How to eradicate rose slug sawflies?

Victoria in Port Hope, ON., asks:

How do I eradicate rose slug sawflies? Each year they devastate my roses, and this year they added mountain ash to their menu. I try to keep up by picking them off daily, and I spray with the appropriate bug spray. Even so, I cannot keep up. The mountain ash is now a skeleton, except for a few of the lower branches!

Judith Adam, horticultural consultant for Garden Making says:

There is no easy solution for eradicating European rose slug sawflies (Endelomyia aethiops) without the use of insecticides that are not permitted in Ontario. The adult sawflies emerge from soil under the roses in early spring and lay their eggs on the undersides of new rose foliage. The larvae appear several weeks later and begin to feed on the leaf tissues, and it would be more effective to spray before the larval stage appears. You could try spraying the undersides of leaves with horticultural oil in mid-spring and again two weeks later, trying to smother the eggs and halt their development.

The larvae will eventually drop into the soil to pupate and begin the cycle again. European sawflies have only one generation a season, but there are other rose sawflies that produce several generations in one growing season. You can read more about this here: www.missouribotanicalgarden.org