Japanese Beetles: They’re Back! – Turfgrass Science at Purdue University

Japanese Beetles: They’re Back!

The first Japanese beetles of the year were observed in central Indiana last week and now is a great time to review the biology and control of these pests.

Japanese beetle adults pupate from larvae in late spring and begin emerging at this time of the year. We expect them to arrive in full force within the next 1-2 weeks and begin feeding on their favorite plants. Plants particularly at risk include those that have been recently transplanted or those that are stressed for one reason or another. Favorites for Japanese beetle include linden, crab apple, plum, and other fruit trees, rose bushes, grapes, and several garden variety vegetables. Though it’s impossible to protect all trees and ornamentals in any particular landscape, a few select plants can be protect by physical (netting or picking off the beetles each day) or chemical controls (Sevin, Orthene, Malathion, Azadirachtin or any of several pyrethroids). Repeat applications of these insecticides are required to provide protection for the two months or so during which these beetles are feeding.

During the next two months, these beetles will mate and the females will lay eggs in the turf. Females look for areas that can be easily burrowed into, and thus irrigated soils of golf courses, lawns, and athletic fields are often infested, especially in drier years. Adult females also prefer to lay eggs in lower cut turf. These eggs hatch into larvae (white grubs) over the next month or so and it is these larvae that feed in the soil and thatch, severing roots, and causing drought stress and potentially thinning and death of the turf. Frequent irrigation and/or rainfall may limit damage significantly, but dry conditions exaggerate the damage from grub feeding. It is at this time that insecticides are effective because the grub control products work only AFTER the grubs have hatched – usually the end of July or the first part of August. Two very effective grub control products include imidacloprid (Merit) and halofenozide (Mach 2). A more recent addition to the professional market is clothianidin (Arena) and results to date show that clothianidin is just as efficacious as imidacloprid and halofenozide. These products are long-lasting and are most effective if applied from late June through July. As with all insecticides, it is critical to follow the label directions exactly when making applications. The insect parasitic nematode Heterorhabditis bacteriophora can also provide very good control when used against young white grub larvae. However, all grub control products, chemical and biological, should be irrigated into the soil with at least 1/4 inch of water immediately after treatment. In cases where irrigation is not possible, timing the application just before a significant rainfall event is recommended and if nematodes are used, applications are best made in the early morning or evening to avoid prolonged exposure to UV radiation (sunlight).

If grub insecticides fail or turf areas are unprotected, symptoms may begin to show up in September. Drought stress, loose turf that rolls up like a carpet, and in the worst case, skunk and raccoon damage are definite symptoms of grub damage. Dylox is the most commonly used insecticide at this time of the year although Merit and Arena can provide good control as late as the last week in August. Regardless, at this late stage, grub control products must be watered in thoroughly and may require 7-10 days to produce results, so don’t rush to a second application.

Grubs are too large to control by October, plus the returning rains will probably limit damage. As the temperatures cool, the grubs cease feeding and move down as much as 1 foot into the soil profile. Grubs contain a form of antifreeze so even the coldest of winters doesn’t affect survival. Thus, insecticides are no longer recommended at this time

As the soils warm in the spring, Japanese beetles move higher in the soil profile and begin feeding to a limited extent. Though we commonly see grubs in the spring in lawns and beds, it is rarely worth the effort to control them. Not only are the grubs large and tough to control, the spring rains limit any damage from root feeding. Plus controlling grubs at this time of the year in a particular property will have no effect on populations later in the summer.

Lean more about white grubs and other turf insect, weed and disease issues at the Turf Field day on July 18th.

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