Grubs in Lawns

The Japanese beetle (Popillia japonica) is an abundant and important landscape pest. Adults are a brilliant metallic green; larvae are typical white grubs that are C-shaped when disturbed.

White grubs eat organic matter, including plant roots. Therefore, damage first appears to be drought stress. Continued feeding will cause the grass to die in large irregular patches. Tunneling of larvae cause grass to feel spongy under foot and the grass can often be rolled back like a loose carpet. Grub populations may or may not cause observable grass injury by predatory mammals such as skunks, racoons, opossums, and moles that dig in the grass in search of a meal.

One of the most frequently asked gardening questions is how to control white grubs in lawns. Many times people apply too much or too little insecticide, treat areas that don't need treatment, or apply insecticides at the wrong time of year. Below is a set of procedures you can follow to successfully control these insects in the lawn.

Grub Sampling

The time to sample for grubs in Rhode Island is August 1 to September 15 – adult beetles generally lay their eggs in July and most larvae should be present by early August, although in dry years development may be slowed. Depending on soil moisture, you may need to sample more than once to make sure you "capture" the grubs developing at your site.

Furthermore, lawns differ in susceptibility to white grubs because of differences in grass species, soil health, irrigation, amount of sun or shade, traffic, etc. A dense stand of grass with a healthy root system can generally tolerate up to 10 grubs per square foot, although skunks, raccoons, birds and moles sometimes damage grass, seeking grubs in lower densities. Also, most annual white grubs seem to prefer grass in sunny areas.

To determine how many grubs you have in your lawn you can use a flat spade to cut back a sample of grass, taking square foot samples (cutting through the grass and thatch on three sides) several places over the lawn area. Peal back the grass and inspect the thatch and upper inch of soil for grubs. Populations of annual grub species that are less than six grubs per square foot can usually be masked by water and fertilizers. When populations reach 10 and 15 per square foot (or more) they can cause significant grass damage.

Control Strategies

White grubs attack grass areas irregularly from year to year. The major factor influencing development of damaging numbers of grubs is soil moisture and rainfall. In general, in years with normal or above normal rainfall, grub populations increase. Well maintained grass next to ornamental plants favored by the adults seems to be more commonly attacked. Also, fescues are not as severely attacked as Kentucky bluegrass and perennial ryegrass.

Option 1–Water Management:

Practically all white grub species require moist soil for their eggs to hatch. The young larvae are also very susceptible to desiccation. In areas where grass can stand some moisture stress, do not water in July and early-August when white grub eggs and young larvae are present. On the other hand, moderate grub infestations can be outgrown if adequate water and fertilizer is applied in August through September and again in May when the grubs are feeding. This latter strategy is not preferred because mammals may dig up the lawn or irrigation bans may occur.

Option 2–Organic Control:

Milky Spore, a powder that contains the bacterium Bacillus popillae, attacks the larvae of Japanese beetles. Each grub that dies from Milky Spore decays and releases new spores. Once all of the grubs are destroyed, the spore will remain dormant but viable in the soil until new grubs come along even years later. Three to five years are needed to provide lasting controls. Milky Spore will not harm beneficial insects, animals, or humans and it is safe to use near water.

Option 3–Chemical Control:

There are three commonly used insecticides for grub control:  (1) Merit and Grub-X (imidacloprid) are slow-acting, long-lasting products that prevent subsequent festations. (2) Dylox (trichlorfon) is fast-acting and is used to spot treat. (3) Mach 2 (halofenozide) is an intermediate material used in late summer.

• Imidacloprid should be applied between May 1 and June 30 in New England, before eggs are laid, to maximize its effectiveness.
• Trichlorfon can be applied as late as early October if the air and soil temperatures are unseasonably mild. It must be well watered-in with at least ¼-inch of water, especially late in the season (i.e., late September – October) when soils are dry. (Dr. Pat Vittum, Turf Entomology, UMass)
• Halofenozide is a molt-accelerating compound, that is, it interferes with a grub’s ability to molt successfully. Therefore, late summer or early fall applications will not be effective because most of the grubs have already molted. Similarly, spring applications are not effective because the grubs that are present in the spring are the same mature grubs that were present the previous fall. Therefore, appropriate timing for applications of halofenozide is late summer (June 21 – July 21) for Japanese beetle in New England, the time when insects are flying. 

 Insecticide Application

It has been found that 95–99% of any pesticide used for grub control ends up in the thatch. If the thatch layer is one inch thick or more, grubs probably will not contact challenging doses of the insecticides.

Synthetic pesticides generally degrade rapidly. However, many are subject to additional degradation by bacteria and fungi and these microbes tend to build up if a pesticide is used continuously. Therefore, it is recommended to use a pesticide only once per season, when needed.

In general, irrigating after an insecticide application is made will improve performance for soil insect control. It is also generally recommended that grass clippings be returned to the lawn for one to two mowings after a grub insecticide application. Do not wait more than 30 days to recheck the grub infestation, especially if the original population was high. If the grub population has not been reduced below six grubs per square foot consider reapplication of another pesticide. Remember, the smaller the grubs the easier they are to kill with insecticides.

Source: Steven R. Alm and R.A. Casagrande, 2002

Useful Websites

White Grubs in Home Lawns (Penn State Cooperative Extension) http://www.ento.psu.edu/extension/factsheets/white_Grubs.htm

White Grubs in Lawns (University of Illinois Extension) http://www.urbanext.uiuc.edu/lawnfaqs/grubs.html