Most summers bring unwanted 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, a plant knowledge specialist at the Arboretum’s Plant Clinic. Morton. “We started to see them on river birches and basil,” she said. “They have a lot of different restaurants they can go to.”
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Their varied tastes are one of the reasons why Japanese beetles are more difficult to manage than some other insects that feed on just one type of plant. “It’s hard to control something that can eat up half the things in your garden,” Yiesla said.
Beetles can fly and emit pheromones into the air that attract others of their kind. If you have Japanese beetles this tends to lead to more Japanese beetles.
Attempting to exterminate them with insecticides – which would also harm many beneficial insects – would ultimately be in vain. “If your garden has good things to eat, more beetles will come right from the bottom of the block,” Yiesla said.
However, there are steps you can take to reduce the damage. Here are some tips from the Clinique des Plantes:
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- Get them early. If you tackle the first beetles as soon as you spot them, you may be able to reduce the beetle crowds. “Try to prevent them from signaling to other beetles that they have found a good source of food,” Yiesla said.
- Do not use traps. “Japanese beetle traps are a great way to invite all the neighborhood beetles to feast on your backyard,” she said. The traps, which employ these 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 can annoy the beetles and encourage them to soar into someone else’s landscape.
- Drop them. For smaller plants like rose bushes, fill a small bucket with water and pat or shake the branch to make the beetles fall into the water, where they will drown. A drop or two of dish soap in the water will prevent the beetles from escaping. “Don’t use insecticidal soap,” Yiesla said. “Insecticidal soap only works on insects at one stage of their life when they have a soft body, like larvae. It will do nothing to an adult beetle protected by a hard shell.
- Stop watering the lawn. Japanese beetles love to lay their eggs in lawns with moist soil. “The eggs hatch into larvae 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-comfortable nursery for Japanese beetles, don’t water the grass. Let the soil dry out and the grass goes dormant during the summer.
- Do not treat lime trees with imidacloprid. In the past, flowering lime trees were regularly treated in the spring with a pesticide called imidacloprid to kill Japanese beetles. Unfortunately, the pesticide also killed the bees. Consequently, this use of imidacloprid on any species of lime (trees of the genus Tilia) is now against the law.
- Keep your point of view. “Japanese beetles won’t kill your plants,” Yiesla said. “Plants may look chewed, but they will survive it. It’s just a problem of ugliness.
For advice on trees and plants, contact the Morton Arboretum Plant Clinic (630-719-2424, mortonarb.org/plant-clinic, or email@example.com). Beth Botts is a writer at the Arboretum.
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