Wild Side: Cabbage whites

Keep an eye out for this frequent Island find.

Cabbage white butterfly. —Matt Pelikan

“Wild Side,” it has to be admitted, has a fascination with the obscure: Many of my subjects are small, rare, or hidden species that require skill and determination to find. While I hope you’re interested in learning about them, they’re creatures that the average person won’t find just by accident.

To counteract that tendency, I’ll write about an insect that you can hardly overlook. I suppose it’s possible you’ve never actually noticed one. But if you spend any time outdoors during the warmer months, you’ve surely at least seen a cabbage white, Pieris rapae.

A medium-size butterfly, about an inch across, this species may be the most wide-ranging butterfly on the Vineyard. Cabbage whites have very broad habitat preferences, and they wander extensively; this is a butterfly that you could encounter, literally, anywhere on the Island.

The cabbage white is also very common — at times, prodigiously so. A native of the Eurasian continent (in England, the same species is known as the small white), Pieris rapae was accidentally introduced (probably by means of eggs or pupae in agricultural products) to the Montreal area. Within just a few decades, it had invaded most of North America. Under the right conditions, hundreds of thousands of adult cabbage whites have been observed flying at once over farm fields. Here on the Vineyard, the species is a regular in yards, and occasionally reaches nuisance levels on crop fields.

As the common name suggests, the cabbage white associates closely with cruciferous plants — that is, the huge array of species in the cabbage family — which serve as food for cabbage white caterpillars. Often, the larval host is cabbage or its close relatives, grown for human food. But this butterfly also uses many of the cabbage relatives, like wild radish, that grow wild as weeds. Like the butterfly, most of these weeds are accidental imports from Eurasia, and many are considered undesirable or invasive.

The secret is mustard oil, the irritating substance that gives these plants their pungent bite. While most insects are repelled by mustard oil (which is no doubt why the plants produce it), cabbage whites love the stuff. Adults sniff the oil out as a way to find appropriate plants to lay eggs on. And the larvae of the cabbage white are stimulated to feed by the presence of mustard oil. It’s a great example of an insect evolving to make a resource out of something that other species avoid.

Cabbage white ranks among the easiest of the Vineyard butterflies for a beginner to identify. Predominantly white, they are our only butterfly of that color except for an uncommon form of the closely related orange and clouded sulphurs. Cabbage whites have dark tips on their forewings, which sulphurs lack, and either one (male) or two (female) black spots in the middle of the forewing.

Underneath, a cabbage white can range from nearly white to yellowish to gray; the exact color appears determined by the temperature at which the butterfly reaches maturity, and early-season cabbage whites, which mature under cool conditions, tend to be much more pigmented than summertime individuals.

In a typical year, I find my first cabbage white in mid-April; it either comes motoring through my yard in Oak Bluffs, or I notice one puttering along a roadside while I’m out and about. The earliest I’ve found one was Feb. 26, and I have a few records from March. But because the habits of this species bring it into close association with agricultural activity, adults may emerge at unusual times in greenhouses or amid the warmth of a compost pile, so finding one on a mild day even in midwinter would not surprise me much.

Once on the wing, this species is steadily present through the warmer months. By late October, cabbage whites are becoming scarce. The latest solid observation I’m aware of was Nov. 17, and after about the first of November, most reports of cabbage whites probably reflect misidentified white-form sulphurs.

The greenish larvae of the cabbage white usually sport a pale stripe down their middle of the back and a row of tiny yellow dots on the sides. In small numbers, they are not a problem; but when the species grows abundant, their chewing can render crops unsellable. This is not a popular butterfly among farmers, or even home gardeners.

In some parts of the world, biocontrol agents — that is, other insects introduced specifically to prey on or parasitize the cabbage white — are used to keep numbers at a sane level. Many native insects probably help farmers out by preying on this species, though the mustard oil Pieris caterpillars ingest makes them unpalatable to many would-be predators.

Cabbage whites were on the wing early this year; I had my first on April 6, and the species will grow steadily in abundance over the coming weeks. It may not be a welcome sight if you’re a gardener, but if you pay any attention at all, you’re sure to see this interesting insect.