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Bug Profile: Japanese Beetles

IMG_6092About two weeks ago, we started noticing copper and emerald droplets all around our farm. They congregated mostly densely on the asparagus, mainly in twos, and when touched, flew away, though not far.

Japanese Beetles, a handsome and destructive bug, is expected to visit Northeastern gardens in July, and this year, showed up in ours exactly on schedule. The beetles, as the name suggests, hail originally from Japan, where they have natural predators and aren’t a huge threat to cultivated plants. However, having hitched a ride across the ocean on the backs of some iris bulbs (bound for the 1916 World Fair), the beetles found a vast land empty of predators and full of gardens. Over the course of the last century, the beetles have been found to have an appetite for over 200 cultivated ornamentals and edibles.

Gardening resources unanimously describe the damage caused by Japanese Beetles as “skeletorizing” – a terrifying, yet accurate, visual for any home gardener. The beetles eat a plant’s soft foliage, leaving only stems and veins to blow sadly in the wind. The beetles also munch on the flowers and fruit of a diverse range of plants. Corn, tomatoes, and beans are among their top favorites. The danger of the damage they inflict is not just physical destruction but opening access points for disease to infect already weakened plants.

The beetles are only active during the day and can fly up to 5 miles. Communicating via pheromones, Japanese beetles can sense when other beetles are nearby, mating and eating, and fly to join the group. They are also able to know when a choice damaged plant is close and will fly toward easy lunch. To control these tricky pests, we suggest:

Picture 1Hand Pick: The best way to keep a Japanese Beetle population at bay is to pick them off by hand and smush or drown them in soapy water. This is best done early in the morning, when they are still sleepy… well, wet with dew and unable to fly quickly. Spread a cloth under the plant that the beetles have occupied, then shake onto the cloth, gather up, and dispose of. Then, cover the plant with row cover for continual protection. Hand-picking applies both to adult beetles and their larvae stage. Japanese Beetle larvae looks like grey to white grubs with brown heads, often crescent shaped, and commonly found living in (and feeding on) plant roots, with the roots of grasses their favorite delicacy.

Chickens and Starlings. If you share your yard with chickens, let them wander where Japanese Beetles are spotted: they will feed on adult beetles as well as underground larvae. Starlings will do the same – you can help them out by turning your soil in the fall and exposing the larvae for them to feast on.

Natural Bug Repellant. Tansies, Rue, Garlic, Chives, and Catnip are said to deter the beetles. Border your garden with these plants to cut down on beetle traffic.

Take care in heat and stillness. Japanese Beetles’ idea of perfect weather is 85 degrees and above and no wind (which is why they become active in the Northeast in July). When such conditions occur, keep in mind that new beetles might be on their way to your garden.

Skip the traps. Multiple studies have shown that the pheromone-filled traps are not effective. They actually attract more beetles into the garden than they are able to catch.

3 thoughts on “Bug Profile: Japanese Beetles”

  • Terre Levin

    You forgot to list milky spore. It's supposed to be applied, preferably in late fall - hopefully you'll have a relatively mild winter, and it's supposed to prevent the beetles from growing and becoming adults. I hope it's true. My own observation is that it is, but that's not scientifically valid.

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  • Mary Lou

    Thanks! As much information as you can provide about their life cycle and natural means of eradication are greatly appreciated. This is a great start.

    At our farm, they congregate on grape vines, roses and a few other plants. They can devastate my grape vines in no time flat.

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  • Sherry Milliken
    Sherry Milliken 08/07/2014 at 4:29 am

    In my garden they will demolish my grapes and have completely "skeletonized" my cherry tree this year, in spite of my daily beetle picking into a bucket with kerosene. I tried the soapy water and some were able to escape by climbing on top of the other beetles and flying off. Maybe my water wasn't soapy enough.

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