Those Pesky Japanese Beetles!

— Written By and last updated by Margaret Green

It is that time of year again when Japanese beetles are flying around and devouring plant material. We have been fortunate over the last couple of years to have low populations of Japanese beetles. This is due to the dry weather that we have been having. Unfortunately, the dry weather has hampered the success of our gardens and landscapes.

Japanese beetles feed on over 275 different kinds of shade and fruit trees, shrubs, flowers, small fruits, garden crops and weeds. The beetles tend to aggregate in response to odor released by damaged plants and a pheromone released by female beetles. Soon after emerging female beetles burrow two to three inches into damp soil and deposit 40 to 60 eggs in small batches. In extremely dry weather, many eggs and larvae perish. In warm wet summers, eggs hatch in about 2 weeks. There is only one generation per year. The larvae burrow through the soil feeding on roots until cold weather forces them into hibernation. The grubs overwinter in soil about six inches deep, and move close to ground level in the spring where they complete feeding and pupating. The peak feeding time and abundance of beetles is in late June or early July.

Spraying pesticides can protect flowers and ornamental plants. However, there are other options for control besides pesticides. They can easily be knocked off the plants in a jar or bucket of soapy water. If insecticides are your weapon of choice carbaryl, malathion, or imidacloprid are useful products. Pyrethrin containing products are slightly more persistent. Carbaryl will protect foliage for about five days and pyrethroid products may give up to two weeks. All products are weather dependent.

Controlling the larvae (grubs) can be done easily by using the right product at the right time. Keep in mind the grubs are feeding in the spring as soil temperatures are warming up near the surface. Therefore, quick acting insecticides like granular carbaryl will work in the spring. In late June and early July the eggs are hatching near the surface, therefore, at this time a granular imidacloprid or bifenthrin product would work better. Milky Spore Disease (Bacillus popilliae) is a soil treatment where the spores infect and kill only Japanese beetle grubs. Milky spore can be applied from July until the first hard freeze to large areas of turfgrass. Milky spore can be an expensive type of treatment and the control is as only as good as the application. No matter which product or approach is selected, be sure to follow the label always.

One thing to keep in mind is that beetle traps will attract beetles. They should only be used to monitor the pest not to control the pest. If you use a beetle bag you will need to empty it every two to three days. Deciding on the best option for managing Japanese beetles and grubs will depend on their activity in your landscape and surrounding area.

Timeline for Japanese Beetles

Japanese Beetle