Living with Japanese Beetles: Strategies for Reducing Populations, Plant Choice, and Plant Protection

By John Ashworth, CSU Extension – Denver Master Gardener since 2014 

In the metro Denver area, Japanese beetles (JB) are unfortunately here to stay. They have steadily spread from the southern fringes of the metro area to Denver and the surrounding suburbs. To limit damage to trees and plants, it’s important to understand their one-year life cycle, learn to interrupt it, and work to reduce the population. 

Credit: (uky.edu)

As this diagram shows, adult JB emerge from the soil in early summer. They’ll damage foliage, buds, and flowers of host plants for about two months. By late summer, females dig into moist areas of turf and deposit eggs near grass roots. Upon hatching, the larvae feed on grass roots until early fall. By late fall, the larvae move deeper into the soil and remain there until spring when they again move closer to the surface and pupate. This new generation of beetles emerges as adults in the early summer.

Gardeners can manage JB in a number of ways:

• Reduce larvae survival in the grass.
• Capture adults.
• Select plants that are less attractive to adults.
• Block adults from attacking.
• Reduce larvae’s survival. 

This post and CSU’s very thorough Fact Sheet will help you develop a multi-season strategy for dealing with JB.

Summer 

Daily Removal 

In mid-July you will start to see adult JB activity, such as skeletonized leaves or deformed buds. Since adults are easy to spot with the naked eye, the most effective strategy is to brush the insects into a jar of soapy water daily, ideally every morning and evening.

Researchers differ on the very best time of day to pick JB : CSU recommends mornings, when temperatures are cooler, while this research suggests that evenings are best because flying beetles find their meals by following the distress pheromones emitted by the plants. It is believed that picking beetles in the evening reduces overnight feeding. In the morning, beetles don’t know where to land! 

It is also okay to squish or stomp on JB, contrary to earlier belief.

Japanese beetle skeletonizing a rose leaf. Photo:  Melissa Schreiner, Colorado State University, Bugwood.org

Protective Barriers

Small plants, like basil, can be protected by a fine mesh toule placed over a hoop. Find this material at fabric stores. Garden centers also carry mesh products which may be helpful for larger plants such as roses, grapes, and other JB targets. 

Basil pot frame and basil pot covered in toule. Photo: John Ashworth

Insecticidal Options

Use water-soluble sprays on larger plants to either repel the beetles or kill them when they attack leaves or flowers. Neem-based sprays repel the beetles, but do not kill them, and need to be reapplied after rain or sprinkling.

Pyrethrin sprays work for short periods of time and can be used on most food crops. 

Safe bio-based spray such as beetleGONE! and beetleJUS! contain the bacterium Bacillus thuringiensis (BT). The beetles ingest the BT as they eat the plant and die in 2-3 days. These options are mostly safe for bees, but are only effective for a few days. Apply when pollinators are not present.

Many commercial chemical sprays (imidacloprid, permethrin, etc.) that are sold as effective for JB elimination are, unfortunately, highly toxic to bees and other pollinators, should not be used around flowering plants, and should be used only sparingly on food crops. 

Plant Selection Can Help

It is important to know what plant varieties the beetles find most attractive. Consider removing and avoid planting those that are JB magnets such as roses, Virginia creeper, silver lace vine, linden trees, rose of Sharon, crab apples, and hollyhocks. 

I have found by growing heirloom Cantare bush beans instead of pole beans, I have eliminated the beetle population in that area of my yard.

I also have had success planting leadplant (Amorpha canescens), a native shrub with attractive flower spikes in May and June. Beetles congregate and mate on the flowers without damaging the plant, and I’m able to pick them off easily. Since the beetles are lured to the leadplant, the nearby hibiscus and basil suffer less damage.

Leadplant. Photo: Krzysztof Ziarnek, Kenraiz, CC BY-SA 4.0, via Wikimedia Commons

Japanese Beetle Traps 

JB traps attract beetles from neighboring yards and increase the problem rather than solve it. They are helpful in confirming the presence of JB and are effective only when an entire neighborhood uses them. If you wish to use a trap, hang it far from the plants you are trying to protect. 

Late Summer 

Adjust Lawn Watering

To help reduce next year’s JB population, reduce grass watering from late July to early August when they are laying eggs as females have difficulty laying eggs in drier soil.  Resume normal grass watering in late August.  

Fall and Winter

Grubs burrow deep into the soil in late fall and remain there until spring. No action is needed at this time.

Spring

Block Adults From Attacking

In mid-May grass can be treated with the mail order biological control grubGONE!, which contains BT. The BT attacks the grubs before they morph into adults.

Readily available chemical insecticides containing imidacloprid, chlorantraniliprole, or chlothianidin kill grubs in the soil and are effective when applied to the lawn in mid-May. Common trade names are Scott’s GrubEx, Merit, Zenith, and Arena. 

Since JBs are very mobile, these treatments reduce the population at large but may not eliminate them from your yard. 

Closing Thoughts

By employing these practices you can reduce JB damage in your yard. In a future post I’ll delve further into springtime actions you can take to minimize grub activity in the lawn.