A Rose Slug by Any Other Name

The rose slug, bane of rose gardeners in this area, is actually the larval stage of the European Sawfly (Endelomyia aethiops, formerly Caliroa aethiops). It is in the order Hymenoptera with bees, wasps, and ants.

If you grow roses in Fairfax, you either have rose slugs or you soon will. And those “problem-free” Knock Out roses are just as susceptible to rose slugs as any other rose.

May is the month when Rose slugs first appear.

The damage appears as irregular translucent or tan spots on the leaves. This is because the young, 1/2 inch-long larvae eat only the top surface of the leaf. Their mouthparts are not large enough to chew through to the other side. Eventually the damaged areas die, and the spots becomes holes. Enough holes and spots produce a very tattered rosebush.

Fortunately for us, there is only one generation of the European Sawfly every year. But the news is not all good on this front because there is a second type of rose slug that shows up a little later in the season to finish off what’s left of the rose foliage. It is called the Bristly Rose Slug (Cladius difformis),the larvae of a different sawfly. Typically, it shows up for dinner in late May and June. It is about 5/8 inch long and has hair-like bristles on the side of its body (you will need magnification to see these). This one chews the undersides of leaves and produces large holes until it consumes all but the biggest veins of the leaves. It can produce more than one generation in a season.

Rose slug infestations generally do not threaten the health of the plant. However, if you are fastidious about the appearance of your roses, you probably want to control the rose slugs. Some of the literature advises that you find the culprits and handpick them from the plants. Considering the extent of the havoc they wreak, rose slugs are not easy to find. I have spent hours scouting my roses only to find about ten of them on nine plants. I could have sworn there was a least a battalion at work judging from the damage they caused. Some experts advise washing the top and undersides of the leaves with a strong spray of water. I have tried that, too, with no noticeable effect.

The 2016 Virginia Pest Management Guide advises the use of insecticidal soap on rose slugs. Insecticidal soap is an effective organic control for soft-bodied insects. It is a contact insecticide; the soap must be sprayed on the rose slugs to have any effect. It has no residual effect on insects after it dries and is not considered a serious threat to beneficial insects. Use a commercially available preparation of insecticidal soap that contains potassium salts of fatty acids or potassium laurate as its active ingredient. The commercial formulations are tested and effective insecticides. Home-made mixes based on dishwashing detergents seldom work because most dishwashing detergents no longer contain potassium salts of fatty acids. The dishwashing detergents now contain more effective detergents for washing your dishes, but they have no insecticidal effectiveness.

The Pest Management Guide also advises the use of carbaryl (active ingredient in Sevin) and deltamethrin. Be careful with these. They work as both contact insecticides and stomach poisons for insects. They will harm beneficial insects if not applied properly. Follow the label directions for best results.

Some of you with experience controlling cabbage worms on broccoli, cabbage, and kale may be wondering whether the botanical insecticide Bacillus thuringiensis kurstaki works on rose slugs. The answer is no. Cabbage worms are caterpillars; rose slugs are not. Bt kurstaki only works on caterpillars.

So, while a rose is a rose, a rose slug is a nuisance.

References
A rose is a rose is a rose, The Phrase Finder
European Rose Slug, Virginia Cooperative Extension
Rose Insects and Related Pests, Clemson Cooperative Extension, HGIC 2107
Pest Management Guide: Home Grounds and Animals, 2016, Virginia Cooperative Extension, Section 4, Home Ornamentals