How to Control Cutworms in the Garden

Do you have holes in your vegetables? For a few weeks now, I have been seeing holes in my vegetables, but no signs of pests. I looked under the leaves, but there is nothing. When this happens, I have to get my detective hat on.

The holes appeared to be from pest with chewing mouthparts and they occurred on several plants (in my garden, they were on broccoli, cabbage, strawberries, and spinach), so I started thinking about what may chew on plants blindly. I thought about earwigs, slugs, caterpillars, and beetles. Since I didn’t see them in the day, I decided to go out at night with my flashlight and check to see if they were out. Apparently when I did it, they had not come out quite yet. When I saw a strawberry plant, chewed down almost completely, I decided to dig in the soil and I found it, a large cutworm.

In this blog, I will discuss the biology of cutworms as well as some organic control strategies.

What are cutworms?

Cutworms are caterpillars that turn into moths. There are several species of cutworms. Mature larvae are quite robust and almost 1.5 inches long. Larvae tend to curl up into a C-shape when they are disturbed.

What do cutworms eat?

They feed at night and hide in the soil during the day so they are hard to locate. They feed on a wide range of plants, including asparagus, beans, cabbage and other crucifers, carrots, celery, corn, lettuce, peas, peppers, potatoes, and tomatoes. They are often a dull brown color, so they blend well into soil.

What damage do cutworms cause?

When they are small, they create small holes in the foliage. As they grow, they create even larger holes and cut the stems off by circling the stem at the ground level and chewing them. This can kill the plant. Damage occurs more often to young seedlings. They can also put holes in fruit. They prefer the cooler weather with fall being the most destructive time of year.

Now that I identified my culprit, I need to treat the rest of the plants because if there is one cutworm, there are others and I wasn’t about to disturb the soil around all my plants. If you want to attempt to find them, you may be able to spot cutworms by running your hand over the soil and feeling for any clump where they have pushed the soil up. If you find cutworms, you need to physically remove them and crush them or drop them into soapy water.

What are some non-chemical controls for cutworms?

Here is a strategy you can use to keep cutworms away from your plants. First, you can place aluminum foil or cardboard collars (e.g. toilet paper tubes) around any transplants. This creates a barrier, keeping the larvae from feeding on the plants. You should push the collar an inch or two into the soil.

What are some organic pesticides that can be used?

Some pesticides you can use for controlling cutworms include compounds containing Bacillus thuringiensis (Bt) or spinosad. Both of these pesticides are approved for organic use and are more environmentally friendly.

Bt is a bacterium that contains a toxic protein. It is a stomach poison so the caterpillars need to chew on the plant material for it to work. Bt needs to be reapplied every 5-7 days. When applied to the leaves, Bt works best when the cutworms are small since they are feeding on the leaves.

For larger cutworms (also works with smaller ones), a drench may be more effective since the Bt is absorbed into the roots where the larvae feed. It is best to apply it closer to the evening since cutworms feed at night. You can use a ready-to-use spray like Captain Jack's Bt spray or Monterey B.t.

Spinosad is made by a soil bacterium that is toxic to the caterpillars. My go-to pesticide is Sluggo Plus since it kills both cutworms and slugs or snails. You apply it on the surface of the soil and it works as a bait, so cutworms feed on the pesticide.

In this instance, I chose to spray Bt on my crops and even though some of my crops have not been touch by the cutworms yet, it seems like all of them are potential targets and on the list.