By Whitney Cranshaw, specialist in entomology, Colorado State University Cooperative Extension
One of the few garden insects that get a "jump-start" on the season are Cutworms. The immature (caterpillar) stage of various moths, cutworms have a voracious appetite for emergent plants in spring. Several different species occur in our region, each with somewhat different habits, but the army cutworm is far and away the most important cutworm of the Rocky Mountain region.
A dull gray or gray-brown caterpillar, army cutworms begin the New Year less than ¼ inch long, semidormant, commonly in lawns or amongst the debris of the previous garden. As spring temperatures warm they renew activity and feed on a wide variety of plants, somewhat preferring broadleaves over the grasses. Army cutworms are active at night and often will cut plants near the soil line, dragging unfinished plants into the soil cracks and other protected sites where they spend the day. Late April and early May is typically the time when cutworm damage peaks.
After it has finished feeding on plants, the army cutworm undergoes a radical transition, pupating in the soil and emerging as the adult moth. This adult form has a common name all-too-familiar to many that live east of the Rocky Mountains--the Miller Moth. An insect of unusual habit, the moths try to tough it out during the summer, alternately seeking sources of energy-rich nectar and cool, humid areas to conserve their energy. This behavior causes miller moths to incrementally migrate upward in altitude, ultimately over-summering in the mountains. The army cutworm does not reproduce throughout the summer, but survivors make a reverse migration in late August and September to lay eggs throughout lower elevations.
Army cutworm prefers to lay eggs in dense vegetation. A lush garden full of weeds can be a particularly favored spot for the moths to lay their eggs. By keeping gardens relatively free of weeds late in the season, serious cutworm problems can usually be avoided. Also, rototilling and some other types of early spring tillage can kill many of the young cutworms.
As plants become established watch out for early sign of cutworm activity, as indicated by cut plants. Individual valuable seedling can be protected with collars that will deter the night-foraging cutworms. All manner of materials may be used for this purpose, such as tops of plastic jugs, aluminum foil, and even cardboard pushed down into the soil around new plants. Some gardeners choose to get through cutworm season just by overseeding, letting the insects do some of their thinning for them.
Photos: Miller Moth, Judy Sedbrook; cutworm, John Capinera
© CSU/Denver County Extension Master Gardener 2010
888 E. Iliff Avenue, Denver, CO 80210
Date last revised: 01/05/2010