Northwest Iowa — If you grow cabbage, cauliflower, broccoli, or other “cole” crops, now is the time to be on the lookout for a garden pest.
That’s from Iowa State University Extension and Outreach horticulture specialist Margaret Murphy. She says you are probably familiar with the adult form of the imported cabbageworm. They are the small, white butterflies that flutter around your garden. Their front wings have small, black spots. As the name suggests, she says females prefer cruciferous vegetables such as cabbage, kale, and broccoli to host their tiny, light-colored eggs. The eggs are often found on the underside of leaves and usually hatch within a week of being laid. The larvae are velvety green caterpillars that develop a faint yellow stripe down the back as they mature. Larvae will reach about an inch in length and have a voracious appetite as they slowly chew their way through a leaf or bore their way into a head of cabbage, according to Murphy.
She says that management of this pest begins with good surveillance. Keep an eye out for the butterfly flittering around the yard. I have already noticed it in the garden so now is the time to start checking cole crops for eggs or larvae. Such crops can tolerate some insect damage but, if treatment is needed, there are a few approaches you can try.
Physical control: Remove caterpillars by hand. She says, “I have tried this, and while it works, the task can get a bit daunting if you have a large crop. I have switched to using floating row covers, which are made of light-weight, gauze-like material. They allow light and water through while creating a barrier to insect pests preventing the female from laying her eggs.” She says that for floating row covers to be effective, however, it is important that the cover be placed over the crop as soon as (or before) you observe the butterfly in your yard. Also, be sure to anchor the bottom of the floating row cover to prevent the wind from flipping up the edges exposing the plants.
Insecticidal control: An organic option for insecticidal control includes Bacillus thuringiensis, commonly referred to as Bt. Other control options include spinosad, insecticidal soap, and carbaryl. Whenever applying any type of insecticide, remember to read the label and follow all the instructions carefully. (To learn more about reducing insecticide impact to bees in the garden, take a look at Iowa State University Extension and Outreach publication, Protecting Bees from Pesticides. It is available to view or download at the Extension store, https://store.extension.iastate.edu/Product/Protecting-Bees-from-Pesticides.)
Lastly, an additional step toward controlling this garden pest is to practice good garden clean up. Remove infested plant material after harvest to reduce the risk of the imported cabbageworm overwintering as a pupa on plant debris.
For more information, contact Horticulture Educator Margaret Murphy at (712) 472-2576 or at mmurphy@iastate.edu.
