A well-timed spray application can be effective in killing the young larvae. However, if the emergence period weather is cold and wet, or intermittent, adult sawflies emerge periodically. Once the residual effectiveness is lost, larvae from newly laid eggs are unaffected by the treatment and new leaves developing after spraying and are not protected. Such delayed spring development also complicates second generation control. A slightly late treatment is more effective than one applied too early.
Precise timing is essential.
Many factors influence the residual effectiveness of insecticides, but generally during a cool season a longer lasting insecticide is preferred. Soil drench treatments with systemic insecticides may protect birches for an entire season, but because of their high mammalian toxicity are recommended only for commercial applicators. Other Birch Leaf Mining Sawflies Three species of European birch leaf mining sawflies causing serious damage to native and introduced birches were accidentally introduced into North America. The BLM was first recorded in 1923. Adults of F. pusilla are the first to oviposit in young tender leaves, followed later in the season by P. thomsoni and H. nemoratus , both of which lay their eggs in more mature leaves.