Most summers bring unwelcome guests to Midwestern gardens: Japanese beetles.
These iridescent green beetles are best known for feeding on roses and linden trees, but in fact they can feast on hundreds of different plants, according to Sharon Yiesla, plant knowledge specialist in the Plant Clinic at The Morton Arboretum. “We’ve started seeing them on river birches and basil,” she said. “They have a lot of different diners they can go to.”
Their varied tastes are one reason Japanese beetles are more difficult to manage than some other insects that feed on only one kind of plant. “It’s hard to control something that can eat half the things in your yard,” Yiesla said.
The beetles can fly, and they emit pheromones into the air that attract others of their kind. If you have any Japanese beetles, it tends to lead to more Japanese beetles.
Trying to exterminate them with insecticides — which would also harm many beneficial insects — would ultimately be futile. “If your yard has good things to eat, more beetles will just fly in from down the block,” Yiesla said.
Still, there are some actions you can take to reduce the damage. Here are some tips from the Plant Clinic:
Get them early. If you tackle the first few beetles as soon as you spot them, you may be able to reduce the beetle mob. “Try to prevent them from signaling to other beetles that they’ve found a good food source,” Yiesla said.
Don’t use traps. “Japanese beetle traps are a great way to invite every beetle in the neighborhood to feast in your yard,” she said. The traps, which employ those enticing pheromones, attract far more beetles than they kill.
Try a water cannon. Spraying an affected tree or large plant with a strong stream of water may annoy beetles and encourage them to fly away to someone else’s landscape.
Knock them off. For smaller plants such as rose bushes, fill a small bucket with water and tap or shake the branch to knock the beetles into the water, where they’ll drown. A drop or two of dishwashing soap in the water will make it harder for the beetles to escape. “Don’t use insecticidal soap,” Yiesla said. “Insecticidal soap only works on insects at a life stage when they have soft bodies, such as larvae. It won’t do anything to an adult beetle protected by a hard shell.”
Stop watering the lawn. Japanese beetles like to lay their eggs in lawns with moist soil. “The eggs hatch into grubs that will eat the roots of your grass plants,” Yiesla said. “Then they’ll turn into adult beetles and start on the rest of your garden.” To avoid making your lawn an extra-cozy nursery for Japanese beetles, don’t water the grass. Let the soil dry out and the grass go dormant during the summer.
Do not treat lindens with imidacloprid. In the past, blooming linden trees were routinely treated in springtime with a pesticide called imidacloprid to kill Japanese beetles. Unfortunately, the pesticide also killed bees. As a result, this use of imidacloprid on any species of lindens (trees in the genus Tilia) is now against the law.
Keep your perspective. “Japanese beetles won’t kill your plants,” Yiesla said. “The plants may look chewed up, but they will survive it. It’s just an unsightliness problem.”