Extension, College of Agriculture & Life Sciences, The University of
Tim C. Knowles, Area Extension Agent, La Paz and Mohave Counties
Description and Biology of the Pest
armyworm (Spodoptera exigua) caterpillars are smooth skinned with few
or no hairs on the body, may be olive green to almost black in color
down the middle of the back, and have a yellow stripe on each side of
the body. They usually have a conspicuous black dot on each side of
the second body segment behind the head and just above the second pair
of legs, a white colored dot at the center of each spiracle, and reach
a length of about one inch. In contrast, the western yellow striped
armyworm has a black dot on its first abdominal segment, a brown colored
dot at the center of each spiracle,
and an inverted Y marking that is white to orange in color
on the front of its dark brown head. The alfalfa caterpillar can be
distinguished from the beet armyworm by its velvety green smooth surface
and a single prominent white stripe along each side. The common armyworm,
which is occasionally found in corn and sudangrass, differs from the
beet armyworm in that it is dark green to light grey with two orange
stripes along each side. Additionally, the common armyworm lacks the
inverted Y-shaped mark found on the front of the head of the western
yellow striped armyworm.
moths live 4 to 7 days and are about 3/4 inch long, dusky, mottled grey
with distinct lighter markings on the forewings, including a rounded
and crescent-shaped spot. They are nocturnal, but may be picked up in
sweep net samples. The female moth lays small masses of pale greenish
or pinkish, striated eggs on the undersides of leaves, covering them
with dirty white hairlike scales. Early instar larvae hatch within 2
to 5 days and usually feed in groups, skeletonizing leaves and spinning
silk over the feeding site. Rate of development depends on temperature
with larvae living 18 to 24 days and pupation lasting 5 to 8 days. A
complete generation is completed within 21 to 35 days, often with 5
or more generations per year. They do not
overwinter in Arizona desert production regions, but continue to feed
on various plant hosts through the winter. The beet armyworm is a pest
of many crops including alfalfa, cotton, and vegetables. Pigweeds (Amaranthus
sp.), and nettleleaf goosefoot are also favored hosts.
Young beet armyworms skeletonize foliage, leaving the veins of leaves
largely intact. Heavy feeding and leaf skeletonization on the tips of
the stalks by early instars can cause distinct flagging visible from
a distance as terminal leaves turn white. Comparatively, the alfalfa
caterpillar eats the entire leaf of alfalfa, and although the alfalfa
weevil also skeletonizes leaves, it is not present in alfalfa during
the summer when armyworm damage occurs.
Early in the season, beet armyworm populations may move to seedling
cotton. The caterpillars eat all but the epidermal leaf layer producing
a windowed effect. Later in the season, they characteristically bore
into the cotton plant terminals and feed on squares, blossoms, and green
bolls. Their damage to cotton fruit is recognized by extensive feeding
and holes chewed in adjacent bracts and leaves.
Biological and Cultural Controls
Many predators and parasites combine to substantially maintain beet
armyworm populations at low levels. Insecticide sprays for other pests
can disrupt this natural control. Populations are sometimes held in
check by parasitic wasps (including Hyposoter sp.), tachinid flies,
predators, and a virus disease. Beet armyworms killed by a virus become
black and limp and often are found hanging from alfalfa leaves. Early
cutting of alfalfa hay can give satisfactory control of the beet armyworm,
if the infestation appears late in the cutting cycle. Watch for beet
armyworm on adjacent crops and on weeds in and around cotton fields.
Monitoring and When to Treat
Insecticide treatments are usually required in alfalfa when at least
5-10 beet armyworm caterpillars greater than ½ inch long are
found per 90 degree sweep (or 15 per 180 degree sweep), up to one week
before alfalfa is intended to be cut. Insecticide treatments may be
necessary when alfalfa hay prices and potential yields are high enough
to warrant the expense, and it is too early to harvest the hay. Once
alfalfa enters summer slump, and hay quality and tonnage decline, growers
may not be able to justify the cost of insecticide applications for
beet armyworm control. Although several synthetic pyrethroid insecticides
are labeled for beet armyworm control in alfalfa, they are effective
only on first to second (and occasionally third) instar larvae. Furthermore,
beet armyworm populations resistant to Lannate (methomyl) have been
found in Imperial and Yuma County alfalfa.
(Lock- On, Lorsban)
|½ pint = 7 days
1 pint = 14 days
2 pints - 28 days
|One application per cutting.
Four applications per season.
||One application percutting
First and second instars only
|0 days (EC)
|Do not apply when bees are working
||Ten applications per crop
Apply at 5 to 7 days intervals
||Apply at 7 to 14 day intervals.
||Toxic to bees
First through third instars only
|<0.1 lb. a.i. = 0 days
>0.1 lb. a.i. = 14days
|0.2 lb. a.i./acre/ cutting limit.
Toxic to bees and fish (see label).
IPM Manual Group. 1985. Integrated pest management for alfalfa hay.
UC Davis Press.
Meister Publishing. 1997. Insect Control Guide. Willoughby, OH.
The information given herein is for educational purposes only. Reference
to commercial products or trade names is made with the understanding
that no discrimination is intended and no endorsement by the University
of Arizona Cooperative Extension is implied.
These suggestions are not intended to take the place of product labels.
The user must accept responsibility to obtain, read, understand, and
follow all product label instructions. Label violations can lead to
civil and criminal penalties, unmarketability of crops, and could contribute
to cancellation of product labeling.
The University of Arizona is an Equal Opportunity/Affirmative
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by the University of Arizona.
Document located http://ag.arizona.edu/pubs/insects/az1047/
Published August 1998
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