FRANKLIN LIVING JULY-AUGUST 2024—

When it comes to the Extension System, regional agents can offer advice for tackling all kinds of issues. One of the issues facing home gardeners this summer is threatening knockout roses.

“I have gotten several calls about rose slugs and sawfly larvae affecting knockout roses in the last week or so,” explained Jayne F. Luetzow, a regional Extension agent for home grounds. “Knockouts are generally pretty pest free but are being affected by these guys.”

Knockout roses are a trademark variety first introduced about 20 years ago, cultivated by rose breeder Will Radler. They are marketed as an easy-to-grow shrub with a long season of blooms. “They can fit into any landscape in most climates,” asserts knockoutroses.com. “Plant them individually as a focal point, among other shrubs, or in large groups to create a colorful hedge. Incorporate perennials and annuals to create a season of color, texture and interest.” A range of colors are available, and the website shares advice on care, from planting and fertilizing to pruning and pests.

When it comes to pests, these roses “are not pest-resistant, but they are extremely tough so even if bugs get after them, they should be fine,” the website reassures. Rose slugs and sawfly larvae can, however, be a threat.

The Extension offers the following control recommendations:

1. Handpick or physically remove rose slugs or sawflies found on the plant, if infestation is not very bad. Tiny, green inchworm-looking critters are the culprit you’re looking for; check the undersides of leaves.
2. Prune out the damaged parts of the plant and throw them in the garbage. “Do not compost,” Luetzow warned. “It is not the best time of year to prune; however, whenever you have a disease or pest, it is best to prune it out to help keep it from spreading.”
3. Use an insecticide. “Always read the label,” Luetzow said. “As a note, slug baits and BT (Bacillus thuringiensis) are ineffective against these guys.” Instead, the Extension recommends horticulture oils and insecticidal soaps, which are safest for the environment and pollinators, or one of the following: Acephate (Orthene); Bifenthrin (Talstar); Carbaryl (Sevin) – “Be careful, as carbaryl can be toxic to our pollinators,” Luetzow warned; or Cyfluthrin (Tempo).

Ultimately, the shrub website says knockout roses should be able to withstand any invasion by these pests. “Rose slugs will chew the leaves of plants, leaving trails where they’ve munched through, but they won’t be detrimental to the overall health of your Knock Out® Roses.”

For more advice from a local Extension agent, call the Franklin County Extension at 256-332-8880.