I’ve had a few calls in the past few days asking about armyworm and black cutworm damage in soybeans. I had the chance to get out to a few fields and found that the damage is being caused by the variegated cutworm (Peridroma saucia). The variegated cutworm is another migrating moth that comes into Ontario in the early spring around the same time as the true armyworm. The females are attracted to low growing vegetation to lay eggs. These cutworms feed on the leaves of the plants and don’t tend to clip them off at the stem like black cutworm. Typically this insect is not a serious problem in soybeans in Ontario but this year it seems to have caused some sporadic damage and in some cases to large sections of field. So far it appears that the damage is being found in no-till soybeans where there was more dense spring weed cover and corn residue in the field which is a perfect hiding spot for the larvae during the day. I have not heard of any damage yet in conventional fields.
The variegated cutworm larvae are grey, mottled and have distinctive yellow dots at the top of the front abdominal segments. Sometime you may find a yellow-orange stripe along the body.
Since there are no thresholds available for this insect in soybeans, use the soybean defoliation chart to help determine if treatment is needed. The larvae feed by eating chunks out of the leaves starting from the leaf margin. This is different from bean leaf beetle damage where small holes are made in the leaf.In 10 areas of the field, pick trifoliate leaves from five plants in the middle of the canopy. Discard the least and most damaged leaflets from each trifoliate collected. Compare the damage found to the cart below to determine the % damage to the leaves. Look under crop residue for the cutworms and determine the size of the larvae.
Soybeans can withstand a fair amount of defoliation in the vegetative growth stage before any yield loss is noticed. If you are finding more then 30% defoliation in the vegetative stages, use of an insecticide may be needed. Matador and Silencer are registered for cutworm control in soybeans and applications should be made in the evening when cutworms are active. Larvae larger then 2.5cm may be more difficult to control with insecticides.
From what I have been seeing, it seems like the larvae are quite large and may be in their final instar before they pupate. If replanting is being considered I think that by the time the beans germinate, the majority of larvae should be in the pupae stage and the risk of feeding will be reduced.
WBC Moth Caught!
In other news… we have caught our first confirmed Western bean cutworm. This year the first confirmed moth was caught in Aylmer. Now the count begins towards the peak flight so keep up to date on the numbers on the Canadian Corn Pest Coalition web site www.cornpest.ca. I’ll be posting additional info on the blog as we go.