Scouting and Treating Western Bean Cutworm

Scouting and Treating Western Bean Cutworm

Photo of a western bean cutworm moth
Western bean cutworm moth
Photo of western bean cutworm egg group just before hatching.
Western bean cutworm eggs immediately prior to hatch.
Photo of western bean cutworm larva, featuring stripes on prothorax.
Larva with stripes on prothorax.
 

 

June 29, 2007

The first western bean cutworm moths of the season were caught late last week in light traps across the state. In the last 10 years, this insect has spread throughout Nebraska and gone as far east as Illinois. As moth numbers increase, mating will commence and the females will begin to lay eggs on corn. The appearance of the first moths provides a signal that farmers and crop consultants should begin to scout fields for the white, dome-shaped eggs.

Life Cycle

Western bean cutworm moths lay eggs in clusters of five to 200 on the top surface of the upper most leaf of a corn plant and on any leaf surface of dry beans. The eggs require five to seven days to develop, during which time the egg color changes to tan and then to purple immediately before they hatch.

After the small, dark brown larvae hatch on the corn plant, they move to the whorl or tassel to feed on the tender yellow leaf tissue or on the tassel itself. Once the tassel emerges or if it has already emerged when the eggs hatch, the larvae will move to the green silks. The developing larvae will feed on the green silks moving down the silk channel until they reach the ear tip. The larvae will feed in the ear tip until they are fully developed. If the infestation on one ear tip is so great that the larvae become crowded, a few individuals may move outside the ear and begin to feed on the side of the ear.

Scouting and Management

Photo of western bean cutworm larvae devouring a corn tip.
Larger western bean cutworm larvae on an corn ear tip.

Even though field scouting for western bean cutworm in field corn should begin when the first moths are caught, control decisions should be made shortly after the moth flight peaks. The moth flight usually peaks between July 10 and July 24. When scouting for western bean cutworms in corn, check 20 plants in at least five areas of each field. Look for eggs on the top surface of the upper most leaf or look for larvae in the tassel. If 8% of field corn plants, 5% of seed corn plants or 5% of popcorn plants have egg masses or larvae, consider applying an insecticide. Herculex soybean varieties appear to control the larvae very well, although not perfectly, and should not need treating.

Western bean cutworm moths prefer to lay eggs in corn plants that are in the late whorl stage compared to those that have completely tasseled. Pay particular attention to later planted fields or those with uneven development. Western bean cutworm eggs that hatch when corn plants are in the whorl stage of growth have a high rate of survival. The larvae are well protected in the whorl or tassel.

Treatment

If an insecticide treatment is warranted in corn, ideally it should be made when 95% of the plants in a field have tasseled. This timing of the application increases the chance that the worms will be exposed to the insecticide resulting in better control. However, since larvae will move to the silk as soon as it is available it is better to treat a day or so early rather than a day or so late. Chemigation has provided very good control of this insect, even at lowest labeled rates, and since chemigation will bring the insecticide to the ear tip the window of treating is a bit wider than for ground or aerial sprays. Asana, Ambush, Baythroid, Pounce, Lorsban, Capture, Mustang, Penncap-M, Warrior, and Seven are all labeled for control of western bean cutworm.

Light Trap Data

UNL entomologists monitor black light traps throughout the summer to track activity of moths that may be economically important. While moth captures cannot predict economic damage in individual fields, this information can be useful as a guide to which insects may be locally abundant. Moth flights are an early warning of economic damage that may occur.

Light trap data are now available online at http://entomology.unl.edu/fldcrops/ for traps at Clay Center, Concord and North Platte.

Keith Jarvi
IPM Assistant
NEREC, Norfolk

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A field of corn.