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Cabbage White Butterflies and Their Caterpillars
Of all the plants in your 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.
Gardeners like us have a garden with a little bit of everything. And maybe you've noticed, like I have, that every kind of plant seems to have its own insect assigned to eating it up. I have already written about the big green hornworms on tomato plants. This article is about the nearly-invisible green caterpillars that are right at this moment munching on your cabbage plants. This is the larvae of that common white butterfly you see everywhere in North American towns and fields -- Pieris rapae, the cabbage white. This butterfly was introduced from Europe about a century ago -- thanks a lot, Europe -- and since then it has spread from coast to coast. It's easily the most common butterfly in North America. And it eats almost everything! You may find it not only on your cabbages, but also on broccoli, kale, and any other leafy green.
The Culprit is a Plain White Butterfly and Its Larva
The cabbage white, Pieris rapae, is a plain white butterfly with small black spots above and a pale yellow ground color below. They're kind of like the tighty whities of the butterfly world -- not much to look at, and everywhere you turn. They begin life as a little egg that hatches into a caterpillar. The caterpillar, highly cryptic on the green cabbage leaf it feeds on, is the one that tears up your garden. The adult just drinks nectar and mates.
(By the way, these are NOT moths -- they're butterflies)
I wish I knew where the mistake of calling P. rapae a "moth" got started. They're butterflies in every sense of the word -- day-flying, fragile bodies, thread-like antennae -- but for some reason people who don't really know their insects will refer to them as moths. Even experienced gardeners and people with some familiarity of proper butterfly names has the habit of calling these common butterflies "moths."
Where Did I Learn About Pieris Rapae?
When I collected butterflies as a kid, I got seriously tired of seeing one kind of butterfly again and again, everywhere I turned. It's the cabbage white, and the reason it's everywhere is that it can eat anything. Another thing that helps the cabbage white survive and thrive is the fact that its caterpillars are really hard to find, even when they're right in front of you. These plain white butterflies mate and lay eggs several times in the course of the summer. Multiple broods means an ever-increasing number of both adults and caterpillars.
Here's the Villain -- a Little Green Caterpillar
Yep, it's probably this little green guy right here. It looks pretty innocent, but believe me, it's not -- if there are enough of them, they can weaken or kill entire plants. And good luck finding them. 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. This becomes increasingly relevant as you decide to try to find all of them and pick them off by hand, which is one of the only ways you're ever going to get rid of the little guys.
Baby Cabbage White caterpillar and the egg it just hatched out of
Eggs and Newly Hatched Larvae of Pieris rapae
We're getting pretty tiny here, but this great photo shows the eggs that the female Pieris rapae laid on the underside of a cabbage leaf, and a group of newly hatched, or "first instar" larvae. You can see how the little guys have already started munching on the first layer of the leaf -- not eating a hole all the way through, but instead leaving little "windows" in the leaf. This is often the first sign that you have a cabbage white infestation.
Control of Caterpillars
These caterpillars can be controlled, but it tkes a particular approach that is not always effective. It's a tricky topic, since the best way to control garden pests is often to pick them off by hand. But since cabbage caterpillars are so hard to see, you might not find many of them. One decent alternative is to dust your plants with fine talcum powder. This clogs up the breathing holes of the caterpillars but doesn't harm the plant. "Insect detergent" is another way to go -- a dilute mix of dish soap and water can kill many small caterpillars and insects. There are insecticides out there if you must, but think hard and do some homework before you decide to go the chemical route.
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.