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February 1, 2006
Pg. 30 ISSN: 0892-8312
Scout for western bean cutworm
By Lynn Grooms
WESTERN BEAN cutworms (WBCs) have been a problem in the western Corn Belt
since the 1940s and last year were discovered to have migrated further east into Illinois
and Wisconsin. Given the severe yield loss that high numbers of WBC larvae can cause,
what can ag dealers and growers do?
First, dealers should help growers look for WBC eggs or larvae just before or at tasseling
and either confirm or deny their presence, says Kevin Steffey, University of Illinois
entomologist.
The University of Nebraska reports that WBC eggs are laid in masses of 5 to 200,
usually on the upper surface of the top leaves. The eggs are about the size of a pinhead.
When first laid, the eggs are white. As they develop they turn tan, then purple just before
hatching.
Newly hatched larvae are about ? in. long and are dark brown. Young larvae are tan with
a darker, faint diamond-shaped pattern on their backs. As the larvae mature, they turn a
plain pinkish-tan or pale brown and reach a body length of 1? in. When the eggs hatch,
the larvae first feed on pollen and then move to the corn ears. The larvae will feed there
for several weeks before they drop to the soil to form a subterranean overwintering
chamber.
Steffey notes that, although moths were captured in Illinois last year, the WBC does not
yet represent a significant threat there. The University of Illinois Extension in cooperation
with Pioneer Hi-Bred International set up a network of traps in 56 Illinois counties last
year, monitoring for the appearance of WBC moths from July 1 to August 15.
Most of the traps were set in northern Illinois. The furthest south that traps were set was
in Pulaski and Massac Counties. According to Steffey, the most moths (not larvae) were
captured in Mercer and Whiteside Counties in northwestern Illinois. In those traps, moths
were caught in triple-digit numbers. Those numbers are small compared to those in
Iowa, where in some counties there are counts of 2,000 moths, he says.
Threshold guidelines
The University of Nebraska and other universities in states where the WBC has existed
longer have established economic thresholds. The commonly accepted threshold is at
least 8% of plants showing WBC eggs or larvae in a check of 10 to 20 plants in five
random locations.
Robert Wright, University of Nebraska entomologist, recommends scouting for WBC
eggs in the late whorl stage and treating with a foliar insecticide if the economic
threshold is reached, or planting a Bt corn, such as Herculex I or Herculex Xtra which
has activity against WBC.
Steffey notes that Illinois growers may consider using Herculex products to protect
against the European corn borer but adds that they should not necessarily buy it for
WBC control alone. That is because the WBC is not yet established in the state, he
says.
"Herculex I or Herculex Xtra provide fairly decent control of WBC larvae. The dilemma is
whether this technology should be purchased only for control of this insect," says Marlin
Rice, Iowa State University entomologist. "The answer is probably no, unless the
farmer has a history of economically damaging infestations. In Iowa, this is most likely to
occur in the northwestern section of the state. In most of eastern and southern Iowa, the
probability of damage from this insect has been very small in recent years, even though
the insect occurs in these areas."
Pyrethroids
Should the economic threshold be reached, there are a number of insecticides labeled
for the control of WBCs. This list includes all of the commonly applied pyrethroids,
Steffey says.
"We have not conducted any insecticide evaluations at Iowa State," Rice says.
"However, the word from the field is that most farmers are using a pyrethroid insecticide,
such as Warrior."
Nebraska's Wright notes that Nebraska growers generally get good results with synthetic
pyrethroid insecticides and chlorpyrifos (for example, Lorsban 4E). "Proper timing is the
biggest factor in getting good control with any product," Wright says. "If growers wait
until the WBC larvae are in the ear, good control is difficult. Proper timing based on
regular scouting is the key to getting good control with foliar insecticides."
"Only field scouting can verify whether eggs were laid in a field or not," Rice says,
adding that pheromone traps cannot be used to gauge the level of infestations in
neighboring fields or for justifying an insecticide application. Traps simply tell a scout that
adult WBCs are in an area.
Winter's effects
Could the colder-than-normal winter weather in much of the Midwest help keep WBC
numbers down this coming summer?
"December 2005 was the coldest December on record in Iowa," Rice says. "However,
most of the northern half of the state, where western bean cutworms are more common,
had snow on the ground nearly the entire month. Snow acts as a great insulation blanket
against bitterly cold air that potentially could harm larvae in the soil. I suspect that the
winter weather will have a negligible effect, at least so far, on this insect. Weather that
has a greater impact would be thunderstorms during late July and early August when
eggs are being laid and larvae are hatching."
"WBCs overwinter in the soil as full-grown caterpillars," Wright adds. "In coarse-textured
soils, they can dig deep enough to avoid extreme temperatures. In finer-textured soils,
they tend to overwinter closer to the surface and may be exposed to extreme cold
temperatures, which reduce their survival." Wright notes that wet weather in winter and
spring, particularly in finer-textured soils, may also reduce the WBC's survival over
winter.