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University of Illinois Extension

White Grubs

White grub larva

White grub larva  

Frequency

5 (1 = rare 5 = annual)

Severity

5 (1 = very little damage 5 = plants killed)

Hosts

All turfgrass species grown in the Midwest may be damaged by white grubs.    

Symptoms

In general, 10 to 12 grubs per square foot or more will eat enough roots to cause dieback of the turf. The turf will turn brown and can be easily turned back like a carpet because few uneaten roots remain. The area not only becomes unsightly, but its usefulness may be reduced in golf courses, football fields, and other athletic turf. Heavily used areas or turf in dry years may become severely damaged with relatively low numbers of white grubs. Similarly, turf areas that are used lightly or irrigated may retain a good appearance with a relatively high number of grubs per square foot. The ability of the turf to replace eaten roots can compensate for high grub numbers.

Various animals are attracted to white grubs for food. Raccoons will roll back large areas of turf a foot or more wide to eat the grubs in the root zone. Armadillos dig several holes per night, each of which will be several inches across and deep. Skunks will typically dig up small, 2- to 3-inch diameter divots of sod to eat the grubs underneath. A single skunk may make 100 of these small holes per night. Insect-eating birds, such as starlings, will peck through the turf to obtain and eat the grubs. When a flock works over an area in this way, the result is a brownish area that on closer inspection is seen as damaged turfgrass plants. All of these animals will damage turf that appears to have as few as 3 to 5 white grubs per square foot, thus causing injury in areas where grub numbers are below as well as above turf-damaging levels.  

Life Cycle

Several species of white grub attack Illinois turfgrass including black turfgrass ataenius (Ataenius spretulus), Japanese beetle (Popilla japonica), masked chafers (Cyclocephala spp.; adults are also known as June beetles and larvae are known as annual white grub), and May beetles (Phyllophaga spp.; larvae known as true white grub or three-year white grub). The information on this page is written to address all of these species of white grubs.

White grub larvae are white and soft-bodied, with brown heads. They have 6 legs and the body is curved into a C-shape. Actively feeding larvae are blackish at the posterior end of the body. They live in the turf root zone where they feed on roots and some decaying organic matter. The larvae pupate underground, later emerging as beetles. White grub adults are heavy bodied, with widened front legs that are used for digging through the soil. The adults dig out of the soil, mate, and fly to other areas. They burrow into the soil to lay their white eggs, which hatch into larvae.  

Management

Scout at the correct time for white grub species to determine the need for control measures. For most species, this is in the first half of August. This can be done by cutting through the turf with a carpet knife, large pocket knife, or other heavy-bladed knife, and pulling back 1/3 to 1 square foot of turf. The turf will naturally separate from the soil in the root region where the white grubs will be actively feeding. Most of the white grubs will be easily seen by their white color, but a few additional ones may be discovered by looking at the thatch underside and by lightly tilling the underlying soil with the knife blade. In most turf areas, the grubs will be in the root zone, although in very dry soils the grubs may have moved down a few inches for moisture.

A quick count of the grubs followed by quickly returning the turf to its original position and tamping it down with your foot should allow the turf to continue growing with no ill effects. Select turf areas that are known to have had grub damage in the past or those that are not under tree canopies. Select areas near pavement that would have attracted the adult females due to warmth, or choose more highly-watered areas, such as near flower or vegetable gardens or along the edges of tees and greens on golf courses. Residential front yards commonly receive more irrigation than back yards and are more likely to have high grub numbers.

Turf areas that have perennial grub damage or highly irrigated turf areas with few trees during dry summers will almost always have treatable grub numbers, particularly if a large adult grub flight was noticed earlier. Even so, scouting a few spots of such turf areas is helpful to reinforce the need for control. Realize that during rainy summers, the adults frequently lay eggs over such large areas that few areas will receive enough eggs and resulting larvae to cause damage. Cooler summers may help increase the occurrence of fungus diseases and other natural grub control agents, and cause a population collapse with little resulting turf damage. Finally, white grub eggs may die under very high summer soil temperatures or with very dry soils resulting in few larvae. This reinforces the need for at least casual scouting to reduce the chance of needless insecticide application.

The control of annual white grubs is primarily accomplished by the application of insecticides into the root zone of the turfgrass to kill the feeding larvae. In general, mortality in excess of 60 percent is considered to be adequate to drop the white grub population below the injury level and thus accomplish the goal of the application.

Insecticide longevity in the soil ranges from about a week to several months, depending on the product. Grubs should be present before a short-lived insecticide is applied. Insecticides that last 3-4 months can take 3 weeks or so to kill the grubs, so treatment will be most effective if applied when the eggs are being laid. An analysis of the numbers of adult white grubs coupled with rainfall and other weather patterns can help you determine whether treatment is needed.

Insecticides will be most effective if they are watered into the root system where the grubs are located. Because insecticide applied as liquid can dry on the turfgrass in as little as 30 minutes, they should be flushed into the root zone with at least 1/2 inch of water within that time. Even insecticides whose labels state that irrigation is not needed will degrade from the ultraviolet light in sunlight if not watered into the soil within a few days by rainfall or irrigation.

Areas where irrigation may take several hours to several days with hose and sprinkler should be treated with granular insecticide. Barring any small rains, this insecticide will remain in the thatch until watered into the root zone with at least 1/2 inch of irrigation. Improper flushing of the insecticide into the root zone as well as improper calibration are common reasons for failure of grub control in residential turf.

Nonchemical management of annual white grubs relies on causing a turf area to be less attractive to egg-laying than nearby areas. Reduced irrigation in late June into July when the adult annual white grubs are flying will make a turf area less attractive. Cool-season grasses, particularly bluegrass, will go dormant under hot, dry conditions, resulting in reduced growth and a brownish appearance. In some areas, this appearance may be unacceptable; but if it can be tolerated, it will likely eliminate the need for insecticide control of white grubs later.

Years with heavy rainfall will cause less irrigated areas to be as attractive to adult grubs as more heavily watered areas. In those years, eggs are usually spread out over too large an area to result in high enough grub numbers to cause damage in most areas, irrigated or not. Because annual white grub adults avoid laying eggs under tree canopies, the presence of trees will reduce egg laying in those areas and result in lower grub numbers.

Insecticidal nematodes, which are tiny worms parasitic to white grubs, have to be applied soon after shipment to wet grass and irrigated into the soil before they dry on the grass blades. They need to be reapplied each year that grub control is required. Although more expensive and time-consuming as well as providing a lower level of control than chemical insecticides, they do provide a non-chemical control alternative.



Filed under plants: Turf

Filed under problems: Insects Damage

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