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Control of Cabbage White Butterflies and Caterpillars
This article is about the nearly-invisible green caterpillars that are responsible for the holes we find in our cabbage, kale, and other cruciferous crops. The culprit is likely the larvae of the cabbage white butterfly, Pieris rapae, that has spread from Europe throughout the world.you see everywhere in North American towns and fields.
Of all the plants in our gardens, cruciferous vegetables like cabbage are among the most frequently attacked by insects. As both a gardener and a dedicated avocational entomologist, I have a two-fold interest in the situation.
A Successful Invasive Species
Since trans-Atlantic commerce became common-place in the 19th century, there has been a constant trade of not just goods but animal and plant species as well. One of the species that made the move from Europe to the New World was the cabbage white. Once it gained a foot-hold in the Northeast, it quickly spread across the entire continent. Today P. rapae is by far the most commonly seen butterfly across the eastern half of the country.
The cabbage white flies earlier in the year than most of its competitors, getting an early start on laying its eggs on cruciferous garden crops often before they have even leafed out. You may find it not only on your cabbages, but also on broccoli, kale, and any other leafy green crop
Cabbage whites are not moths...
In some areas cabbage white butterflies are called "cabbage moths." This is inaccurate: they are butterflies in every sense of the word—day-flying, fragile bodies, thread-like antennae. Even experienced gardeners and people with some familiarity of proper butterfly names has the habit of calling these common butterflies "moths."
Identifying the Cabbage White and its Caterpillar
The cabbage white, Pieris rapae, is a plain white butterfly with small black spots above and a pale yellow ground color below. They begin life as a little egg that hatches into a caterpillar. The caterpillar is very well camouflaged on green cabbage leaves. The adult, like most adult insects, has one function: to mate and lay eggs. The adult butterfly just drinks nectar and mates, but the caterpillar exists to eat and accumulate fat. For this reason, it's the caterpillar that most gardeners seek to control.
Control of Cabbage White Caterpillars
There are several ways to approach control of the cabbage white and its caterpillars.
Pick them off by hand
The best way to control garden garden caterpillars is often to pick them off by hand. But since cabbage caterpillars are so hard to see, you might not find many of them. It's worth a try, though, especially if you only have a few, or are against using any killing agents.
Wash with mild detergent
There are insecticidal detergent washes out there, but a mix of dish soap and water can kill or drive away cabbage white caterpillars (along with many other insects)
This insect control agent is literally dirt, made up of the ossified crystalline shells of diatoms. There is such a thing as "food grade" diatomaceous earth, but you will just need a bag of the garden-variety kind. The little shard of the diatoms kills most insects that crawl around in them. This method is cheap and effective, and doesn't use chemicals. However it will kill many beneficial insects, so use thoughtfully!
It Can be Very Difficult to Spot Cabbage White Caterpillars
The green caterpillars of the cabbage white butterfly are the exact same color as the leaves they feed on. They lay low and often sit parallel to the main leaf vein, making them even harder to spot. In addition, they have a covering of fine white hairs that soften shadows and blur the lines that you might otherwise see as you search for them. This camouflage has evolved over millions of years (and is still evolving), so it's no wonder that predators like wasps, birds, and gardeners have trouble seeing cabbage white caterpillars.
Eggs and Newly Hatched Larvae
The above photo shows the eggs that the female Pieris rapae laid on the underside of a cabbage leaf—and a newly hatched, or "first instar," caterpillar. If you have sharp eyes, they eggs are actually easier to find that the full-grown caterpillar: they are tiny yellow cones like the one pictured here. They're laid one at a time on the underside of the leaf by the female butterfly. After a week or so, they hatch into the tiny caterpillar you see in this photo.
If You Do Decide to Pick Cabbage White Caterpillars Off By Hand . . .
Get ready for a little frustration. Cabbage white caterpillars are harmless and not protected by stinging spines or hairs. But they ARE extremely hard to spot as they sit on the food plant.
These insects, like so many others, have evolved what's called "cryptic coloration" to evade predators, of which you now are one. This means that even though they're there, you'll likely have one heck of a time seeing them thanks to their almost supernatural ability to blend in with the leaves they rest on. Like tomato hornworms, cabbage butterfly caterpillars blend in with the leaf they on to an amazing degree.
So get your reading glasses and get ready to spend some time. You'll likely not get all of them, but even a few can make a difference in the happiness of your plants.
Other Natural Control Mechanisms
As you try to control the cabbage white caterpillars and the butterflies that you see in your garden, please bear in mind that they provide food for a wide array of predators. Parasitic wasps will also attack and kill large numbers of the caterpillars, as will birds, lizards, mice, and many kinds of predatory insects.
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This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.