It never ceases to amaze me how all it takes is one rose in a garden in order for the Bristly Roseslug to find it and attack. The roseslug is the transparent (when young) or green (when older) larval caterpillar of a sawfly (wasp), Endelomyia aethiops. The larvae are no more than one-half inch in length and, if you’re lucky, can be found while eating leaves and promptly squished. Younger larvae skeletonize the leaves, as seen in the photo at left, while older larvae chew holes in the leaves, often along the leaf edges.
In addition to hand-picking, you can try spraying the plant with a strong jet of water. Once the larvae are knocked off the plant, they can’t climb back up. For protection, try a 2%-3% solution of horticultural oil in water as long as the plant isn’t in bloom. Treatments for severe infestations involve systemics, such as acephate (Orthene) or carbaryl (Sevin), which are nerve poisons. Systemics should be used with extreme caution, and only if treating for other pests where damage can be severe, such as Japanese beetle.
Roseslugs tend to emerge early in the season (late May, early June) and have only one generation per year in Zones 5-6. Careful monitoring, combined with hand-picking or water blasting, should minimize seasonal damage.