CO-Horts Blog

Wednesday, April 17, 2019

Battling Japanese Beetles

Posted by Lisa Mason, Arapahoe County Extension

If you live in certain parts of the Denver Metro area, you are probably very familiar with this pest. Littleton and other parts of Denver are inundated with this insect starting around mid-June.

Japanese beetle population levels in Colorado. Source: Colorado Department of Agriculture

This invasive beetle has two unique challenges: 1.) Japanese beetles cause damage during the larval/grub stage in turf grass, and adults cause damage by feeding on a variety of landscape plants and trees, and 2.) adult beetles feed on blooming flowers at the same time pollinators and beneficial insects are pollinating those flowers, which limits control options.

Japanese beetles were first introduced in the United States in 1916 and has spread throughout the US via nursery stock. The beetle was brought to Colorado in the 1990s but only recently have populations grown considerably. The larval stage feeds on the roots of turf grass, and the adult beetles feed on vegetation, including popular plants such as roses, crabapples, Virginia creepers, and linden trees. They can feed on over 300 plant types.

Japanese beetle life cycle. Source: University of Minnesota
A Japanese beetle larva. Source: David Cappaert,
An adult Japanese beetle. Source: David Cappaert,

You won’t see adult Japanese beetles appear until around mid-June, but now is the time to start planning for their return. Here are some things you can do:

For now and longer-term:
  • Keep your plants as healthy as possible. A healthy plant will be able to sustain more damage. Practicing good cultural care will help reduce the stress on your plants.
  • Keep turf as healthy as possible, too. A strong root system will be able to tolerate more feeding from grub.
  • We are providing a perfect environment for Japanese beetles: well-irrigated turf provides their habitat and a wide variety of plants provides their diet.
    • Consider planting other cultivars or plant varieties in the landscape. Research which cultivars are not susceptible to Japanese beetles.
    • Be creative with plant selection! Evergreens, annuals, perennials, some vegetables, and many tree varieties are not susceptible to the beetle.
  • Research your options for biocontrols and pesticides.
    • Choices can be limiting depending on if pollinators are foraging on the same flowers that the Japanese beetles are feeding on. Pollinator-safe options include Bacillus thuringiensis var. galleriae (marketed as beetleGONE!, beetleJUS!) and chlorantraniliprole (marketed as Acelepryn SC).
    • Look for plants that do not produce flowers or their flowers bloom at a different time than when the adult beetles are active. For example: crabapple trees bloom in the early spring when adult beetles are not active and other treatments may be an option.
    • Remember the label is the law! If you chose to apply pesticides, the directions must be followed exactly.
    • For a complete list of control options on foliage and turf and how they affect pollinators, see the Japanese Beetle factsheet.

When the beetles arrive:
Hand picking Japanese beetles off roses. Source: Lisa Mason
  • Hand-pick Japanese beetles to your hearts content! You can squish them or dump them in a bowl of soapy water. This effort may seem tedious, but keep in mind each female beetle can lay 40-60 eggs. Hand-picking will also reduce the volatile attractants produced by plant wounding.
  • Think twice before using Japanese beetle traps. Research shows that traps can attract more beetles to the area. As they fly towards the trap, they may find new food sources along the way. If you choose to use traps, place them at least 30 feet from any vegetation that the beetles will eat.
  • Make sure you have grubs in your turf before treating the turf with products. Checking can save you time and money. Grub damage looks very similar to other turf issues. Try doing a “tug test”. If you tug on your turf and the grass pulls out of the ground immediately, then you have may have grubs that have been feeding on the roots. If the grass is rooted into the ground, you may have another issue.
  • If you do have grubs, consider drying out the lawn when the grubs are active. They need well-irrigated soil to survive.

A native bee and Japanese beetles on a rose. Source: Lisa Mason
Are geraniums a plausible solution? Not likely. Research indicates that if Japanese beetles feed on geranium flowers (not leaves), they become paralyzed and rarely recover. However, geraniums are not a very favored host plant and if more attractive plants are nearby, such as roses, Virginia creeper, or lindens, then they would not visit the geraniums.  Also, if they do feed on geraniums, they mostly feed on the leaves, which are not toxic.

Japanese beetles are here to stay, unfortunately. With some longer-term landscape planning and persistent treatment efforts, we can try to disrupt the perfect environment we have created for them. 

More questions on Japanese beetle? The Japanese beetle factsheet is a great resource! You can also call your local county extension office.

Best of luck battling the beetles this summer! 

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