PEST ALERT: Japanese beetles have found their way to California!
If you ever lived on the East Coast, you’ve probably seen the devastation caused by Japanese beetles. These shiny green and bronze pests skeletonize leaves and can completely defoliate smaller trees and shrubs. If that weren’t bad enough, their larva attack from underground, feeding on root crops and lawn roots.
Japanese beetle identification
Adult Japanese beetles (Popillia japonica Newman) are just under ½” long and slightly less wide. Males tend to be a little smaller than the females. Japanese beetles are easily recognized by their metallic green body and shiny bronze outer wings. They are clumsy flyers. Up close, five small tufts of white hair can be seen on either side of the body. Beetle larva are about 1” long and white, with a small copper-colored head and a larger copper-colored rear end. Like many other grubs, they rest curled up in a C-shape.
Japanese beetle lifecycle
Japanese beetles go through complete metamorphosis. Female beetles burrow into the top 2-4” of soil, normally in turf and lawns, to lay 40-60 eggs throughout an area. These eggs hatch as larva in midsummer. Larval beetles go through 5 molts (or instars), feeding heavily on turf roots and root crops for several months. In the final instar, they reach a pupal stage. The pupae are reddish-brown to tan and ½” wide. The larva often burrow deeper into the soil for winter. Damage to lawns is often the first sign of an infestation. Mature beetles emerge in late spring and early summer to begin feeding above ground and to look for mates.
Destructiveness of Japanese beetles
Adult Japanese beetles attack over 200 garden plants. Every part of the leaf is eaten except the veins, causing skeletonization. Favorite foods include tomatoes, grapes, peppers, roses, cherries, peaches, pears, raspberries, corn, blueberries, beans, and strawberries. For a complete list of host plants, see the Wikipedia page on Japanese beetles. Larval forms of the beetle feed for several months on lawn roots and some root crops. The first sign of Japanese beetle infestation may be dead areas of a lawn. A drench test can be conducted to see if grubs are the cause of the problem.
To perform a drench test, mark off a one square yard area of lawn (3’ x 3’) that includes both healthy and unhealthy grass with a rope or other clear marker. Mix 2-4 tablespoons of liquid dish soap with one gallon of water in a watering can. If the soil is especially dry, two gallons may be needed. Apply the solution evenly within the area. The soapy water will bring insects to the surface. Over the next ten minutes, check the area for visible signs of grubs and other insects.
How to control Japanese beetles
Pheromone traps are not recommended as a control for Japanese beetles. Research has shown that pheromone traps actually attract 25% more beetles than are captured. The majority of attracted beetles end up feeding on plants near the trap, rather than entering it. Beetles can smell the pheromone attractant from 1 kilometer (0.62 miles) away, so this method is counterproductive in areas with heavy populations. It may be effective locally, as we try to nip this potential tidal wave in the bud. Traps should be checked weekly.
Habitats can be modified by adding plants that are resistant or unattractive to Japanese beetles. According to Held (2004), in “Relative Susceptibility of Woody Landscape Plants to Japanese Beetle,” Journal of Arboriculture 30(6), pp. 328-335, dogwood, forsythia and hydrangea are just a few plants that Japanese beetles find distasteful. For a more complete list, see the North Dakota State University page on Japanese beetles.
Biological control can be achieved by introducing nematodes. Specifically, Heterorhabditis bacteriophora and Steinernema glaseri species have been effective. These nematodes are available commercially. They attack the grubs and should be applied in August.
Milky spore disease is also effective against Japanese beetles. This bacteria (Paenibacillus popilliae) is eaten by the grubs and then causes fat depletion, resulting in dead grubs. Milky spore is not available for sale in some states, but it can be used in California.
Insecticides have been used to control Japanese beetles, but timing is critical and the results may be a mixed bag. Systemic insecticides take time to work and must be applied repeatedly.
Japanese beetles were first found in the U.S. in 1916. Since that time, they spread west to the Rocky Mountains. Unfortunately, in 2015, a male and female Japanese beetle were found in Sunnyvale, CA. It was hoped that that was the extent of the infestation, but we won’t know for sure util a few months or even years have passed. If you think you see one of these destructive pests, please the call the Pest Hotline at 1-800-491-1899.
I hope this information inspires you to grow more of your own food. You can ask your garden questions on my Home page.