Japanese Beetle Basics

Japanese Beetle (Popillia japonica)                 

History— a pest insect species native to Japan and N. China.  It was accidently introduced to New Jersey in 1916 and began spreading throughout the east coast and mid-west. Found in metro Denver sometime between 1995 -2005 and Pueblo around 2009. 

Mandatory successful Japanese beetle eradication in Palisade, CO started in 2004 with full cooperation from local residents, businesses, and government agencies.

Plant quarantines are in place in Colorado for receiving and moving nursery stock.

Permanent eradication of Japanese beetles in our area is not probable because of urbanization and the banquet of plants they find in Colorado. Irrigation practices which keep turf grasses green and growing through the hot summer months facilitate a hospitable place for adult beetle egg laying.

Ongoing and appropriate landscape management practices are the best tools for both the adult beetles and their offspring—grubs (larvae).

Biology and Life Cycle—one-year complete life cycle from egg, larva, pupa, adult. Adults usually emerge in June, females begin egg laying until September. Each female lays up to 60 eggs as an adult (not all at once) in nearby grass turf, they seek well-irrigated areas. Cool-season grasses like Kentucky blue grass, ryegrass and fescue are their preferred turf for egg laying. Grubs live in turf roots – fall, winter, spring, emerging as adults in June for the cycle to begin again.

Japanese beetles are attracted to over 300 plant species and will eat grass roots, leaves, fruits and flowers of many ornamental plants plus agricultural plants. They are very mobile and will fly to plants they prefer, up to five miles. The volatile oils released from chewed plants attract more adults to those plants. They are very actively feeding, mating and egg laying in the heat of the day. They begin feeding at the tops of plants. In the cool of mornings and late afternoon they are sluggish and easier to manually flick in to a soapy container to their death.

It is OKAY to squish adult beetles on the spot; their remains (pheromones) do not attract more beetles to the plants to feed. What attracts more beetles to plants are the volatile plant oils that plants release as they are being chewed.

Management Strategies—a variety of management approaches are best, there is no perfect insecticide or tool to kill all adult beetles and grubs. With the two very different life stages, a good strategy is to learn management for both adult beetles and grubs. Special attention and care should be made when applying products to plants visited by both Japanese beetles and pollinator insects, particularly bees. The short list includes roses, silver lace vines, Virginia creeper, raspberry, linden, gaura, rose of Sharon, hollyhock.

Larvae/White Grub Management for Lawns

Adult Japanese Beetle Insecticide and Biological Options on Leaves and Flowers

Deterrents—egg and grub survival is dependent on soil moisture during the egg laying period (peak is June through August). Avoid over-irrigation during this period and consider reducing irrigation, but not to the point of harming existing plants that need watering during this time. Resume regular (if not extra) watering in late August to September to keep soils moist for regrowth of roots, especially if grubs have caused root injury (dead areas). Maintain a taller lawn in the egg laying period. Higher grass has more root mass that can tolerate root damage from chewing grubs. A short lawn with less root mass is more susceptible to grub damage.

Prevention— Limiting plants they favor in the landscape will reduce Japanese beetle visits.

Physical Controls—remove adult beetles regularly when they are most sluggish (early morning or evening). Benefits to hand picking - 1) immediate reduction 2) will reduce attractiveness of chewed plants which should result in fewer beetles drawn to the plants. Discard in trash or compost. Dead or crushed beetles do not attract more live beetles, or serve as a repellent when chopped and sprayed over plants.

Beetles can be fed to chickens, ducks and goats. Be sure to rinse off any liquids the adults were trapped in before feeding to animals.

Traps—are very effective in capturing adult beetles, but are only recommended to use for detection in new areas. Research shows that once beetles are well established over a wide area traps only serve to draw in more beetles which means more surrounding plant damage.

Lucky you if your neighbors use traps!