I posted a BYGL Alert on May 4 titled, “Roseslugged Roses.” The subject was the European Roseslug (Endelomyia aethiops, family Tenthredinidae, order Hymenoptera). You can read the posting by clicking this hotlink: https://bygl.osu.edu/node/2630
This Alert focuses on another non-native, the Bristly Roseslug Sawfly (Cladius difformis (= C. pectinicornis). Although this sawfly and the European Roseslug have “slug” in their common names, they don’t share the slug-like body plan of other “slug sawflies,” and the larvae don’t cover themselves in shiny, slug-like slime.
Some online resources state that the roseslugs prefer one side of the leaves over the other. In general, the bristly roseslug is most often found on the lower leaf surface. However, I’ve observed this sawfly as well as the European roseslug feeding on both the upper and lower leaf surfaces. This means inspections must include both sides of the leaves.
The slug sawflies consume all of the leaf tissue except the leaf veins and the epidermis on the opposite leaf surface. The intact leaf epidermis may turn white, causing leaves to appear bleached, or leading to a subtle “windowpane” effect. The effect is most dramatic when the sawfly larvae feed on the lower leaf surface. Eventually, the dried epidermis drops from the leaf, leaving holes.
Both roseslugs have brown to reddish-brown head capsules. The European roseslug has a semi-translucent, yellowish-green body, free of obvious bristles. The bristly roseslug sawfly’s body is also semi-translucent, light green, and covered with short bristles, as the common name implies. This sawfly can also cause damage to raspberries and strawberries.
However, the most important difference between the two sawflies is the number of generations per season. The European roseslug only has one generation per season. Based on what I’m seeing on Knockout roses in southwest Ohio, this sawfly is finished for the season. Although their damage remains obvious, it’s mostly confined to the lower portions of the plants, and the injured leaves are being shed or covered by new growth.
The bristly roseslug sawfly has multiple generations per season. The exact number in Ohio has not been documented. However, generations eventually overlap, resulting in almost continuous feeding throughout the season by different instar larvae. In the past, I’ve observed the cumulative damage producing “see-through” rose plants.
Management depends on accurately identifying the true sawfly culprit. Only the multi-generational bristly roseslug is worthy of control measures. However, applying topical insecticides to blooming roses is problematic.
Contrary to popular belief, roses with open blooms do attract pollinators, particularly hoverflies (family Syrphidae). Keep in mind that Diptera (flies) is the second most important insect order of insects that visit and pollinate flowers worldwide, second only to Hymenoptera (bees and wasps), and ahead of Lepidoptera (moths and butterflies).
Hoverflies are common visitors to rose blooms, and they provide a twofer. The adults are pollinators, and the larvae are effective predators of soft-bodied insects, particularly aphids.
Foliar sprays with “standard” insecticides (e.g., pyrethroids) risk killing non-target insects, including beneficials like hoverflies visiting flowers. Even biorational insecticides that have lower impacts can present a risk.
For example, products containing spinosad (e.g., Conserve, Entrust) are effective against sawfly larvae and have a lower impact on bio-control agents. Chlorantraniliprole (e.g., Acelepryn) is also effective and presents a minimal risk to pollinators. However, even though both are low-risk insecticides, they are not no-risk insecticides. They can have significant sub-lethal impacts on hoverflies.
Soil drench applications of systemic insecticides reduce the risk of direct impacts on non-targets like hoverflies. Lengthy protection is provided by products with the active ingredients imidacloprid (e.g., Merit), dinotefuran (e.g., Safari), and thiamethoxam + cyantraniliprole (e.g., Mainspring Xtra).
Horticultural oils are effective but require direct contact. The same is true for insecticidal soaps (e.g., M-Ped). However, don’t mix your own insecticidal soap! DIY soap concoctions frequently include ingredients that are not tested and labeled for application to plants and commonly produce phytotoxicity. On the other hand, they can control plant pests by killing infested plants.
Finally, while sawflies may look like caterpillars (Lepidoptera), they are related to wasps, bees, and ants (Hymenoptera). Products based on the bacterium Bacillus thuringiensis var. kurstaki (Btk) that kill caterpillars will not affect sawfly larvae.
Selected References
Dunn, L., Lequerica, M., Reid, C. R., & Latty, T. (2020). Dual ecosystem services of syrphid flies (Diptera: Syrphidae): pollinators and biological control agents. Pest management science, 76(6), 1973-1979.
https://scijournals.onlinelibrary.wiley.com/doi/abs/10.1002/ps.5807
Marchiori, C. H. (2022). Life history of Tenthredinidae family (Insecta: Hymenoptera). Int J Sci Technol Res Arch, 2(1), 68-081.
Hong, C., and Day, E. (2026). Horticulture & Forest Crops, 2026 Pest Management Guide. Virginia Cooperative Extension, Virginia Tech, Virginia State University, Pub. 456-017.
www.pubs.ext.vt.edu/content/dam/pubs_ext_vt_edu/456/456-017/ENTO-635.pdf
Marchiori, C. H. (2022). Life history of Tenthredinidae family (Insecta: Hymenoptera). Int J Sci Technol Res Arch, 2(1), 68-081.
Raguso, R. A. (2020). Don’t forget the flies: dipteran diversity and its consequences for floral ecology and evolution. Applied Entomology and Zoology, 55(1), 1-7.
https://link.springer.com/article/10.1007/s13355-020-00668-9
