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.
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.