What Are Japanese Beetles?

Adult Japanese Beetle Appearance

Japanese Beetle Larvae (Grubs) Appearance

Japanese Beetle Pupae Appearance

Pupae undergo pupation within an earthen cell formed by the last larval stage. They are approximately 1/2 inch long and 1/4 inch wide. Their coloration starts pale cream and gradually changes to metallic green as they age, preparing to emerge as fully developed adult beetles.

1. Egg Stage

Females lay eggs in soil of healthy, well-irrigated lawns that get lots of sun. Eggs hatch into larvae within 10 to 14 days.

2. Larval Stage

Grubs go through three growth stages (instars), becoming progressively larger. By late fall, they burrow deeper to overwinter. In spring, they return to feed for 4 to 6 weeks.

3. Pupal Stage

Pupation occurs within an earthen cell near the soil surface and lasts 1 to 3 weeks. The beetle transforms from larval to adult form during this stage.

4. Adult Stage

Adults emerge in late June or early July, live for 30 to 45 days. They feed on plants and mate. Females periodically burrow into soil to lay eggs throughout summer.

States with Established Populations

Wisconsin, Minnesota, Iowa, Missouri, Nebraska, Kansas, Arkansas, Oklahoma, Texas, South Dakota, Washington, North Dakota, and sporadic locations in California, Oregon, and Nevada.

Why They Spread

• Favorable climatic conditions
• Wide variety of host plants
• Absence of natural predators
• Turfgrass expansion (breeding grounds)

Adult Beetle Damage

Adults cause significant damage to leaves, flowers, and fruits of more than 300 plant species. Preferred plants include maples, roses, grapes, corn, plums, peaches, birches, willows, apple, cherry, elm, raspberry, basil, marigold, and soybean.

Larvae (Grub) Damage

Grubs chew on grass roots, reducing the grass's ability to take up water and nutrients. This results in dead patches that can be rolled back like carpet due to lack of root support.

Economic Impact

Costs an estimated $460 million per year to control. Damages pastures and crop fields, reducing yields and threatening staple crops.

Ecological Impact

Reduces food for wildlife during feeding stages. Stressed plants become susceptible to other insects and diseases, creating cascading ecosystem effects.

Social Impact

Severely affects lawns, golf courses, sports fields, and parks. Brown patches impact recreation and aesthetics. Animals digging for grubs cause further destruction.

Physical Removal

Handpick or knock beetles into soapy water. Check plants daily. Removing damaged leaves reduces chemicals that attract more beetles.

Physical Barriers

Fine netting protects plants from beetle access. Do not cover blooming plants that need pollination until fruit begins to set.

Traps (Limited Use)

Not recommended for home gardens. Traps attract more beetles than they capture, leading to increased damage. Useful only for monitoring new infestations.

Less Preferred Plants

Plant beetle-resistant species: boxwood, clematis, chrysanthemum, conifers, daylily, geranium, ginkgo, forsythia, lilac, magnolia, red/silver maple, oak, redbud, rhododendron.

Low-Risk Insecticides

• Neem oil: Deters beetles, requires repeat applications
• BTG: 1-2 weeks protection, safe for bees

Residual Insecticides

• Chlorantraniliprole: 2-4 weeks, low risk to bees
• Pyrethroids: 2-3 weeks (toxic to bees)

Systemic Insecticides

• Applied as soil drench, trunk spray, or injection
• Caution: Neonicotinoids highly toxic to pollinators

Biological Insecticides

• Parasitic nematodes: Heterorhabditis bacteriophora, applied after eggs hatch during cool, overcast conditions
• Milky spore bacteria: Paenibacillus popilliae, specifically targets Japanese beetle grubs
• Tiphiid wasps: Natural biocontrol agents that target grubs in spring and late summer

Preventive Insecticides

Apply mid-June to mid-July before adults lay eggs. Chlorantraniliprole is effective and low risk to bees. Products with imidacloprid or clothianidin are effective but hazardous to pollinators.

Curative Insecticides

Apply late July to mid-September when damage is noticeable and grubs are small. Trichlorfon, clothianidin, and carbaryl are effective but toxic to bees. Mow flowering weeds before application.