Most of us weren’t living in Riverton, New Jersey, in 1916, but that’s the place and year when Japanese beetles, Popillia japonica, first showed up in America. Back then they had no idea then that these rather attractively colored, half-inch long, metallic-green and copper beetles would turn out to be one of the most devastating landscape pests to lawns and scores of ornamental plants.
It took them awhile, but Japanese beetles have made it to Colorado, and their summer feeding frenzy has gotten notice — lots of notice.
The assumption is that Japanese beetle larvae from Japan hitchhiked on some nursery stock to New Jersey. In Japan they aren’t that much of a problem due to factors that have kept numbers down, including an effective parasitic fly that preys on adult beetles, cooler temperatures and reduced larvae habitat
Japanese beetles are happy and flourishing in their adoptive home country, and have become a pervasive pest on the East Coast, the South and parts of the Midwest. Their expansion to our area includes localized infestations in central Denver, Englewood and Littleton. Pueblo and parts of Boulder and Jefferson Counties have reported beetle activity. Will they find your landscape and take up residence? Probably, but it’s anyone’s guess how soon that will happen. In the meantime, get to know this enemy pest and be ready.
They love turf areas and grasslands for their larvae to develop, and the smorgasbord of surrounding host plants to munch. Without effective natural enemies, they have the ultimate free for all — the uninvited guest that never leaves and eats the backyard each and every summer for two months. And before leaving, they deposit their young (in the form of an egg, then larvae) to feed on turf roots, causing more plant damage and a guaranteed return visit next year.
Adults emerge from turf areas sometime in late June to July and immediately spend the next six to eight weeks dining on your (and their) favorite florae. They chomp on close to 300 landscape plants, including annuals, perennials, trees and shrubs. The short list includes: roses, Virginia creeper, grape vines, raspberries, zinnias and green beans; linden, elm and fruit trees; and even weeds like common mallow. It’s easy to spot their damage — lacy patterns or skeletonized foliage on leaf surfaces or devoured flowers or petals. Being very mobile and flying up to five miles, they love to feed in groups and signal their fellow beetle friends to come and join the free banquet. Japanese beetles start eating at the tops of plants and work downward. They are sun worshippers and feed the heaviest in temperatures between 85 and 95 degrees.
Japanese beetles have a one-year life cycle – adult beetles live for 30 to 45 days each summer, eating, mating and laying eggs before dying off in the fall. Each female lays between 40 to 60 eggs in nearby turf roots.They are attracted to well-irrigated lawns, so as beetle numbers grow, home landscapes can have high grub numbers, which means dead lawn areas. Grubs spend 10 months feeding on turf roots just a few inches below the soil surface; they go deeper in colder temperatures. In the spring they move closer to the soil surface and pupate before emerging as adults for the cycle to begin again.
Controls for Japanese beetles and larvae:
- The reality is there’s no quick kill fix like a homemade soap spray (we wish). Stay on top of removal from the moment they arrive or more will show up. Try low cost or free cultural options first. Flick or tap them into a jar of soapy water during the cooler parts of the morning or evening when they are sluggish. Never squish them on the plant or ground; any remaining smushed body parts will attract more beetles to the plant.
- Pet turtles and chickens will gladly snack on Japanese beetles or larvae, but they can’t reach leaves on trees or vines.
- A lightweight cover cloth can protect vegetable or other plants, but it will need to be kept in place for most of the summer.
- Some say bird predators like starlings eat both adult beetles and larvae. (But they may peck away and rough up the lawn, too, as will skunks and other critters if grub numbers are high.)
- Let the lawn go dormant or reduce watering in their summer egg-laying months to reduce numbers or dry them out. Resume regular watering in the fall. Low mowed grass is easy for females to lay eggs in, so keep the grass tall, at least three inches.
- When adding new plantings, consider ones that aren’t favored by Japanese beetles. A few include redbud trees, lilac, mockorange, juniper, boxwood, yews, coreopsis, columbine, coral bells, hosta, pansy and most herbs (but they like basil).
- Japanese beetle traps, commercial or homemade, are not recommended. They attract more beetles not only to the trap but to surrounding plants and yards.
- Organic and synthetic controls may repel or kill adults and larvae. Unintended drift, over application or disrupting pollinators or beneficial insects should be avoided. Carefully read and study all container labels and apply when recommended — usually early morning or late evening. Read about safe pesticide application tips: https://www.colorado.gov/pacific/agplants/environmental-protection-pesticides-safety-and-work-group-information
- Because Japanese beetles are mobile, correctly timed fall or spring soil treatments to kill the eggs and larvae will not be entirely effective unless neighbors and neighborhoods collectively work together. There are both chemical and organic turf products to consider. It is a good idea to check for multiple larvae in the lawn before treating.
- The Palisade area of the West Slope started eradicating Japanese Beetles in 2004 following an area-wide plan to chemically treat lawns and ornamental plants. Without treatment, their grape, wine and peach industry could have been negatively impacted. Read their plan at http://www.wci.colostate.edu/shtml/JapaneseBeetle.shtml
- Plant-based insecticides like neem oil must contain the active ingredient azadiractin to repel Japanese beetle adults. It works for three to four days and disrupts their hormonal balance. Beneficial insects do not eat plants, so they not impacted, however, this product can be harmful to fish and other aquatic animals.
- A product with white kaolin clay (non- toxic) mixed with water and sprayed on plants acts as a barrier and irritant. It’s mainly used on fruits, beans and other vegetables. Wash off the product with water at harvest time.
- Homemade solutions containing garlic, pepper or soaps are not very effective; some commercial formulations may help a little when sprayed on adult beetles. Pyrethrum is the extract from the chrysanthemum plant and is a quicker acting adult knock-down product. Spray the beetles, plus the tops and bottoms of leaves. Do not use when pollinators or beneficial insects are in the area.
- Pyrethroids are synthetically produced compounds that do not come from flowers. They may offer protection from adult beetles for a couple of weeks after an application. Never spray when pollinators or beneficial insects are in the area.
- Biologic nematodes (small round worms that kill some insects like grubs) in the genus Heterorhabditis can be used as a soil drench during cool weather. It must be watered in and applied when beetle larvae are present. Check with reputable mail order companies.
- Milky Spore, a bacterium that produces “milky disease” in Japanese beetle grubs, applied to turf grass may reduce grub numbers as the bacterium builds over several years. Milky spore is not harmful to humans, pets, beneficial insects, wildlife or aquatic life.
- There are several other chemical lawn products that provide good control of grubs, read about them on this fact sheet from Whitney Cranshaw, entomologist from CSU http://extension.colostate.edu/topic-areas/insects/japanese-beetle-5-601/
- An additional resource: United States Department of Agriculture – Managing the Japanese Beetle: A Homeowner’s Guide. It includes plant lists that Japanese beetles favor and avoid. https://www.aphis.usda.gov/publications/plant_health/2015/japanese-beetle-handbook.pdf
More from Betty Cahill: http://gardenpunchlist.blogspot.com/
