Japanese Beetle

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Beginner

Scientific Names
Popillia japonica

Identification

"c"-shaped, translucent, white grub
Reddish-brown pupae
Oblong, metallic green-to-bronze, with coppery red wings.
Small white tufts on the sides and tips of the abdomen.

Often Confused With
June beetles, European chafer and rose chafers

Period of Activity
July-August

Scouting Notes
Take note of Japanese beetle populations while scouting for foliar diseases and other insect pests.

Thresholds
There are no thresholds for this pest.

Advanced

Scientific Names
Popillia japonica

Identification
Japanese beetle larvae are translucent white, “C”-shaped grubs. They are approximately 2 cm long at maturity. The pupae are light reddish brown, 13 mm long and 6 mm wide.

The adults are 10-13 mm long, metallic green to greenish-bronze, with coppery red wings and small white tufts on the sides and tip of their abdomen. The adults occasionally aggregate in large numbers on the fern.

Often Confused With
June beetles, European chafer and rose chafers

Biology
Japanese beetle has a host range in excess of 300 plants and has one generation per year. It over-winters as larvae, deep in the soil profile. As the frost leaves the ground, the grubs begin moving closer to the soil surface to feed briefly on grassy roots and then pupate. After spending 3 to 4 weeks in the pupal stage, they emerge as adults.

Adults emerge in greatest numbers during early July and live for up to 6 weeks, during which they spend extended periods aggregating, feeding and mating repeatedly. They are most active on warm, sunny days. A temperature of 21°C and a relative humidity of 60% are ideal for beetle flight. When temperatures exceed 35 °C and relative humidity is greater than 60%, flight ceases. Beetles can fly up to 1.6 km; even flights of 8 km have been noted with a good wind. Beetles usually feed and mate during the morning and return to the soil in the late afternoon and evening.

Females lay eggs in several small batches in grassy areas. These egg-laying episodes occur multiple times between bouts of feeding and mating. Each female may lay a total of 40 to 60 eggs over time. Egg-laying continues until late July and August.

Eggs hatch in about two weeks. Moisture is crucial to egg hatch and larval development. Under dry conditions, eggs and larvae fail to develop. If there is adequate moisture, newly hatched grubs feed on fine plant roots in the upper 5-10 cm of soil. In drier or cultivated soils, grubs will be found lower in the soil profile.

In September, as soils begin to cool, grubs move deeper into the soil and remain where the soil is about 10 C at a depth of about 15-25 cm. In cultivated soils, this may be deeper; up to 25-30 cm.