Japanese beetles in yards and gardens (2024)

• Japanese beetle grubs spend the winter underground in the soil of lawns, pastures, and other grassy areas.
• In spring, grubs move up near the soil surface to finish feeding and pupate into adult beetles.
• Adult beetles start to emerge from the ground in late June or early July. They can fly up to several miles to feed.
• Adults feed primarily in July and August, although some may be active into September.
  • Beetle-damaged leaves emit feeding-induced odors that attract other beetles (like sharks to blood).
  • This often results in large clusters of beetles feeding and mating on particular plants while neighboring, equally attractive plants are only lightly infested.
• Virgin females produce a sex pheromone for mating that is highly attractive to males.
• After mating, females tunnel underground in the soil one to three inches to lay eggs.
  • Females will lay eggs several different times during July and August, totaling as many as 60 eggs.
  • Dry soil conditions can reduce egg survival, resulting in fewer adult beetles the following year.
  • The eggs hatch in about two weeks and the grubs feed mainly on the roots of lawn grasses.
• Grubs go through three different growth stages (instars) during the summer becoming progressively larger with each stage.
• As the soil starts to cool in the fall, the nearly mature, full-sized (third instar) grubs dig deeper in the soil, where they spend the winter.

Look for beetles in your yard and garden starting in late June and early July. Start management when they first appear. Damaged leaves attract more beetles so minimizing beetles on plants should mean fewer beetles will be attracted to them.

Japanese beetles feed for six to eight weeks so it is important to continue management until their numbers decrease. Once they are present in large numbers, managing them becomes more difficult.

Most feeding is finished by mid to late August.

Physical removal

Physically removing beetles can be a practical and effective management practice for smaller landscapes or a few plants, especially when only small numbers of Japanese beetles are present. Handpick or knock the beetles into a bucket of soapy water to kill them.

Check your plants daily and remove any beetles that you find to minimize feeding damage. Remember beetle-damaged leaves emit air-borne chemicals that attract more beetles. By physically removing them, you’ll reduce the number of new beetles attracted to your plants.

The best time to remove Japanese beetles is in the evening or in the morning when beetles on the plants are still cool and sluggish. However, anytime that it can be done is still useful.

Physical barriers

In some cases, it is possible to protect plants with fine netting to prevent beetle damage. However, do not cover plants in bloom that require pollination (i.e. fruits) as this will prevent pollinators from reaching them. Instead, handpick beetles until the plant is done blooming and starting to set fruit, then cover it.

Traps

Don't use Japanese beetle traps. Hanging a trap in a home garden is not an effective way to protect plants. And they may attract more insects to your yard.

The traps attract beetles using synthetic female sex pheromone and a blend of chemicals with a strong floral odor. They were developed by researchers to monitor for the presence of Japanese beetles so that management strategies could be implemented.

While these traps can collect an impressive number of beetles, research at the University of Kentucky has demonstrated that more beetles fly toward the traps than are caught. This usually results in more damage to nearby gardens and landscape plants than would have happened if no traps were present.

Using less preferred plants

Although Japanese beetles feed on many different kinds of plants, there are some that they seldom damage. When choosing new plants for your landscape, consider using a less preferred plant.

Plants usually not damaged by Japanese beetles include boxwood, clematis, chrysanthemum, conifers (e.g. arborvitae, spruce, fir, pine), daylily, geranium, ginkgo, Japanese tree lilac, forsythia, common lilac, magnolia, red and silver maple, oak, white poplar, redbud, rhododendronand yew.

Biological control

Two natural enemies of Japanese beetles have been released in Minnesota. The fly Istocheta aldrichi lays eggs on adult Japanese beetles in summer, whereas the wasp Tiphia vernalis parasitizes grubs in the spring. Although both natural enemies became established here, neither is very abundant and they have little impact on Japanese beetle populations.

Is the four o’clock flower toxic to Japanese beetles?

The four o’clock, also called marvel-of-Peru (Mirabilis jalapa), has been reported by some gardening sites and blogs to not only halt the beetle’s garden invasion but kill them in the process. But these sources offer no evidence of its effectiveness.Four o’clocks are grown as annuals in Minnesota.

• USDA pamphlets in 1940 and 1963 listed four o’clock as lightly fed by Japanese beetles, but did not mention any toxic effects to the pests.
• A 2013 Colgate University study did not observe dead beetles when they were only fed four o’clock plants. Some varieties of four o’clock were more heavily fed on than others, however.

This plant does affect humans. The sap from four o’clocks is a mild irritant, causing itching on the skin. Also, eating the plant may cause stomach upset or other intestinal issues as well.

If physical removal and barriers are not practical or you wish to supplement non-chemical management, you may choose to use an insecticide to protect valued plants. Remember that insecticides can pose significant risks to beneficial insects, including pollinators, as well as birds, fishand mammals.

Consider using a professional pesticide applicator, especially for trees. SeeHow to hire a tree care professional.

Use low risk insecticides when they are effective for protecting your plants.

Low risk insecticides

Products containing pyrethrins (e.g. Pyola®) are somewhat effective contact insecticides provided they are sprayed directly on the insect. Repeat applications are necessary. Avoid spraying bees and beneficial insects as these products are toxic to these non-pests as well.

Neem oil is effective for several days but repeat applications are necessary. Neem oil helps deter Japanese beetles but is less effective when large numbers are present. This product is low risk to bees and other beneficial insects.

Bacillus thuringiensis galleriae (e.g. BeetleGone, BeetleJus), derived from soil bacteria, is moderately effective against Japanese beetle adults, giving one to two week’s protection. This product is not toxic to bees and other beneficial insects. Bacillus thuringiensis galleriae is only available from online sources.

Residual insecticides

Several effective, longer lasting insecticides are available for treating Japanese beetles. Chlorantraniliprole (Acelepryn®) provides two to four weeks protection, and is low risk to bees. Pyrethroids, including bifenthrin, cyfluthrin, lambda cyhalothrin, and permethrin, last about two to three weeks. Carbaryl or acephate will provide one to two weeks’ protection

Caution: pyrethroids, carbaryl, and acephate are toxic to bees and other pollinators.

Systemic insecticides

A systemic insecticide is absorbed by the plant and moved through its tissues and is applied as a soil drench or injection, a trunk spray or a trunk injection. This can be useful to avoid pesticide drift, especially when treating large trees.

Trunk injections should be done by a certified tree care professional.

Imidacloprid and dinotefuran, both neonicotinoids, are available to residents. They are applied to the soil and only one application is needed per year. Do not apply to soil within 4-5 feet of pollinator attractive plants.

Caution: Imidacloprid and dinotefuran are very toxic to pollinators. Either avoid applying these insecticides to bee attractive plants or wait until the plants have finished blooming before treating them.

Professional pesticide applicators have access to chlorantraniliprole (Acelepryn®). This product is long lasting and is a low risk to bees. Professional pesticide applicators can also apply acephate (Lepitech) systemically as a soil drench. Acephate is toxic to bees so applications should not be made near bee attractive plants until after bloom.

If your lawn has a recent history of grub damage, you may wish to treat with a preventive insecticide in June or early July (see below) to help ensure that the lawn is not damaged again.

If you adopt a "wait and see" approach, and discover in summer that your lawn has become infested, a curative insecticide can be applied in late July to mid-September when the grubs are still relatively small (1/2 inchor less). Yellowing or browning grass in August is an early symptom of white grub damage. There are other possible causes for discolored turf so check under the grass to make sure it is due to white grubs.

Once white grubs are nearly full-sized (about 1 inchlong), and the turf has begun dying in patches that pull easily from the soil, you can still apply a "rescue" treatment with a fast-acting curative insecticide in September, but expect only partial control. Once the grubs have stopped feeding and started to move downward in late fall, insecticides are not effective against them.

Do not treat in springbecause the large grubs are hard to kill, they feed for a relatively brief time and rarely cause damage in the spring.Treating in spring is no guarantee that the lawn will not be re-infested again in mid-summer.

Parasitic nematodes, such asHeterorhabditis bacteriophora, are available. They generally do not affect beneficial insects. They need to be appliedafter the eggs have hatched and the grubs are present. If using nematodes, apply them during cool, overcast days or in the evening and water before and after application as they are susceptible to drying out. While they can be effective, results have been inconsistent.

Milky spore bacteria, Paenibacillus popilliae (formerly known as Bacillus popilliae) infect only Japanese beetle grubs and have no effect on beneficial organisms. Although these bacteria occur naturally in the soil and may infect a small percentage of the grub population, applying commercial milky spore products has not been shown to provide any benefit in modern university research trials.

Bacillus thuringiensis galleriae (Btg) is a bacterial strain that produces a toxin that affects beetle adults and larvae. Products containing Btg (grubGONE!®, grubHALT!®) have provided inconsistent (i.e., variable) levels of grub control in recent university trials.

Preventive insecticides are applied as insuranceagainst grub damage. Preventive treatment may be warranted if a lawn has a recent history of grub infestation, or if the adult beetle flight is particularly high in a given summer.

The best timing is during the month or so before the adult beetles first emerge and start laying eggs (mid-June to mid-July in Minnesota). Chlorantraniliprole (such as Scott's Grub-Ex®) is an effective, preventative insecticide that is also low risk for bees.

Products containing imidacloprid or clothianidin are also effective if applied preventively, but they can pose a hazard to bees foraging on flowering weeds or nesting in treated lawns.

To minimize their hazard to pollinators

• Mow any flowering weeds, like clover and dandelions, just before or right after the pesticide application.
• Avoid areas being used by ground-nesting bees.

Homeowner products for preventive grub control usually have the words "season-long" grub control on the packaging.

Curative means treating white grubs when they are feeding and damage is noticed. If turf damage has been sporadic the last few years, it may be worth waiting to see if they are a problem. Watch closely for symptoms of turf damage.

Effective curative insecticide are trichlorfon, clothianidin, and carbaryl. All three are toxic to bees. Clothianidin, in particular, is systemic; i.e., it can be taken up by the roots and move into the nectar and pollen of flowering lawn weeds.

To minimize the hazard of curative grub insecticides to pollinators, mow any flowering weeds just before or right after the pesticide application. Avoid areas being used by ground-nesting bees. Homeowner products for curative grub control usually have the words "24-hour" grub control on the packaging.