We would not suggest pre-treating the plants as it may expose harmless insects to the pesticide (even if the formulation used is considered organic) while potentially also wasting pesticide that isn't having any impact. Any pesticide whose instructions mention that a pest needs to be present upon treatment is relying on direct contact with the pest in order to work; the residues left behind likely don't have any pest-controlling abilities. (Only a systemic pesticide will remain in the plant's tissues through absorption, but these may contaminate pollen or nectar as well since they persist in the plant for so long, so systemics are a last resort for pest control.) Spinosad has the ability to seep into the foliage that the spray contacts but cannot be transported throughout the plant to growth produced after treatment or leaves which were missed by the spray. How long a treatment lasts can depend on the individual product's formulation since spinosad is used in a variety of different pesticides.

If you can't physically remove the larvae when they appear (granted, the youngest larvae are hard to spot) to reduce pesticide use, then we suggest only treating after you start to see damage and can confirm the sawfly larvae are present.

Miri