Though curses aren’t usually the words usually associated with seeing beautiful butterflies soaring around your garden, if you are a home vegetable gardener or part of a community garden that donates produce to food pantries, there is one butterfly that you may dislike.
No, it is not the butterflies whose larval host plants are dill, parsley and fennel. Many people who have butterfly gardens purposely grow extras of these plants as host plants for the butterfly larva. By following the rule “one for me, and one for the birds and butterflies.” you can have your share and the butterflies/caterpillars can have theirs. However for vegetable gardeners, the sight of pretty white butterflies flitting around members of the brassica family (ex- kale, cabbage, mustard, turnips, etc) can mean only one thing: an invasion of hungry larva caterpillars that will soon damage their crops.
Cabbage white butterflies, also known by butterfly-lovers as “summer snowflakes,” are found in two sizes, the Small Cabbage White Butterfly (Pieris rapae) and the Large Cabbage White Butterfly (Pieris brassicae.) The small cabbage white butterfly, though still considered an agricultural pest, is not as voracious a feeder as the Large Cabbage White Butterfly and will be the focus of this article.
The Small Cabbage White Butterfly is found throughout Europe, Asia and North Africa. It was accidentally introduced to Quebec around 1860 and spread rapidly throughout North America. By 1898 it had spread to Hawaii and by 1929 to New Zealand. Often, one of the first butterflies to appear in the spring, it lays eggs on the underside of a leaf. The eggs are laid singly and are yellow making them difficult to spot. The eggs hatch after about five to fourteen days and then the damage to members of the mustard family begins. Using their powerful mandibles, the larva munch holes in the leaves. Sometimes they will even eat into the heart of a cabbage, leaving a shell in its place. The larva then pupate, to start the whole cycle again.
Thankfully there are safe biological and barrier controls for this pest butterfly. In the mid 19th century the Australian government introduced parasitic wasps to control the damage produced by both species of butterflies. However this approach is only suitable for large commercial growers. There are other insects however that can help. These include ladybird beetles, lacewings, and some species of insect-eating birds. A physical control might include covering the plants with mosquito netting or other barriers. Be sure to secure all the edges.
Perhaps the easiest organic method of control is to use Bt (Bacillus thuringiensis), a naturally occurring bacteria that kills a caterpillar but leaves beneficial insects unharmed. When the caterpillar eats a treated leaf, it will get an upset stomach, stop eating, and die within four days. Just be sure not to apply it in wet weather as the spray will wash off.
By using Bt or other methods of control, you should be able to “have your cabbage and eat it too.”