Updated: Apr 2, 2021
A long time ago, in a galaxy far far away we’re talking Riverton, New Jersey, 1916 an unassuming beetle was found living in a crate of iris bulbs at a local nursery. Shipped directly from Japan, before the establishment of border inspections on imported goods prior to entry, the bulbs are now believed to have been carrying the larvae of a diabolical little creature that would leave a path of destruction in its wake for well over the next 100 years. In less than a centruy, the Japanese beetle made its way West, moving long distances in nursery stock sold in trade and about 1 to 5 miles per year on its own, from plant to plant. They first appeared in Colorado in the early 2000s, leaving scientists and experts caught off guard by the ability of the pest to establish itself in our region. An insect that likes moisture and humidity, it was thought that the Japanese beetle would never become a problem in the semi-arid Colorado climate. However, our urban landscape areas are oases of green, islands of irrigated plant material that the beetles, unfortunately, thrive in. Adult Japanese beetles are intriguing scarabs with bronze colored wings and tufts of white hair adorning each side of their shiny, metallic-green bodies. But, like the villain in any good summer blockbuster, behind their beauty hides a destructive bent; they feed voraciously on countless varieties of plants. Gregarious by nature, they gang up by the dozens on plants, leaving skeletonized leaves and ravaged flowers. At a half-inch long, they’re easily spotted in early to mid-summer as they chomp down on our gardens.
The Life Cycle
Japanese beetles complete their life cycle in one calendar year but fully-grown adults only live for just 30-45 days, on average. In Colorado, adult beetles leave behind the safety of their soil homes in mid to late June, with peak emergence occurring in mid to late July. They are most active in the heat of the day and are voracious feeders on ornamental and agricultural plant leaves and fruits. If an adult is bothered or threatened while feeding, it will feign death and drop to the ground to avoid predators. They have a high rate of reproduction: females feed, mate and lay eggs every 24-48 hours. At each egg laying, female beetles deposit 1-5 eggs 2-4 inches deep in soil. Larvae hatch within 10 to 15 days (depending on ground temperature), feed on the roots of the turf grass and begin to move vertically and horizontally, in response to moisture and temperature. During the summer months, the larvae will inhabit the surface of the soil, moving lower as temperatures decrease, moving back up again in March, to pupate in May.
The Japanese beetle is the most widespread turf-grass pest in the US. Although they will seldom kill trees or shrubs, they can slowly weaken them, making them subject to secondary pests and disease. The estimated damage and cost of controlling Japanese beetles in the United States is over $460 million each year. They eat the foliage and flowers of over 200 species of plants including: grape, crab apple and apple, rose, linden, Norway Maple, birch, cherry, plum, apricot, peach and raspberry, with Virginia Creeper and strawberries being particular favorites.
Management of the Japanese beetle is complicated because adults and grubs (larvae) are very different from one another and cause injury to a variety of hosts. Control of just one life stage will not necessarily guarantee control of the other.
Utilize plants that are not susceptible to Japanese beetle whenever possible! Plants that are resistant to Japanese Beetle are: lilac, spruce, chokecherry, elderberry, Bur Oak, Bittersweet Vine, boxwood, forsythia, hydrangea, juniper, mockorange, pear, pine, smokebush, snowberry, Silver Maple, Red Maple, sumac, spirea, yew; as well as coreopsis, larkspur, foxglove, Coral Bells, hosta, poppy, columbine and pansy. Many vegetables are not attractive to them. A few plants act as repellents: catnip, garlic, chives, tansy and annual geraniums.
Reduce irrigation to turfgrass areas. Japanese beetle adult females seek out well irrigated Kentucky bluegrass or other turfgrasses in which to lay their eggs. Try to make this area as unattractive to the pest as possible.
Hand pick or physically remove adults. Research suggests that this is actually a very effective management strategy as it reduces feeding-induced plant volatiles. It’s the plant volatiles emitted when a beetle feeds that attracts more and more. So by picking the beetles off and dropping them into a bucket of soapy water each evening, there are fewer beetles which leads to less feeding and less volatiles released to attract more beetles. Japanese beetles drop readily when disturbed to they can easily be collected by holding a jar or container beneath filled with soapy water.
Use of biological controls. Many biological controls for Japanese beetle have been researched.
Currently, the use of entomopathogenic nematodes (microscopic round worms that only parasitize insects) has been shown to be effective for controlling grubs. Cost of implementation is high and results are inconsistent when compared to conventional controls. Available in many garden centers or by mail order, Steinernema glaseri and Heterorohabiditis bacteriophora may be effective controls if attention to pre and post irrigation is noted.
An additional biological control called Milky Spore, a bacterial disease of grubs, can also be purchased for Japanese beetle control, however, research has not shown this product to be very effective.
Alternative controls such as products containing azadiractin – a derivative from neem seeds, are also labeled to target the Japanese beetle. Look for trade names such as Bioneem, Aazatin or Azasol. While these products may provide a short, but effective impact on adults they must be reapplied frequently.
Animals such as chickens feed on Japanese beetles.
Use of labeled pesticide controls. Insecticide treatment of turf to control larvae (grubs) has been the most common and largely the most effective management strategy employed against Japanese beetles. Use insecticides for grub control in early summer, for control of adults, applying when feeding and damage are observed. Always read and follow pesticide label directions. Many of the insecticides that control Japanese beetle are also highly toxic to bees and other pollinators. Pesticide labels will contain information describing how to use the product while protecting pollinators. It is essential and the law that all applicators of pesticides follow the pesticide label. Avoid application of pesticides to all blooming plants when pollinators are active in the area.
Contact a licensed tree care or landscape company. Response time is critical when it comes to saving your landscape from the mouths of these eating machines. If you see signs of Japanese beetles, contact Plant Escape immediately, so that our trained horticulturalists can begin evaluation and treatment of your plants, shrubs and tress, ASAP.