Since they were accidentally imported to New Jersey in 1916, voracious Japanese beetles have gradually spread north to Ontario, west to Colorado and Oregon, and south to Alabama and Georgia. They are the insect most often reported in The Big Bug Hunt in North America, and it’s almost time for the yearly party to begin. Japanese beetles feed for only about six weeks in summer and spend the rest of the year as grub-shaped larvae in the soil. Exact emergence dates vary with climate, but in general the risk period for garden plants is mid-June to early August.
Japanese beetles (JBs) have few natural enemies in North America, but they can feed on over 300 plants, including large landscape trees like lindens and crabapples. Among garden plants, JBs are partial to beans, corn, grapes, hollyhocks and others such as hibiscus, roses and zinnias. Among attractive trees like lindens, Japanese maples and crape myrtle, the beetles find some varieties more palatable than others.
Organic gardeners can do several things to control Japanese beetles, and the weather can be helpful, too. JB females lay eggs in grassy areas about 3 inches deep, and the hatchlings eat grass roots in fall, when hot, dry weather can be murder on the eggs and shallow-feeding larvae. The grubs also can be killed by severe winter cold when there is no insulating blanket of snow.
Hand Picking Japanese Beetles
Hand picking the beetles in the morning, when they are cool and wet with dew, can make a big difference in how many beetles you see in your garden. Use a broad bowl or cake pan loaded with an inch of soapy water, and jiggle and brush the beetles into your collection container. Should they become overly interested in a bushy plant like your prized Japanese maple, place an old sheet on the ground under the plant and shake individual branches to dislodge the beetles. Drown your captives in soapy water.
Growing tall, upright varieties also can make JBs easy to spot and collect. They prefer to feed on plants high up, where they can easily attract the company of their fellows, so you will often find them at eye level on sweet corn, pole beans, and grapes, where they are easy to hand pick. Tall zinnias are easier to defend compared to smaller ones, and bright colored zinnias may be less attractive than soft pastels.
Protecting Plants from Japanese Beetles
Roses are probably JBs’ favorite food, and they especially fond of the flower petals, which are rich in digestible sugars. If JBs appear just as your favorite rose is poised to bloom, protect it with a light covering made from tulle (wedding net) held in place with clothespins.
You might also use flowers as a trap crop as part of your Japanese beetle control plan. When planted away from the vegetable garden, flowers including hollyhocks, four o’clocks, marigolds and zinnias can become popular JB hangouts. You will still want to hand pick the beetles to help reduce local populations.
Organic Pesticides for Japanese Beetle Control
US organic standards permit the use of several pesticides that have plant-based pyrethrum as their active ingredient to control Japanese beetles in dire situations, and they do give fast knock-down of actively feeding JBs. In addition, there is a special strain of Bacillus thuringiensis, the galleriae strain, which produces a toxin that kills JBs after they eat it. Sold as BeetleJUS and BeetleGONE, this remedy has more residual activity compared to pyrethrum.
Whatever you do, don’t treat trees, roses or other plants with systemic pesticides, which can in turn kill bees and other pollinators. Plants that produce poison leaves also produce poison nectar and pollen.
Trapping Japanese Beetles
Also use traps with care, because they are often not as effective as they appear to be. Pheromone-baited JB traps will capture loads of beetles, but they may attract beetles that would otherwise seek company elsewhere. To reduce this risk, place traps as far as possible from your vegetable garden, and preferably downwind of it. You also can use a line of several traps to intercept beetles coming to your garden from a heavily-infested landscape tree on some unknown neighbor’s property, but traps are simply not effective in most situations.