Rose slugs are numerous this year throughout Illinois, causing rose foliage to be lacelike. The green larvae are more numerous on the undersides of the leaflets. Although causing window-feeding when young, the larvae are now large enough to eat holes in the leaflets and even cause defoliation.
Bristly rose slug larva and damage on rose.
The bristly rose slug is green, with fine, hairlike spines, and grows to about 1/2 inch long. Rose slug looks like bird manure when young but when older looks like the bristly rose slug without the bristles. Both are present at this time of year. Our reports indicate that the bristly rose slug is the most common this year.
Realize that although these insects look superficially like caterpillars they are sawfly larvae, and Bacillus thuringiensis kurstaki will not be effective against them. Acephate (Orthene), bifenthrin (Talstar), carbaryl (Sevin), and cyfluthrin (Tempo) are effective. Insecticidal soap will also be effective with very good coverage. Avoid getting the insecticide on flowers, although most rose varieties have had the nectar and pollen bred out of them and are not attractive to pollinating insects. Species roses and particularly some single-flowered varieties will attract pollinators, which could be killed by insecticide sprays on the blooms. Because carbaryl is more likely to cause bee kills, avoid spraying blooming roses being visited by pollinators with Sevin. (Phil Nixon and Morton Arboretum)