by Kathryn Hamilton, EMGV
At this time of year, Japanese Beetle “Season,” my favorite gardening tool is a plastic fork. When disrupted, beetles are supposed to fold their hind legs and fall. You are supposed to be able catch them with a container of soapy water. In my experience that’s true only about half the time. Sometimes they must be pointed in the right direction; other times they need to be fished out from between the layers of a rose petal, and at still other times, they must literally be pried off the flower. Apparently, even Japanese beetles have a survival instinct!
Japanese beetles (Popilla japonica) are attracted to the foliage, fruits, and flowers of nearly 300 different plants, among them: roses, crape myrtle, hibiscus, purple-leafed plums, grape leaves, and geraniums. If you are a Piedmont gardener, chances are very good that you have encountered these pests in recent weeks.
The good news is that Japanese beetles are unlikely to destroy established trees or shrubs. Skeletonized leaves and flowers will grow back once the beetles disappear. The better news is that within 30 – 45 days of their onset they will be gone.
Here are some takeaways to help you cope with Japanese Beetle “Season.”
1. Only one generation occurs each year. They typically emerge in early June and are gone by mid-July. You may see an isolated beetle during the rest of the year but ground zero is late May until early July.
2. Beetles emerge when the temperature is “just right.” Scientifically this equates to approximately 1,000 growing degree days. (Here’s a scientific explanation of growing degree days.) If the weather heats up faster beetles are likely to appear sooner. Weather conditions also determine the grub population, their larval stage. Damper weather typically means more grubs. More grubs mean more beetles.
3. Japanese beetle traps should not be considered control devices. Designed to attract beetles, rather than trap them, they can increase the beetle population in your back yard. Furthermore, if not emptied every couple of days, the beetles will rot inside, releasing an ammonia which repels them. Instead of going into the trap, they are likely to tap your hibiscus.
4. Japanese beetles aggregate in response to odor released by damaged plants and a pheromone released by female beetles. I usually cut the least-damaged roses and leave one or two that have been attacked. They are always one of my best beetle-harvest sites.
5. Where practical, cover the plant with light netting.
6. Just say “no” to things they don’t like. But who could imagine a garden without roses?
7. Rose experts advise picking your roses and bringing them inside. They can beautify your property the rest of the year.
8. Another tip, before cutting a rose to bring inside, be sure the sepals have fallen or the rose will not bloom.
9. You can, of course use insecticides, a discussion of which is outside of this posting. However, any insecticide will have some negative effect on other insects, including those which are beneficial. The North Carolina Agricultural Chemicals Manual lists pesticides and their relative safety for bees. Should you opt for an insecticide, prune all the flowers first. There are label restrictions against using most insecticides on flowering plants and when pollinators are present. Read the label for detailed restrictions.
10. Insecticides only treat the exposed petals. So, if a bud opens throughout the day, the unprotected petals are just another meal. And, unless they are systemic, insecticides must be reapplied after a rain.
My bottom line strategy: I cut my best roses and leave a few that have already been attacked as traps. Several times a day, I am single-mindedly devoted to search and destroy missions, first dumping the beetles into a container of water soapy water then finishing with a flush down the toilet.
Japanese Beetles on Ornamental Landscape Plants
Insects: Japanese Beetles