Japanese beetles (a.k.a. scarab beetles), those shiny metallic green mini-monsters, are a very destructive bug that can truly wreak havoc on plants, flowers, and roots in your garden. They begin feeding in mid to late June in temperate zones on more than 300 varieties of host plants, including field crops, ornamental trees and shrubs, garden flowers and vegetables, lawn turf, pastures, and golf courses.
One recent tool in the war against these invasive intruders is Japanese beetle traps, sold commercially and marketed to gardeners. The traps, however, can actually attract more beetles to an area than there were before, thus compounding rather than alleviating the problem. The long and short of it is that for most home garden applications, Japanese beetle traps are not a viable solution.
Unfortunately, the most effective Japanese beetle control method involves the use of harsh chemical insecticides but these can be dangerous to other insect species (including beneficial ones) as well as humans, wildlife, and pets. One major benefit to using traps is that the chemicals they contain won't harm plants, animals, or other insects. Another bonus is that they're designed to be hung above ground so that children and pets can't get at them. If safety is a major concern, you might want to at least give traps a try before moving on to more extreme measures.
How Japanese Beetle Traps Work
Most Japanese beetle traps consist of a ventilated bag or box which contains two chemical attractants: a sex pheromone and a floral lure. Japanese beetles spend their days feeding in groups and mating. The combined chemical attractants do an effective job of luring beetles in large numbers in about a .62 mile (1 kilometer) radius.
The major drawback is that according to studies, lure traps tend to attract far more beetles than they actually trap-about 25 percent more. In other words, when you hang a trap in your yard, you're inviting every Japanese beetle in the neighborhood but only three-fourths of them will end up in the trap itself. The beetles that avoid the trap will subsequently treat your well-manicured landscaping as a full-service buffet.
When Beetle Traps are Effective
Japanese beetle traps are not entirely without merit, however. They can be used effectively as a survey tool to determine whether the numbers of the pests in a specific area warrant control. They also work well for managing isolated beetle populations and have been found to be effective deterrents in those places in which a single owner is able to control a large area, such as an orchard. (A three-year test with mass trapping systems placed throughout blueberry and elderberry orchards in Missouri trapped 10.3 million adult beetles and decreased the number of adults on plants throughout the season from low to very low levels.)
Neighborhood associations can work together to control a Japanese beetle infestation but it takes cooperation and commitment. Beginning in mid-to-late June, if you and your neighbors hang traps throughout the infested area, you may be able to stop the bugs from migrating from yard to yard. Unfortunately, to be effective, the traps need to be monitored weekly at a minimum, as well as cleaned and maintained with fresh lures. Since trap cleaning is a fairly disgusting chore, if everyone doesn't keep up with their end of the bargain, it's far from a perfect solution.
Pesticides and Other Deterrents
If you decide to apply insecticides, you'll need to start when the beetles are first observed and you may need to reapply the pesticides multiple times throughout the season. In addition to pesticides, there are biological and physical controls you can try to lessen the Japanese beetle population, such as shaking visible beetles into a bucket of soapy water to drown them. You can also treat your lawn with a diluted solution of dishwashing liquid and water which forces the larval-stage beetles hiding underground to come up for air, making them vulnerable to birds and other predators.
Japanese beetles can sometimes be selective with what they eat. If you're planning a landscape design, select plants that the scarabs don't have a taste for. Plants most resistant or unattractive to the Japanese beetle include American bittersweet, dogwood, forsythia, hydrangea, lilac, paper birch, pine, silver maple, spruce, white poplar, and yew. If you plant enough of these, it may be an incentive for beetles to find somewhere else in the neighborhood to dine.
If you have existing plants that are Japanese beetle favorites, you might want to consider if it makes economic sense to remove and replace them, versus treating them with chemicals. For example, if you have a flowering cherry tree, consider replacing it with a Kousa (Japanese) dogwood; if you have a linden, plant a red maple instead.
Biological Warfare: Geraniums and Nematodes
Planting geraniums as sacrificial victims for your Japanese beetles can be another effective deterrent. Scarab beetles are attracted to geranium petals and eating them is an intoxicating experience. So intoxicating, in fact, that the blissful beetles become paralyzed and are easily consumed by predators. Those that shake off the stupor will simply return to gnaw on your geraniums again, often to the exclusion of other, less toxic plants.
Insect warfare, in which nematodes-specifically Heterohabditis bacteriophora and Steinemema glaseri-are introduced to garden soil is another method worth considering. Nematodes actively seek and attack groups of grubs, however, they must be applied in August, near dawn or dusk to be effective.
- Adesanya, Adekunle W.; Held, David W., and Liu, Nannan. "Geranium Intoxication Induces Detoxification Enzymes in the Japanese Beetle, Popillia Japonica Newman." Pesticide Biochemistry and Physiology 143 (2017): 1-7. Print.
- Knodel, Janet J.; Elhard, Charles, and Beauzay. Patrick B. "Integrated Pest Management of Japanese Beetle in North Dakota." North Dakota State University Extension Service, 2017. Print.
- Oliver, J. B., et al. "Insecticides and Their Combinations Evaluated as Regulatory Immersion Treatments for Third-Instar Japanese Beetle (Coleoptera: Scarabaeidae) in Field-Grown and Containerized Nursery Plants." Journal of Entomological Science 52.3 (2017): 274-87. Print.
- Piñero, Jaime C. and Dudenhoeffer, Austen P. "Mass Trapping Designs for Organic Control of the Japanese Beetle, Popillia Japonica (Coleoptera: Scarabaeidae)." Pest Management Science. 2018. Print.