The Japanese beetle, Popillia japonica, is an invasive pest in North America that causes significant damage to turf and ornamental plants. First found in the United States in 1916, it is now the most widespread turf-grass pest in the country. Homeowners often ask how long they must contend with the visible, leaf-eating adults each year. The adult stage, which causes the most noticeable damage to foliage, is actively present for approximately six to eight weeks.
The Visible Adult Flight Season
The emergence of the metallic green and bronze adult beetles depends primarily on local soil temperatures, but generally begins in late May to early June in warmer regions. Peak activity, when the population is most numerous and feeding is most intense, occurs from late June through the end of July. This active phase lasts between four and eight weeks in any given location before the population gradually declines.
Individual adult Japanese beetles live for about 30 to 45 days. During this period, they are voracious feeders, consuming the foliage of over 300 plant species and leaving behind characteristic “skeletonized” leaves where only the veins remain. Feeding is most extensive when temperatures range between 70°F and 95°F. Females repeatedly leave plants to burrow into moist soil to lay a portion of their 40 to 60 eggs, a cycle that continues until their lifespan ends.
The Complete Annual Life Cycle
While adults are only visible for a couple of months, the Japanese beetle is present year-round, completing a single, 12-month life cycle. The cycle begins when females lay eggs in the turf during mid-summer (late June to early August), preferring moist grass areas. These eggs hatch within about two weeks, and the newly emerged larvae, known as grubs, immediately begin feeding on grass roots.
The grubs go through three stages (instars) and are the most damaging stage to turfgrass, often causing brown patches that can be rolled back like a loose carpet due to the lack of roots. Around October, the grubs stop feeding and burrow deeper into the soil to overwinter. They remain inactive until the soil warms in the spring, when they migrate back up to the root zone to feed briefly.
In the late spring (May to early June), the grubs enter the pupal stage, the non-feeding transition phase before adulthood. This pupation lasts about two weeks, after which the fully formed adult beetles emerge from the ground, starting the entire process over. This complete cycle explains why the visible adult population returns every summer, as the next generation has been developing underground for the preceding ten months.
Acute Management During the Active Phase
When adult Japanese beetles are actively feeding during their six-to-eight-week flight season, homeowners can employ several immediate management strategies. For smaller infestations or lower-growing plants, the most direct and chemical-free method is hand-picking or knocking the beetles off plants. The collected beetles should be dropped into a container of soapy water; the soap breaks the water’s surface tension and prevents them from escaping.
For high-value plants, physical exclusion using fine mesh netting or row covers can effectively prevent feeding damage. If chemical intervention is necessary, contact insecticides containing ingredients like pyrethrins or the bio-insecticide Bacillus thuringiensis galleriae can be applied to the foliage, though repeat applications may be needed.
The use of pheromone-baited traps is generally not recommended for home gardens. The powerful attractant lures more beetles into the immediate area than the traps can capture, potentially increasing damage to nearby plants. If traps are used, they should be placed at least 30 to 40 feet away from the plants one is trying to protect.