Controlling Japanese Beetles

Japanese beetles are a very destructive pest in both the adult and larvae stages. The larvae, called white grubs, feed on plant roots and organic matter in the soil resulting in dead patches of turf that can be picked up like carpet. The adults, a brilliant metallic green beetle, eat the leaves and flowers of of over 300 plants.

The adult Japanese beetles normally emerge from the soil during the last week of June through July and immediately begin feeding on host plants. After feeding and mating for a day or two the females burrow into the soil to lay eggs 2-4 inches below ground level. They repeat this process for about a month laying a total of 40 – 60 eggs. The eggs hatch in 8 – 14 days and the larvae begin to feed on roots. They dig deeper into the soil in late fall to over-winter and return to the surface in the spring.

Beetles can be controlled by several methods depending on their life stage.

Adult Control
Traps and insecticides are the most popular controls used adult beetles. Traps need to be kept away from plants that attract beetles or the feeding damage can be increased. Also, attracting the beetles may result in larger grub populations not reduced. Several over-the-counter sprays are available for adult control including Sevin and pyrethroids. During heavy adult activity sprays may need to be applied every 5 -10 days.

Grub Control
Biological controls include bacterial milky disease and entomopathogenic nemotodes. Both these controls have only been proven to be marginally effective. They require time to build up in the soil and insecticides against the grubs should not be used during this period.

The grubs are best controlled when they are small and actively feeding near the soil surface, usually late July to mid-August. Several insecticides have proven to be effective including Dylox, Sevin, Merit and Mach2.