Appearance
- Eggs: Tiny, white to yellowish-white eggs are laid in the soil during the summer. Usually deposited 3 to 4 at one location.
- Larvae: Wrinkled, thick, white grubs with yellowish-brown heads, up to 1 1/4" in length, that curl up into a C-shape when exposed to light.
- Adults: Shiny, dark metallic green beetle with coppery wing covers, broadly oval in shape, up to 1/2" long, with a row of white spots along each side of the abdomen, and hairy legs.
Life Cycle/Habits
- Grubs hatch in late summer and overwinter in the soil. They move closer to the surface in spring and feed lightly on plant roots before pupating.
- Adults start emerging in mid-June and feed on plant foliage, stems, corn silk, and fruits through August.
- In July, beetles seek out moist soil in which to lay eggs, with grassy sites most preferred. Acidic soils favor egg and grub survival.
- Grubs hatch in August and begin feeding on the roots of various grasses; peak feeding is in September-October.
- One generation a year.
Host Plants
- Adults feed on asparagus stems, the foliage, and silk of corn, the foliage of okra, bean, rhubarb, grape, raspberry, and blackberry, some tree fruits, and hundreds of ornamental plants, and trees.
- Grubs feed on the roots of corn, beet, beans, asparagus, tomato, and onion, as well as many grasses.
Signs/Symptoms
- Both grubs and adults are damaging to plants.
- Beetles feed voraciously on flower petals and leaf tissue between the veins, skeletonizing leaves and leaving them with a lace-like appearance.
- Leaf death often results. When beetle populations are high, significant foliar damage leads to reduced plant vigor and yield.
- Grubs do the most root damage as they enlarge in September and October, but plants rarely sustain enough injury to show ill effects.
Monitoring
- Be alert for foliar damage starting in mid-June.
- Skeletonized leaves look lacy, may curl, and will brown and die.
- Grub root-feeding is rarely serious or evident except in lawns. Very high grub numbers combined with drought may result in rootless sod which can be rolled up like a carpet.
Prevention/Control
- Handpick adults or tap infested leaves over a container of soapy water – the beetles will fall in and drown.
- Avoid baited traps: they attract extra beetles that will increase damage.
- Row cover can be used on low crops.
- Early or late corn plantings circumvent feeding injury.
- Neem and pyrethrum sprays are "organic" insecticides that may be effective.
- There are many predators, including grackles, starlings, moles, shrews, and skunks, but few native insects prey on Japanese beetles.
- Lawns irrigated in summer attract egg-laying adults. Maintain neutral soil pH to lessen egg and grub survival.