Understand the Lifecycle of Japanese Beetles to Help Combat Them

Tips for What Methods to Use at Each Stage in the Lifecycle

Japanese beetles follow a predictable life cycle, and understanding the timing of their different stages is crucial for effectively managing their impact on plants. The life cycle of Japanese beetles consists of four main stages: egg, larva (grub), pupa, and adult. Here's a breakdown of their life cycle, when they hatch and begin to eat plants, and what you can use at each of those stages to protect your plants:

Japanese beetle lifecycle illustration by APHIS employee Joel Floyd

Egg Stage

Japanese beetles usually lay their eggs in mid-summer, typically from late June to early August. Female beetles prefer to lay eggs in well-irrigated, grassy areas with moist soil. A single female beetle can lay up to 40 to 60 eggs during her lifespan.

Tips to Control at the Egg Stage

The egg stage is challenging to address directly, but you can take some preventive measures to reduce egg-laying and subsequent larval development:

  • Remove Attractive Egg-Laying Sites: Japanese beetles prefer to lay eggs in well-irrigated, grassy areas with moist soil. Minimize irrigation in your lawn during their egg-laying period to make your garden less attractive to egg-laying females.
  • Encourage Predators: Attract natural predators like birds and toads to your garden. They can feed on adult beetles and help reduce their population before they lay eggs.

Larval Stage (Grubs)

After about two weeks, the eggs hatch into small, C-shaped, white grubs that start feeding on grassroots and organic matter in the soil. These grubs remain in the soil, usually at depths of 2 to 4 inches, throughout late summer and fall.

Tips to Control at the Larval or Grub Stage

To control the grub population and prevent them from developing into adult beetles, you can use the following methods:

  • Beneficial Nematodes: Apply beneficial nematodes, such as Heterorhabditis bacteriophora or Steinernema feltiae, to the soil. These microscopic organisms parasitize and kill the grub larvae.
  • Milky Spore Disease: Introduce milky spore bacteria (Bacillus popilliae) into your lawn. Milky spore is a natural bacterium that targets and kills Japanese beetle grubs but is harmless to humans and pets.
  • Btg - Bacillus thuringiensis var. galleriae: Bacillus thuringiensis (Bt) is a naturally occurring soil bacterium typically used as a microbial insecticide. The Bt strain registered for the Japanese beetle is for use on the grub stage only. Bt is a stomach poison and must be ingested to be effective. Apply it to the soil as you would insecticides. Effectiveness is similar to that of insecticides. Check with your county extension agent office regarding the availability of Bt var. galleriae.
  • Manual Removal: If you spot large concentrations of grubs in specific areas of your lawn, you can consider manually removing them by hand.


Pupal Stage

During late fall, the grubs burrow deeper into the soil to overwinter as pupae. The pupal stage lasts for about 2 to 3 weeks, and the transformation from grub to adult beetle occurs within the pupal case.

Tips to Control at Pupal Stage

During the pupal stage, the beetles are dormant within the soil, making control at this stage difficult. Nematodes are an option. Focus on preventive measures and addressing adult beetles once they emerge.

Adult Stage

As spring approaches, usually from late May to early June, the adult Japanese beetles emerge from the soil. These vibrant beetles are about 3/8 to ½ inch in length with shiny, metallic green bodies and coper-brown wing covers.

Tips to Control at the Adult Stage

Controlling adult Japanese beetles is essential to prevent damage to your plants, including roses. Here are some methods to combat them at the adult stage:

  • Handpicking: Regularly inspect your plants, especially roses, and manually remove adult beetles. Drop them into soapy water to prevent them from returning.
  • Neem Oil: Apply neem oil on affected plants. Neem oil acts as a repellent and disrupts the feeding and mating behaviors of adult beetles.
  • Floating Row Covers: Use floating row covers to physically block adult beetles from reaching your plants.
  • Companion Planting: Consider planting repellent herbs like garlic, chives, and catnip around your roses to deter adult beetles.
  • Pheromone Traps: While pheromone traps can attract and capture adult beetles, they should be used with caution. Placing them too close to your garden can attract more beetles to your plants.
  • Encourage Natural Predators: Attracting birds, toads, and other insect-eating animals to your garden can help keep adult beetle populations under control.

Once the adult beetles emerge, they become voracious eaters and immediately start feeding on plants. They are most active on warm, sunny days and prefer to feed on the foliage of plants with soft, tender leaves. Roses, being a favorite of Japanese beetles, are among the first plants to fall victim to their appetite.

The feeding period of adult Japanese beetles typically lasts for 6 to 8 weeks, from early June to late July or early August, depending on the region and local climate. During this period, they can cause significant damage to plants, defoliating foliage and leaving behind skeletonized leaves.

Understanding when Japanese beetles hatch and begin to eat plants is vital for planning your pest management strategies. Implementing preventative measures before their emergence and employing organic control methods during their feeding period can help protect your plants, including your beloved roses, from the devastating impact of these invasive pests.

Do you have a method that works well for you? Please share it with us using this email address social@jacksonandperkins.com