Pest Profile: Japanese Beetles

Every July they visit. Thousands upon thousands of them. Hungry invaders that will stop at nothing to make sure any joy we get out of our summer gardens is destroyed swiftly and completely. I speak of Japanese Beetles, the scourge of all rose gardeners and anybody who enjoys having a nice lawn.

These little copper and green beetles were brought into this country in a shipment of iris bulbs from Japan in the early 1900s. They have since spread over the majority of the country east of the Mississippi river, and feed on a wide range of hosts, including the entire Roseaceae family, which includes apples, peaches, roses, raspberries, and other flowering and fruiting plants. Blueberries are another favorite food of these little pests.

Japanese beetles have a one to two-year life cycle, starting as eggs and then larva, which feed quite happily on tender roots of lawns and garden plants. The larvae often attract moles and voles, which do further damage to the underbed. July brings a hatch of shiny hard-shelled beetles which proceed to eat seemingly everything in sight, before disappearing to lay more eggs in late summer. We can usually get one good round of roses in before they are munched beyond blooming, and then a late-season bloom after they’ve gone.

Beetle control consumes quite a bit of our efforts here in the northeast, and we do have a reasonable arsenal at our disposal. The simplest method is to apply bacteria commonly called Milky Spore to lawns and gardens. The bacteria are consumed by Japanese beetle grubs, which then die within a week or two, releasing more bacterial spores into the soil. Fewer grubs mean fewer adults (and fewer moles/voles), and if the breeding cycle can be broken, our gardens will be much happier. There has been some debate about the effectiveness of Milky Spore in this area, as the coldness of our winters is perhaps detrimental to the bacterial populations. I have not found this to be true; people I’ve talked to have consistently reported fewer mole trails in their lawns in the spring, and seemingly fewer adult beetles, at least for a few weeks. The trick, of course, is to get the entire neighborhood to treat their properties, and fight the beetles together.

“Beetle Bags”, or pheromone traps are a good way to attract more beetles to your yard. They are effective in that they will trap and eventually kill the clumsy flying beetles, but overall will attract more insects than they catch, making them not terribly effective. Insect killing sprays work to knock down the beetles that are on the plants already, and often have lasting potency for up to a week afterward. Natural repellents include catnip, onions and garlic, and tansy. For small numbers, they can be effectively controlled by a soapy water spray, or hand-picking into a cup of soapy water. The resulting liquid after the beetles have died can be sprayed onto the plants as an additional repellent.

Besides the Milky Spore, we do have another natural defense against these predators. The tiny Tachinid fly will attach its eggs to the heads of Japanese beetles. When the eggs hatch, the larva bore into the beetle to feed, devouring it from the inside out. If you’re in the garden and see a beetle with tiny white spots on its shiny green head, leave it alone. The pest is done munching on your plants and its demise is quite imminent. Organic pest controls are essential to preserve the Tachinid fly population, as they are insects themselves, and broad spectrum insecticides that kill the beetles will also kill these tiny predators.