With summer comes a variety of garden pests. One to watch for this time of year is the white cabbage butterfly. It is sometimes referred to as the imported “cabbage worm” by gardeners since the worm does the major damage to cabbage family (brassica) plants.
They overwinter here and exist in Minnesota all year. However, the numbers seem to get higher in late June and later. The adult butterflies lay eggs on the undersides of the leaves of cabbage and other brassica crops like: broccoli, cauliflower, kohlrabi, kale and turnips. They also like flowering cabbage and nasturtiums. One to two weeks after eggs are laid, green caterpillars emerge and immediately begin to feed on leaves and eat ravenously. In addition to the leaf damage, what goes in does come out as disgusting frass. One often sees the frass before the caterpillars because their camouflage is so effective. Hatchlings can go undetected in the heads of cabbage, cauliflower and broccoli until produce is unusable.
It is a real challenge to prevent damage. A couple of insecticides are available that control the insects effectively and have less negative impact on pollinators than insecticides from the past. They are Bt, short for bacillus thuringensis and spinosad. Both are biological insecticides that are bacteria that attack the insects but are not known to harm other organisms. Combined with frequent close inspection for the worms, the crops can reach harvest with only one or possibly two applications of this biological insecticide.
Many gardeners today do not trust any claim of safety when it comes to an insecticide. Alternative remedies involve mixtures of garlic powder and cayenne pepper or chili powder, combined with water and sprayed on leaves. There is little research to support the effectiveness of homemade mixtures, but this is not to say they have no effect. We just don’t know how effective or reliable they are.
Hand picking caterpillars is effective. However, it is difficult since eggs are most often scattered individually and not laid in groups. Planting red-leaved cabbage and other brassicas makes spotting the caterpillar easier. Some gardeners cover the plants with fine netting or row cover material to keep butterflies from gaining access to the plants. It’s a challenge to secure the row cover tightly along the ground but it is an effective way to avoid the pest damage.
Another possibility is interplanting with garlic to repel butterflies because as I mentioned earlier the numbers increase in late June.
I have reduced damage significantly by planting early. I use a cold frame to warm the soil in late March and set out transplants around April 20. Cabbage, cauliflower and broccoli all tolerate light frost and can finish producing their main heads by the end of June which is just ahead of the increase in butterflies. The important thing to remember is that any cultural practice is just part of the equation and none will work all of the time. It takes a combination of strategies and experimentation to find solutions to garden pests.
To learn more about pests that invade our gardens in mid-summer, join the Pine County Master Gardeners at the Pine City Community Garden July 22 for the next program in our Summer Series. The programs begin at 6 p.m. and are open to everyone at no charge. Also, online information on garden pests is available at www.extension.umn.edu and go to yard and garden.