Most Knockout roses in our area have bounced back from the winter kill by resprouting from the base causing the plants to be smaller and more compact this year. However few, some in our area had to be replanted the one’s that didn’t had a little help from Mother Nature with their garden pruning duties. A good renewal pruning can rejuvenate the knockout roses and keep their flowers numerous and shape compact. Renewal pruning is removing all the canes to 1-2’ off the ground in early spring.
Despite making it through a brutally cold winter, knockouts and other rose varieties are being eaten by the larvae of sawfly. 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
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.
Rose Slug Picture Phil Nixon
It may be important to identify what kind of sawfly is infested your rose bush because some have more generations per season than others.
European roseslug sawfly (Endelomyia aethiops) produces only one generation per year
Bristly roseslug sawfly (Cladius difformis), can produce two to six generations per year. Bristle like hairs covering the body. Looks like bird droppings in first larval stages.
Curled roseslug sawfly (Allantus cinctus) generally produces two generations per year. Curl up the body when at rest.
For more information on organic pest management please visit University of Illinois Horticulture Blog called Flowers, Fruits and Frass at http://web.extension.illinois.edu/lmw/eb255.