Japanese beetles (Popillia japonica) are invasive insects in North America that were brought over from Asia in the early 1900s. Since then, they have spread to more than 30 states in the U.S. and into Canada. Adults can cause unsightly damage to many ornamental plants and their offspring are significant pests of turfgrass.
Controlling Japanese beetles can be difficult due to their life cycle and the lack of natural predators outside of their native range. To protect your plants it is important to understand their life cycle and which life stages to target depending on the plants you’re trying to protect. In this article, we will give you a full overview of everything you need to know about Japanese beetles, the most effective ways to control them, and dispel some of the misinformation commonly found online.
Japanese Beetle Life Cycle
Japanese beetles undergo complete metamorphosis consisting of four stages; egg, larva, pupa, and adult. The life cycle begins with the emergence of adults who have been overwintering in the soil. Emergence is dependent upon temperature and will begin anywhere from May to early July depending on how far north you live. Males typically emerge a few days before females and tend to be slightly smaller.
Unmated females release potent sex pheromones as they emerge from soil which attracts any nearby males. Once mated, females return to the soil, digging 2 to 4 inches deep, to deposit 2 to 3 eggs. She will once again emerge from the soil 1 to 4 days later, feed and mate again. This process can occur more than fifteen times with each female laying up to 60 eggs in her life.
Japanese beetle eggs are white, small (1.5mm), and hatch 10 to 14 days after being laid. Japanese beetle larvae are grub worms with a creamy white color with a yellow-brown head. Their body is covered in brown hairs and small spines and is segmented into 13 sections.
Larvae develop through 3 instar stages, molting between each instar as they grow larger. First instar larvae are small (2-3mm) while third instar larvae reach up to 1 inch long. They feed primarily on grass roots and organic matter but can utilize the fine roots of other plants. During the third larval instar, when temperatures begin to cool, they cease feeding and remain 2 to 6 inches deep in the soil to overwinter.
In spring, when soil temperatures begin to rise, Japanese beetle larvae will begin feeding on roots again. After 4 to 6 weeks of feeding, they are ready to pupate near the soil surface. Pupation takes 1 to 3 weeks, after which adults emerge from the soil and the life cycle continues.
Damage Caused By Japanese Beetles
You may be wondering “what do Japanese beetles eat?” so you can protect your vulnerable plants. Adults feed on more than 300 host plant species, many of which are commonly used for landscaping and gardening. This includes popular plants such as roses, crepe myrtles, maples, fruit trees, and corn.
They will feed on flowers, leaves, and soft-skinned fruits. When feeding on leaves, they typically skeletonize them leaving only the leaf veins behind. Feeding damage typically begins at the top of a plant, where beetles can easily land, and moves downward. The chemicals released by plants due to feeding damage actually attracts more beetles to the area.
Larvae, on the other hand, primarily feed on the roots of grasses and can cause considerable turf damage in pastures, yards, and on golf courses. In small numbers, turf typically recovers quickly. When populations are large, the damage done to the root system limits the ability of the grass to take up water which can lead to significant dieback.
How To Prevent Japanese Beetles
Japanese beetle prevention is difficult, if not impossible. If you have plants such as roses in your yard, and Japanese beetles are in the area, you are likely to deal with these beetles sooner or later. The only guaranteed prevention is to exclude beetles from plants using netting during peak Japanese beetle activity from May to August.
Plants That Repel Japanese Beetles
There are many misleading reports regarding the use of certain plants and plant extracts to repel Japanese beetles. Studies have shown mixed results when using both companion and repellant plants to protect desired plants from Japanese beetles. For instance, garlic extracts have been shown to have no deterrent effect against Japanese beetles on linden foliage.
Another study showed that rue, geranium, and garlic chives used as companion plants with roses provided no protection to roses from the feeding damage of Japanese beetles. Some of these treatments increased feeding damage on roses.
The same researchers tested repellant plants including crushed red pepper, fennel seeds, crushed spearmint, cedar shavings, osage orange fruits, and fleshy ginkgo seeds which yielded similar results. Some plants thought to be repellants increased the amount of damage done by Japanese beetles.
There are many natural predators of Japanese beetles in their native range which is why they are not considered a major pest in Asia. In North America, some insects and mammals will feed on Japanese beetle larvae but not enough to control their populations. The only known predator of adult Japanese beetles is the fly Istocheta aldrichi which can parasitize them.
There are several nematodes, ground beetles, and ants that will feed on Japanese beetle grubs or eggs. Small rodents and mammals will also dig up and consume grubs, but they can increase damage to lawns. Two wasps, Tiphia vernalis and Tiphia popilliavora, will parasitize Japanese beetle larvae and kill them.
How To Get Rid Of Japanese Beetles
Getting rid of Japanese beetles can be a difficult task. It requires the use of multiple techniques together aimed at controlling both adults and larvae. Timing is extremely important when managing Japanese beetles. Make sure you understand their life cycle and keep an eye out for them emerging in late spring to time treatments correctly.
Japanese beetle traps are easily found online and in gardening stores. These traps claim to control Japanese beetle populations by capturing them. However, research has shown that these traps are only effective at eliminating Japanese beetles when populations are small or have just recently become established in the area. Keep them as far away from plants you want to protect as possible to minimize feeding damage from incoming beetles.
These traps attract the beetles using synthetic sex pheromones and plant semiochemicals. Often, they attract way more beetles than they capture and can lead to more damage than if no trap were used. The best use for these traps in an area where Japanese beetles are already established is for monitoring. They can help you determine when adults begin to emerge in spring so you know when to start other management techniques.
Pesticides can be sprayed for Japanese beetle control, but should only be used if heavy infestations were experienced in previous years. Many insecticides that impact this pest will also kill natural enemies and may lead to secondary pest outbreaks such as mites, aphids, or whiteflies. You should limit the use of insecticides to highly valued plants or plants you know are preferred hosts of Japanese beetles. Consider using some of the other methods outlined in this article first or along with the use of insecticides.
Effective insecticides for Japanese beetle adults include pyrethroids, organophosphates, carbamates, and neonicotinoids. These are readily available at garden centers in various forms. When selecting an insecticide, make sure it is labeled to control Japanese beetles and that it can be used on the plants you need to treat. Also, make sure you follow the directions for the best results.
How To Get Rid Of Japanese Beetles Naturally
Research has shown that removing flowers from roses during the peak of adult beetle presence can reduce the overall damage to plants. You can limit grub damage to grasses by reducing or stopping irrigation since females prefer to lay eggs in moist soil.
Milky spore (Paenibacillus popilliae) is a bacteria that occurs naturally in the soil and can infect and kill Japanese beetle larvae. However, studies have shown that the manufactured form of the bacteria is not effective in managing Japanese beetle larvae. Part of the issue may be that milky spore requires a continual source of larvae to infect over several years to become established in the soil.
Two other strains of soil bacteria, Bacillus thurigiensis subspecies japonensis (Btj) and Bacillus thurigiensis subspecies galleriae (Btg), are much more effective for Japanese beetle control. Btj works best when applied to soil when first and second-instar larvae are present. Btg can be applied to plants and has been shown to reduce feeding by adults for up to 2 weeks.
Home Remedies To Get Rid Of Japanese Beetles
There are many home remedies online for Japanese beetles, though their effectiveness remains unproven by research. While there may be some truth to spraying soapy water and other concoctions to kill Japanese beetles on contact, they are not going to provide the long-lasting protection needed to control these pests.
Unless you are dealing with a very small number of Japanese beetles, and you don’t mind spraying each individual to control them, it’s best to stick to the research-proven methods for controlling these damaging insects.
How To Get Rid Of Japanese Beetle Grubs
Grubs can be much more difficult to manage than adults since they are harder to get to. Tilling the soil can be an effective management tool against grubs but you can’t till your lawn every spring and fall. The only real management options for grubs are nematodes and insecticides.
Just like with adults, the use of insecticides for Japanese beetle grubs should not be the first option you consider. Limit watering your lawn if possible to reduce its attractiveness to adult females laying eggs. If you have experienced significant damage in the past, or notice an unusually high population versus previous years, chlorantraniliprole and neonicotinoid insecticides work best.
Nematodes For Japanese Beetle Grubs
Three species of nematodes are known to feed on scarab beetle larvae including Japanese beetle larvae. Steinernema kushidai, Steinernema glaseri, and Heterorhabditis bacteriophora are the best nematodes to use for Japanese grub worm control and they can be purchased online. They enter the larvae through their spiracles or other openings and transmit the bacteria Photorhabdus spp. which kills the grubs in a couple of days.
It is best to release nematodes later in the season when grubs are in their second or third instar stages. Nematodes are easily killed in hot, dry weather. It is best to release them on overcast or rainy days to ensure they make it into the soil without dying.
Japanese Beetle vs June Bug
Though Japanese beetles and June bugs are both in the family Scarabaeidae, there are a few differences between them. The table below summarizes their main differences.
|Japanese beetle||June bug|
|Scientific name||Cotinis nitida||Popillia japonica|
|Adult size||Less than ½ inch long||1-inch long|
|Adult appearance||Glossy copper elytra, green body, glossy, white tufts of hair along the margin of elytra||Matte green to brownish elytra, orange-yellow margin on sides elytra and pronotum|
|Feeding habits||More than 300 host plant species, primarily on leaves and flowers, sometimes on fruit||Sap and fruits with soft skins|
Frequently Asked Questions
Japanese beetles are attracted to an area by certain plant volatiles (for food) or sex pheromones (for mating). On average, they will travel 1 to 7 km to these scents when searching for food or mates. Also, since Japanese beetles are not native to North America, there’s a lack of natural predators to help control their populations.
Just like any creature, Japanese beetles aren’t all bad even though they can be a significant pest to humans in their non-native range. There are reports of people feeding them to backyard poultry and humans consuming them as an alternative protein source.
Adults and larvae are both polyphagous and feed on more than 300 host plant species, many of which are found in gardens or used for landscaping. Adults will consume leaves, flowers, and fruits. They will also skeletonize leaves, leaving the leaf veins behind. Larvae live underground, feeding primarily on turfgrass roots which can lead to significant damage.
Japanese beetles have mandibles, like all beetles, which they use to feed on plants. They technically can use their mandibles on humans or pets, but their jaws are not strong enough to break through the skin. There has been no report of Japanese beetles causing physical harm to humans.
There isn’t much known about what Japanese beetles do at night. Studies have shown that they will continue feeding, to a lesser extent, and will rest on plants they feed on during the day. Contrary to popular belief they only build nests for laying eggs, not for nocturnal hiding or resting.
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