Western Bean Cutworm Adults Found in Northwestern
Kelly Cook and Kevin Steffey, Extension Entomologist,
University of Illinois
This article first appeared in “The Bulletin” No.
17 Article 1/July 15, 2005
As we described in the article "Preparing to Monitor for
Western Bean Cutworms," published in the May 27, 2005, issue
of the Bulletin, the western bean cutworm, a midsummer pest of
corn, has been moving steadily eastward across Iowa. Professionals
and graduate students at Iowa State University have been tracking
its movement. In 2004, it was discovered in southeastern Iowa
as well as in Warren County, Illinois, and Harrison and Putnam
counties in Missouri. The findings of western bean cutworms in
Illinois and Missouri in 2004 were new state records, and the
moths were captured during only one night of trapping.
The discovery of western bean cutworm adults in Illinois in 2004
prompted us to establish a network of pheromone traps across Illinois
to monitor for the presence and distribution of this pest in our
state. The network is a cooperative effort between University
of Illinois Extension and Pioneer Hi-Bred International. Traps
have been placed throughout northern and much of central Illinois,
and in some selected counties in southern Illinois. Operation
of most of the traps began on July 1. The traps will be checked
two or three times each week unless they begin to capture large
numbers of moths, after which some of the traps will be checked
Since July 1, 2005, western bean cutworm moths have been captured
in pheromone traps in eight counties-Adams, Lee, Mercer, Rock
Island, Warren, Whiteside, Will, and Winnebago. The moths captured
were identical to those in the photographs that Dr. Marlin Rice,
Extension entomologist at Iowa State University, supplied for
our cooperators. If the capture in Will County is verified, the
eastern edge of this insect's distribution now is Indiana. The
traps will continue to operate through July and into August. All
captures will be reported in a database developed and maintained
at Iowa State University http://www.ent.iastate.edu/trap/westernbeancutworm/.
If you visit the Web site, you will notice numerous western bean
cutworm moth captures already made in Iowa and Missouri.
What does this mean for corn producers in Illinois? The confirmation
of western bean cutworm moths in Illinois counties indicates that
cornfields in those counties will need to be scouted. Future captures
in other Illinois counties will create the same obligation. Female
moths currently are laying egg masses on host plants, including
field corn, sweet corn, tomatoes, nightshade, and ground cherry.
In corn, females lay eggs primarily on the upper surface of leaves.
Fields that are tasseling or near tasseling or fields that have
upright leaf characteristics are attractive to western bean cutworm
adults. Eggs within masses range from 5 to 200 in number, but
the average number of eggs per egg mass is about 50. Eggs are
white when first deposited, but they turn tan and then dark purple
as they mature. Larvae hatch from eggs 5 to 7 days after oviposition.
Larvae feed on corn plants for about a month before pupating.
The larvae are very mobile and may infest plants in the same and
adjacent rows in an area 6 to 10 feet in diameter. Young larvae
can be found feeding on flag leaves and tassels. As they mature,
larvae feed directly on the silks. As both corn and larvae mature,
the larvae feed on ears and developing kernels.
When should scouting for western bean cutworms begin?
Scouting should start now, if you are in a county in which western
bean cutworm moths have been positively identified, or in the
near future, if and when the first moths are found in your area.
It is especially important to note that of the current available
transgenic hybrids, only those that contain the Cry1F toxin (Herculex
I Insect Protection) are labeled for control of the western bean
cutworm. YieldGuard hybrids do not control this insect.
To look for western bean cutworms in cornfields, examine the
upper surfaces of plant leaves for egg masses and/or small larvae.
Also look at tassels before pollen shed. Look at 10 consecutive
plants in at least 5 different areas of the field. Egg laying
varies with plant growth stages, so sample hybrids with different
maturities within a field separately.
When should I treat corn to control western bean cutworm
larvae? Entomologists at the University of Nebraska indicate
that an insecticide treatment may be warranted when 8% of plants
have egg masses and/or small larvae. This is a nominal threshold
based on experience, and it is the most widely used threshold.
Timing of an insecticide application is critical. If larvae have
hatched, apply an insecticide after 95% of tassels have emerged,
but before larvae enter the silks. Control is more difficult after
larvae have moved to the silks. Once larvae have moved inside
the husk, insecticides are ineffective and larvae will feed until
they mature. If larvae have not hatched and the corn plants have
tasseled, time the insecticide application to coincide with the
hatch of larvae. If the eggs are purple, hatch usually occurs
in about 24 hours.
According to entomologists at the University of Nebraska, control
decisions associated with western bean cutworms need to be made
shortly after peak moth flight. The moth flight in Nebraska usually
peaks between July 10 and July 24. If you want to read the experts'
words, visit the most recent issue of Crop Watch, the University
of Nebraska's weekly newsletter.
More information about the western bean cutworm can be found
on the Western Bean Cutworm Factsheet at http://www.ipm.uiuc.edu/fieldcrops/insects/western_bean_cutworm/index.html.
We'll continue to provide updates on the distribution of this
insect in Illinois. Sign up to receive “The Bulletin”
for weekly updates on this and other pests at http://www.ipm.uiuc.edu/bulletin/index.php.