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MT 200005 Agriculture
Pale Western and Army Cutworms in Montana
by Sue Blodgett, Greg Johnson, Will Lanier and Judee Wargo*
*Authors are, respectively, integrated pest management specialist,
department head and integrated pest management assistant, Department of
Entomology, Montana State University-Bozeman, and MSU Extension agent,
Chouteau County, Fort Benton, Montana.
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.
Figure
1. A pale western cutworm (left) and an army cutworm are shown with a penny. (Photos
in this publication are by P. M. Denke)
Figure
2 An army cutworm feeds on a canola seedling.
Figure
3 (below, left). An army cutworm burrows into soil at the base of a plant.
Army Cutworm
Life Cycle
The army cutworm begins laying eggs in late August and cancontinue 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 cutworms. 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, pale mid-dorsal stripe
is usually present. The head is pale brown with brown to dark brown freckles.
Figure 5. A pale western cutwom is shown in the soil beside a
plant base.
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 1/8 inch, and difficult to detect.
Larvae are subterranean and feed on plant stems below the surface of the
ground. Mature larvae measure approximately 11/2 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 under-surface 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 are 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,
cutworms 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 cutworms 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 Control
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. Entomol.
7: 414-436.
Copyright 2000 MSU Extension Service
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in furtherance of cooperative extension work in agriculture and home economics,
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File under: Insects and Pests
B-18 (Field Crops)
Issued April 2000 4892000400MS