Two pests that are starting to appear in Ontario fields include cereal leaf beetle and alfalfa weevil.
Cereal leaf beetle: Both adults and larvae can cause damage by chewing long strips of tissue between the leaf veins, though most of the damage is done by the larvae in June. The larvae look somewhat like slugs or a drop of black oil on the leaf, as they are covered in a wet fecal shield to keep from drying out. One adult or larvae per stem warrants control, especially just prior to heading and into early heading when protecting the flag leaf stage is critical. Control options can be found here.
Alfalfa weevil: These green larvae have quite an appetite. Small larvae begin feeding within the leaf buds but as they grow, they can feed on the leaves, leaving nothing left but the leaf veins. Thresholds are dependent on the height of the crop and can be found here. Cutting is always the preferred method of control unless the crop is not ready to cut. Registered insecticides can be found here, though keep in mind that they are detrimental to both the parasitoids that help to control alfalfa weevil and the pollinators that may be foraging the crop. Follow steps to protect pollinators including spraying after 8pm when temperatures are cool (as long as there is no temperature inversion) when honey bees are less likely to be foraging. Contact your local beekeeper in advance so that they can take steps to protect the hives. For more information on protecting pollinators during spraying, click here