How to Control Rose Slugs

These tiny maggots can be hard to see, but they can do crazy damage to your rose bushes.

by Jessica Walliser
PHOTO: Jessica Walliser

I don’t often face pest issues in my garden, but when I do, I manage them organically. So far this year, I’ve only spotted a handful of Japanese beetles, and there’s been nary a Colorado potato beetle in sight, making it a pretty good year for the vegetable garden (at least as far as pests are concerned).

However, my two roses have faced a big challenge. They spent the better part of this summer being destroyed by some very hungry little green insects. By the time I noticed the damage, the leaves were skeletonized with nothing but their mid-ribs remaining. The little green insects are a type of sawfly larvae that feed primarily on roses.

With tiny green bodies that measure a mere 1/8 to 3/4 inch and light-brown heads, they are caterpillar-like in appearance, but aren’t true caterpillars. (Officially speaking, they’re maggots because they are larval flies.) Although there are sawfly species that attack many different plants—from hibiscus to pines to hollyhocks—this particular species is commonly known as rose slugs.

Rose slugs are most frequently found on the undersides of leaves, and because of their small stature, they can be difficult to find. Once I spotted the culprit, I began hand squashing them in earnest, but because they’re so tiny, it takes a huge amount of time and energy to control them in this manner. In other seasons, I also relied on several beneficial insects, which parasitize and prey upon sawfly larvae to help me manage their populations by always inter-planting my roses with plants like sweet alyssum, wallflower, lemon balm, cilantro and other flowering herbs. These plants attract many species of parasitic wasps and tachinid flies that naturally control sawfly larvae by using them as hosts for their own maturing young. But this year, we had to pull two massive shrubs out of that bed, and I never got around to planting any sweet alyssum and cilantro around my roses. I suspect that’s been a big part of the problem.

When hand-squashing wasn’t effective, I turned to an organic product control with the active ingredient spinosad. I only ever use these products as a last resort, when the damage is extreme and the health of my plants is truly at risk. Spinosad is a fermented bacterial product that is labeled for use on many common garden pests. I am always careful to follow label instructions and apply any products only when bees are not active (early morning or late evening). For spinosad to work against sawfly larvae, the tops and bottoms of all leaves must be covered.

Today marks three weeks since I made that single application of spinosad. My roses now look beautiful and have even managed to re-grow a whole new set of leaves. They’re blooming their heads off, and I haven’t spotted a single sawfly larva since. Success!

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