General Symptoms of White Grub Damage to Turfgrass
White grubs are among the most destructive insect pests of turfgrass. They feed below the soil surface on the roots and rhizomes of all commonly used turfgrass species and cultivars, and are capable of eliminating the entire root system of the plant. Where abundant, white grubs are capable of destroying large areas of turf in a short period of time.
Sampling TechniquesSampling for white grubs should begin early in the predicted grub activity period, and before signs of injury are visible (once damage appears, considerable root injury will already be present). Since white grubs do not distribute themselves evenly throughout the turf, it is essential that the entire turfgrass area be sampled in a consistant, uniform pattern. Enough samples should be taken to assure a reasonably accurate estimate of white grub numbers in the sampled area. At each sample site, cut two 1/4 ft (6" X 6") sections of turf on three sides, peel back the sod and examine the upper 2 inches of root zone for the presence of white grubs. Examination is aided by shaking or breaking the sample, and by probing through the soil and roots with a sharp instrument such as a pocket knife or screwdriver. Turfgrass managers with access to a golf course cup cutter can substitute 4-inch (0.1 ft) diameter turf-soil core samples. If no white grubs are detected but damage is present, examine the turf for other causes of injury such as disease, excessive thatch, moisture stress, heat damage and/or sod webworm or billbug feeding.
Management StrategiesIn most years, the first week in August is the optimum time to control white grubs in turf -- if sampling indicates that corrective measures are justified. At this time, most eggs should have hatched, and the small grubs will be feeding near the soil surface where they are more easily controlled. Damage from these grubs will not become apparent until later in the season when grubs are larger and the turf is stressed by hot, dry weather.
Treatment GuidelinesThe following thresholds are estimates of the average number of white grubs necessary per unit area of turf to produce visible injury and are provided to assist the turfgrass manager in making treatment decisions. Remember that the condition of the turf, its value, and the damage caused by birds and animals searching for grubs, may alter these thresholds. In general, if white grub numbers exceed these thresholds in non-irrigated turf, an insecticide application is justified. Irrigated areas should be able to withstand substantially more white grub pressure before visual injury occurs. Treatment decisions should be based on average numbers of white grubs detected in the sampled area. If white grub numbers exceed threshold levels in only a few isolated patches, consider controlling these grubs with spot-treatments.
Insecticidal ControlEffective chemical control of white grubs depends on moving the insecticide down to the root zone where the grubs are feeding. This is best accomplished by applying 1/2 to 3/4 inches of water immediately after application. Repeat irrigation every four or five days to continue moving the insecticide into the soil. This also keeps the crown and root area moist to encourage recovery of the turf. If conditions have been very hot and dry and grubs are deeper in the soil, a pretreatment irrigation of 1/2 inches applied 48 hours before the insecticide application should encourage grubs to move closer to the soil surface and enhance the level of white grub control. Thatch plays an important role in reducing the efficacy of turf insecticides applied for white grub control. If the thatch layer exceeds 1/2 inch, a light aerification and increased post-treatment irrigation will enhance insecticide penetration and should improve white grub control. In many cases, some level of white grub activity will remain even after treatment. However, proper management should minimize damage from reduced infestations. Appropriate fertilization and watering to encourage a healthy turf will enable the grass to recover and reestablish in damaged areas. In some cases, a second insecticide application may be necessary to achieve an acceptible level of white grub control. This can be determined by resampling the area in question.