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URI Master Gardeners and Hotline volunteers answer questions, by the thousands each year, on 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.
Step 1. Determine the extent of your grub infestation: Not all lawns have grub infestations that warrant control. Our best estimate is that in our area only 10-20% of lawns have damaging populations - and these infestations are often limited to part of a lawn. 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 turf, seeking grubs in lower densities. To determine how many grubs you have in your lawn you can use a flat spade to cut back a sample of turf. Count the grubs in the top 3 inches of soil and replace and water the turf. If you take a 6" X 6" sample (1/4 of a square foot), a grub density of more than 2-3 per sample probably warrants treatment. Take a dozen or so samples throughout the lawn area to determine which areas may need treatment. The time to sample for grubs in Rhode Island is between August 1 and 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. Since grubs are found in patches and do not generally infest the entire lawn, the more you sample, the more confidence you will have in treating only areas infested with grubs. Step 2. Grub Identification: There are several white grub species that cause similar damage to lawns in Rhode Island. Japanese beetles and Oriental beetles are the most common species, but we find quite a few Asiatic garden beetles and European chafers as well. It is best to get your grubs identified to the species level to optimize choice of insecticide against these pests. The URI Cooperative Extension Education Center personnel can do this for you. If you want to try it yourself, you'll need a hand lens and some reference illustrations. Illustrations are available on the web at: http://ohioline.osu.edu/hyg-fact/2000/2510.html. Step 3. Treat in the Fall: It is tempting to treat white grubs in the spring when damage is apparent, but spring treatments are generally ineffective. The large grubs present in the spring have already done most of their feeding and caused their damage and are now difficult to kill with insecticides. It is better to overseed damaged areas in the spring and wait until late summer to sample for a new generation (concentrate sampling in previously-damaged areas - they tend to return to the same area of your lawn). We recommend treatments between August 1 and September 15 only to those aesthetically important areas of your lawn that exceed 8-10 grubs per foot. Note that (as discussed below) two commonly used insecticides: Merit (imidacloprid) and Mach-2 (halofenozide) must be applied early in the season before eggs are laid in order to be effective against the fall larvae. With these products, treatment decisions should be based upon past history of grub infestation. Step 4. Available Products: The choice of products for use against white grubs changes every year as older products are withdrawn and new ones become available. Dursban was used for years against white grubs, but it will no longer be sold for this purpose. Similarly, Diazinon will not be available for sale for turf use after 2003. If you have already purchased these materials, you can use existing stocks on your lawn. Apply these products in the fall. When selecting insecticides, it is important to look at the active ingredients list for the chemical name for the product. Some companies have used the same trade names for products with very different ingredients. Other choices for fall application include: Dylox (trichlorfon) and Sevin (carbaryl). Dylox is a fast-acting material but is susceptible to alkaline hydrolysis. That means that it degrades in very hard or alkaline water or in a high pH soil very rapidly. One-half of the active ingredient will be degraded in 30 minutes at a pH of 9. Many public water supplies in Rhode Island are at pH 9 or higher. You also would not want to lime the lawn just before or after a Dylox treatment for the same reason. (If you are determined to attempt grub control in the spring - perhaps to reduce bird or mammal damage to the lawn, Dylox is one of the more effective materials available.) Mach-2 (halofenozide) is an insect hormone mimic that is most effective against Japanese beetle grubs. It should be applied when beetles are flying (June 21-July 21). Low toxicity to non-target organisms. Soil half-life: 129 days. Imidacloprid (Trade Name: Merit) is an effective chemical treatment for grubs but, as noted above, it must be applied before eggs are laid. Treatments between April 1st and August 15th are generally effective. All chemical controls must be watered in to be effective. Read the label for specific instructions, but generally 1/4-1/2-inch of post-treatment irrigation is required. Non-chemical controls: Milky spore disease (Doom, Japidemic) is a slow acting bacterium used to control the grubs of the Japanese beetle. It is not intended for use where grub populations are high. Once treated, however, bacteria are supposed to cycle in your lawn as new grubs die and release more bacteria. Research data are inconclusive on the effectiveness of this material. Nematodes are becoming available for white grub control. The most promising results to date have been with Heterorhabditis bacteriophora (the so-called 'HB' strain) at a rate of at least 1 billion per acre or Steinernema glaseri at a rate of 2 billion per acre. It is critical that the soil temperature be close to 70o F and that you water these in with 1/4 inch of water. Several studies have shown very good control; others have demonstrated poor control. Research is continuing and product availability is limited. For additional information see the web site: http://www.oardc.ohio-state.edu/nematodes. Step 5. Long-term suppression: Since grubs are not found as often in shaded lawns, plant shade trees and select shade-tolerant grasses for long-term suppression (see URI's Sustainable Tree and Shrub List).
By Steven R. Alm and R.A. Casagrande, 2002
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