The Control of Japanese Beetles

You know it is summer in Denver when we start handpicking Japanese Beetles (Popillia japonica) off plants in our yards. By midsummer, these voracious eaters overwhelm many neighborhoods frustrating homeowners and gardeners to no end. The question is, what to do? Before we get into that, let’s discuss where the Japanese Beetle comes from and its lifecycle.

Quick Facts…

  • Adult Japanese Beetle attack over 300 commonly grown plants.
  • They put Colorado’s fruit and wine industry at risk.
  • They defoliate Elms, Birch, Lindens, some Maples, and fruit trees.
  • Japanese beetle larvae or grubs feed on the roots of turfgrass.
  • Control options include handpicking and the use of certain insecticide products.
  • Traps do attract and capture adults but are not that effective.
  • Insect parasitic nematodes and other biological controls show promise in controlling these insects.

Japanese beetles first came to the eastern United States about 100 years ago. They spread to other parts of the country inside nursery stock. Experts thought that the beetle would not thrive in Colorado because of our dry climate. However, the widespread installation of sprinkler systems across the city transformed our urban landscape into a perfect breeding ground for the bug. In 2003, the insect first appeared in Cherry Hills Village, and within ten years, the insect now has permanent, reproducing populations.

Japanese Beetle Life History

Japanese Beetle lifecycle

A Japanese Beetle lives one year. Adults emerge from the soil in early June and starting feeding and mating on foliage and flowers of their host plants. Throughout their 4-8 week lifespan, the mated females lay eggs in grass lawns. They seek out moist soil and dig 2-3 inches down, then bear a small cluster of eggs among the grass root system. Each female beetle repeats this process throughout the summer, laying a total of 40-60 eggs.

Once hatched, the larvae seek out nearby plant roots for food. The grubs become full-size by September. High infestations cause dead spots on lawns. The larvae continue to feed into the fall and dig deeper into the soil to overwinter. When the weather starts to warm the following spring, the grubs pupate into adults emerging a few weeks later to start the cycle over again.

The Japanese Beetle Double Whammy

It is a double whammy because both the larvae and adult stages damage plants. Adult damage is evident since the insects feed on leaves, buds, and flowers. They eat on the soft tissue between leaf veins leaving a feeding pattern called “skeletonization.” Unfortunately, the insect highly favors roses and Virginia Creeper in Denver. The Japanese Beetle also attacks Linden and Elm trees. Large scale infestations defoliate trees. Their grubs feed on grass roots eliminating the ability of the plants to acquire water. Amazingly, severe infestations cause brown patches, which can be rolled back like carpet since the grubs ate away the roots. Damage on individual plants may be patchy or concentrated when large numbers of beetles appear.

Control of Adult Japanese Beetles

Japanese Beetle Trapping

Traps use floral-based compounds to lure the bugs into the devices. Mandatory mass trapping was a component of the successful Japanese beetle eradication in Palisade, Colorado. However, the piecemeal application of traps is not useful, and Ross Tree does not recommend traps for Japanese Beetle control. The big joke at the office is the best way to keep Japanese Beetles out of your yard is to give your neighbors traps as presents.


hand picking japanese beetles

In Denver, handpicking becomes a morning routine for people determined to control these insects in their yards. It is best to pick them in the morning before they become active. Most people place the beetles in a jar with soapy water. Handpicking is time-consuming but prevents feeding damage.

Insecticide and Biological Controls

Controlling Japanese Beetles with insecticide products is complicated because the pests are active at the same time as flower pollinators. Most control products are highly toxic to bees and persist after application. Be sure to use in the early morning or dusk when bees are not active. There are systemic products available on the market as well. Homeowners concerned about bees should contact a licensed tree service company like Ross Tree to manage the Japanese Beetles in their yard. Parasitic nematodes soil drenches do control Japanese beetle grubs in lawns. This Denver Post article discusses promising Japanese Beetle control products currently on the market. 

Japanese Beetle Grubs in Lawns

Japanese Beetle grubs

One way to improve the resistance to the Japanese Beetle grubs in lawns is to encourage the growth of deep roots systems. A well thought out watering, fertilization, core aeration, mowing regiment makes lawns more resistant to grub damage. Homeowner seeing beetle activity should consider treating their lawns to prevent grub damage. Here are some control ideas:

  • Allowing the grass to grow taller increases root growth, making the turf less susceptible to the larvae damage.
  • Conversely, lawns mowed short are more susceptible to damage.
  • Dryness kills the eggs and hatchlings, so watering enough to sustain the grass during the hottest months but allowing the turf to dry disrupts the Japanese Beetle breeding cycle.
  • In the spring, watering your grass longer, but less frequently, promotes deeper root systems.

Whether to treat Japanese Beetle yourself comes down to a value judgment by the individual. Many turn to professionals because it’s complicated for the following reasons:

  • The larvae and the adult of beetle are both harmful to plants.
  • Product control of the insect is problematic because of the possibility of harming bees and other pollinators.
  • Correct timing and application are critical to the success of product treatment.
  • State and the Federal Governments regulate the use of these products, so follow all application instructions precisely.
  • Improper use is hazardous and may lead to water contamination.

Are you expecting a sizeable Japanese Beetle infestation this year? Protect your plants and lawn by being proactive by calling Ross Tree at 303-871-9121 or fill out this request service form.