Sawfly Larvae on Roses
In the tradition of the world’s great dynasties, centuries of breeding and pampering have established roses (Rosa spp.) as botanical royalty. Today, thousands of rose cultivars bring their star quality to sunny, well-drained, organically rich soils in U.S. Department of Agriculture plant hardiness zones 2 through 10. Throughout that range -- and despite their pedigree -- roses face leaf-munching sawfly larvae as invasive in the garden as paparazzi at a royal wedding.
Although they resemble caterpillars more than slugs, bristly roseslugs (Cladius difformis) are nether one. They're the offspring of rose sawflies, tiny, stingerless wasps named for the adult females' saw-toothed egg-laying appendages. The shiny-black, orange-marked insects use them to cut pockets along the edges of rose leaves before depositing a single egg in each one. Look for the adults beginning in early spring. Bristly roseslug wasps produce up to six generations each year, with the final one overwintering as pupae in the soil around the plants.
Bristly roseslugs have orange-brown heads and pale-green bodies covered with short, stiff hairs. Newly hatched larvae strip the green, chlorophyll-containing cells from a rose's foliage. If you notice tiny, transparent "windows" of skeletonized tissue scattered across the undersides of the leaves, suspect a bristly roseslug infestation. As the larvae mature, they chew completely through the foliage, causing large holes before dropping to the soil, pupating and emerging as adult wasps to repeat the cycle. Heavy or prolonged feeding may compromise a rose's photosynthesis, slowing its growth and reducing its vigor.
Starting in spring, inspect both sides of your roses' leaves regularly for bristly roseslugs. The simplest way to manage a small infestation is to prune the affected leaves, pick the pests off by hand and squash them. If you're squeamish and wearing gloves doesn't make the task more palatable, knock them off with a branch or a hose them off with a blast of water. Use the water option in early morning so your roses dry before nightfall. Otherwise, their wet foliage could invite fungal infection.
Insecticidal Soap Control
Control heavy sawfly feeding by spraying your roses with membrane-disrupting insecticidal soap. Make a soap solution of 2 1/2 tablespoons of non-detergent, liquid soap per gallon of water and spray heavily enough to coat both sides of the roses' foliage. Because the soap kills only the roseslugs it hits, additional applications may be necessary as new larvae hatch through the summer. To prevent potentially damaging soap buildup, hose the roses down with clean water two or three hours after spraying them.
- Manual of Woody Landscape Plants, Fifth Edition; Michael A. Dirr
- David Austen Roses: Climate Information
- University of California Statewide Integrated Pest Management Program: Rose
- University of California Statewide Integrated Pest Management Program: Sawflies
- Fine Gardening: Pest Patrol: Rose Slugs
- Sacramento Rose Society: Rose Sawflies
- University of California Statewide Integrated Pest Management Program: Seasonal Development and Life Cycle -- Sawflies
- Oregon State University, Pacific Northwest Nursery IPM: Roseslug
- University of Minnesota Extension: Insect Pests of Roses
Passionate for travel and the well-written word, Judy Wolfe is a professional writer with a Bachelor of Arts in English literature from Cal Poly Pomona and a certificate in advanced floral design. Her thousands of published articles cover topics from travel and gardening to pet care and technology.