Japanese Beetle Resistant Roses

May 15, 2021Blaine HowertonGardens & Landscapes0

Rita Jokerst, Horticulturist, Gardens on Spring Creek

Japanese beetles are the scorn of any rose gardener, and we at the Gardens on Spring Creek are disappointed to report these pests are officially here in the Northern Colorado area. These concerning beetles do not just target rose gardens – they attack a wide variety of landscape, edible, and ornamental plants as they damage plants in two different ways. 

The larvae (or grubs) feed on the roots of turfgrass, thereby producing drought-stress symptoms in large swathes of off-color, unhealthy-looking lawn. The grubs’ presence in turn attracts further damage by other critters. Skunks, raccoons, and many birds will dig into lawns infested with Japanese beetle larvae to feed upon them. Secondly, the highly mobile adults damage plants above ground, chewing on the leaves and flowers of many, many plants. So, what preemptive actions can a gardener take against these pests? Follow the data and plant wisely!

In 2016 and 2017, Colorado State University Professor Whitney Cranshaw evaluated Japanese beetle damage on roses at the War Memorial Garden in Littleton, Colorado. Over the course of a growing season, seven observations were made and included beetle damage, ranked on a scale of 0-3 (no damage to heavy damage) and flower visitation by bees, ranked 0-3 (no visitation to high visitation). Studying both the beetle damage and how preferred a plant is by bees is important because many of the go-to insecticides that will successfully control the beetles can also harm globally declining bee populations. 

Below are some takeaways from Dr. Cranshaw’s research. “Not recommended” roses had both high levels of beetle damage and high bee visitation, making Japanese beetle control difficult and insecticide application unwise. “Maybe” roses had no bee visitation and varying levels of beetle damage, meaning they could be effectively treated with insecticidal controls without risking negative impacts to our local bees. “Recommended roses” had no beetle damage, therefore not in need of any interventive action as Japanese beetles move into our region. Asterisked roses can be found on the grounds at the Gardens on Spring Creek.