How to Get Rid of Japanese Beetles Without Traps That Backfire

The Short Answer

To get rid of Japanese beetles in a home garden, knock them into soapy water early each morning, cover small or prized plants with fine netting while the adults are flying, and leave the bag traps on the store shelf. The adults are metallic green with coppery-brown wing covers and about half an inch long, close to the size of a coffee bean. Each one lives about 30 to 45 days, and in much of the Midwest and East the feeding runs from mid-June through August. Grub treatments protect a lawn, but they seldom reduce the beetles on your roses.

Why Bag Traps Make the Damage Worse

The traps sold in garden centers use two baits. According to University of Kentucky Entomology, one mimics the scent of virgin female beetles and the other is a sweet food-type lure that draws both sexes. The pair is powerful enough that a single trap can pull in thousands of beetles in a day.

The trouble is where those beetles end up. Kentucky’s own research showed that traps attract many more beetles than they catch, and Penn State Extension puts the capture rate at 75 percent of the beetles attracted. Kentucky adds that susceptible plants along the flight path and in the vicinity of traps are likely to suffer much more damage than if no traps are used at all. University of Maryland Extension cites research showing a 31 to 40 percent increase in damage to adjacent plants when traps were used close to susceptible species.

Kentucky’s conclusion is that in most landscapes traps probably do more harm than good. Maryland, Wisconsin and Purdue extension publications all decline to recommend them for control.

When Japanese Beetles Arrive and Leave

Timing depends on the state. Penn State reports that adults emerge from the soil in mid-June to early July in Pennsylvania. Maryland Extension says emergence in central Maryland begins in mid-June and peaks in early July, with beetles active for about 4 to 6 weeks. University of Wisconsin Extension gives late June or early July, and Purdue describes adults in Indiana as most active from mid-June through August.

Because emergence is staggered, the season in a garden lasts longer than the 30 to 45 days any one beetle lives. Kentucky notes that the beetles are most active on warm, sunny days.

The first arrivals matter more than their numbers suggest. Kentucky reports that odors given off by beetle-damaged leaves seem to be an important factor in drawing more beetles to a plant. Removing the early feeders keeps a shrub from becoming the gathering point for the neighborhood, so begin checking roses, grapes and lindens a little ahead of the mid-June emergence dates above.

Hand-Picking That Reduces Damage

Kentucky says removing beetles by hand may give adequate protection for small plantings, especially when beetle numbers are low. Maryland’s method is to hold a container of soapy water under the branch or stem where the beetles are feeding and knock them in.

Three habits make it work:

1. Go out early in the morning. Kentucky advises shaking beetles off while they are sluggish, and Penn State notes that in cool temperatures they fall off instead of flying away.
2. Keep at it through the flight period. With emergence spread over weeks, new beetles keep arriving after the first ones are gone.
3. Start with the plants they favor. Maryland notes that the beetles feed on more than 275 species but especially on plants in the rose family, and Penn State names roses and grapes as favorites.

The feeding is easy to recognize. Wisconsin describes adults feeding on the upper surfaces of leaves, eating the soft tissue between the veins and leaving a lace-like skeleton. If the holes in your plants look different, the leaf damage identification chart sorts out the other common causes.

Netting, Sprays and What to Expect From Them

A barrier is the only tactic that keeps beetles off a plant entirely. Kentucky suggests covering highly valued plants such as roses with cheesecloth or other fine netting during the peak of beetle activity. University of Wisconsin Extension recommends nylon insect screens over smaller plants, including vegetables, herbs and small shrubs and trees, from late June to early September. Close the edges, and remember that a covered plant cannot be visited by pollinators. Our guide to row covers for pest protection compares the fabrics and netting used for this on vegetable beds.

Sprays that suit an organic garden have limits. Wisconsin says insecticidal soaps and horticultural oils such as neem oil have very limited effectiveness against Japanese beetles. Our comparison of neem oil and horticultural oil covers what those oils are better suited to. Neem products that contain azadirachtin work differently, as a repellent. Purdue notes they can reduce defoliation when applied regularly, though no more than weekly, during beetle flight, and Kentucky measured about 3 to 4 days of feeding deterrence per application.

Bees visit many of the same flowers. Colorado State University Extension warns that azadirachtin is hazardous to bees if sprayed on them directly and should not be applied while bees are visiting. The same publication rates Bacillus thuringiensis var. galleriae, a bacterial insecticide sold for beetles, as a very low hazard to bees. Read the label of any product for the crops and timing it allows.

Plants Japanese Beetles Favor and Plants They Skip

Depending on the source, Japanese beetles feed on about 300 species (Kentucky), more than 275 (Maryland) or over 350 (Wisconsin). They still show strong preferences, and Kentucky publishes lists of both ends.

Often severely attacked	Seldom damaged
Roses	Common lilac
Grape	Boxwood
American linden	Hollies
Flowering crabapple	Flowering dogwood
Norway maple and Japanese maple	Red maple and silver maple
Gray birch	Magnolias
Cherry, plum and peach	Tuliptree
Rose-of-Sharon and hollyhock	White, red, scarlet and black oak

If one shrub is stripped every July, replacing it with something from the right-hand column ends the yearly fight. Where the favorite stays, plan to net it or pick it daily.

Lawn Grubs: What Treating Them Does and Does Not Do

Kentucky describes the rest of the life cycle. Females begin laying eggs in turf soon after they emerge and mate, about 40 to 60 eggs each over a lifetime. The young then spend roughly 10 months in the soil as white grubs feeding on grass roots, and most pass the winter 2 to 6 inches below the surface.

Killing those grubs helps the lawn. It does little for the garden, because Kentucky notes that adults are highly mobile and can infest new areas from several miles away.

Check before treating. Purdue’s guidance is that insecticide applications are generally not required when soil sampling turns up 5 or fewer grubs per square foot. Above that level, the options differ in how well they are supported:

• Insect-parasitic nematodes applied as a soil drench can give good control of Japanese beetle grubs in lawns, according to Colorado State. Purdue advises checking a sample of the mix under a magnifying glass to confirm the nematodes are alive and moving before applying.
• Preventive grub insecticides work on young grubs. Colorado State places the application just before or shortly after egg hatch, which in Colorado is typically mid-June to early July.
• Milky spore is a bacterial disease of the grubs. Purdue states that commercially available products have not proven effective, and Colorado State notes that it will not produce an immediate reduction in beetle numbers.

Soil moisture matters as well. Kentucky notes that midsummer rainfall and adequate soil moisture are needed to keep eggs and newly hatched grubs from drying out, and Colorado State describes the egg and earliest grub stages as quite sensitive to drying. Its advice is that if the top couple of inches of lawn soil can be allowed to dry a bit while eggs are being laid and hatching, in July and early August, many may be killed. Heavy lawn watering in those weeks works in the beetle’s favor.

The same pick-first approach applies to other large chewing pests of the vegetable garden, including tomato hornworms.