We have some roses at work that I noticed looked a bit brown. Upon closer inspection, the leaves looked skeletonized, like the damage a Japanese beetle does, but this was sucked dry, but not chewed through. It’s also a bit too early for Japanese beetle. Keep looking…
The rose sawfly has one generation a year, with larvae appearing in mid to late spring. The larvae fall from the plants and tunnel into the soil by mid-June, but it’s later this year. They remain dormant underground until next spring, when the adults emerge and lay eggs on the new rose foliage to begin the cycle over again.
Larvae can be effectively controlled with a neem oil product or an insecticidal soap. Spray only the leaves (both sides), in the morning as neem oil can possibility hurt pollinators (More research needs to go into that). The strategy is to find larvae while they are still small and before damage becomes severe, like our roses! There is no need for control after the larvae have finished eating and left the plants, give or take mid-July.
One last note, these are not caterpillars, they are actually primitive wasps, so Bt or Bacillus thuringiensis will not work.
© Ilex Farrell – Midwestern Plant Girl