Japanese Beetle Invasion

Photo: David Cappaert, Michigan State University, Bugwood.org
Photo: David Cappaert, Michigan State University, Bugwood.org

Stunning in their metallic shades of vibrant copper and emerald green, Japanese Beetles might seem like welcome immigrants to our state. However, anyone who has lived in other parts of the country knows how destructive these voracious scourges can be. Until recently, Japanese beetles were unknown in Colorado. Unfortunately, they are now prevalent both in southern Denver and in locations along the Western Slope. It is only a matter of time before they spread southward to El Paso county. Gardeners here can prepare by learning how to identify these beetles and protect their landscapes.

Identification is the first step in controlling pest populations. Since there are a number of indigenous insects that superficially resemble these beetles, close examination is helpful. Japanese beetles have an oval shape, and range from 8 to 12 mm (5/16 to 1/2 inch) long. Their bodies are metallic green, and the wing covers, which don’t quite cover the tip of the abdomen, are a lovely coppery brown. The antennae have lumps at the ends which may spread out into fans. White spots on a black background encircle the abdomen. Most look-alikes are either much larger or smaller, or lack the white spots and/or the green body. If in doubt, the El Paso county master gardener help desk is happy to help. Preserve the insect in alcohol if you can’t bring it in right away.

Photo: Doug Stone, Mississippi State University, Bugwood.org

Japanese beetles lay eggs in the soil; these hatch into larvae in mid-spring. The larvae, or grubs, eat roots in areas of high moisture, and can severely damage plants, especially lawns. Because the white grubs resemble the larval form of many other beetles, they are very difficult to identify. When spring weather warms, the larvae pupate. Adults emerge between late June and the end of July.

Adult beetles feed on leaves, leaving large, irregular holes in the spaces between the leaf veins. While any plant is at risk, they favor lindens, roses, crabapples, Virginia creeper, grapes and beans. Fruit, with its high sugar content, is particularly relished. (Some resistant plants include conifers, lilacs, ash trees, and Euonymus species such as burning bush.)

Control takes advantage of the beetles’ preferences and life cycle. To prevent lawn damage, let your grass get a little thirsty. During the month of July, reduce your watering schedule to one deep soaking every seven to ten days. This won’t harm your turf, and the beetles will seek somewhere soggier to lay their eggs.

If you see adult Japanese beetles in your yard, hand picking can be effective. Covering susceptible plants with netting will keep beetles out. Traps are available, but are only 30% effective. They’re best used for detection, not control.

The best time to treat for grubs is when they are small, during the summer months. Apply a commercial grub control, following the directions on the label. Water well to get the pesticide deep into the soil, where the larvae live. Acelepryn is a non-toxic chemical that kills Japanese beetle larvae, but doesn’t hurt other animals.

Organic options are somewhat limited. While effective in areas with more rainfall, milky spore disease does not control Japanese beetles in Colorado. One biological control that does work here is the parasitic nematode, Heterorhabditus.

While Japanese beetles will be one more challenge to gardening in Colorado, help is available. Xeric landscapes are particularly unfriendly to these moisture-loving pests, giving us yet another reason to conserve water in our landscapes.

For more information on the appearance, distribution, and control of Japanese beetles, see Colorado State University Fact Sheet 5.601, or contact the El Paso county extension help desk at CSUmg2@elpasoco.com or 719.636.8921.

4 thoughts on “Japanese Beetle Invasion

  1. I discovered what I believe to be Japanese beetles on my grapes in Pueblo. I reported this to Fort Colllins and the Pueblo extension service with specimens. I purchases two traps. I have been getting about 50 beetles per day in the trap in my yard. I am moving the other trap to other locations to see how far the spread is. The extension service also has 5 traps to monitor spread in the city.

    My house is near the park, golf course and Arkansas river. With the number of beetles I have seen I would suspect that there must have been a population in this area for several years. My concern is that they will spread along the Arkansas and Fountain water courses and become established in places where there is enough water for them to survive dry spells. Perhaps a long dry winter with temperatures below zero for extended time will eliminate them, but coordinated efforts to monitor them and control them may be more effective.

  2. In Englewood and Denver, the Japanese Beetles are devastating roses! The beetles love Virginia Creeper as a habitat but will eat roses before they’ll eat the creepers. And they are voracious eaters. Pull out all your Virginia Creeper or any other of their favored habitat. It’s amazing how many beetles the vines will accommodate.
    Patrol for beetles in the morning. They come out of the lawn in the morning to feed, and are still sluggish and more easily captured. You can control them by hand, by picking them off the plants and popping them into a jar of soapy water. Look into the petals and underneath flower heads and leaves.
    Getting the beetles right away is very important since they emit a pheromone attracting other beetles to an area where desired food is available. Soon, you’ll be overrun.
    Wear white clothing while you’re hand eradicating, so they don’t see you and fly off. It does work. I read it somewhere, tried it and found it to be true.
    Insecticide applied in late Summer, when the eggs are newly works best. A number of pesticides are available against Japanese beetles. Some ingredients to look for on pesticide packaging include carbaryl, acephate, and permethrin.
    We’ve been trying to get the extension office to conduct a public awareness campaign, but that most likely will not happen. Their information on the beetles is also lacking and in some areas, inaccurate. They also don’t seem very concerned about the invasion. The person I spoke with at the office was not at all helpful.
    Do an online search and bring up information from other states, particularly East Coast and Midwest. They’ve been dealing with these pests for years and have the best information.

  3. I forgot to say that “Insecticide applied to the lawn”. For options of control on your plants you’re most comfortable with, check online.

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