White grubs are the larval stage of beetles, such as the Japanese beetle and European chafer. These C-shaped larvae feed directly on turfgrass roots, causing patches of lawn to wilt, brown, and die. Successful control depends entirely on precisely timing the application to coincide with the most vulnerable stage of the grub’s annual life cycle. Understanding the pest’s development is necessary to select the correct product and apply it during the short window when it will be most effective.
The Grub Life Cycle and Timing Windows
The life cycle of the most damaging turf grubs in Michigan, primarily the European chafer and Japanese beetle, generally spans one year. Adult beetles emerge from the soil in early to mid-summer, with the European chafer appearing in June and the Japanese beetle in July. After mating, the females burrow into the soil to lay their eggs, typically during July and August.
The eggs hatch into tiny larvae in late July or early August, and these first-stage grubs begin feeding on grass roots near the surface. They grow rapidly, entering their second and third larval stages throughout late summer and fall. This period, from late August through October, is when the most significant turf damage occurs because the grubs are actively feeding.
As soil temperatures drop, the nearly full-grown grubs burrow deeper below the frost line to overwinter. They remain dormant until the soil warms again in early spring, typically March or April. They briefly migrate back to the root zone to feed before pupating. By late May, the grubs stop feeding, transform into pupae, and emerge as adult beetles in early summer to restart the cycle.
Optimal Timing for Preventative Treatment
Preventative grub control is the most effective strategy because it eliminates pests before visible damage appears. This approach relies on applying systemic insecticides early enough so the chemical is present in the grass roots when the eggs hatch. The optimal timing window for a preventative application in Michigan is generally from mid-June through July.
Products containing Imidacloprid, Thiamethoxam, or Clothianidin should be applied during the mid-June to mid-July window. These insecticides target the newly hatched grubs in late summer. Applying them close to the egg-hatch timing maintains potency when the small larvae begin feeding in August. This ensures the chemical is present in the root tissue, poisoning the grub upon its first bite.
For products containing Chlorantraniliprole, the application timing shifts earlier, ideally to late April or May. This active ingredient moves more slowly through the soil to the root zone where the grubs feed. Applying it early provides the necessary time for the insecticide to be fully absorbed by the turf before the eggs hatch. Regardless of the active ingredient used, immediate irrigation is necessary after application. At least one-half inch of water is required to move the insecticide down to the root level.
Identifying and Treating Active Infestations
Curative treatment is required when preventative measures were missed or failed, and signs of an active infestation are present. Damage typically appears in late summer or early fall as irregular brown patches that do not respond to watering. A tell-tale sign of severe infestation is turf that feels spongy and can be easily rolled back because the roots have been severed.
Increased activity from foraging animals, such as skunks, raccoons, or birds, also indicates a grub problem, as they dig up the lawn to feed on the larvae. When these signs appear, a curative insecticide must be used to stop the feeding grubs before widespread root loss occurs. The most common active ingredients for this purpose are Trichlorfon or Carbaryl, which are fast-acting compounds designed to kill the larger, actively feeding larvae.
The best time to apply these curative materials is from late August through October, when the grubs are still feeding near the soil surface before they burrow deep for the winter. Curative treatments can also be applied in early spring, from March to May, when the grubs briefly return to the root zone. Spring treatments are less consistently effective because the grubs are larger and preparing to pupate, making them less susceptible to the insecticide. Curative applications require prompt watering to ensure the chemical reaches the root zone and contacts the grubs while they are still feeding.