The time to treat will be before you notice the first damage or at least as soon as possible. The following is based on an assumption of a proper identfication that the larvae you note are Sawfly Larvae. Here is some info from the Univ of IL:The sawfly larvae are whitish to pale green with light tan heads and obvious black eyes. Fully grown larvae are a little over one inch long. Although sawfly larvae look similar to caterpillars, they have more than five pairs of prolegs. Not being true caterpillars, they are not controlled with Btk. Spraying the foliage with carbaryl, Sevin, or a labeled pyrethroid will provide control. Take care when treating to avoid times when pollinating insects are present.
