Lawn grubs are one of the biggest complaints among homeowners. Damage can range from spots of brown in the grass to entire lawns dying. Grubs also attract other pests that feed on grubs. Part of effective lawn treatment for grubs involves identifying the grubs and the signs of their presence. Taking samples directs treatment methods to ensure the best resolution to the grub problem.
Grubs are the larvae of beetles. The grubs found in lawns, according to Penn State University, are usually dirty white with a brown head, soft body and six legs. Grubs are found lying on their sides in the form of a "c" when exposed. The size of the grub depends on the species of beetle it grows into and the age of the grub. The full-grown Japanese beetle grub and northern masked chafer grub reach a length of 1 inch. Further identification can be made by observing the raster (hairs, spines and bare spaces) of the grub along the underside near the rear end. The Japanese beetle grub that has a V-shaped pattern and the northern masked chafer has a random pattern.
There are signs that lawn treatment for grubs may be needed. Spongy areas of grass, sod that can be rolled up similar to carpet or patches of dead turf from April to May or September to October are signs of grub infestation. A sudden appearance of skunks, crows or moles feeding on the lawn also are signs of a grub problem.
Determine if treatment is necessary by sampling the area at the proper time during the year. Cornell University suggests looking for grubs in mid-August when grubs are still growing. Penn State University suggests using a garden spade to remove 1 square foot of turf up to 4 inches deep in multiple locations to determine if treatment is needed. The sod can be replaced and packed firmly back into position to restore it.
Set up the sampling area by sketching a general map of the property and determining the high-priority areas to check for grubs. Mark sampling areas with an "x" 10 feet apart as a guide to where to look for signs of grubs. Cornell also suggests considering areas where grubs have appeared before to check for re-emergence of grubs. After cutting the sod, peel it back and count the grubs found underneath the sod mat. More than six grubs per square foot means treatment should be considered; finding more than 10 grubs per square foot means treatment is needed immediately.
Natural treatments can be either cultural or biological. Cultural treatment of grubs can be maintained through underwatering of the area. Grubs require moist soil to hatch and grow. Maintain a drier than normal lawn during the months of July and August to hinder the eggs from hatching, suggests the Missouri State University Extension Service.
Biological methods include using nematodes (parasitic worms) that thrive inside the grubs and kill them, or a bacterial agent used to introduce a disease to control Japanese beetle grubs. Nematodes must be handled with care and used by the expiration date on the packaging. There are two forms of nematode: Heterorhabditis spp. and Steinernema spp. with the former being preferred for better performance, according to Penn State University.
Chemical treatments can be either curative or recuperative. Curative applications should be conducted when grubs are small and feeding near the lawn surface. According to Missouri State University, the best time period for curative applications is early August through mid-September. Curative treatments are short-lived, and therefore proper timing of treatment can range several weeks depending on the species and soil conditions in any given year. Penn State University cautions against curative application treatments during spring because the treatment will be ineffective when beetles lay eggs later in the year.
Recuperative treatments are insecticides mixed with water to eliminate existing grub populations. These applications are useless if the water mixture does not reach deeper than 1/2 inch under the soil. This takes the chemical to the thatch layer and stimulates the grubs to move up to the insecticide.