OKLA. CITY —
First, let me say that I hate white grubs. Not only for the damage they cause but also for the damage caused by the skunks, armadillos, moles and raccoons that dig up my yard searching for these tasty morsels.
White grubs are the larval stage of several beetles. Most grubs assume a “C” shape when they are dug out of the soil, and they have a well-developed brownish head and three pairs of well-developed legs. Grubs go through three stages or instars during their development and they feed on the roots of grasses. All managed grass areas are potentially susceptible to grub attack. When grub numbers are high enough, the grass can be lifted like a throw rug because the grubs have eaten the roots. At lower numbers, the grass may appear unthrifty.
The simplest method of sampling for grubs is to make cuts on three sides of a 12-inch square of turf with a stout knife or a shovel, pry this flap back, and carefully inspect the root zone and the upper 1-3 inches of soil (deeper in dry soil). If grubs are present, they will be found in this area. Sample several areas and determine the average number of grubs per square foot.
In Oklahoma, usual turfgrass grub damage is from masked chafers and May/June beetles. Masked chafers, have a one-year life cycle. Eggs are laid in the soil during the summer and the adult beetles emerge from the soil the next spring or summer. Damage from annual white grubs is most common in September and October as they feed ravenously on turfgrass roots before winter. The area should be treated if 8-10 larvae per square foot of these annual grubs are found.
May and June beetles usually have a two- to three-year life cycle. It appears that many species in Oklahoma have a two-year life cycle. The first and second instars are present for only a short time. Most of the life cycle is spent as a third-instar grub. Because nearly a year is spent in the third instar, considerable damage can be done by these grubs. In the spring, when these grubs move back to the root zone to feed after winter; most of their damage occurs. Populations of more than seven grubs per square foot can result in complete loss of roots. The usual treatment threshold is four to five grubs per square foot. Controls should be aimed at the first and second instar grubs because they are easier to kill and cause less damage.
There are two chemical control strategies targeting white grubs: preventative and curative treatments. Both strategies can make use of systemic products that make the host plant toxic to grubs. Preventative products are applied from late June to mid July to provide control of newly hatched white grubs. Larvae feed on roots of protected plants consuming lethal insecticide. Preventative products include Merit, MachII and GrubX. In general, preventative treatments are only recommended for clearly demarcated areas with a history of white grub infestations. Treating the entire lawn preventatively requires more insecticide, money, and time than is required for effective white grub management.
Curative treatments are applied in locations where white grubs are known to occur from site sampling or visible damage. Curative applications are made any time white grub populations are high and/or damage is present, but are more common in early fall for annual grubs or in spring when large multi-year white grubs are active. Insecticides used for curative 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 before treatment can improve efficacy of these insecticides.
RAY RIDLEN is a horticulture/agriculture educator for the Oklahoma County OSU Extension Service. He may be reached