Margaret Lauterbach: How to fight cutworms in your garden

Special to the Idaho StatesmanApril 24, 2014 

Cutworm

Cutworms and army cutworms (which might climb plants) are now out in our area's farm fields, according to Pacific Northwest Pest Alert Network. Our city gardens will be next for those hungry critters.

Eggs for cutworms and army cutworms, both hairless caterpillars, were laid last summer by moths. Army cutworms are the offspring of miller moths, those annoying invasive moths that arrive in hordes in summer, banging against lights and screens.

After they hatched, the worms grew, feeding in fall and winter when conditions were right, and this month and next they emerge from soil to feed. They're ready to devastate our gardens now, before they pupate, and then emerge as moths, continuing the cycle.

Some cutworms are fairly easy to control. In my vegetable garden, for instance, we find only one at a time, even though it has hatched from a cluster of eggs. The common cutworms might separate in search of food.

There might be three generations per year, but after spring planting, garden seedling stalks grow sufficiently tough that cutworm damage is not even noticed. Their damage is definitely seen in spring.

Some folks guard against cutworm damage with paper collars around seedling transplants, a seedling transplanted in a bottomless paper cup, or a toothpick or nail thrust into the soil next to the trunks of the transplants. Just be sure there's no cutworm in the cup's soil.

I don't use any of these methods, but when I transplant into the garden, I trowel a fair-sized hole and watch for cutworms. By now they're pencil-thick, faintly striped in drab colors, a little over an inch long, and curved into the shape of a C. At this time of year, they're within two or three inches of the soil's surface. When I see one, I smash it.

They don't much move around in soil and don't travel far once they're out. To feed, they literally "belly up" to a stalk and eat through it as far as possible or girdle it near soil line. Gardeners find cut off, wilted seedlings lying next to ragged stumps.

Gardeners in the North End of Boise find many more cutworms than I (my garden is on the second Bench in the south side). Since parents are night-flying moths, they might be attracted to ornamental lights that are more prevalent in the North End than in my area.

Army cutworms are a different species from common cutworms (Peridroma saucia). Army cutworms (Euxoa auxiliaris or E. ochrogster) also emerge in spring, preferring broadleaf plants in their diet (right now that's alfalfa), but willing to gorge on grasses too (along with the regular cutworms and other destructive creatures). E. ochrogster caterpillars are dark red and faintly striped.

I think army cutworms, actually moth larvae, occur in groups rather than singly, like the regular cutworms. Once they emerge from pupas, the moths migrate to the high country to feed on wildflower nectar, hiding in rock piles during daylight hours. They're a high content of fat, so bears scrape them out, feeding heavily on them.

If you must use an insecticide, apply a pesticide in evening after irrigating to bring them closer to the surface so these night-feeding worms will ingest the toxin. BT (Bacillus thuringiensis) apparently isn't very effective on these caterpillars when mature, but it will control immature ones. Other pesticides with residual effect include carbaryl (Sevin), Cyfluthrin and Permethrin. Be sure to read and follow label instructions carefully. Or use a flashlight to help you catch and kill them by hand.

Some beneficial biological control is available. Ground beetles, brachinid wasps, birds and tachinid flies assist in controlling cutworms, but infestations might be heavy, depending on weather conditions.

For some of us gardeners, loss of an irreplaceable seedling is a "heavy infestation."

Send garden questions to melauter@earthlink.net or Gardening, The Statesman, P.O. Box 40, Boise, ID 83707.

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