Despite making it through a brutally cold winter, knockouts and other rose varieties are being eaten by the larvae of sawfly stated Horticulture Educator, Kelly Allsup. The larvae, not a slug, is yellow- green and velvety looking, 1/2" long, found on the underside of the rose leaves or in flower buds. The leaves are left riddled with feeding holes between veins. They excrete slimy substance all over their bodies resembling slugs but are not slugs. After larvae feed they drop to the ground to pupate. Adults emerge in spring and lay their eggs on the bottom sides of rose leaves
Reports of rose slugs damaging roses throughout central Illinois have surfaced for the past few weeks. Inspect the roses in the garden for this pest. For light to moderate infestations of this garden pest are not detrimental to plant health and do not need chemical option. Heavy infestations can cause plant damage and should be addressed with mechanical and chemical options.
For light to moderate infestations use forceful water sprays to knock off pest or remove damaged leaves and larvae.
For moderate to heavy infestations apply chemicals with the active ingredients of spinosad, bacillus thuringiensis kurstaki, or potassium of fatty acids aka insecticidal soap. The active ingredients will on the label of the chemical. To avoid killing bees and other pollinators, don't apply these pesticides to flowers and treat at night.
For more information on roses and rose pests visit University of Illinois Extension's "Our Rose Garden" website at http://urbanext.illinois.edu/roses. It is also available as a mobile app at http://web.extension.illinois.edu/state/apps.cfm. The rose garden app features information about various roses, how to plant and prune roses, how to protect roses in the winter, a gallery to keep track of favorite roses with notes, information on diseases and pests, and includes several University of Illinois Extension YouTube videos about rose care.