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How to identify it
- The larva is grayish-brown to gray or greenish gray with irregular lengthwise stripes, the diagnostic one being a narrow dark gray stripe on either side just above the spiracles. Mature larvae are about 1.2 to 1.4 inches (30 to 35 mm) long and 5 mm wide by the High Plains guide, and 1.5 to 2 inches by UC IPM, which calls it somewhat larger than the variegated cutworm.[2][3]
- Like other cutworms it is a smooth-skinned caterpillar with three pairs of true legs and five pairs of abdominal prolegs that curls into a C when disturbed; it may pass through seven larval stages instead of the usual six.[1][3]
- Adults are nondescript grayish-brown moths with irregular spots or bands on dark gray or brown forewings and lighter hind wings; they fly in late summer and autumn (early August to October in Manitoba, peaking in August).[2][3][1]
Look-alikes
| Look-alike | How to tell them apart |
|---|---|
| Redbacked cutworm | Redbacked and darksided cutworm larvae are often mixed in the same field and easy to confuse; the redbacked has two broad dull-red stripes down its back, while the darksided has a narrow dark gray stripe along each side.[1][2] |
| Variegated cutworm | The variegated cutworm is yellow to brown with a row of four to six yellow or pink diamond-shaped spots down the back and is a little over an inch long; the dark-sided cutworm is larger and gray to greenish gray with lengthwise stripes.[3] |
| Dingy cutworm | Another species that turns up in damaging prairie cutworm populations alongside redbacked and darksided larvae.[1] |
Where it lives
A cutworm of the Canadian Prairies and the US High Plains that also reaches California, where UC IPM lists it among the cutworms of asparagus.[1][2][3]
Biology and life cycle
The darksided cutworm overwinters as eggs. Females deposit them in the soil in late July and early August, and they stay dormant until air temperatures warm the following spring.[2][1]
Larvae feed and grow until about the end of June and then pupate in earthen cells near the soil surface. In laboratory rearing, larvae took an average of 87 days to pass through their seven larval stages. The moths emerge in late summer and autumn, giving one generation a year.[2][1]
Larvae feed mostly at night and rest just under the soil surface by day. An Ontario study found older larvae just below the surface in wet weather but 3 to 4 inches (8 to 10 cm) down in dry weather, with younger larvae resting 1/4 to 1/2 inch deep near the base of the host plant.[2][1]
Many predators and parasitoids attack prairie cutworms: ground beetles, rove beetles, stiletto fly larvae and wolf spiders, plus bee flies, tachinid flies and braconid, ichneumonid and encyrtid wasps. The encyrtid Copidosoma bakeri was the most common wasp parasitoid of cutworms found in Manitoba from 2012 to 2016, and wet soil during the larval stage promotes fungal diseases.[1]
Damage and signs
- Young larvae feed on leaves, sometimes climbing plants and chewing small transparent windows in young foliage. Older larvae cut plants off anywhere from an inch below the soil surface to two inches above it, so wilted, broken or dead plants are the usual sign.[2][1]
- Darksided and redbacked cutworms do their greatest damage in row crops with low plant populations per hectare, where each cut plant matters.[1]
- In asparagus the larvae eat small holes in the tender tips of new spears at night, and one-sided feeding makes the spears curl; damage to the fern is generally rare.[3]
Health risk
The sources treat the darksided cutworm only as a crop pest and mention no effect on people.[2][1][3]
Control
Prevention
Check fields early in the season. The High Plains guide suggests control when there is more than one cutworm per square foot or the stand is approaching the lower limit for an acceptable plant population; Manitoba gives nominal thresholds by crop, such as 25 to 30 percent stand reduction in canola or one or more small larvae per three feet of soybean row. Confirm cutworms by digging an inch or so into the soil around a damaged plant or spear.[2][1][3]
Treatment may not be necessary when most larvae are already full grown (1.2 to 1.4 inches), because they are near the end of their feeding period.[2]
Sanitation
Control weeds in and around the field and incorporate field trash and the previous crop's residue thoroughly to reduce overwintering; these practices are acceptable in organically certified crops.[3]
Traps and physical control
Young cutworm larvae can be starved before spring seeding by letting volunteer growth reach 3 to 5 cm (1.2 to 2 inches), cultivating, and then seeding 10 to 14 days later.[1]
Biological control
Ground beetles, rove beetles, wolf spiders and a range of parasitic flies and wasps reduce cutworm populations, and disease and parasites were the main factors ending a redbacked cutworm outbreak; minimum tillage fields in Manitoba carried a greater diversity of cutworms and their parasitoids than conventionally tilled fields.[1]
Pesticides
Foliar insecticides and seed treatments are available; apply foliar sprays in the evening, when the larvae feed, and consider treating only infested patches. The High Plains guide lists Bacillus thuringiensis, beta-cyfluthrin, carbaryl, chlorpyrifos, cyfluthrin, esfenvalerate and gamma-cyhalothrin among labeled products, several flagged as extremely hazardous to bees.[1][2]
Common questions
- How do I tell a darksided cutworm from a redbacked cutworm?
- Look at the stripes: the redbacked cutworm has two broad dull-red stripes along its back, while the darksided cutworm is grayish-brown with a narrow dark gray stripe along each side just above the spiracles. The two are often mixed in the same field.[1][2]
- When does the darksided cutworm do its damage?
- In spring and early summer. Eggs laid in the soil the previous July and August hatch when the soil warms, and the larvae feed until about the end of June before pupating, so it is an early-season pest with one generation a year.[2]
- Why are my asparagus spears curled with holes in the tips?
- UC IPM attributes night feeding on the tender tips of new spears to cutworms including the dark-sided cutworm; one-sided feeding curls the spear. Dig an inch around a damaged spear to find the larva.[3]
Sources
- Manitoba Agriculture, Government of Manitoba, “Cutworms in Field Crops (fact sheet, May 2024).” www.gov.mb.ca/agriculture/crops/insects/pubs/cutworms-in-field-crops.pdf (accessed 2026-11-10)
- High Plains IPM Guide (Colorado State University, University of Nebraska, University of Wyoming and Montana State University), hosted by Bugwood, “Darksided Cutworm (F. B. Peairs), High Plains Integrated Pest Management Guide, Bugwood wiki.” wiki.bugwood.org/HPIPM:Darksided_Cutworm (accessed 2026-11-10)
- University of California Statewide IPM Program, “Cutworms, UC Pest Management Guidelines: Asparagus.” ipm.ucanr.edu/agriculture/asparagus/cutworms/ (accessed 2026-11-10)
- iNaturalist, “Euxoa messoria taxon page (order and family).” www.inaturalist.org/taxa/219965 (accessed 2026-11-10)