It takes about three years to get established and provide significant control. This soil bacterium enters the insect's gut and bloodstream multiplies, and eventually kills the hosts. Milky spore is probably the most readily available organic grub control. Apply insecticides soon after the adults lay their eggs and the grubs are less than ½” in size when controlling existing grubs. Spring applications when grubs are large and mature grubs are not very effective at killing these. Proper timing and application of the right product are critical for effective control. The adults can fly up to two miles traveling in from surrounding untreated spaces. Managing the grubs can help reduce damage to the lawn but won’t necessarily reduce the beetles feeding on your edible and ornamental plants. Enjoy the short break and be ready for an increase in the future. You may have a large influx of these insects for several years followed by one with very few. Their populations do rise and fall over time and can vary throughout the area. And the cycle begins again as the soil begins to warm in spring. As soil temperatures drop in fall the full-size grubs move down through the soil for winter. The eggs hatch in several weeks and the small grubs begin to feed on grass and other plant roots. They prefer moist lawn areas and mulched garden beds. Once mated the females lay a small batch of eggs 2 to 4” below the soil surface. The adults feed for 6 to 8 weeks or more.
Much less math involved in that approach. Or you can just watch for their appearance in your garden. The daily numbers are added for a sum of the growing degree. This number is calculated by adding the high and low temperatures for each day, dividing by two, and subtracting the base of 50˚F from that. The beetles start emerging at 1000 growing degree days using a 50˚ F base temperature. This figure corresponds to the accumulation of warm temperatures needed for the beetles to reach the point where they are ready to emerge from the soil. Or you can use growing degree days to calculate the beetles' emergence from the soil. Your local University Extension Office can help you narrow down the time to begin watching for this pest in your region. The adult beetles emerge sometime in May in warmer regions and later in June or July in colder areas of the US and Canada. After several weeks they stop feeding, create a space in the soil and pupate, transforming into the adult beetle. As soil temperatures warm to 50 degrees in spring they move closer to the soil surface and start feeding on plant roots. These insects spend the winter 6 to 8” or deeper in the ground as grubs. Understanding the Japanese beetles’ lifecycle will help you manage this pest. These grubs look very similar to other grubs of insects like the May/June beetles and chafers. New hatchlings are about 1/8” long and grow to about an inch. Japanese beetles start life as a white C-shaped grub. They have coppery-brown wing covers and tufts of white hairs along the edges of their back making them easy to identify. These shiny, metallic-green beetles are slightly less than ½ inches long. This is often when I get the call to diagnose the lawn problem. More damage occurs when skunks and raccoons move into the area digging and dining on these tasty grubs. At this point, the grass can easily be lifted from the soil. As the population increase, larger areas are damaged and the roots destroyed. You may notice small patches of the lawn turning pale and dying just like drought stress. The immature beetles are grubs that feed on the roots of lawn grass and ornamental plants. Large populations can cause more severe damage resulting in brown and falling leaves. Some gardeners living with this pest refer to the riddled leaves on otherwise healthy plants as Japanese lace. Japanese beetle adults eat the leaf tissue between the veins resulting in a lacey appearance. Some western states have quarantine restrictions in place to help restrict the spread of this pest. Japanese beetles first arrived in the United States in 1916 and are currently found in most states. When immature, the grubs feed on the roots of lawn grass, vegetables, and ornamental plants. Those are just a few that are on my list as well. But if you are battling these insects, you have a list of your own like hibiscus, cannas, and many more.
Norway and Japanese maples, birch, crabapples, purple-leaf plum, rose, mountain ash, linden, grapes, and basil are among their favorites. These voracious beetles eat and mate in broad daylight feeding on the leaves and flowers of 350 different types of plants. Controlling Japanese Beetles in the Lawn and Garden By Melinda Myers - horticulturist and gardening expert
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