Japanese Beetles

Japanese beetles (Popillia japonica) may be beautiful with their metallic green and coppery-bronze sheen, but to rose lovers, they’re nothing short of devastating. These invasive pests are a particular menace to roses, skeletonizing leaves and destroying blooms seemingly overnight. For members of the Rose Society, understanding the life cycle of Japanese beetles and employing effective, environmentally responsible control methods is key to preserving healthy, thriving gardens.

The Life Cycle of the Japanese Beetle

To effectively manage Japanese beetles, it’s important to understand their life cycle:

1. Egg Stage (Mid-Summer)
   After mating in early to mid-summer, female beetles lay eggs in moist soil—often in turfgrass areas. These eggs hatch in about two weeks.

2. Larval Stage (Grubs: Late Summer to Spring)
   Once hatched, the beetle larvae (white grubs) feed on the roots of grasses and other plants through the late summer. As temperatures drop, they burrow deeper into the soil to overwinter. In spring, they return to the upper soil layers to resume feeding before pupating.

3. Pupal Stage (Late Spring)
   By late May to early June (depending on your location), the grubs transform into pupae.

4. Adult Stage (Early to Mid-Summer)
   Adults emerge from the soil and begin feeding on roses, grapes, linden trees, and over 300 other plant species. They typically live for 30–45 days, during which they feed, mate, and lay eggs.

Control Strategies

There is no one-size-fits-all solution for controlling Japanese beetles, but an integrated approach yields the best results—especially when timed with their life stages.

Cultural Controls

• Handpicking: In small gardens, handpicking beetles early in the morning (when they're sluggish) and dropping them into soapy water is effective.

• Exclusion: Fine mesh netting or row covers can protect prized rose blooms during peak beetle activity.

• Trap Use with Caution: Commercial Japanese beetle traps can attract more beetles than they catch. This form of control is not recommended.

Soil Treatments

• Milky Spore and Beneficial Nematodes: These biological treatments target grubs in the soil. Apply in late summer or early fall when grubs are actively feeding near the surface.

Targeted Insecticides

• Use caution with broad-spectrum insecticides, like Sevin, which can harm pollinators and beneficial insects. Always follow label instructions, and spray early in the day or at dusk to avoid impacting bees.