One of the biggest mistakes people make when purchasing any of the Knockout Rose varieties is to think they are immune to problems.
Knockouts are highly adapted roses that have been cultivated to be highly resistant to both pests and diseases but can still suffer from issues like the rose slug.
Rose slugs (aka roseslugs or roseslug sawflies) are actually not slugs at all but sawfly larvae.
Three different species are known to target roses:
- Bristly Roseslug (Cladius difformis) – ¾” inches long and usually pale green covered in bristles; reproduces multiple times per year and usually feeds on the undersides of leaves
- Curled Roseslug (Allantus cinctus) – nearly 1” inch long and pastel green, which eat holes into the leaves; named for their habit of coiling under the leaf they’re feeding on
- European Roseslug (Endelomyia aethiops) – A little under 1/2 “ inch and prefers feeding on the tops of leaves; common in Europe but may infest unchecked plants shipped to the US
Due to how these larvae feed by either eating holes into the rose leaves or only eating a layer (resulting in brown spots), the damage can often be mistaken for disease.
If you suspect your Knockout Rose bushes are infested with rose slugs, treating the plant quickly is important.
Left alone, these “slugs” (rose sawflies) can defoliate the plant or severely weaken it to the point it becomes vulnerable to other pests or diseases.
More on Knockout Rose Diseases
How to Get Rid of Rose Slugs on Knockout Roses
There are several ways to combat these pests.
Here are some of the most common remedies and some pros and cons of each.
Chemical Insecticides
In a pinch, many gardeners turn to chemical options, and there’s no doubt that chemical control insecticides can stop a rose slug infestation in its tracks.
Unfortunately, it can also kill beneficial insects, such as pollinators, ladybugs, and parasitic wasps.
It can also produce superbugs over time, so this remedy should be used only cautiously.
insecticides with imidacloprid or acephate, cyfluthrin, bifenthrin, carbaryl, malathion,
Hand-Picking
If you don’t mind a little close contact, you can pluck the larvae off with your fingers or a pair of tweezers, dropping them into a bucket of soapy water.
This method is relatively effective if you have the time, but it’s too much work for most gardeners to pick through each leaf looking for tiny, well-camouflaged pests.
Hosing Down
Sometimes, you can kill two birds with one stone – or, in this case, two care tasks with one garden hose.
Simply use a pressure nozzle or stick your finger over an open hose and spray down the leaves, allowing the water pressure to knock the rose slugs off.
Obviously, this method can result in a bit of overwatering if you take too long, and the larvae aren’t actually harmed, so this method may not be the best solution unless you’re just trying to buy a little time until a pesticide or other product arrives.
Maintain Your Roses
Sometimes, simple grooming can do more for your plant than you realize.
Removing damaged and diseased leaves and branches makes the plant less attractive to adult sawflies.
Likewise, proper fertilization and mulching can make it more difficult for sawflies and their larvae to find a home.
While this won’t necessarily stop an existing infestation, proper care of your roses can greatly reduce the risk of a new infestation.
Insecticidal Soap and Neem Foliar Sprays
These two options can be great remedies but are a little time-consuming.
Neem foliar sprays work by suffocating insects on contact and dissipate in 45 minutes to an hour, leaving behind no residue.
When applied around dusk or dawn every other day for 14 days, it can eliminate many plant pests, including rose slugs.
Insecticidal soap is often added to the neem spray or used instead of neem.
In both cases, all plant surfaces must be thoroughly saturated for the best results, which may be a bit more work than many gardeners would like.
However, the neem can also combat surface fungal infections, so it’s not a bad idea to spray your roses every 2 weeks as a preventative.
Neem Soil Soak
Perhaps the king of natural remedies, neem soil soaks employ pure, unmodified neem oil to create a soil drench that is almost too good to be true.
As the roots soak up the neem, its main ingredient, Azardchtin, permeates the plant sap and becomes a systemic pesticide lasting up to 22 days.
This won’t harm beneficial insects (even those in the soil) but will poison any pest that chews on the leaves or drinks the sap.
It can also fight off several forms of microbial and fungal infection.
While this method doesn’t kill the rose slugs instantly, it prevents them from reaching adulthood, causes sterility, and even makes them starve to death.
It can take a couple of weeks to see the results, but those results are devastating and won’t lead to superbugs like chemical equivalents.
Apply every 2 to 3 weeks as a preventative.