Nematodes, beetles, and spores… oh my!

by Jenevieve Hughes | July 17, 2014

Have you noticed a sudden prevalence of this pest in your garden?

When one of our Master Gardeners told us she’d been fielding inquiries at her county extension office about a particular insect eating everything in sight, we decided it was time to investigate the July scourge of the Japanese beetle.

The culprit is apt to ransack your garden and defoliate a number of vulnerable plants in the process, from rose bushes and crape myrtle to birch trees and grapes.

Popillia japonica typically arrives after July 4th to wage war in your garden, and it can be a real battle keeping this critter under control.

All things considered, these iridescent insects aren’t half bad-looking. With their metallic-green bodies and copper-colored wings, they appear to change color slightly depending on your angle of view or the sun’s illumination. Their wing casements (or elytron, from a Greek word meaning shield), are actually quite eye-catching. We don’t mean to sound shallow, but looks-wise, you could do a lot worse.

Japanese beetles are thought to have arrived in North America a century ago on an imported shipment of irises. In their native habitat of Asia, they face a greater threat from natural predators, which helps keeps their numbers down. Here in North America, however, they are prone to attack several species of plants and trees, including apple, linden, pin oak, cherry, plum, apricot, and peach!

These beetles are members of a broad family commonly called “scarab beetles”, which were highly venerated in ancient Egypt, where the sacred insects were frequently depicted on amulets imprinted with hieroglyphics. It’s a good thing the ancient Egyptians didn’t attempt any ornamental perennial borders or they probably would’ve felt a lot differently.

In early July, the adult beetles will emerge from your soil and spend the next 30-45 days eating all sorts of woody and herbaceous plants. Once they have stuffed themselves silly in your garden, the females move into a nice grassy patch of your yard to lay eggs in the soil. By September, grubs hatch and begin to feed on turf grass roots, and as temperatures drop in late fall, they’ll furrow down into the soil to hide out under the frost line.

The good news is that there is only one generation per year, so adults won’t emerge again until next summer, and there are several ways to treat the problem, while minimally impacting the environment. Master Gardeners Irene Skrybailo (Promisek at Three Rivers Farm) and Lea Anne Moran (Hill-Stead Museum) both weigh in with some organic solutions.

Two popular treatments include beneficial nematodes and milky spore, which can be purchased from a local garden center specializing in organic products. Irene says the goal here is to target grubs while still in the ground so the timing of the application is critical. Let’s break down a few specifics.

Beneficial nematodes are natural microorganisms that can wipe out grubs if applied properly. Use a hose sprayer or watering can to apply, and aim for early morning or evening, as ultraviolet rays and heat can destroy nematodes on the spot. Water the soil after application to further keep nematodes away from heat or light and to facilitate their transport through the soil. Most importantly, don’t apply them in conjunction with a chemical pesticide or they won’t stand a chance! And be patient, they can take a few weeks to take effect.

You can also apply milky spore to your soil. Milky spore is a bacterium, which, once eaten by the grubs, will multiply with successive generations and take the grubs down as it goes. After about two or three years, the amount of ‘good’ bacteria in your soil will have increased and will continue to act against the grubs. Avoid using other methods at the same time, says Lea Anne, as this may reduce the production of ‘good’ bacteria essential for healthy soil. And you’ll have the best luck if you can get your whole neighborhood to use it — a beetle-busting block party, if you will — so that you aren’t fighting the battle alone.

For adult beetles in the midst of their mid-summer madness, Irene says pheromone traps offer a “bring the boys to the yard” approach, where you’ll essentially invite all the beetles from your garden (and those in your neighbors’) to come boogie down in a scented bag. But while these traps can be an effective way to attract the insects, she warns that this method could bring more beetles to your garden than you bargained for, and there’s a good chance that they’ll prefer your plants to the trap!

Want a sure-fire solution? Lea Anne suggests an early morning walk through your garden with a pail or bottle of soapy water. Look for beetles on the top leaves of any affected plants, and tap the leaf over the bottle until the beetles drop into the soapy water.

You can also add natural repellents such as catnip, garlic, tansy, and chives to your garden for an anti-beetle approach that is as fragrant as it is pretty.

When in doubt, our gardening gurus feel strongly that the best methods for controlling garden pests are those with the least negative impact on the environment.

If at all possible, choose an earth-friendly approach!