True armyworm (TAW) larvae have been spotted in some winter wheat fields in Chatham Kent and Lambton County. This is in addition to cereal leaf beetle (CLB) larvae reaching threshold in some cereal fields, in some cases causing significant damage. So far reports of CLB reaching threshold have been from fields in the Sombra, Petrolia, Arva/Thorndale and Seaforth areas but scouting is advised in all fall seeded cereal crops for both of these pests to avoid any surprises. Oats are particularly vulnerable to CLB injury so ensure those fields are prioritized.
True armyworm: The best time to scout for true armyworm is shortly after dusk when larvae are actively feeding. Examine 10 areas of the field, assessing the number of larvae per 30 cm x 30 cm (1 ft2). Pay particular attention to the border area directly adjacent to other grassy host crops. During the day, if it is cloudy and overcast, you might be lucky enough to see larvae on the head of the plant but on sunny days, they will be down on the ground among the crop debris or under soil clods. Brown frass may also be present on the plants and on the soil surface. Birds diving into your field is a good indication that there are good eats there so take a look. If you do find larvae, look for any white eggs that may be attached to the backs of the armyworm larvae. This is a sign that the larvae have been parasitized by one of its parasites which have done the job for you. Avoid treating with insecticides when large numbers of parasitized larvae are present as they have already been controlled by parasitoids or when larvae are close to 2.5 cm in length, as insecticides will no longer be effective and the larvae will soon stop feeding.
Chemical control is only warranted on TAW if there are 4 to 5 un-parasitized larvae (smaller than 2.5 cm) per square foot. If a significant amount of wheat head clipping is occurring, spray may be warranted if larvae are still actively feeding, are smaller than 2.5 cm and as long as pre-harvest intervals have not been reached. A reminder that lambda-cyhalothrin products (Matador, Silencer, Voliam Xpress) are not to be used on field crops this year. Product options left can be found here on the OMAFRA Crop Protection Hub (True Armyworm) More information on true armyworm can also be found in Ontario Crop IPM – Wheat (gov.on.ca).
Cereal leaf beetle: Cereal leaf beetle infestations have caught some by surprise. With most of the winter wheat crop in early heading stages, the next few weeks are the most critical to stay on top of any CLB injury to the flag leaf. In the early heading stages, control is only warranted if feeding is taking place on the flag leaf in several areas of the field and only if CLB is still actively feeding. Once out of early heading stages, the risk has passed and spring cereals will be more at risk and require our focus. In spring cereals, control is warranted if an average of three larvae per tiller are found before boot stage. One CLB adult or larvae per stem warrants control after boot but prior to heading.
As mentioned above, lambda-cyhalothrin products (Matador, Silencer, Voliam Xpress) are not to be used on field crops this year. Malathion 85E and 500 are the only products left with cereal leaf beetle on the label. We are aware that there is currently a significant supply shortage for Malathion in Ontario that could last until at least the end of June. Some retailers may have limited supply, so calling around to find some may be necessary. If you are already using Delegate for true armyworm control, you will see some level of control of cereal leaf beetle too.
Although bees don’t pollinate cereal crops, they may be fly through fields or forage on flowering weeds within or along the field borders. Follow all label precautions and buffer zone requirements and time insecticide applications to minimize bee exposure. Plan to apply these insecticides in the evening after 8pm, when bees are less likely to be foraging. This allows the spray to dry before bees may be foraging the next morning. If there is an expected temperature inversion in the evening, avoid spraying to reduce the risk of drift. Contact any beekeepers with hives within 5 km of the field in advanced, to ensure their hives can be located strategically, temporarily protected or relocated where feasible.