Life History

Japanese beetle has one generation per year. Adults may begin to emerge from the soil in early June and are usually most abundant in early summer – from late June through early August. As adults, Japanese beetles can be found feeding and mating on foliage and flowers of their host plants. Mated females seek areas where soil is suitably moist to lay a small cluster of eggs among plant roots. A total of 40-60 eggs may be laid by each female beetle, during her four-to-eight-week life span. Upon hatching from the eggs, the grubs (larvae) seek out nearby plant roots and feed. During the time Japanese beetles are in the egg and earliest grub stage they are quite sensitive to drying and may die if soils temporarily dry out during this period. Larvae continue to feed until soil temperatures drop to about 60°F (16° C) at which time the larvae move deeper into the soil where they remain through winter. Activity resumes as soils warm in spring and, after a feeding period of about 4-6 weeks, the larvae form an earthen cell and pupate. A few weeks later the pupal stage is completed, and the new adults emerge. 

Japanese Beetle Injury

Japanese beetles can cause injury to plants in both the adult and larval stages. However, the type of injury caused by adults and larvae are very different. Injury by the adults is more obvious and is usually the primary concern. Adults feed on leaves, buds, and flowers of many commonly grown vegetable and landscape plants. Feeding is usually restricted to the tender tissues between the larger leaf veins, which results in a characteristic skeletonizing type of injury. More generalized feeding occurs on softer tissues such as flower petals. Rose flowers are particularly susceptible to Japanese beetle injury.  

Japanese beetle larvae feed on the roots of turf grasses, limiting the plant’s ability to acquire water. It is likely there will be increasing turfgrass damage in areas where this species becomes established, adding to the damage done by native white grubs present in Colorado turfgrass (e.g., masked chafers, May/June beetles). 

Management

Control of Adult Japanese Beetles:

Hand picking: Hand picking beetles can often be effectively employed in small plantings. The beetles are easily picked or dislodged from plants or can be removed by shaking infested plants over a collecting container in early morning when temperatures are cool. 

Insecticides: There are several insecticides that can be used to help control damage by adult Japanese beetles. These different insecticides vary considerably in features such as how long they can persist and control beetles, what plants they can be used on, whether they move systemically in the plant, and their hazard to desirable insects, notably pollinators. Insecticides that are highly toxic to bees and can persist long enough to kill insects for days are hazardous to pollinating insects that visit the flowers. Some insecticides, which are less toxic to bees or persist for only a short period, can be used on flowering plants if applications are made during early morning or dusk–when bees are not active and visiting plants. A couple of insecticides do not have restrictions for use on plants in bloom because they have very little, if any, toxicity to bees.  

Control of Japanese Beetle Grubs in Lawns:

Cultural Controls: Mowing height affects the susceptibility of lawns to grub damage. Turfgrass that is moved higher has a larger root mass, so it can tolerate root damage better. Lawns mowed shorter, will have smaller roots, so they are more susceptible to grub damage.  

Allowing the top couple inches of soil to dry a bit when eggs are being laid and hatching—July and early August—can kill eggs and early stage larvae. However, if grubs have already caused some root injury, then watering may need to be increased to keep soils moist enough to promote regrowth of roots. Anything that can improve growing conditions – watering, fertilization, core aeration, mowing – will allow turfgrass plants to better tolerate root damage white grubs produce. 

Biological Controls: Soil drench applications of certain kinds of insect parasitic nematodes can provide effective control of Japanese beetle grubs in lawns. (These organisms are discussed in more detail in Extension Fact Sheet 5.573, Insect Parasitic Nematodes). Certain nematodes in the genus Heterorhabditis (e.g., Heterorhabditis bacteriophora, H. megadis) are effective and available for purchase. Applications of Heterorhabditis nematodes are made as a soil drench, preferably during cool, overcast periods, and must be immediately watered into the turfgrass. They should be applied when Japanese beetle larvae are present and active.  

Another biological control option for Japanese beetle control is milky spore (Paenibacillus popilliae), a bacterium that produces “milky disease” in Japanese beetle grubs. Milky spore is applied to turfgrass where Japanese beetle grubs are active and may infect some of the grubs, producing a chronic infection that reduces survival and reproduction. Applications of milky spore powder will not produce immediate reductions in number of Japanese beetles, but if the bacterium becomes established it will provide long-term suppression of the beetles. In areas of the eastern United States, where milky spore has long been widespread, it annually infects a small number of grubs, resulting in some reduction of the Japanese beetle populations (less than 5%). 

Insecticides for Grub Control: Several insecticides are available that can provide excellent control of Japanese beetle grubs in lawns. Most commonly available are insecticides that are applied preventively to kill young grub stages. Applications of these types of products are best made just before eggs hatch or shortly after (typically mid-June to early July). There can be some risk to pollinators if insecticides are applied to lawns that have flowering plants attractive to bees. If flowering plants are present, the site should be mowed immediately before treatment to remove the attractive blooms.