October 29, 2015 — Kenosha News
Question: I was wondering if you could tell me the best way to prevent the Asian beetles from eating my roses. Every year they return. This year I tried Neem oil without success. Thanks for your time. J.S.
Answer: I think you are referring to Japanese beetles. Japanese beetles are 1/2 inch long, metallic green with copper colored wing covers. (Asian multicolored ladybird beetles which are a type of ladybug and don’t eat roses.) Japanese beetles appear in late June to early July and can be found in our yards into September.
Japanese beetles devour the foliage and flowers of more than 300 plants but roses, lindens, grapes and raspberries are favorites. Damaged leaves look like brown lace.
Early emerging beetles attract later emerging beetles to your yard so removing and destroying the first beetles you see in June can reduce later populations and reduce feeding damage on your plants.
For lesser populations of Japanese beetles or on smaller plants such as roses, I prefer hand picking and drowning the bugs in soapy water. Many foliar applied insecticides will kill Japanese beetles. Neem oil (azadirachtin) and spinosad (spinosyn) are both effective organic insecticides for Japanese beetles. – Recommended synthetic insecticides contain carbaryl, acephate, cyfluthrin, rotenone, or bendiocarb. – Repeat sprays are necessary for organic and synthetic insecticides. Follow the label directions. Often you need to apply every 5 to 7 days for several weeks. Only heavily attacked or highly visible smaller plants should be treated. Healthy untreated trees and shrubs will survive feeding damage.
Systemic insecticides, which are applied in spring and taken up by the roots, work fairly well for killing Japanese beetles eating foliage. However, systemic insecticides don’t translocate well into the flowers so aren’t helpful in reducing flower damage.
Do not use Japanese beetle traps which draw more beetles into your yard than would be present without the trap. Traps may be used in a large landscape to attract beetles away from the garden or specimen plants.
I’ve dealt with Japanese beetles in my own yard and, as a horticulture educator, observed Japanese beetle population patterns for 20 years. When Japanese beetles move into a neighborhood, the populations can be extremely high for two to four years. After that, the beetles don’t disappear but the numbers and damage are much lower. So don’t give up on your roses.
Barb Larson is horticulture educator for the Kenosha County University of Wisconsin Extension. She holds a master’s of science in horticulture from the University of Wisconsin-Madison. If you have a plant or gardening question, email her at firstname.lastname@example.org or call 262-857-1942.