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Japanese beetles ransacking local gardens-what to do about it


August 28, 2019

Courtesy Photo Japanese beetles leave behind leaves and flowers that look like demented lace, and they’re not choosy about what they gobble.

Japanese beetles—those colorful, iridescent compact munching machines that have been decimating gardens up and down the Front Range this year—have reached record numbers in our area, and will continue to multiply and thrive unless a combined effort from all of us helps curb their proliferation.

If you don’t know what these destructive insects look like, you can track them by their trail of indiscriminate feasting. They leave behind leaves and flowers that look like demented lace, and they’re not choosy about what they gobble. Follow the destruction and you’ll soon find beautiful scarab beetles with green heads and iridescent coppery shells. If you’re a bug lover, you’ll be enchanted with their look until you realize how destructive they are and will continue to be. Then you’ll move quickly from entomologist to vengeful bug murderer.

Colorado was invaded by the beetle in the 1990s, and the numbers have grown steadily since. Gobbling up over 300 species of fruits, ornamental plants and grasses in the Colorado landscape, these wee beasties need to be controlled by both commercial and private parties in order to bring the numbers of the invasive pest down each year.

To understand the “how” of Japanese beetle (JB) management, it helps to know why these moisture and humidity-loving beetles thrive in our semi-arid climate. A look around at the verdant lawns, lush gardens and well-watered road medians in Boulder County provides the answer: We’ve created a perfect environment for the life cycle of this invasive pest. They love our well-irrigated landscapes, and they adore such plants as roses, Lindens, Virginia Creeper, hollyhock and grapes—all very common in our area.

The most common and very time-consuming method of dealing with the bugs involves plucking them from plants and chucking them into a container of soapy water, or squashing them immediately. Be quick about both methods, because they can fly away fast. This method takes care of the JB on your plants, but what about your neighbors’ gardens?

As mentioned, these little munchers fly. So if you’re plucking and squashing, you might want to encourage your neighbors to participate in the same activity.

This method is fine and dandy for the immediate defense of your prize-winning roses, but considering how rapidly the JB has spread along the Front Range, it seems that we need to take a multi-pronged approach to their management. A concerted effort that involves cultural, biological and possibly chemical controls.

The cultural effort needs to include educating ourselves and our neighbors about the JB and its life cycle. The adult JB burrows into the ground and lays its eggs only in moist soil around July. Its fat, white grub then lives deep in the soil until the next year’s late spring, when it emerges and gets ready to fly up to a mile for a meal.

If we all participate in watering less during the critical underground life stages of the JB grub July-September; growing lawn grass longer between mowings; aerating lawns with good old-fashioned lawn spike shoes; and promoting healthier root growth in lawns, as the JB grubs eat root masses, and a healthier root mass can tolerate their excessive munching. Also worth consideration: planting fewer plants the JB loves.

The next step—biological control—happens all year long, and while it isn’t for immediate control of the JB, we’re playing the long game here. Plus, it’s safe for the bees.

Beneficial nematodes are actually parasites that enter through the body walls and natural openings of the JB grub, and introduce a bacteria into the grub which then kills it. The handy nematodes then eat up the bacteria and the decomposing body ot the grub. Nematodes must be applied to your landscape in the spring and fall. They can be purchased at garden supply stores and online.

Photo by Dani Hemmat Colorful and iridescent, Japanese Beetles have showed up in droves on the Front Range this year and are leaving a wake of destruction in their path.

Before you’ve deposited those handy nematodes, use Milky Spore. This is a bacterium that is lethal to the JB grub, and has no known effects on beneficial insects or food crops. You can apply it dry to your soil, then spray on your nematodes for a one-two punch to the JB life cycle. Know that these methods take a year or two to establish better grub control, but they are effective and worth it.

As far as chemical controls go, we don’t want to use anything that would harm beneficial insects, like our beloved and endangered bees. Pyrethrins, chemicals that are found naturally in some chrysanthemum flowers, are commonly used to control such insects as fleas, can be used to kill the JB, but they are highly toxic to bees. Natural is not synonymous with safe.

Neem oil is a natural repellent that, when sprayed on the desirable plants, discourages feeding. It doesn’t harm anything, but when applied before the JB is spotted or right at the first sighting, it helps keep them off your plants. Neem must be applied regularly and consistently to do its job. It’s sort of like spraying liver on chocolate cake. It just won’t be tasty after that.


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