By Nate Royalty, Ph.D.
What does it look like?
White grubs are the larvae of adult scarab beetle species. The most commonly occurring grubs in lawns are Japanese beetles, Oriental beetles, green June beetles, European chafers, northern and southern masked chafers and black turfgrass ataenius. There are, however, up to 200 total types of beetles that produce white grubs as their immature offspring. Size of the grub depends on the development stage and species, but the mature larva (third-instar) of most lawn pests is about 1/2 inch to one inch long. Grubs are “C”-shaped and creamy white in color, with a brown head and six legs. To accurately differentiate between species, one must examine under magnification the raster pattern, which is a specific configuration of hairs on the underside of the tip of the abdomen.
Host material and range
One of the most destructive insects that infest turfgrass, white grubs feed on the roots of grasses and ornamentals. They are found in all regions of the country. The life cycle of the white grub is typically one year, although some species have multiyear lifecycles, and others may have multiple generations in a single year, particularly in warmer climates. In the northern United States, most of the economically important species lay eggs from late May through late July. A female beetle can lay from 15 to more than 300 eggs, depending on the species. The eggs take one to two weeks to hatch. The larvae feed from late spring through early fall, and molt twice. As winter approaches, the grubs dig deeper and hibernate four to eight inches below the surface. In spring, they move back to the surface, pupate, and then adults emerge. In the southern United States, grub activity may begin earlier and extend later in the year because of the warmer temperatures.
White grubs tunnel through the soil, feeding on organic manner as they move. The destruction of roots may result in spongy turf, and the destruction of roots can cause discoloration and wilting of turf foliage, resulting in dead patches if the injury is severe enough.
The true test of the severity of infestation is the number of grubs discernable in a one-square-foot area of turf. A count of five or more insects in the soil is a good indication that treatment is necessary. However, it often is difficult to detect early-instar grubs, and by the time accurate sampling is done, it may be too late for any treatment to prevent damage.
Control the moisture, if possible, as most grubs prefer to lay eggs in moist soil. Most grubs also prefer turf that is high in organic matter. Promote the development of a healthy root system in the lawn; healthy turf will reduce the impact of grub feeding. Dry turf is more susceptible to grub-related wilting and discoloration, so ensure that the turf is not under drought if possible.
Although the insects feed close to the surface twice during their life cycle, they are much easier to control when young. Liquid or granular applications of imidicloprid, applied from late May through mid-August, will provide effective control of first- and second-instar grubs. Post-application irrigation is recommended to move the product through the thatch and into the soil where the grubs feed. Also, remove excessive thatch prior to application; the organic material in thatch will prevent the insecticide from moving into the soil.
What can you do?
Consider treatment if the turf in question has a history of heavy grub populations, or if adults are prevalent. Sample your lawn by cutting an area of turf and lifting the flap to count the number of grubs. Begin treatment if you find more than a few grubs in a square foot section.
Also, watch for animals digging in the lawn. Birds, moles and other animals feed on large third-instar grubs; the digging is a warning sign that there are heavy grub populations present. If this digging occurs, a rescue treatment with trichlorfon is necessary.
Nate Royalty, Ph.D., is product development manager — insecticides for Bayer Environmental Science.