HPIPM:Cutworms

Author: Sue Blodgett, Frank B. Peairs[1] revision, (Michael J. Brewer Frank B. Peairs Jay D. Donahue original)

Identification (and life cycle/seasonal history)

Pale western cutworm, Agrotis orthogonia Morrison, and army cutworm, Euxoa auxiliaris (Grote), are the two most consistently damaging species, although variegated and dingy cutworms occasionally cause damage in this region. Army cutworm feeding may occur during warm fall periods, causing fall damage to alfalfa as well as early in the first cutting. Pale western cutworm overwinters in the egg stage and therefore occurs later in the spring. Cutworm outbreaks tend to be separated by several years in which they cause very little damage. Cutworm larva avoid sunlight, feeding at night or on overcast days, making populations difficult to sample.

Army cutworm larvae are nearly two inches long when fully grown, generally colored light gray with lighter markings and a pale stripe running down the back. Adults are small moths, with a wingspan of about 1.5 inches. The army cutworm is the primary "miller" moth species which are a nuisance in early summer, entering homes in outbreak years.

The army cutworm has one generation per year and spends the winter as a partially grown caterpillar, feeding during warm periods throughout the winter. Fall outbreaks occasionally occur, especially in warmer regions of the High Plains. In the spring it feeds more frequently and development proceeds more rapidly. After development is complete, a small pupation chamber is built several inches below the soil surface. Adults emerge soon afterward.

Army cutworm or miller moths migrate from cropland into the Rocky Mountains to spend the summer aggregated at high elevation sites. Army cutworm moths have been seen aggregated at 12,000 feet elevation in the Snowy Range of Wyoming and Beartooth Mountains of Montana. The aggregations have been documented as a food resource for grizzly bears. As day length shortens, adults fly back to the Plains traveling hundreds of miles. Pheromone traps are used to detect their fall activity and to forecast next year’s populations. Eggs are usually laid in the soil in late summer through fall. Larvae feed above ground on the young stems of grasses at night, retreating under the soil during the day. The larvae overwinter as partially grown caterpillars, completing their development in the spring. Pupation takes place in the soil as summer begins.

Pale western cutworm had one generation per year and spends winter in the egg stage. It hatches in the spring, feeding below ground at night. This the more damaging of the two species because it clips stems, rather than defoliating above ground plant parts like the army cutworm. Following feeding it pupates in the soil and remains there for the summer, emerging in the fall to mate and lay eggs coinciding with the army cutworm flight. The pale western cutworm adult activity may also be monitored with pheromone traps in order to predict larval populations the following spring.

Plant Response and Damage

Cutworms cause economic damage by cutting plants off at the soil surface (pale western and dingy cutworm species) or by foliar feeding (army cutworm, variegated, and darksided cutworm species). The damaging larval stage occurs in the spring. Young alfalfa seedlings have small reserves of food to regenerate top growth cut off by cutworms. Older established plants are less likely to be killed, but yields can be reduced or growth slowed under high densities of cutworm larvae.

Management Approaches

Cultural methods

Growers should avoid rotations from grass hay or grain into alfalfa if cutworms have been problems in the past. Adequate irrigation and fertilization of new stands may help the young plants grow past the most vulnerable stage for damage from cutworms.

Biological control

Soil moisture promotes fungal pathogens that kill many larvae in wet years. Pale western cutworm is particularly susceptible to wet periods that favor parasites, predators and disease causes populations to decline sharply. The number of days with at least 0.25 inch of rainfall ("wet days") is used to determine the potential impact of rainfall on the pale western cutworm population. If 12 or more "wet days" occur during the spring (March-June), the pale western population will be reduced to the point that it will take two or more dry springs to allow the population to rebuild to significant levels. If there are 10 or fewer "wet days," the pale western cutworm population is likely to increase, and the potential for damage the next year is increased.

Chemical control

There are two methods to monitor cutworm activity. Adult activity may be monitored in the fall when moths are flying, mating and laying eggs. Larvae may be sampled in April and May. Larvae hide in loose soil at the base of the plants or under soil clods during the day and can be detected by sieving soil or visual inspection. They move deeper into crevices in the soil under dry conditions. At night, larvae can be found feeding above the surface.

Chemical control may be justifiable when cutworm density reaches three to four per square foot in mature stands. For new stands, two larvae per square foot cause economic damage. Under heavy pressure or high residue conditions, control may be difficult.

Product List for Cutworms:

Insecticide	Product per Acre (Fl oz. or oz. product)	Preharvest Interval, remarks
FastacR,1,2	2.2 - 3.8	3 days. 12 hr REI. Extremely Hazardous to Bees! Do not apply to alfalfa in bloom. No more than 3.8 fl oz per cutting, 11.4 fl oz per season.
Baythroid XLR,1	0.8-1.6	7 days. 12 hr REI. Extremely Hazardous to Bees! Maximum of 2.8 oz/ A applied per cutting and total 11.2 oz/A applied per season. Do not apply to seed alfalfa. Do not apply to mixed alfalfa grass stands.
carbaryl1,2	See labels	7 days. 12 hr REI. Most formulations are Extremely Hazardous to Bees! Do not apply to alfalfa in bloom. Sevin XLR+ is safe for bees if applied at <1.5 lbs ai/acre when no bees are in the field. Do not apply more than once per cutting
chlorpyrifos 4ER,1,2	See labels	14 days (1 pt), 21 days (over 1 pt rate). 24 hr REI. Do not make more than 4 applications/year or one application per cutting. Extremely Hazardous to Bees! Do not apply when bees are present. Minor phytotoxicity may occur on rapidly growing foliage.
chlorpyrifos + gamma cyhalothrinR,1,2	See labels	See labels for preharvest intervals and specific use restrictions. 24 hr REI. Extremely Hazardous to Bees! Do not apply when bees are present. See labels for additional restrictions for individual active ingredients.
Cobalt AdvancedR,1	11-26	21 days. 14 days to graze. 24 hour REI. See labels for additional restrictions for individual active ingredients. Extremely Hazardous to Bees!
cyfluthrinR,1,2	See labels	7 days. 12 hr REI. Extremely Hazardous to Bees! Maximum of 0.05 lb a.i./acre applied per cutting and total 0.2 lb ai/acre applied per season. Do not apply to seed alfalfa.
Belt1	2 - 4	0 days. 12 hour REI. Army cutworm only. No more than 4 fl oz product/cutting or 12 fl oz product/season.
gamma cyhalothrinR,1,2	See labels	1 day forage, 7 day hay. 24 hr REI. Extremely Hazardous to Bees! Do not apply when bees are present. Do not apply to alfalfa in bloom. Do not apply more than 0.03 lb ai/A per cutting. Do not apply more than 0.12 lb ai/A per season.
lambda cyhalothrinR,1,2	See labels	1 day PHI forage, 7 day PHI hay. 24 hr REI. Extremely Hazardous to Bees! Do not apply when bees are present. Do not apply to alfalfa in bloom. Do not apply more than 0.03 lb ai/A per cutting. Do not apply more than 0.12 lb ai/A per season.
BesiegeR	5 - 8	1 day for forage, 7 days for hay. 24 hr REI. Make applications when bees are not actively foraging by applying during early morning or evening hours. See labels for additional restrictions for individual active ingredients. Do not apply more than 9 fl oz product per cutting or 31 fl oz per season.
permethrin R,1,2	See labels	0 days if 0.1 lb ai/A or less, when rate is greater than 0.1 lb ai then 14 day. Do not apply more than 0.2 lb ai per cutting. Extremely Hazardous to Bees! Do not apply when bees are present.
zeta cypermethrinR,1,2	See labels	3 days cutting, grazing. 12 hr REI. Extremely Hazardous to Bees! Do not apply when bees are present. Maximum of 4.0 oz per cutting, 12.0 oz per season.
StallionR,1	2.5 - 11.75	7 days. 24 hr REI. Extremely Hazardous to Bees! Do not apply to alfalfa in bloom. See labels for additional restrictions for individual active ingredients. No more than 11.75 oz per cutting, 35.25 oz per season
RRestricted use pesticide 1Labeled for chemigation 2Generic active ingredient, several formulations.

The information herein is supplied with the understanding that no discrimination is intended and that listing of commercial products, necessary to this guide, implies no endorsement by the authors or the Extension Services of Nebraska, Colorado, Wyoming or Montana. Criticism of products or equipment not listed is neither implied nor intended. Due to constantly changing labels, laws and regulations, the Extension Services can assume no liability for the suggested use of chemicals contained herein. Pesticides must be applied legally complying with all label directions and precautions on the pesticide container and any supplemental labeling and rules of state and federal pesticide regulatory agencies. State rules and regulations and special pesticide use allowances may vary from state to state: contact your State Department of Agriculture for the rules, regulations and allowances applicable in your state and locality.