It's a Bad 'Worm'; It's a Benign Beetle; It's Both!
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Q. I'm having problems with some type of 'worm' that bores into the stems of my young seedling plants well below the soil line. I first found them last year when I noticed some tomato seedlings starting to wilt and die, so I pulled them out. When I did, I noticed holes in the stems with the worms inside. The plant stems weren't even 3/8 of an inch in diameter, and this worm was literally inside the stem. They do not fit your description of cutworms, and I am otherwise unable to identify them. They look like a meal worm but are much smaller and more elongated. Any insight would be greatly appreciated.
- ---Matt in Fairless Hills, PA
The 'cutworms' that Matt mentions (soon to be covered in depth in an upcoming Question of the Week) are caterpillars that also lurk underground and attack a variety of plants, including tomatoes—but they sever the young seedlings at the soil line; they don't bore into them.
And while squash vine borers (another caterpillar pest covered in depth in a previous Question of the Week) do burrow into plant stems, they only prey on squash; and only on types of squash whose vines are hollow—like zucchini and pumpkins. And squash vine borers tend to do their damage later in the season, when the plants are nice and big and the heartbreak of losing them is much more severe.
And while "worm" is often erroneously used as part of the common name of many caterpillar pests (like the corn earworm, tomato hornworm, cabbageworm, cutworm, etc.), Matt's "worms" didn't look like caterpillars—they seemed more like a cross between an earthworm and a millipede. (Note: There are no real worms that damage plants. If plant damage is involved, the 'worm' is a caterpillar.)
So it's a good thing that while I'm scratching my head, Matt persisted in sending the photos around until someone correctly identified the pest as a wireworm—the larval form of the click beetle, an unusual insect that, when flipped on its back, will right itself with a loud 'click'. Then I remembered this pest from my time as Editor of ORGANIC GARDENING magazine.
Although the adult click beetle will do a little nibbling on plant leaves and flowers here and there, it isn't considered a serious garden pest; it's more of a delightful 'natural wonder' for kids who discover the living click toys. But the wiry larval form can do serious damage, and to a surprisingly large variety of plants. Up to an inch and half long when fully grown, they're shiny, segmented, leathery, and reddish brown in color. They bore into all sorts of underground plant parts, including tubers, bulbs, corms, seedlings, roots, and the crowns of perennials. They're VERY serious problems on corn, potatoes and gladiolas.
The adult beetles (which are only half to 3/4 of an inch long) lay the eggs of the new generation in soil early in the Spring, and the larvae hatch in about a week, making this prime wireworm time. Unlike the larval forms of most insects, wireworms live in this juvenile stage for a long time—at least two or three years, and up to six. They stay close to the surface when the soil is cool in the Spring, but burrow down deep to avoid excessive heat in the summer and freezing cold over the winter. Wireworms are typically worst in new gardens that were previously lawns, and/or gardens with drainage problems.
Since they are close to the surface right now, you can cultivate the soil to expose them to your tender mercies and/or those of birds. Allowing chickens access to your garden beds before they're planted for the season will rid you of wireworms, as well as cutworms and other soil dwelling pests—and lots of weed seeds. It also feeds the chickens in the most natural way—super natural, in fact!
The warmer the soil gets, the less risk that wireworms will be feeding, so delay your plantings as long as possible if these pests have been a problem in the past (or if your garden was a lawn last year). If your soil drains poorly, lighten it up with double-digging and/or perlite or a soil-free mix. (These pests are much less likely to be a problem in raised beds, whose drainage is generally ideal.)
An old and popular trick is to trap them in cut potatoes and carrots buried a few inches deep in the soil a couple of weeks before you put your plants out. Mark the trap spots well, check the roots and tubers every few days and destroy any wireworms you find. Beneficial nematodes watered into the soil may also help control these pests—as well as beetle grubs, flea larvae and other nasty things that lurk below the surface.