Pale Western and Army Cutworms in Montana

 

by Sue Blodgett, Greg Johnson, Will Lanier and Judee Wargo

 

 

Two major cutworm species, the pale western and the army cut­worm, are serious but sporadic pests for Montana producers. Out­breaks can occur when cutworm populations are high and weather conditions are favorable for sur­vival. However, outbreaks do not necessarily occur in successive years. An area-wide adult survey has been used in Montana to assess moth activity of pale western and army cutworms, which has helped in the forecast of damaging cut­worm larvae populations.

 

Origin and Distribution

Both cutworm species are native to North America. Army cutworms are distributed throughout the semi­arid region of the Great Plains, extending to eastern Kansas with reports of occurrence as far east as Illinois and Michigan. They occur in Alberta as far north as the Peace River District and occasionally in Saskatchewan and Manitoba. Pale

western cutworms occur from Alberta to Arizona and New Mexico, extending east to western North and South Dakota, Ne­braska, Kansas and the panhandles of Oklahoma and Texas.

 

Army Cutworm

 

Life Cycle

The army cutworm begins lay­ing eggs in late August and can continue through October. Up to 3,000 eggs are deposited per fe­male on or just beneath the soil surface. Eggs hatch in the fall fol­lowing rain or snow. This cutworm species overwinters in the larval stage. Larvae become active in late winter or early spring and are par­ticularly damaging to winter wheat. Army cutworms feed above ground so evidence of feeding damage indicates their presence. However, they feed during night, from dusk to dawn, staying below ground during the day except on very over­cast days when they can be found feeding above ground. This noctur­nal behavior plus the small size of the early instar larvae (1.5 mm) makes them difficult to detect even though the feeding damage to foli­age is obvious. When larvae are abundant and food supply is short, the larvae will move en masse to adjacent fields, hence the name army cutworm.

Mature larvae (40 to 50 mm) burrow into the soil and construct earthen cells in which they pupate. Adult moths, also known as mill­ers, emerge from these cells in May and early June. An interesting phenomenon with this cutworm species is its seasonal migration to high elevations in the Rocky Mountains. Adult moths are active at night at these higher elevations, feeding on alpine flower nectar. During the day they form dense aggregations under stumps, logs and other structures that offer them protection from direct sunlight. Wildlife researchers report that these dense aggregations of cut­worm moths are an important food source for grizzly bears in the ab­sence of high-quality forage alter­natives in July and August. Adult moths return to the plains in late summer and begin laying eggs. Dry periods during August—Octo­ber are detrimental to egg hatch and larval survival of army cut-worms. Army cutworms have one generation per year.

 

Description

Adult army cutworm moths range from light brown to dark grayish-brown and are heavy-bod­ied. The forewing of the adult has a prominent circular spot and kidney-shaped marking. The hindwing is grayish-brown with a whitish fringe. Larvae are greenish-brown to greenish-gray with the dorsal (top) side darker than the ventral (un­der) side. A narrow, pate mid-dorsal stripe is usually present. The head is pale brown with brown to dark brown freckles.

 

Pale Western Cutworm

 

Life Cycle

The pale western cutworm has a life cycle similar to the army cut­worm. Moths deposit up to 300 eggs per female in the upper two inches of loose soil in cultivated ground in early fall. Some hatching may occur in the fall but most eggs hatch the following spring. The newly hatched larvae are small, approximately li inch, and diffi­cult to detect. Larvae are subterra­nean and feed on plant stems be­low the surface of the ground. Ma­ture larvae measure approximately li,2 inches and become less active, burrowing into the soil where they form an earthen cell. They pass most of the summer in the soil as a prepupa, with pupation occurring in August. Adult moths emerge from the cell during late summer, mate and lay eggs soon after. Pale western cutworm has one genera­tion per year.

Dry weather favors pale west­ern cutworm survival. Excessive precipitation in the spring can drown larvae in low-lying areas or force larvae to the surface of the soil where they are exposed to attack by predators and parasites. Wet weather produces conditions favorable for the increase of patho­gens. A method of predicting cut­worm outbreaks is based on the number of wet days (i.e., days on which more than a quarter inch of precipitation falls) in May and June. More than 10 wet days in­crease cutworm mortality; less than 10 wet days favor cutworm survival.

 

Description

The adult pale western cutworm is a gray to brownish white, heavy-bodied moth. Distinct markings on the wings are absent and the under­surface of the wing is white. Lar­vae frequently take on a character­istic C-shape when disturbed. The general body color of the larva is pale yellowish-gray with a distinct white mid-dorsal line. The head is yellow-brown with two distinct vertical black dashes that form an inverted V.

 

Adult Monitoring

Pheromone traps can be used to monitor adult flights of both cut­worm moths. Pheromones are spe­cies-specific scents that are pro­duced by females to attract males for the purpose of mating. For example, traps baited with army cutworm pheromone attract only male army cutworm moths. Traps are monitored weekly from early August through early October when moths are locating mates, mating and laying eggs. The cumu­lative moth catches for the dura­tion of the trapping period indicate the relative abundance of each cutworm species in a locale and can be used to forecast the poten­tial for damaging larvae popula­tions the following spring. An ad­vantage of using pheromone-baited traps is that captures are predomi­nately single species and time spent sorting and identifying adult moths is minimized.

Separate traps with the appro­priate lure (bait) are used for the army and pale western cutworms.

An example of adult pheromone trapping results is shown in Fig­ure 4. Cumulative trap catches are shown for participating counties. When 200 pale western cutworm moths or 800 army cutworm moths or more have been trapped the potential for a damaging larval population of these species is great. However, these thresholds are highly influenced by environmen­tal conditions that occur between moth flight and damage by larvae.

 

Larval Monitoring

    Larvae of either species gener­ally are not active above ground on bright, sunny days. They can be found just below ground by scrap­ing the soil surface lightly. Small larvae are well-camouflaged and can be difficult to see in the soil. Larval densities can be assessed by digging and screening the soil from one foot of row or a square foot at different sites in the field. Soil samples should be dug to a depth of at least 3 inches. Soil can either be sieved through a fine mesh screen or placed on a piece of white paper; the white backround can aid in detecting cutworm larvae. However, soil sieving is both labor and time intensive and has limited practicality as a routine sampling method. Examining plants for signs of cutworm larval activity and for cumulative larval damage is a more practical approach to assess­ing larval populations. Damage by cutworms can be distinguished from winter kill by looking carefully for evidence of feeding damage.

 

Damage

Both pale western and army cutworms can feed on and damage wheat, barley, alfalfa, canola, peas and sugar beets. Bare spots in the field in early spring may indicate cutworm activity. The army cut­worm is a climbing cutworm and feeds on above ground foliage. Damaged leaves indicate damage by army cutworms. Plants may recover from army cutworm feed­ing if defoliation is not severe.

Pale western cutworm larvae are subterranean and feed on plants below the soil surface. Because pale western cutworms cut stems, if the growing point is destroyed. the plants cannot recover from damage. Pale western cutworm infestation results in stand reduc­tion. Damage appears as dead or wilted tillers or plants.

As mentioned earlier, moisture can influence cutworm mortality. Records indicate that soil packing from compaction also forces larvae to soil surface.

 

Decision-making

Guidelines

Early detection of damaging cutworm populations is critical. Adult monitoring provides valu­able insight on the potential for damaging army and/or pale west­ern cutworm populations in an area. Fields should be monitored periodically beginning in late win­ter or early spring for cutworm larvae. Treatment thresholds are directly related to the health and vigor of the growing crop. If the crop is experiencing moisture stress then the effects of cutworm dam­age will be more dramatic.

Small Grains: If plants are not drought stressed and appear healthy and vigorous then insecticide treat­ment is justified when there are 4 to 5 army cutworm larvae per square foot. Treatment is justified for pale western cutworms when 2 to 3 larvae per foot of row are present. However, under drought conditions, treatment may be justified at lower cutworm larvae popu­lations. Prior to making a decision to treat a field it is important to determine the size of the larvae. Large larvae indicate completion of the feeding or damaging stage of cutworms. When this happens the majority of the damage has been done and a treatment may not be cost effective.

Alfalfa: In mature stands 3—4 cutworm larvae per square foot warrant making an insecticide treatment. In new seedling stands 2 cutworm larvae per square foot cause economic damage.

Canola: The economic thresh­old is quite low because seedling canola plants have little ability to compensate for damage. Treatment can be justified at 0.3 to 0.5 cut­worm per square foot.

Sugar beets: Army cutworms can seriously damage seedling sugar beets when they move from adjacent fields or grassy borders into emerging beet fields. Because sugar beets emerge and grow slowly during early establishment, cut-worms can drastically reduce a sugar beet stand in a short period of time. Considerable problems with both pale western and army cutworms have resulted where winter cereal cover crops are grown during the winter and beets are direct seeded into cover crop. When the cover crop is killed the cut-worms readily move to seedling sugar beets.

Very low cutworm densities, less than 1 cutworm per 20 feet of row, can cause significant damage. Large larvae can consume several plants each night and, if present in large enough numbers, can com­pletely destroy a sugar beet stand in only a few nights.

 

Management

 

Chemical options for control

Several insecticides are regis­tered for cutworm control in small grain, alfalfa, sugar beet and canola. For the most current list of regis­tered insecticides, refer to the High Plains Integrated Pest Management

Guide for Colorado, Western Ne­braska, Montana and Wyoming. Bulletin No. 564A, or to your local county Extension agent.

 

Cultural management strategies

     Tillage may cause some mortal­ity of eggs (pale western) and small newly hatched larvae (army) al­ready in soil. However, tillage is not compatible with no-till and conservation tillage production systems.

 

Biological Con trot

Both species of cutworms are attacked by wasp parasites belong­ing to the families of Ichneumoni­dae, Braconidae and Chalicidae and fly (dipteran) parasites, of Tachinidae and Bombylidae. The following pathogens have been recorded from army cutworms: Beauveria sp., Isaria sp., Metarrhizium anisopliae (Metschnikoff), Sorosporella uvella (Krassilstischik) and an entomopox virus. Insectivorous birds are the most common verte­brate predators feeding on larvae.

 

References

 

Burton, R. L., K. J. Starks and D. C. Peters. 1980. The army cut­worm. Oklahoma State University Agric. Expt. Stat. Bull. B-749. 35 pp.

Jacobson, L.A. 1971. The pale western cutworm, Agrotis orthogonia Morrison (Lepi­doptera: Noctuidae): a review of research. Quaest. Entomot. 7:414-436.

 

Copyright C 2000 MSU Extension Service. This document may be reprinted if no endorsement of a commercial product, service or company is stated or implied, and if appropriate credit is given. To use these documents in electronic formats, contact the Ag/Extension Communications Coordinator, Communications Services, 416 Culbertson Hall, Montana State University-Bozeman, Bozeman, MT 59717; (406) 994-5132; E-mail - APBTK@Montana.edu.

 

 


Figure 1 A pale western cutworm is shown in the soil beside a plant base.

Figure 2 A pale western cutworm (left) and an army cutworm are shown with a penny.  

 

Figure 3 Top - army cutworm feeds on a canola seedling.

 

Bottom - army cutworm burrows into soil at the base of plant

 
Figure 4 Pale western cutworm population as trapped in several Montana counties (Multiple county listings indicate multiple trap sites in those counties.) Traps were monitored every week for eight weeks. Potential for economic damage from pale western cutworm is anything greater than 200.