A New Pest Heading this Way - Western Bean Cutworm

A new pest is on its way to Ontario. Whether it arrives this year or next, it is something we need to be prepared for. Western Bean Cutworm (WBC) is native to North America, residing in Arizona and Nebraska but has started to spread out to new areas since 2000, successfully overwintering further north and east. Michigan and Ohio have both captured moths in the last two years, with Michigan reporting significant damage in some counties by late fall of last year due to a late flush of moths that crossed Lake Michigan. Ontario is the next region for them to expand to. Though arrival of moths in the first year doesn't necessarily result in extensive damage, we do need to get out and scout should we start capturing moths in our traps, so that we can stay on top of things.

The Pest

Despite its name, Western Bean Cutworm is a pest of corn (field, sweet and seed) as well as dry beans (not soybeans) and to a lesser extent tomatoes and nightshade. Unlike other cutworms, this pest prefers to feed on the fruit of the plant, ie, corn ears and bean pods.

Adult WBC moths are easy to identify from other corn pests. Each wing of the moth has a white band running along the edge or margin of the wing and has a spot or "moon' and boomerang-like mark (Figure 1).

Figure 1. Adult Western Bean Cutworm. Note the distinct markings on the wings (indicated in photo).
Photo credit: Marlin Rice, Iowa State University

WBC larvae are tan to pink in colour and do not have warts or spots (tubercles) on them like European corn borer do. The only distinguishing marking WBC larvae have is on their pronotum, the shield-like structure just behind the head of the larvae. The WBC's pronotum has two broad dark brown stripes (Figure 2).

Figure 2. Western Bean Cutworm Larva. Note the two broad brown bands on the pronotum (indicated by arrow).
Photo credit: Marlin Rice, Iowa State University

The Life Cycle

WBC overwinter in other regions as larvae in soil chambers. Adult moths emerge and are actively flying in late June, early July laying their eggs on the upper leaf surface of the upper leaves of the corn plants. Hybrids that hold their leaves upright are most preferred. Adults are attracted to fields in the whorl to pretassel stages of corn and only prefer the dry bean crop to lay their eggs in once the corn crop is in tassel or beyond. Eggs hatch within a week. Unfortunately the larvae are very mobile and can disperse from the original egg site to other plants in the vicinity so many plants can be impacted from one egg mass.

In corn, young larvae feed on the tassels and silks until they are large enough to tunnel into the ear and feed extensively on the kernels. Entry holes can sometimes be seen on the outside of the husk though they can also enter through the silk channels (Figure 3). Once they are in the ear control is impossible. Unfortunately western bean cutworms are not cannibals and therefore multiple larvae can feed on the same ear, leaving little left to harvest when infestations are high (Figure 4). Additional impact to quality can be expected from ear rots and secondary pests that may come in and feed on the damaged ears. Estimates on yield loss in corn from other jurisdictions indicate that a field infestation of one larva per ear can cause a 3.7 bu/ac loss.

Figure 3. Entry holes from WBC larvae.
Photo credit: Chris DiFonzo, Michigan State University

Figure 4. Feeding damage to corn ear by WBC larvae.
Photo credit: Chris DiFonzo, Michigan State University

In dry beans, the moths lay their eggs on the underside of the leaves, opposite of what they do on corn. Larvae hatch and can disperse to several neighbouring plants. Initially they may feed on the leaves and flowers but as they grow, they mine into the pod of the plant and feed directly on the bean seed. Once in the pod it is too late to control them as insecticides will not reach the larvae inside.

Ontario Monitoring

An extensive trapping network initiated by Iowa State University has established across the Midwest states and now Ontario. Adults were not captured in Ontario traps in 2007 though a larger network of traps will be started next week in Ontario to increase detection efforts. Overall we should have approximately 18 traps sites spanning across Southern Ontario. Results will be provided in CropPest.

If moths are captured, alerts will be sent to growers and reps on when and how to scout for the larvae and what options are available for control. In terms of transgenics, only Herculex Bt corn (Cry 1F) hybrids have been found to give some protection from of WBC.

Stay tuned for future CropPest articles that detail scouting techniques, thresholds and management strategies.