grey. The chrysalis matches the leaves and is often found along the midvein.
There are actually several different white butterflies that lay eggs on plants in the cabbage family. Collectively, they are called ‘cabbage whites’. Native species of cabbage whites include:
The imported cabbageworm, Pieris rapae, has largely replaced these native cabbage butterflies.
Damage caused by imported cabbageworm
As soon as the eggs hatch, fuzzy green caterpillars start feeding, and feed they do! They start by eating their own eggshell and then they chew round holes in leaves at an alarming rate. Healthy plants can look tattered in only a few days. Leaving trails of brown fecal pellets on leaf surfaces, larvae may also burrow into cauliflower and broccoli heads, making them inedible.
The first sign of cabbageworm infestation is random, round holes in the leaves of cabbage, kale, cauliflower, broccoli and other members of the Cruciferae or cabbage family. You may also notice greenish brown fecal pellets littering leaf surfaces.
Since cabbageworms are active year round in California, the damage they cause can be extensive. A single adult cabbageworm can fly over 7 miles in a single day, so you really need to be on the alert.
How to control imported cabbageworms
Unlike many other invasive pests, imported cabbageworms actually have some enemies already. Tachinid flies and a handful of parasites feed regularly on cabbageworm eggs and larvae. Speaking from experience, I can tell you that those natural controls will not be enough once cabbageworms have found your crops. You can spray Bacillus thuringiensis (Bt) or spinosad for a measure of organically acceptable control. Handpicking larvae and extermigating eggs really are really your best control measures. And get rid of any nearby weeds from the cabbage family. Adult cabbageworm butterflies feed on nectar from flowers, preferring blooms of purple, blue, and yellow.
Personally, in addition to regularly monitoring plants for the presence of cabbageworms, I have trained my dog to chase cabbage butterflies out of my yard. It helps, and it's pretty fun to watch.
You can grow a surprising amount of food in your own yard. Ask me how!