Colorado roses have been attacked by Japanese beetles for several years, leaving behind ravaged foliage and deformed flowers. Curiously though, some gardeners report that their roses suffer little to no damage from the insect. Are some rose cultivars less enticing to the Japanese beetle than others? This was the focus of Colorado State University’s 2016 observational study conducted at Littleton’s War Memorial Rose Garden. In the first year of this multi-year study, the following rose cultivars were not found to be damaged by Japanese Beetles: Angel Face, Debut, Hondo, Joseph’s Coat, Mardi Gras, Picotee, Popcorn, Prima Donna, Ralph Moore, Singin’ in the Rain and White Lightnin’.
Conversely, the following cultivars were observed to have the highest levels of Japanese beetle destruction: Pink Promise, Honey Perfume Whisper, Love and Peace, Day Breaker, Strike it Rich, Cherry Parfait, Eureka, Starry Night, Rainbow Knock Out, Lady Elsie May, Carefree Delight and June Lover.
Adult Japanese beetles destroy flowers at the same time bees are gathering pollen, making their impact even more significant. For this reason, CSU’s study identified roses with high Japanese beetle susceptibility and high visitation by bees. Topping this list were Rainbow Knock Out, Lady Elsie May and Strike it Rich. Also in this group were Prominent, Home Run, Easy Does It, Apricot Nectar, Gemini, Starry Night, Baby Boomer, Sweet Diana, Julia Child, Cathedral, Betty Boop, Mon Cherie and Cloud Dancer.
Early findings suggest that gardeners may be able to lessen damage to roses by planting cultivars that are less attractive to Japanese beetles. It also underscores the importance of close monitoring and care of plants to reduce the effects on pollination. In the future, additional information on why some cultivars are preferred over others is likely to emerge.
This CSU publication provides comprehensive information on caring for plants infested with Japanese beetles, the larvae stage effecting turf and more. An additional bit of advice – it has recently been found that crushing the Japanese beetle does not attract more beetles. So, while we’ve previously been advised to handpick and drown the insect in soapy water, feel free to stomp on them too. Since they are night feeders, they are easiest to find around dusk, when they are about to feast on your plants.
Written by Linda McDonnell, a Denver County Master Gardener
Note: The study cited in this post was presented to the Denver County Master Gardener Association by Dr. Whitney Cranshaw of Colorado State University in May 2017.
Photos courtesy of http://www.pixabay.com