White Grubs

White grubs are the larval stage of several scarab beetles, including May or June beetles, masked chafers, and Japanese beetles.  Adult May or June beetles (Phyllophaga spp.) are oblong, robust insects.  There are many species in Oklahoma, but most are shiny, reddish brown or dark brown, and measure 3/4 to 1 1/4 inches in length.  They are often incorrectly referred to as “Junebugs”.  Adult masked chafers (Cyclocephala spp.) resemble May/June beetles but are smaller and yellowish brown.  White grub larvae are white, C-shaped grubs with distinct, brown heads and three pairs of legs near the front end.  Adult Japanese beetles (Popillia japonica) are about the same size as masked chafers, but the body is metallic green with bronze wing covers.  A row of five white tufts of hair are found along each side of the body next to the outer edges of the wing covers.  The larva is a typical white grub with a brown head and three pairs of short legs and measures about 1 inch when fully grown.

Life Cycle

May or June beetles (multi-year white grubs) have life cycles ranging from 1 to 3 years, depending on species and location.  It appears that many species in Oklahoma have two-year life cycles.  Adults emerge from the soil, some species as early as early April and others extending into mid September.  Adults of most species are most common in May and June, hence their common name.  After mating, females enter the soil and deposit about 50 eggs in small, earthen cells over a period of one to three weeks.  Larvae hatch three to four weeks later and begin feeding on dead organic matter, later moving to the roots of plants.  In fall, larvae move deeper into the soil for the winter.  The following spring they move back to the root zone to feed; most of their damage occurs during this time.  At maturity, they move deeper into the soil to pupate.

Masked chafers and Japanese beetles (annual white grubs) have one-year life cycles.  They overwinter as large larvae deep in the soil.  Larvae migrate upward in March and April and resume feeding until May; some damage may appear during this period.  They then move deeper in the soil to pupate.  Adults emerge during June and July and mate.  Females enter the soil and lay 10-30 eggs, which hatch two to three weeks later.  Damage from annual white grubs is most common in late August through September and October as they feed ravenously on turfgrass roots, developing into large larvae very rapidly.

Hosts

White grubs feed beneath the soil surface on the roots of grasses and other plants.

Damage

Aboveground symptoms of white grub damage are browning and dying of the grass in localized spots or in large irregularly shaped areas.  Where infestations are heavy, the grass roots may be entirely eaten away and the turf may be rolled back like a carpet.  Damage may be severe in September and October when grubs are reaching maturity.   White grubs are rarely a problem in bermudagrass but typically damage cool season grasses like bluegrass and fescue.

Inspection and Control

Examine the soil around the grass roots.  Dig in brown areas near the edge of green, healthy areas of turfgrass.  If the soil is dry, you may need to dig 6-10 inches deep to find larvae.  The field should be treated if five or more May or June beetle larvae per square foot are found.  Higher numbers (eight to ten per square foot) of annual white grubs can be tolerated as they are smaller than multi-year white grubs and more likely to feed on dead organic matter, especially when fully grown.  Studies on annual white grubs in Kentucky have indicated it takes at least eight or nine larvae per square foot to damage moisture-stressed Kentucky bluegrass turf.

There are two chemical control strategies targeting species of white grubs: preventative and curative or rescue treatments.  Both strategies can make use of systemic products that make the host plant toxic prior to and during egg hatch.  Preventative applications typically are applied in late May and can provide season-long control of newly hatched white grubs.  Larvae feed on roots of protected plants and consume a lethal dose of the insecticide.  Insecticides used for preventative white grub applications include neonicotinoid products containing thiamethoxam, imidacloprid, and clothianidin and products containing the insect growth regulator, halofenozide.  Preventative products are rapidly absorbed by turfgrass roots and tend to be most effective against young larvae.  In general, preventative treatments are not recommended unless the area to be treated has a history of white grub infestations and is clearly demarcated.  Treating the entire field preventatively requires more insecticide, money, and time than is required for effective white grub management.

Curative/rescue treatments are applied in locations where white grubs are known to occur from pest records for the site, monitoring efforts, or visible damage.  Curative/rescue applications are made any time white grub populations are high and/or damage is present, but are most common in late summer/early fall for infestations missed earlier in the season or in spring when large multi-year white grubs are active.  Insecticides used for curative/rescue treatments include trichlorfon, carbaryl, and clothianidin.

Turf should be watered thoroughly before treatment unless adequate rainfall has provided soil moisture.  This will bring white grubs closer to the root zone where the chemical will have a chance to work.  Following insecticide application, turf should be lightly irrigated (1/4 to 1/2 inch of water) to move the insecticide into the root zone where white grubs are found.  Finally, dethatching a field before treatment can improve efficacy of these insecticides by reducing the amount of chemical bound by thatch.