The bane of many Maine gardeners’ summers has returned: Japanese beetles emerging from the ground to chew through crops, trees and ornamental plants.
Mainers have battled these beetles for almost a century, but populations are increasing around the state with help from milder winters. Beetles appear in July, lay eggs and stay active into September.
Their grubs hatch from eggs in the summer and hibernate in the soil before emerging as adults the following June or July. In the grub stage, they feed on plant roots and can destroy lawns.
The adults won’t necessarily kill plants they feed on, but they eat through leaves, fruits and flowers while other destructive pests are also emerging, such as cucumber beetles and squash bugs.
Japanese beetle reports have started trickling in over the last week, and appear unevenly distributed across the state, according to Jim Dill, a pest specialist at the University of Maine Cooperative Extension.
It’s hard to say yet how the season will look — last year turned out average overall, despite an early possibility for a population explosion — but Mainers in some areas have already told him they’re “clobbered.”
There are multiple approaches to controlling the beetles which gardeners can combine, but because the insects are strong flyers, it’s crucial to enlist the neighbors too.
Experts recommend vacuuming up the bugs or knocking them off plants into a bucket of soapy water, where they’ll drown. This is easier in the mornings and evenings, when the bugs are more sluggish.
Beetles with white dots on their bodies should be left alone, however, according to Dill. Those dots are the eggs of parasitic flies whose larvae will tunnel through the beetle’s shell and eventually kill it. Leaving them will help the parasites spread and keep future populations down.
Another popular option is pheromone traps, which attract more beetles and then kill them. Gardeners are divided over them, some believing they draw in additional bugs, but others have success. They are best placed at least 50 feet away from plants of concern to avoid bringing more bugs to them, and should be emptied regularly, according to Dill.
The university extension doesn’t recommend using milky spore, a bacterial treatment that kills beetles, though some gardeners say they’ve had success with it.
Longer term, treating plants with kaolin clay, neem oil, pyrethrin or beneficial nematodes can help, especially when beetles are in the grub stage. Plants can also be covered with netting.
It’s too late this year, but lawn treatments in June or earlier can reduce the number of beetle grubs before they emerge.