Quick Facts...
- The western bean cutworm passes through five developmental stages
– egg, larva, prepupa, pupa, and adult (moth).
- Infestations averaging several larvae per ear can result in the reduction
of grain yield by as much as 30 to 40 percent.
- Larvae from a single egg mass may infest nearby bean plants within
a 10 to 12 foot circle.
- Some naturally occurring biological controls, including birds, insect
predators, and pathogens have been observed in the field.
Western bean cutworm, Striacosta albicosta (Smith), has been a
pest of dry beans for many years. More recently it has been recognized
as a serious pest of dryland and irrigated field corn in northeastern
Colorado.
Description
The western bean cutworm passes through five developmental stages –
egg, larva, prepupa, pupa, and adult (moth). The dome-shaped eggs, which
are slightly larger than the head of a common pin, are laid in flat, irregularly
shaped masses (Figure 1.), usually ranging from 15 to 50 eggs per mass.
When first laid, eggs are white with a thin, red ring around the top.
Eggs darken with age, first to brown and then to purple or black just
before hatch.
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| Figure 1: Western bean cutworm egg mass. (Photo by F.B.
Peairs) |
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| Figure 2: Western bean cutworm larva. (Photo by F.B. Peairs)
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| Figure 3: Western bean cutworm adult. (Photo by J.L. Capinera).
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| Figure 4: Western bean cutworm damaged pods (left) and seeds
(lower right). (Photo by H.F. Schwartz). |
Young western bean cutworm larvae are dark brown with faint diamond-shaped
markings on their backs. They lighten in color as they mature and are
gray to pinkish brown when fully grown. Mature larvae (Figure 2.) are
about 1 1/2 inches in length and have three short dark stripes running
lengthwise on the first segment behind the head. The stripes behind the
head can be used to distinguish western bean cutworm larvae from corn
earworm larvae, which also are found feeding in ear tips. The prepupae
and pupae occur in the soil and are rarely seen.
The body of the adult (Figure 3.) is about 3/4 inch long and brown in
color. These moths have a wingspan of about 1 1/2 inches and are marked
with creamy white stripes on the leading edge of the forewings. A white
spot is found just behind and about half way along the white stripe, and
a kidney-shaped spot is located near the end of the stripe. The light-colored
hind wings are not distinctly marked.
Life History
Western bean cutworms complete a single generation each year. Fully-grown,
nonfeeding larvae (prepupae) overwinter in the soil in earthen cells found
at depths of three to nine inches. Pupation occurs in May and early June.
The moths emerge between mid-July and early August. They are active at
night and are attracted to lights. Eggs are laid shortly after the moths
emerge. Egg masses are deposited on the top surface of upper leaves of
corn or on the undersides of bean leaves. The eggs hatch in five to seven
days. The majority of western bean cutworms feed until mid-September.
Mature larvae enter the soil and construct the overwintering cells for
the prepupal stage.
Damage: Corn
Western bean cutworm eggs are laid in the upper third of the corn plant,
on the upper sides of the leaves. In erect leaf varieties eggs are commonly
found on lower leaf surfaces as well. Newly hatched western bean cutworms
move to one of two places on the corn plant, depending on the stage of
development of the corn. If corn has not tasseled, larvae feed on pollen
in the developing tassel within the flag leaf. If corn has tasseled, larvae
feed on silk in the ear. If silk feeding persists during pollen shed,
pollination may be poor. Plant to plant movement is common and larvae
from a single egg mass may infest nearby plants within a six to 10 foot
circle.
Once the ear has formed, it becomes the major feeding site for western
bean cutworms, which feed on developing kernels. They reach the kernels
through the silk channel or by chewing through the husks. Infestations
averaging one cutworm per ear result in a loss of about four bushels per
acre. Infestations averaging several larvae per ear can result in the
reduction of grain yield by as much as 30 to 40 percent. Yield reduction
in silage corn usually is not as severe, but kernel loss may result in
reduced ensilage quality.
Damage: Dry Beans
Western bean cutworm moths lay their eggs on the undersides of dry bean
leaves. Larvae from a single egg mass may infest nearby plants within
a 10 to 12 foot circle. Young larvae feed on tender leaves and flower
parts, causing little noticeable damage. Economic damage occurs when larvae
chew through pod walls and feed on developing seeds (Figure 4.). Feeding
activity is mostly nocturnal or on cloudy days. Larvae hide in the soil
at the base of bean plants during sunny periods. Yield losses of eight
to 10 percent due to western bean cutworm feeding have been estimated.
Management
Few effective alternatives to chemical control currently are available.
Tillage has been suggested, but not tested. Some naturally occurring biological
controls including birds, insect predators, and pathogens have been observed
in the field. Some resistance is available in dry beans, but not in acceptable
varieties. Bt corn hybrids containing the Herculex I event are labeled
for control of Western bean cutworm, however, the other available events
will not provide any control.
Properly timed insecticide treatments based currently accepted economic
guidelines remain the only highly effective means of control in both corn
and dry bean. Both aerially applied and chemigated treatments have been
effective in university tests. Currently labeled insecticide products
are given in the High Plains Integrated Pest Management Guide (www.highplainsimp.org).
Treatment Guidelines: Corn
Western bean cutworm eggs are laid on upper leaf surfaces on the upper
part of the plant. Fields should be scouted closely because once the larvae
move into the ear, controlling them will be difficult. Look for eggs and
larvae on upper leaf surfaces in the upper third of the plant. Prior to
pollen shed, tassels should be inspected for larvae. Check 10 consecutive
plants at several locations (at least five) per field, using enough locations
so that all areas of the field are represented.
Chemical control should prove economical if at least eight percent of
the plants have egg masses or small larvae. If most eggs are hatched,
treat when the crop is at least 95 percent tasseled and before the larvae
have started to feed on the silks. If most eggs have not hatched and the
crop is completely tasseled, then treat when most eggs have reached the
purple stage.
Treatment Guidelines: Dry Beans
Scout adjacent corn, where it is much easier to find eggs and larvae,
because beans are likely to be infested and require treatment if nearby
corn is infested. Direct treatment decisions in dry beans can be based
on larval counts, light trap catches or pheromone trap catches. Larval
counts are difficult to obtain in dry beans because of crop growth habit
and the insect’s habit of hiding in the soil during the day. Treat
irrigated dry beans if two or more larvae are found per row foot prior
to the onset of pod damage. Use a guideline of one western bean cutworm
per row foot in dryland production systems.
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| Figure 5: “Milk jug” pheromone trap.
(Photo by G.L. Hein) |
Light traps and pheromone traps are used to monitor adult activity and
provide useful information on the need for and timing of insecticide treatments.
While both traps provide similar information, pheromone traps are cheaper,
simpler to operate, and more efficient. Also treatment decisions in dry
beans can be based on pheromone “trap catches”, as described
below
Treatment Guidelines Based on Pheromone Trap Catches
1. Material
A. ScentryTM pheromone lures
(www.scentry.com/page5.html)
B. Milk jug traps (Figure 5.)
C. 4:1 mixture of water and antifreeze plus
several drops of dish soap
D. Data sheets
2. Procedures
A. Place two traps per field in late June
or early July.
i. Place traps
on opposite sides of the field.
ii. Best locations
are bean field borders near dense vegetation.
iii. Pin lure
to lid of milk jug.
iv. Place
trap on a post at about four foot high.
v. Fill base
of trap with antifreeze mixture.
B. Monitor weekly until peak flight,
then at least
three times per week until
catches start to decline in early August.
C. At each visit, using a small sieve to
remove moths, count and record catch.
Replenish antifreeze
mixture.
D.When average moth catch per trap starts
to decline, determine:
i. The date
of peak capture.
ii. The total
number of moths caught per trap for the season through the
date of peak
capture.
3. Treatment guidelines based on the average catch of the two traps per
field from the beginning of the flight through the peak capture date are
as follows:
A. If fewer than 700 moths were caught then
the risk of significant western
bean cutworm damage is low.
B. Catches between 700 and 1000 moths indicate
low to moderate risk. Consider
treatment if there is substantial activity
in adjacent corn fields; or pod feeding
is noted about three weeks after
peak flight.
C. Catches of more than 1000 moths indicate high risk. Risk goes up as trap
catches increase above this level. Risk can be confirmed by scouting
adjacent corn and by checking ;for pod damage.
4. Treatments are most effective if applied 10 to 20 days after peak flight.
This allows time for all eggs to hatch, but is early enough to avoid damage
from large larvae.
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