Roseslug sawflies (order Hymenoptera, family Tenthredinidae) were once generally considered only nuisance pests of roses in Ohio. The Common Roseslug Sawfly (Endelomyia aethiops) (a.k.a. Rose Skeletonizer) was most often encountered followed occasionally by the Curled Roseslug (Allantus cinctus). The common roseslug has only one generation and the curled roseslug two generations. These sawflies would come and go so quickly they seldom caused appreciable damage.
However, in recent years, these relatively innocuous sawflies have been largely supplanted in Ohio by the more damaging Bristly Roseslug Sawfly (Cladius difformis) which has multiple generations per season. Damage from this sawfly starts in the spring and only ends with the first frost. The expanding numbers with each new generation may produce heavy defoliation by the end of the season.
I found symptoms yesterday on hybrid tea and Knock Out roses in southwest Ohio that was consistent with damage caused by a roseslug sawfly; however, I could not find any larvae to make a positive identification. The culprit could have been the common roseslug and they have completed their development for the season. Or, the damage could have been produced by curled or bristly roseslugs and we are in between generations.
Early instar roseslug larvae feed by removing one leaf surface and the mesophyll beneath. The corresponding epidermis on the opposite leaf surface remains intact and turns white producing a characteristic "windowpane" symptom. Eventually, the "windowpanes" drop out to produce holes. Later instars feed between the main veins to directly produce holes in leaves. Heavy feeding damage by early and late instars may combine to produce "see-through" leaves. We have commonly observed this type of damage from bristly roseslugs over the past few years in southwest Ohio.
You must look closely to spot the pale green semi-transparent sawfly larvae. Despite their common name, the larvae of roseslug sawflies resemble tiny caterpillars and look nothing like the glistening, elongated pear-shaped "slug sawflies" which do resemble tiny slugs. As their common name indicates, bristly roseslug sawfly larvae are covered with short, hair-like bristles that can be best seen with a hand-lens.
Control and prevention of further damage depends on a proper identification of the roseslug culprit. Only the bristly roseslug is worthy of control measures because it continues to produce damage throughout the season. Biorationals such insecticidal soaps are effective, but direct contact is necessary. Products containing spinosad (e.g. Conserve, Entrust) are effective against sawfly larva and will also have less impact on bio-control agents. Chlorantraniliprole (e.g. Acelepryn) is also effective and presents a low risk to pollinators. Soil drench applications of systemic insecticides such as imidacloprid (e.g. Merit) or dinotefuran (e.g. Safari) are effective and provide lengthy protection. Although roseslug larvae look like caterpillars, products based on strains of the bacterium Bacillus thuringiensis (Bt) that are specific to controlling moth caterpillars (order Lepidoptera) will have no effect on these primitive hymenopteran larvae.