June 30, 2006

Western bean cutworm moths found

Figure 1. Western bean cutworm.

Figure 2. A mass of western bean cutworm eggs.

Western bean cutworm are difficult to control once they reach the silks of the ear.

In dry edible beans use at least two milk jug pheromone traps per each 130-160 acre field.

Generally western bean cutworm has been considered a crop pest in the western High Plains, but in the past few years, moths have been captured as far east as Illinois. It is suspected that drought conditions throughout the Midwest have allowed greater survival of this insect and a temporary expansion of its territory. Given these observations, it is important that farmers throughout Nebraska, including eastern Nebraska, check their fields for western bean cutworm infestation this year.

Corn producers

Producers and crop consultants should be checking corn fields for western bean cutworm egg masses now to determine if an economically threatening population could become established. Western bean cutworm moths lay eggs in masses of 5 to 200 on the upper surface of the upper most exposed leaf of corn plants (Figure 1). These eggs are dome shaped and are white when first laid. As the eggs develop, they change from white to tan then to a dark purple right before they hatch. Once the eggs have hatched the small brown larvae will move to the tassel to feed until pollen shed begins, at which time they will migrate to the green silks.

Although all corn fields are acceptable for moth ovipostion, those that are late in development or have an uneven rate of development are usually preferred. Fields should be checked for eggs once a week from now until the peak in the moth flight by searching ten consecutive plants in at least five locations of a 130-160 acre field. The moth flight will usually peak in mid to late July. If 8% of the plants have egg masses, an insecticide treatment is recommended. Treatment applications should occur when 95% of the plants have tasseled and the majority of eggs have hatched or are about to hatch. This timing is important to optimize the amount of control while still affecting the larvae before they reach the silks of the ear. Control is much more difficult once the larvae begin to feed on the silks.

Dry edible bean producers

Western bean cutworm infestations of dry edible beans are difficult to detect in the egg stage. Treatment thresholds are based on either the number of larvae per foot of row or the number of moths collected in pheromone or scent traps. Producers should use at least two milk jug type pheromone traps for each 130-160 acre field of dry beans. These traps should be monitored daily from now until the peak of the moth flight. If the total accumulated number of moths is less than 700, there is a low risk of economic injury due to western bean cutworm feeding in adjacent beans. If the accumulative number is between 700 and 1000, there is a moderate risk of injury to beans. If the accumulative is greater than 1000, the risk of injury to beans is high.

Under a moderate risk, producers should use infestations of nearby field corn as an indicator of whether they need to check beans for feeding damage. If pod feeding is significant, consider an insecticide application. If the risk is high, pods should be examined and if damage is noticeable, treatment should be considered. Western bean cutworm feeding damage is minimal until larvae begin to feed on green pods. Field scouting activities should be timed such that plants are inspected and pods are protected by making any necessary insecticide applications two to three weeks after the peak in the moth flight (about the end of July).

Western bean cutworm management

Numerous insecticides are registered and effective for control of western bean cutworm larvae; however, products containing Bacillus thuringiensis (Bt) or transgenic corn plants that carry the Cry1Ab Bt gene do not control western bean cutworm. Genetically modified corn plants that carry the Cry1F Bt gene will control a high percentage of western bean cutworm larvae.

If spider mites are present in field corn, first generation pyrethroid insecticides, such as those with permethrin as the active ingredient, should be avoided since they can kill spider mite predators; however, if one of these products is selected, a miticide should be added to prevent a potential increase in mite infestations.