Authors: Pamela Opfer and Dan McGrath, OSU Dept. of Horticulture
Black Cutworm (Agrotis ipsilon) & Variegated Cutworm (Peridroma saucia)
Black Cutworm larvae (Photo from University of Illinois) Black Cutworm adult moth (Photo by Ken Gray)
Variegated Cutworm larvae Variegated Cutworm adult moth (Photos by Ken Gray)
How to ID Pest
Black Cutworm: Larvae are about 30 to 40 mm long when mature. They are gray with a lighter brownish colored stripe down the back. The head is dark brown or black. Adults are brownish-gray with a spot and a light silvery band on the front wings. The wingspan is about 35 mm.
Variegated Cutworm: The most distinguishing characteristic of the variegated cutworm is the 4 to 7 pale yellow, circular spots on the back of the larva. Its general body color is variable, but usually brown. The underside of the caterpillar is cream colored. There is a narrow, orange-brown stripe along the side. The adult moths have grayish brown forewings and have a pale oval marking near the wing edge, adjacent to a darker kidney-shaped marking. Adult moths also have small dark notch markings on the edge of their forewings.
Lifecycle
Black Cutworm: Adults emerge in April and begin laying eggs. Migration and egg laying continues into June. Eggs are laid on seedling host plants. Eggs hatch in 3 to 7 days and the first stage larvae feed on the foliage for a few days before molting and moving down into the soil. Larvae feed beneath the soil surface or at the soil surface for about a month, then pupate. Adults emerge in one to two weeks, mate, and lay eggs for another generation during late August or September. Very few black cutworms survive the winter in the Willamette Valley. There are two overlapping generations each year.
Variegated Cutworm: They overwinter as half grown larvae in soil or under plant debris in or around crop fields. Larvae begin feeding in April and mature in late April and May. Larvae pupate in earthen cells in the soil. Adults emerge in May and early June and deposit eggs on the undersides of leaves. Eggs hatch in 4 to 7 days and larvae begin feeding on plant foliage. Larvae feed for 4 to 6 weeks and then pupate in the soil. Summer generation adults emerge in late August and deposit eggs. Larvae hatching from these eggs feed until cold weather and then become inactive and overwinter. There are two overlapping generations each year.
Crops Affected & Damage
First stage larvae of black cutworms feed briefly on foliage of host plants before moving into the soil to fee on roots and crowns of crops. The older larvae feed on underground plant parts during the day. Second and third stage larvae feed at the soil surface. Seedling plants such as corn, and beans are cut off at the soil surface. Damage to older plants causes wilting.
Variegated cutworm larvae feed on foliage of vegetables crops such as beans. Heavy infestations may cause complete defoliation of plants.
Black Cutworm damage to corn (Photo from University of Illinois)
Scouting & Monitoring
When scouting for black cutworms, plan to scout all fields at least once a week for a 3- to 4- week period following corn emergence. Pay particular attention to fields that were planted late or had an early season weed infestation. Check fields for leaf-feeding, cutting, wilting, and missing plants. Examine a minimum of 250 plants (50 plants in each of five locations) in a field. When injured plants are found, dig around bases of plants for live cutworms. Sometimes larvae will be found beneath clods, in the planter furrow, or in soil cracks. Determine the larval stages by using the head capsule gauge. Note leaf feeding caused by cutworms that are too small (1st to 3rd instars) to cut plants. Check these fields again in 24 to 48 hours. Record the number and size of larvae found per 100 plants, the percentage of injured plants, and the plant stage (the average number of leaves with collars visible per corn plant). Fields most at risk to economic injury are in the 1- to 4- leaf stage of plant development. (Illinois factsheet)
Pheromone traps can be used to monitor for cutworms, but use them in combination with field scouting. Pheromone traps capture male moths only.
Wire cone trap with pheremones (Photo by Pamela Opfer)
Interpreting flight data
Pheromone moth trap counts in excess of two black cutworm moths per trap per day indicate significant egg-laying pressure. It is useful to check moth counts prior to making a pesticide application decision at planting. When significant moth counts are detected, intensify your field scouting during crop emergence.
Check sweet corn fields at emergence. If you catch a black cutworm infestation early, post-plant insecticide applications can rescue an unprotected planting. Frequently, the damage is most serious at the edges of a field, but crop loss can also occur in a spotty pattern throughout the field.
As few as two or three cut plants or leaves in 32 m of row (10 feet) at several sites throughout a field may warrant the use of an insecticide.
Risk Assessment
Control Methods
Larvae of the black cutworm can be difficult to control with insecticides because of their subterranean habits. It is important to detect and treat larvae as early as possible. Foliar applications of insecticides will reduce the population of young larvae because even early stage larvae that feed near the soil surface will feed on treated foliage at night.
Biological Control: Cutworms are attacked by a number of predators, parasites, and diseases. Many of these natural control agents are not effective on black cutworms because of their subterranean nature. Several strains and products of (Bacillus thuringiensis) (Bt) are available.
Cultural Control: Cutworms are most injurious in weedy fields with high plant residue and weedy field boundaries. Clean tillage and clean field boundaries reduce risk of black cutworm damage by removing overwintering sites. Weed control is also important. Lambsquarters and wild mustard attract egg-laying females and provide a source of food for larvae. Fall tillage can help destroy overwintering pupae.
Chemical Control: Please consult the PNW Insect Management Handbook for pesticide recommendations.
References & Citations
Berry, Ralph E. 1998. "Black Cutworm Factsheet." Modified from Insects and Mites of Economic Importance in the Northwest. 2nd Ed. p. 221. Accessed on December 7, 2011. (http://mint.ippc.orst.edu/blackcutfact.pdf).
Berry, Ralph E. 1998. "Variegated Cutworm." Modified from Insects and Mites of Economic Importance in the Northwest. 2nd Ed. p. 221. Accessed on December 7, 2011. (http://mint.ippc.orst.edu/vcfact.pdf).
"Black Cutoworm Factsheet." Oregon State University. Accessed December 9, 2011. (http://insects.ippc.orst.edu/pdf/reb99.pdf).
Cook, Kelly A., Susan T. Ratcliffe, Michael E. Gray, & Kevin L. Steffey. 2004. "Black Cutworm." Integrated Pest Management: University of Illinois Extension. Accessed on December 7, 2011. (http://ipm.illinois.edu/fieldcrops/insects/black_cutworm/).
Hollingsworth, Craig S. (Ed.). 2011. Pacific Northwest Insect Management Handbook. Corvallis: Oregon State University.
"Variegated Cutworm Factsheet." Oregon State University. Accessed December 9, 2011. (http://insects.ippc.orst.edu/pdf/reb97.pdf).