Grubs are the C-shaped white larvae of various beetle species found in Michigan. Adult beetles lay eggs just below ground level in lawns from late June through September. Although grubs will feed on a wide variety of plant roots including ornamental trees, shrubs, and garden plants, their most significant damage is done to turf grass roots.
Grubs can cause serious damage to drought-stressed turf and can even harm irrigated turf. Drought-stressed turf may be killed when grub populations reach as little as 5 - 10 grubs per square foot. Regularly irrigated turf may be able to withstand up to 20 grubs per square foot before permanent damage occurs.
Turf grass that is being damaged by grubs first appears off-color, as if under water stress. Irrigating causes a short-lasting recovery or may provide no response at all due to the damage to the root system of the turf. The turf can usually be easily pulled back to reveal the grubs. Large populations of grubs kill the turf in irregular patches. Proper irrigation in the late summer and early fall can minimize or prevent turf damage if grub populations are not too high.
A secondary concern of grub activity is skunks, raccoons, and birds that feed on the grubs. These nuisance animals can tear up large patches of turf as they search and dig for grubs.
GRUB CONTROL IN LAWNS
The best control of grubs is obtained by targeting the young larvae in late summer or early fall. Preventative treatments such as Merit (Imidacloprid), or Mach 2 or Grubex (halofenazide), are applied in June or July.
Curative treatments can applied after grub eggs hatch in mid-August and up until the end of October.
Lawns should be irrigated after a grub control treatment is applied to move the grub control product into the root zone of the turf.
Grubs burrow deep into the soil to overwinter when the weather begins to cool in late October or early November. Grubs move back up to the turf roots in the spring and will do some feeding in April and May. Attempting grub control in the spring isn’t usually recommended, however, because of a combination of the size of the grubs and the minimal amount of feeding they are doing.
Lawn Grubs. Photo: Steven Katovich, USDA Forest Service, Bugwood.org
Japanese Beetle adult feeding. Photo: Chuck Bargeron, University of Georgia, Bugwood.org