White Grub Management

grub drawing

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.
turf damagegrubs
After hatching from eggs, white grubs begin feeding on the roots and underground stems of turfgrasses. The first evidence of injury is localized patches of pale, discolored and dying grass displaying symptoms of moisture stress. Damaged areas are small at first, but rapidly enlarge and coalesce as grubs grow and expand their feeding range. Turf in such areas will have a spongy feel under foot and can be easily lifted from the soil surface or rolled like a carpet, revealing the C-shaped white grubs underneath. Damage is most apparent in mid-August through early September when white grub feeding activity is greatest. Inadequate irrigation and drought stress may compound damage to turf by white grubs. Egg-laying females are generally attracted to vigorous, well-watered turf, and adequate moisture is essential for eggs to complete development. Once eggs have hatched, however, white grubs will feed on either drought stressed or well-irrigated turf. Several animals, especially skunks, raccoons, and moles, are highly attracted to turf insect infestations, and signs of their foraging in an area are strong indications of white grub activity. Flocks of birds, particularly starlings, feeding in the turf provide additional evidence of a possible infestation.
cutting turfcup cutter

 

Sampling Techniques
Sampling 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 Strategies
In 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 Guidelines
The 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.

TREATMENT THRESHOLD
Grub Species Number/Square Foot Number/4-Inch Core
Masked Chafer 8 - 10 1
May/June Beetle 3 - 5 1
Japanese Beetle 8 - 10 1
Black Turfgrass Ataenius 30 - 50 3 - 5

 

Insecticidal Control
Effective 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.

Entomology News at Nebraska

Breaking Entomological News...

  • Dr. Susan WellerDr. Susan Weller, director of the University of Nebraska State Museum and professor of Entomology, has been elected a fellow of the Entomological Society of America (ESA). The society noted Weller’s internationally-known research on arctiine moths and other Noctuoidea, and her administrative leadership in promoting entomology and science education. See the article at UNL News for more about Weller's recognition. Congratulations Susan!
  • Marilyn Weidner & GracieWe are pleased to announce that Marilyn Weidner has been named winner of the IANR Exemplary Service Award! Weidner has roles in administrative support, personnel management, data documentation and archiving, review preparation, promotion and evaluation tracking, payroll for 15 departments, and much more. Well deserved Marilyn!
  • Rogan Tokach winning scholarshipCongratulations to Rogan Tokach (Master's student co-mentored by Dr. Autumn Smart and Dr. Judy Wu-Smart) on being awarded a $10,000 national Christi Heintz Memorial Scholarship by Project Apis m!
    Well done, Rogan!
  • Steve Spomer with Beetle CollectionsSpomer nets 40-year Butterfly & Beetle Legacy. Best wishes to Steve Spomer who is retiring this December after 40 years in the Entomology Department. Spomer has identified nearly 700 insect species, including the Salt Creek Tiger Beetle, and taught countless University of Nebraska-Lincoln students. 
  • Bridget Gross with Bees.Bridget Gross, master's student in Entomology, mentored by Dr. Judy Wu-Smart and Dr. Doug Golick, was featured in the Nov. 11th IANR Student Spotlight. Bridget presented her master's degree seminar last month and graduates in December.

  • Check out the Emerald Ash Borer Resource Center, and our Emerald Ash Borer Look-Alike Insects Sheet - Be Sure Before You Treat!