Western Bean Cutworm Management Strategy for Field CornNew Pest for the Corn and Edible Bean Crop in OntarioWestern Bean Cutworm (WBC) has been spreading east and north from Nebraska, Arkansas and Colorado since 2000. Trap captures in 2008 found moths to be present for the first time in Ontario though no reports of damage were documented. A larger trapping network of over 150 traps has been established in 2009 to determine the geographical spread and level of presence in Ontario. Moths are starting to be found again in 2009 indicating that we need to start scouting and be aware of the management strategies for the corn crop. IdentificationDespite its name, Western Bean Cutworm is a pest of corn (field, sweet and seed) as well as edible beans (not soybeans) and to a lesser extent tomatoes and nightshade. Unlike other cutworms, this pest feeds 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). 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).
Life CycleWestern bean cutworm is native to North America, though it has resided mainly in the Southwestern U.S., until its recent range expansion northeast across the Midwest and now into Ontario. WBC over-winter in other regions (though it is expected to successfully over-winter in Ontario) as larvae in soil chambers. Adult moths emerge and start actively flying in late June through early July. They lay eggs on the upper leaf surface of the upper leaves of the corn plants and prefer hybrids that hold their leaves upright. Eggs take approximately one week to transition from freshly laid to hatching. Eggs are creamy white, laid in clusters of 5 to 200 eggs and are shaped like small cantaloupe (Figure 3). Egg masses turn a purple colour one or two days before hatch (Figure 4). Adults prefer fields in the whorl-to-pretassel stages of corn. Once the corn crop is in tassel or beyond, they prefer to lay their eggs on the dry bean crop. 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 both up and across corn rows. High-risk fields include no-till fields and fields with sandy soil.
DamageAdults begin actively flying in late June, early July and lay their eggs on the upper leaf surface of the corn plants, preferring hybrids that hold their leaves upright. The adults prefer fields in the whorl to pretassel stages of corn and prefer the bean crop to lay their eggs in once the corn crop is in tassel or beyond. Young larvae feed on the tassels and silks until they are large enough to tunnel into the ear and feed extensively on the kernels (Figure 5). Entry holes can sometimes be seen on the outside of the husk though they can also enter through the silk channels. Unfortunately western bean cutworms are not cannibals and therefore multiple larvae can feed on the same ear, leaving little left to harvest (Figure 6). Estimates on yield loss from other jurisdictions indicate that a field infestation of one larva per ear can cause a 3.7 bu/ac loss. Additional impact to quality can be expected from ear rots and secondary pests that may come in and feed on these damaged ears.
Scouting Techniques for CornScout 20 plants in five areas of the field from the end of July to the end of August. Focus on the top three-to-four upper leaves of the plant. Look for egg masses and young larvae. If egg masses are found but are still white (newly laid), flag the plant and return in a few days to determine if the eggs are turning purple and ready to hatch. Eggs hatch one or two days after turning purple. It is important to observe egg hatch to know when young larvae will be active since this is the stage that insecticide application is most effective on. Newly hatched larvae will initially climb to the top of the plant to feed on the tassel but only for a short time before migrating down the plant to feed on the ear. They do not tunnel into the stalk of the plant like European corn borer. Pheromone traps can be used to monitor for moth flight, which will indicate when eggs are being laid in the field and when to initiate scouting efforts. Contact the provincial entomologist for pheromone trap configurations, supply sources and monitoring protocols. Action Threshold for CornSpray is warranted if 5% of the plants have eggs or small larvae. If the eggs have hatched, spray at 95% tassel emergence or if tassels are already emerged, when most of the eggs are expected to hatch. Spray is only effective on small larvae, prior to them entering the ear. Management Strategies
Western Bean Cutworm Trapping Network and Results Maps:Other Resources
Contact Information:Tracey Baute Field Crop Entomologist-Program Lead
For more information: Toll Free: 1-877-424-1300 Local: (519) 826-4047 E-mail: ag.info.omafra@ontario.ca
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