In our blog on May 7, 2020 I wrote about the PW Oso Easy Italian Ice Rose as the winner of Shrub Madness 2020. On June 16, 2016 I wrote about “June is National Rose Month (Part I)”…..reminder to us we need to continue that series. This article was on the many selections of Oso Easy Landscape Roses. Low maintenance shrub roses might be the biggest development in the landscape in the last 20 years. Whether they are Oso Easy, Knockout, Flower Carpet or Drift Roses, these plants add season long color without the maintenance of your typical, older type roses like Hybrid Tea, Floribunda and English Roses. Shrub roses were bread to withstand typical rose diseases such as black spot and powdery mildew. They also need very little pruning. We recommend cutting them back with a shear to about 12” to the ground in the late Fall. These plants are typically insect free as well.
Several years ago, I removed six Knock Out Roses due to two reasons: 2 years in row these plants were fed on by Japanese Beetles and also because they were receiving less and less sun due to 2 ornamental trees. I removed them and replaced with a Bigleaf Hydrangea.
I have a bed of mixed Oso Easy Italian Ice Rose and Pink Carpet Rose that usually looks fabulous this time of year. This year, with the late frost in May and the lack of rain they have just not gotten to their usual mature size. I have been applying rose fertilizer as usual, and yesterday noticed the plants were having issues with the leaves. After careful inspection, found several of these small caterpillar-like insects eating on the leaves.
After some research, I discovered they are actually rose slugs. They are not a true slug, but the larval stage of flying insects known as sawflies. They secrete a slimy substance over their body surface that makes them resemble small slugs. Rose sawflies are yellow-green in color and can grow to a ¾ inch maximum length. As their names suggest, rose sawflies feed on the leaves of rose shrubs, although they can also feed on, cherry, plum, ash, hawthorn, cotoneaster, and other species.
Sawfly larvae feed on the surface of leaves of their respective host plant, removing the soft tissue leaving behind the papery, translucent surface and veins. Heavy defoliation gives plants a brown scorched appearance. In general, light to moderate infestations are cosmetic in nature and rarely harm the host plant. Heavier attacks, however, can weaken plants when leaf loss stresses them to the point of vulnerability to other insect and disease attacks.
The Missouri Botanical Garden suggest these Integrated Pest Management Strategies.
- Check plants for signs of infestation. Early detection can often result in simple cultural control measures. Begin looking for sawfly larvae in mid-spring (rose sawflies). Inspect both upper and lower surfaces of the leaves. For light infestations, remove the infested leaves and destroy the larvae. A forceful spray of water out of a garden hose can also provide control by knocking off and killing many of the soft-bodied larvae. Be sure to aim the water at both upper and undersides of leaves. Continue checking plants throughout the growing season.
- Support natural enemies of sawflies by responsible pesticide usage. Insects such as parasitic wasps, insectivorous birds, small mammals, predaceous beetles, as well as fungal and viral diseases all assist in keeping sawfly populations lower. Restraint in the use of pesticides allows beneficial species to assist your control efforts.
- Use an Insecticide. Chemical controls are also available, but should only be used when necessary, not routinely as a preventive measure. Horticultural oil, insecticidal soaps, neem oil, bifenthrin, carbaryl, malathion, permethrin, cyfluthrin, imidacloprid, and acephate can all be used to control sawflies. Apply pesticides only when larvae are actually present, before infestations reach critical levels. Always be careful to read the label directions fully before applying any pesticide, and follow directions completely. Not effective: Bacillus thuringiensis (Bt), a commonly used biological insecticide that offers control of many caterpillars, is NOT effective against sawfly larvae.
I decided to take immediate action last night by dowsing the plants with Bonide’s Rose Rx 3 in 1 Concentrate. The larvae were killed on contact last night as I brought one into the workbench to study. I returned this morning only to find more. I am assuming these were either on the under side of the leaves that I missed or were new larvae from the ground. Now that I am aware of the issue, close monitoring will take place to make sure the already weakened plants from the late frost will not be decimated from these leaf eating insects.
Bonide’s Rose Rx 3 in 1 Concentrate is an oil extract from the Neem Tree. The bodies of these insects absorb the neem compounds as if they were the real hormones, but this only blocks their endocrine systems. The resulting deep-seated behavioral and physiological aberrations leave the insects so confused in brain and body that they cannot reproduce and their populations plummet. The National Research Council have found that the freely feeding and caterpillar-like larvae of sawflies are target insects as well for this product. In this group, neem’s antifeedant and growth regulatory effects are both important.