Although they are not as abundant as past years, I have picked more than a few Japanese beetles from my rose bushes this summer. Japanese beetles are a major pest to both yard and garden. The larvae (or grubs) live in the soil and feed on the roots of turf grass. The adult beetles feed on the foliage of over 300 ornamental plants.
Japanese beetles were accidently introduced into New Jersey in 1916. They are not a problem in Japan because they are controlled by natural predators which do not exist in the U.S. Nor are they a major problem in the Eastern states which have soil inhabiting protozoans that control the grubs. These protozoans do not live in the soils of the Midwest. The beetles were first identified in Minnesota in 1968, but traps set by the Minnesota Department of Agriculture captured only three beetles between 1969 and 1979. A half million were trapped in 2000 and in 2001 over 1 million were trapped in 15 counties — 99 percent of them in Hennepin and Washington counties. By 2009 the Japanese beetle was established in 19 counties.
The adult Japanese beetle is about 3/8” long with a dark green metallic head and metallic dark tan wings. The adults emerge from the soil in early July to feed, mate and lay eggs. They live about 60 days and are active for 6-8 weeks. During those two months each female lays about 60 eggs. They can fly long distances to feed so where you see the adults is not necessarily where the grubs overwintered in the soil. They feed in the full sun at the top of plants and attack over 300 species of plants. They have a special fondness for vines, roses, raspberries and grapes, and linden, elm and apple trees. Beetle-damaged leaves emit an odor that attracts other beetles and the insects themselves emit an attraction pheromone, which is why once you have one or two suddenly you have hundreds. The pheromone is not produced after dusk, which is when the females lay their eggs in the soil.
The grubs feed on grass roots and are about 1” by September. As the temperatures cool they move deeper into the soil and overwinter 2-10” inches in the ground. When temperatures rise to about 50° in May the grubs start to feed again. In June the grubs turn to a pupa stage and in July the full grown beetles emerge to start the cycle again.
As with most non-native pests, the beetles’ population can grow to epidemic proportions because there are no natural predators here, so it is up to us to try to control them. Non-chemical methods include handpicking, or using a barrier such as cheese cloth to protect a specific plant. Beetle traps are available but are not recommended because they tend to attract more beetles than they can trap. The lowest impact way is to remove them by hand and destroy them. You can brush or shake them off your plants but they will just fly back or on to some other plant. The best method is to shake or brush them into a bucket of soapy water, which will kill them. Do this in the morning and/or early evening before they have a chance to lay their eggs. This method can be effective when the beetles are present in low numbers. For a large infestation you may have to use a barrier such as cheese cloth to protect the plant they are attacking or use an insecticide.
There are a variety of chemicals that can be used on the adults. Low impact products are NEEM oil which is effective on low or moderate numbers, or Pyola for larger infestations. These products are anti-feedants, which will protect your plants but does nothing to prevent the beetles from moving on to some other plant. They are also short-lived and need to be applied every two weeks or after any rain. Both products can also be toxic to bees so they should be applied when the bees are not active. There are systemic insecticides which are easy to apply and long lasting which prevent the beetles from feeding with death following, but these pesticides are highly toxic to bees. I discourage you from using anything that threatens our bee population.
So what is the collateral damage in our battle with the beetles? Certainly some very chewed up and unattractive plants. But as unattractive as your perennials might be right now they are unlikely to suffer any permanent damage unless they are completely defoliated. Mature trees will not be killed by one year of feeding but young or already stressed trees should be protected. Our main objective in the management of the Japanese beetle is to reduce their numbers by catching them before they reproduce.