Current and upcoming insect pests of Ontario field crops
June 25, 2020 By OMAFRA Field Crop Team
Wheat pests are still a concern
True armyworm larvae are in many winter wheat and spring cereal fields across southern Ontario – thankfully not always at threshold. Some fields have populations at threshold (4-5 unparasitized larvae smaller than 2.5 cm per square foot) or above and with minimal feeding damage. Larvae sometimes choose not to do as much feeding and move on to other nearby (usually younger) fields, or are already done feeding and are getting ready to pupate.
It pays to scout the same fields multiple times before making a management decision. Base your decision not only on the number of larvae present but also whether feeding damage is progressively getting worse. Most winter wheat fields south of London are already advanced enough in crop stage that larvae are no longer be a concern.
Defoliation and (most importantly) flag leaf-feeding have a bigger yield impact before anthesis has occurred. Any defoliation after that will have less of an impact. Head-clipping by larvae is another concern, though there are few reports of that happening this year.
Spring cereals will be the next crop focus for armyworm scouting over the next few weeks, as well as mixed forages. There have been no reports of armyworm in central and eastern Ontario yet, though it is expected. We experienced significant moth flights in April and May in eastern North America and expect there was a prolonged egg-laying period. This will result in a variety of larval sizes in fields.
Some wheat fields have two or more pests present at the same time. Cereal aphids or cereal leaf beetle (CLB) and armyworm larvae are active in some fields. Cereal aphids are more of a concern in a younger crop. It takes 12 to 15 aphids per stem prior to heading and 50 aphids per head once the crop has headed. More aphids are required to have the same impact on yield that armyworm or CLB can have. It only takes one CLB adult or larvae per plant prior to heading and four to five armyworm per square foot to impact yield. Focus on these two pests, even if aphids are present, to make a spray decision. Flag leaf-feeding again is the ultimate decision maker.
Cabbage seedpod weevil have been observed in spring canola. They lay their eggs in developing pods during the early bloom stages. They are often more abundant at field edges. Scout using a sweep net in a few areas of the field. The action threshold is 25-40 weevil per 10 sweeps at 10 to 20 per cent bloom.
Which pests are next?
With this hot, dry weather, two-spotted spider mites are the next major concern. As wheat starts to dry down for harvest, mites will be moving to other host crops. Plan to scout soybeans, dry beans, and even high-value, younger corn fields for infestations over the next month. For tips to help with scouting and management decisions, check out this Field Crop News article from 2016.
Finally, soybean aphids are starting to pop up in a few fields. Hopefully some of the natural enemies present in cereal fields now will spread to soybeans to help provide control. The free Aphid Advisor app is available to help guide you on spray decisions once the soybeans reach the reproductive stages.