Now that the strangest summer in living memory has ended, many new gardeners are still enjoying the food produced by their COVID victory gardens. The summer pests faced by all gardeners are also enjoying the bounty. One, the cabbage white butterfly (Pieris rapae), is fluttering about in many a garden on these foggy or sunny early-fall days. Non-gardeners often find them charming, but if you are growing food crops, you will want to learn a bit about this butterfly in order to save some of the crops you are harvesting now as well as the ones you plant for fall and winter.
Very few butterflies damage plants we eat, but the larvae of this one will chow down heartily on all crops in the cabbage family. That includes broccoli, cauliflower, Brussels sprouts, collards and kale (known as “cole crops”), as well as turnips and radishes — all crops that Bay Area gardeners can plant in the late summer, and, in some locations, into fall. The caterpillars of cabbage white butterflies, known as cabbage worms, will devour the leaves of these plants. While they will feed on plants of any age, young plants are particularly endangered, since one caterpillar can devour an entire seedling in just a couple of days.
During World War II, victory gardens in England suffered so much from this pest that people called them “Hitler’s allies.” Gardeners who know what these caterpillars do to crops are often irate to the point of wanting to chase and kill butterflies. However, besides not being good for your blood pressure, that is not the best control method. To reduce damage, we need to learn a little about this pest.
As a butterfly, it has a four-part life cycle. The adult flutterers mate, then the female, (identifiable by the two black dots on her forewings) lays pale yellow eggs on the undersides of the leaves of preferred crops. (The eggs look like tiny rockets ready for launch.) In five to seven days, these hatch into tiny velvety jade green, very hungry caterpillars. After they have eaten for a couple of weeks, when they are an inch long or a bit longer, they form a chrysalis (a cocoon), often on the same plants they have been eating. This can last from about 10 days to the entire winter season before new adult butterflies fly forth.
The most susceptible stage of this pest is the egg. If you have only a few plants to protect, your best bet is to examine leaf undersides every few days and brush off the tiny eggs. While you’re examining the plants, if you encounter any caterpillars, remove them, too. Also, while you’re at it, brush off any other tiny eggs you find on the leaves, as they will mostly hatch into something that is up to no good, and crush or wash off any aphid infestations. If you have too many plants for frequent examination, inspect a few of them for a while to monitor them, and then use Btk spray, according to the package label, starting about a week after you start seeing eggs. (Bacillus thuringiensis kurstaki is a bacterium, harmless otherwise, that will infect and kill the larvae.)
The reason you are seeing so many of these pest butterflies in your late-summer and possibly early-fall gardens is that they had all spring and summer to multiply in the gardens and in areas where wild mustards grow. Now there are more butterflies, and the wild mustards have succumbed to summer’s drought, so watered gardens are all that’s still green. However, if you can protect your cabbage-family crops for a couple of months, winter dormancy should come to the rescue. Usually, in October, a series of nights in the 40s will kill cabbage whites or drive them into dormancy until sometime in March. In this we are more fortunate than a tropical place, like Hawaii, where the mild climate lets them breed all winter. (Another reason to want to avoid climate change.)
Because our normal winter puts the butterflies into dormancy, gardeners who get seedlings growing early in the year have some time to escape damage. Radishes might succeed if sown as early as January, and cole crop seedlings can usually be planted out by February, giving the plants weeks of safe growth before the first eggs or damage appear.
Finally, while it probably won’t do much to prevent crop damage, if you would like to help citizen science, and get out some of your hostility toward this voracious pest, you can send some (dead) butterflies to the Pieris Project, a Sacramento research program. They do genetic research on the critter and would love to have more specimens. You will find complete instructions for catching, preparing and sending in your catch at their website, www.pierisproject.org.
Pam Peirce is the author of the regional gardening book “Golden Gate Gardening,” for mid- and Northern California locations both coastal and inland. She blogs at goldengategarden.typepad.com, where she lists appearances and often writes more on topics covered in this column. Email: [email protected]