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Nature's Ways: Cabbage White Butterfly Is a Serious Agricultural Pest

by and on July 17, 2019 5:00 AM

Several rather nondescript white butterflies flutter around in circles as if playing with one another. Every few minutes one or more of them stop to sip nectar from a purple New York ironweed flower. Soon they fly over my garden, one butterfly landing briefly on a broccoli plant — then on to another and yet another broccoli.

This small, but pretty, butterfly is a common sight at this time of year. While innocent-looking, this butterfly can be a minor pest in the garden and a serious economic threat for commercial vegetable farmers.

The butterfly is appropriately named the “cabbage white,” for its larvae feed on the leaves of many members of the mustard family (cabbage, broccoli, cauliflower, Brussels sprouts). Pennsylvania, among other regions in the U.S., lists the cabbage white as one of our most common butterfly species. According to "Butterflies of Pennsylvania" — a field guide, by James Monroe and David Wright — this species is most visible June through the first week of September.

Adult cabbage white butterflies have a wingspan of 1.5-2 inches. When viewed from above, their wings are a creamy white color. The butterflies’ forewings usually have black tips, with the females having two black spots on each forewing and males having only one. The black spots are also visible when the butterflies are at rest with their wings erect — showing their undersides. The wing bottoms often appear yellowish-white or greenish-white, rather than creamy white.

The black-and-brown-colored mourning cloak butterfly is usually the first species to be seen each spring, but the cabbage white is not far behind. The butterfly overwinters in the pupal stage on host plants. Adults emerge in late March and early April, mate and lay football-shaped eggs singly on the undersides of outer leaves of newly-planted broccoli, cabbage, cauliflower, kale, turnips or garlic mustard. The eggs hatch in four to eight days, depending on the air temperature.

The larvae or caterpillars are velvety-green with a faint yellow stripe down their backs and a row of yellow spots running laterally along each side. They feel fuzzy to the touch. The larvae eat the leaves of their host plants, growing and maturing in two to three weeks. Mature larvae are just over one inch long. They pupate on the host plant where they fed, and the life cycle begins again.

The entire development from egg to adult requires three to six weeks, which is not even considered a short life cycle in the insect world. The length of the cabbage white life cycle depends largely on temperature and larval food availability. There are three to five generations each year, with the population usually peaking in July or August.

Adult butterflies feed on the nectar from many species of flowers. During the summer, I often see them flying over hay fields, for red clover is one of their favorite plants. While the butterflies may be pretty flitting around in a hayfield, it is the larvae which cause the problems.

The larvae are commonly known as “broccoli worms” or “green worms.” If you harvest a head of broccoli from your garden and find a “worm,” you could easily pick it off and also wash away any droppings. However, some people find this distasteful and refuse to grow broccoli for that reason.

A “broccoli worm” found in a commercial vegetable farm is a more serious matter. Birds Eye, Green Giant and other companies are not interested in their customers finding even one cabbage white larva in broccoli. If a contaminated load of harvested broccoli arrives at a vegetable processing plant, the entire load is rejected. This loss could mount up to thousands of dollars for the farmer.

Like many of our insect pests, the cabbage white was introduced into North America from Europe circa 1860. This species was first spotted in Quebec and quickly spread all over the country, reaching Pennsylvania by 1870.

In addition to being a pest, the species had out-competed and diminished populations of one of our native butterflies — the checkered white.

Fortunately, the larvae of cabbage butterflies can be controlled chemically with dusts or sprays, such as Sevin, or organically by an application of products containing Bacillus thuringiensis (Bt) — the same bacteria used to control gypsy moth larvae. You can also cover your broccoli plants with a sheer fabric or fine netting to prevent the butterflies from laying their eggs.

Not much tastes better than broccoli fresh-picked from the garden. Along with snow peas, broccoli is usually my first garden crop of the summer. Early-harvested broccoli rarely has any larvae.

For me, I am going to enjoy watching the butterflies and eating fresh broccoli. A few tiny green “worms” will not ruin my love of this vegetable. (Pro tip: Soaking the broccoli in salt water and then rinsing it eliminates most, if not all, of the larva.) Besides the appealing taste, eating broccoli also helps to prevent cancer.

This story was produced by the staff at the Centre County Gazette. It was re-published with permission. The Centre County Gazette is a weekly publication, available at many locations around Centre County every Thursday morning.

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