Japanese Beetles

1011 N. Woodlawn

Kirkwood, Missouri

63122

314-965-3070

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The Japanese beetle is a highly destructive plant pest of foreign origin. It was first found in the United States in a nursery in southern New Jersey 80 years ago. In its native Japan, where the beetle's natural enemies keep its populations in check, this insect is not a serious plant pest. The same cannot be said for St. Louis.

Both as adults and as grubs, Japanese beetles are destructive. Adults feed on the foliage and fruits of several hundred species of trees, shrubs, vines – and they love roses!  You’ll see damage in the form of skeletonized leaves and large, irregular holes in leaves. The grubs develop in the soil, feeding on the roots of various plants and grasses and often destroying turf.

To treat this pest, it’s important to understand its life stages.  The adult is a little less than 1/2 inch long and has a shiny, metallic-green body and bronze-colored outer wings. You are most likely to see the adults in early summer.

During the feeding period, females intermittently leave plants, burrow about 3 inches into the ground – usually into turf – and lay a few eggs. This cycle is repeated until the female lays 40 to 60 eggs.  

By midsummer, the eggs hatch, and the young grubs begin to feed. Each grub is about an inch long when fully grown and lies in a curled position. In late autumn, the grubs burrow 4 to 8 inches into the soil and remain inactive all winter. This insect spends about 10 months of the year in the ground in the larval stage.

In early spring, the grubs return to the turf and continue to feed on roots until late spring, when they change into pupae. In about 2 weeks, the pupae become adult beetles and emerge from the ground. This life cycle takes a year.

Japanese Beetle Life Cycle – Know When to Treat
If you have a Japanese beetle problem, there’s no quick fix.  But don’t give up – there are ways you can treat the beetle in the spring, before it emerges; in the summer, once the actual beetle is visible; and again in the fall, when it’s in grub form.

Spring Treatment
Timing is important, because you want to catch the grub as it rises in the ground, making it susceptible to treatment.  Use a product like Bayer Advanced Season-Long Grub Control Ready-to-Spread Granules on your lawn on flower beds.  In St. Louis, this treatment is most effective in mid-April to early May.

Summer Treatment
Use Your Fingers – Once the metallic beetles have emerged and are seen on your plants, then pick them off!  You can flick them into a cup of soapy water, or, if you’re aggravated enough, squish them between your fingers. 

Spray
You can also use Sevin or similar bug spray, such as insecticidal soap; Orthene Turf, Tree & Ornamental Insecticide; Bayer Advanced Garden Multi-Insect Killer Concentrate; and Ortho Bug-B-Gon Garden & Landscape Insect Killer Concentrate.

Mechanical Traps
Mechanical traps use a pheromone and floral scent to attract beetles. As a result of their clumsy flying and the design of the trap, they end up caught.  Because the traps actually attract more beetles than they capture, be sure not to put them near your garden or your favorite plants. Put traps at the borders of your property, away from plants the beetles may damage.  Trap placement should be timed to coincide with the emergence of adult beetles, between early June and July.

Late Summer/Fall Treatment
This is a key time to treat the grubs, before they go deeper underground for the winter. In early to mid-August, treat your lawn and garden beds with Hi-Yield Dylox granules.

Biological Controls
Homeowners who choose biological methods to control Japanese beetle populations can successfully use parasites, nematodes, fungi, or other biologically based approaches. While they take a little longer to produce the same results as insecticides, biological controls last longer. More importantly, they do not affect nontarget or potentially beneficial organisms.

Milky Spore -- Milky spore is the common name for spores of the bacterium Bacillus popillae. Upon ingestion, these spores germinate in the grub's gut, infect the gut cells, and enter the blood, where they multiply. Milky spore disease builds up in turf slowly (over 2-4 years) as grubs ingest the spores, become infected, and die, each releasing 1-2 billion spores back into the soil. Milky spore disease can suppress the development of large beetle populations.

Nematodes -- Insect-eating nematodes – microscopic parasitic roundworms – actively seek out grubs in the soil.  Heterorhabditis bacteriophora is commercially available and may be purchased in lawn and garden shops or through biological mail-order catalogs.