In the Garden | Watch out for voracious cabbageworms

In the Garden | Watch out for voracious cabbageworms

Recently, my wife, Amanda, noticed that something was chewing on the nice stand of kale she planted in our vegetable garden.

Initially, I brushed it off to the usual, acceptable amount of insect damage kale can withstand and still produce a harvestable crop. Typically, kale has some insect visitors that prematurely harvest some of the foliage, but we're always OK with a little damage as long as they leave enough foliage for us to harvest throughout the season.

Amanda closely watched the kale patch as the insect attack increased steadily over the next week or so. Eventually, she was able to collect and identify a familiar green caterpillar that we both should have thought of initially.

Our kale was infested with cabbageworms, which are quite common throughout Illinois each year.

These small green caterpillars may seem insignificant, but they can grow rapidly and consume plants as quick as The Very Hungry Caterpillar itself. I have mostly observed this pest on kale, likely because it is the primary cole crop we plant each year, but it is common on other members of the Brassica family, as well, such as Brussels sprouts, collards, cabbage, broccoli and kohlrabi.

There are three different cabbageworms found in Illinois: the cabbage looper (Trichoplusia ni), the imported cabbageworm (Pieris rapae) and diamondback moth larva (Plutella xylostella). In my case, our infestation was from the imported cabbageworm, although symptoms and treatment of each pest are quite similar.

Each spring, the imported cabbageworm emerges from a pupa after overwintering in crop debris or other plant material near the soil surface.

If you have paid attention to the insect population in your garden, you would probably recognize this common, whitish-yellow moth with black dots on each wing. They are quite abundant in the spring, first appearing about mid-April and laying eggs on suitable hosts in the Brassica family.

Within three to seven days, the larva hatch and begin feeding on the underside of leaves.

In the beginning, the larva are small and the damage is minute, but these tiny neon-green caterpillars rapidly mature, going from a tiny speck to an inch long in less than two weeks. As they grown larger, they consume a considerable amount of plant material, which is why my wife's kale suddenly looked terrible. Thankfully, controlling these little guys isn't too difficult and kale is tough, so it will recover.

Recommended control measures should start when the moths first emerge. The simplest solution is to exclude the moths with floating row cover, which is easy on a small scale.

For larger infestations or after eggs hatch, it may be necessary to spray a pesticide in order to save the crop. There are several non-selective synthetic pesticides labeled for control, but the best option is to use more selective and organic approved products containing Bacillus thuringiensis (Bt).

Non-selective, synthetic pesticides certainly work, but also kill a good amount of beneficial insects. However, Bt is selective in that it only kills Lepidopteran caterpillars and saves the good guys. It is actually a naturally existing soil bacterium, "discovered" in 1901, which can be sprayed on foliage.

As the caterpillars consume the foliage, they consume Bt, which shuts down their digestive tract, leading to death in just a few days. Feeding typically stops within a few hours of consuming Bt. The only drawback is that it must be applied about once a week for effective control.

To control our infestation, my wife has been picking off the caterpillars daily, which has significantly reduced the population.

We were out of Bt products, but I bought some at the end of last week and began spraying. I also plan to install a floating row cover to exclude future generations of moths this summer from laying eggs on our kale plants that will hatch into hungry caterpillars.

With a little luck, we'll be harvesting kale again in no time.

Ryan Pankau is a horticulture educator with the University of Illinois Extension, serving Champaign, Ford, Iroquois and Vermilion counties.

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