Japanese Beetle is one of the most damaging insect pests in the Eastern and Midwestern US, but until recently, Coloradans were spared that challenge. It entered the US in 1916, but took until 2003 before a population was established in Colorado. This first infestation was in the Palisade area on the Western Slope. Even though eradication efforts were mostly successful there, established populations have been found since 2005 in Pueblo, southern Denver, Englewood, at DU and at Denver Botanic Gardens. Populations are being seen now in Boulder and Jefferson County.
Japanese Beetle is a scarab type beetle about a half inch long, with a metallic green body and copper-colored covers on its wings. In its larval stage, it is a white C-shaped grub, an inch long at maturity. According to our recently retired state entomologist, Whitney Cranshaw, adults emerge from soil or turf in late June, July and early August. They only live 4-8 weeks, but can cause significant damage in that time. After mating, the female lays eggs in moist turf, the eggs hatch into grubs that eat the roots of grass. When the numbers of grubs is large, they can kill areas of the lawn. The grubs are mature by September and they burrow down and go dormant over the winter. In spring they feed for 4-6 weeks, then pupate for a few weeks before emerging as beetles.
Japanese Beetle grub is the most widespread turf-grass pest in the US. The adult beetles 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, raspberry, some vegetables and some ornamental shrubs. Virginia Creeper and strawberries are particular favorites. Veggies that are eaten are: green beans, okra, corn, asparagus and basil.
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.
How can we manage Japanese Beetles to reduce their damage in our gardens and farms?
Like most pests, Japanese Beetles are attracted to weaker plants, so we can support plants through soil health with organic matter, micronutrients, minerals, sufficient water and supplemental biology like mycorrhizae and beneficial bacteria. Plant resistant varieties and natives and xeriscape plants. The beetles are very vulnerable when the grubs are small. So xeriscapes that are watered deeply and infrequently will dry out the grubs and reduce their populations. Bluegrass can be allowed to go dormant in July to save water and seriously reduce grub population. Then water normally in September to bring turf out of dormancy. Or just follow CSU advice and “water as deeply and infrequently as possible.” Dr. Michael Klein, a Ohio State U authority on Japanese Beetles has stated that “compulsive overwatering is often to blame for grub damage.” He also recommends cutting grass to a 3” height, making it less attractive to egg-laying females. And he suggests that we do everything we can to prevent adult damage early in the season, because early feeding attracts LOTS of other beetles.
Non-toxic solutions for the grubs
Milky Spore is a bacteria that only affects grubs and must be eaten to work. Apply a teaspoon amount every 4’ and water in for 15 minutes. It is best applied in early Aug/Sept when the soil temperature is above 65 degrees. Montana Cooperative Extension says it does not provide adequate control in the western states, but we have at least one customer reporting very good results with Milky Spore.
Nematodes of two varieties are effective but do not work in dry conditions, may not overwinter and are pricey.
Dr. Mike Klein recommends the aerator sandals called Spikes of Death. He says that whereas they are not effective for aeration, if you wear them while Dirty Dancing on your lawn in late Spring when the grubs are near the surface, studies have shown a better knock-down than from chemical insecticides.
Non-toxic solutions for the adult beetles
These may not give total control: insecticidal soap, diatomaceous earth, neem that contains its active ingredient azadirachtin.
Of course hand ‘picking’ is effective. A bucket of soapy water can be held under the plant while knocking the beetles off into the bucket. This is best done when the beetles are slower, in the early morning or in the evening.
We have been advised that baiting a trap with Japanese Beetle Pheromonal Bait …
Some of our Boulder customers have had success with a thyme based spray with Wintergreen called Garden Insect Killer by Liquid Fence.
Lower toxicity solutions
Pyola-a combination of pyrethrins and canola oil, and other pyrethroid products do break down faster than more toxic pesticides, but they are still lethal to bees, birds, fish (and cats?). There are many variations.
Higher toxicity solutions
The Colorado Dept. of Agriculture says, “Historically this insect is a target for large amounts of insecticide use.” Organophosphates as well as Neonicotinoids are used. The Palisade Colorado population was nearly eradicated using two different neonics: Merit and Arena. But because neonics kill or undermine the health of most insects including bees, earthworms, lady bugs and beneficial insects, they are not a sustainable solution. And because they are systemic and always in the plant, they do not work in an IPM (Integrated Pest Management) program. Birds that feed on the beetles can also be killed.
Using floral or sex-attractant scents do attract the beetles, but are not considered a solution since they usually increase insect damage.
Japanese Beetle is yet another foreign pest that does not have natural enemies here. Since it is relatively new to us, we will have to watch, experiment and learn more to keep their populations below damaging levels.
New sightings of Japanese Beetle should be reported to the local CSU Cooperative Extension office.