White Grub Control
Extent of Damage Due to White Grubs
Damage from white grubs in lawns can show up anytime after mid-August. However, our experience in the recent past has been that grub damage does not become obvious until September or even into October. Damage from white grubs is usually localized. It is typical to have severe damage in irregular and isolated spots.
White grub damage may first appear as drought stress (gray-green discoloration and wilting in the hot sun). More severe damage causes the turf to die in large irregular patches that can be rolled back like a loose carpet. High populations of grubs may go unnoticed until discovery by raccoons or skunks. Raccoons, skunks and crows will turn over large patches of loose turf, eat the grubs and leave behind a torn-up mess.
Rainfall and soil moisture are critical factors affecting the extent of grub damage. Adequate moisture in mid-summer will favor beetle activity and grub development. If plentiful rainfall or irrigation continues through August and September (when grubs are actively feeding) damage may not be noticeable because the grass continues to grow and masks the root injury symptoms. Healthy turf can sometimes tolerate 20 or more grubs per square foot before showing signs of injury. The onset of dry weather can lead to “sudden” appearance of grub damage symptoms.
Treatment using an Integrated Pest Management (IPM) approach
It is very difficult to use an IPM approach to white grubs in turfgrass. Turfgrass often has a very high value – either monetary value (golf courses and sod farms) or aesthetic/emotional value (home lawns). White grub infestations are highly variable from year to year and from place to place. Damage is spotty, localized and impossible to predict.
Monitoring, one of the keystones of IPM is not practical for white grubs under most circumstances. Inspection for grubs requires cutting and lifting flaps of turf and looking for grubs below the thatch level. This is difficult, time consuming and potentially damaging. Research has indicated that the number of samples necessary is too large to be practical.
Studies at Cornell University have shown that over 70 percent of all grub control treatments were applied needlessly because there were no grubs in the lawn. Many homeowners are frightened into applying grub controls because of advertisements on TV, in plant centers, or because of horror stories they have heard about grub damage. Most grub treatments are not only expensive but hard to justify from an environmental standpoint. There are 3 approaches to grub management in the home lawn, depending on your tolerance for damage, comfort with pesticides and willingness to spend the cash.
Grub Management Techniques
The Golf Course Approach
Treat every part of the lawn, every year because you might have white grubs and heavy use of high-price insecticide is preferable to ANY white grub damage. The available insecticides for grub prevention are imidacloprid (Merit®, Grub-Ex®) and halofenozide (Mach 2®, Grub-B-Gon®). These must be applied before early August to prevent damage. A compromise modification of the Golf-Course Approach is to treat only those areas of previous damage. Grubs tend to return to the same areas in successive years, so it is logical to treat the areas where you had grubs last year or the year before.
Watch the lawn carefully during August - September for early signs of damage (wilting, turning brown). Apply a curative insecticide such as trichlorfon (Dylox, Bayer Advanced 24-Hour Grub Control) only where needed and when needed. The risk is that you might still lose some sod, especially if summer rainfall or irrigation keeps the grass growing and vigorous through July and August. Damage symptoms may not appear until after it is too late for effective treatment (late September through late October). Unfortunately, raccoons and skunks are much better at locating grub populations than we are and the first hint of a grub problem in your turf is likely to be that your lawn was "plowed" by varmints overnight.
The Do-Nothing Approach
Count up how many years you DID NOT have grub damage. Divide the cost of replaced sod by that number of years. If the yearly-averaged cost of sod is less than the price of insecticide, do nothing and take your lumps in the occasional year when damage occurs. This approach is much easier to follow if your attitude is "it's just grass, anyway."
White grub management decisions are difficult and frustrating. There is no one right answer for everyone.
When to Treat
Treatment for white grubs in late summer is problematic. It is not an automatic decision to choose to use an insecticide for white grubs. By September white grubs are fully-grown and thus harder to kill. The best treatment may kill only 60% of the grubs. Severe damage to turf may have already occurred. If raccoons have found the grubs they will continue to return and cause additional destruction. In many cases it may be preferable to repair the damage through seeding or sodding without treating. If the old loose sod is still green it may reattach with adequate watering.
Insecticide treatments after early October are not effective and are not recommended. If you do treat it may not be necessary to treat the entire lawn. Treat grub "hot spots" determined by observation or sampling. Presently, trichlorfon (Dylox or Bayer 24-Hour Grub Control) is the fastest-acting, most effective homeowner insecticide for curative grub control. By the time damage is apparent it is much too late for preventive white grub products such as Merit and Grub-X. These must be applied before August 1. Insecticides must be watered in to be effective. Use at least ½ inch of irrigation immediately following treatment and continue to water damaged turf to promote recovery.