MSU Extension Turf

In some parts of Michigan that were extremely dry in September and October, lawn care professionals have received a flood of calls from their customers about dead patches of turf in their lawn. Most of the problem is due to the fall drought that turned lawns brown in October. However, dry conditions have also weakened turf root systems, making lawn more susceptible to white grub damage.

The grub damage appears either as dead patches of turfgrass that pulls up easily (like sod), or as mounds of turf dug and turned-over by skunks or raccoons. In both cases, plenty of C-shaped white grubs were found in the root system. In drought lawns, five grubs per square foot can cause visible damage. Past experience and records indicate a tight relationship between fall drought and reports of grub damage in home lawns. In a year with more typical rainfall in September and October, you may have just as many grubs in your lawn, but you won’t see any damage because the turf root system is dense and growing rapidly, perhaps even growing faster than what is being consumed by grubs.

The turf pests causing this problem are the larval stages of Japanese beetle and European chafer. Adult beetles lay their eggs during summer. The tiny (2.0 millimeters) white grubs hatching from those eggs begin feeding in July and early August, growing larger each week until they reach a length of nearly 1.0 inches sometime in September. The most feeding damage to turf roots occurs in September and October when the grubs are large and the soil is still warm enough for them to remain active. In years like this when relatively warm temperatures persist into November, grub injury may continue until early December. European chafer grubs tend to remain active three to four weeks longer than Japanese beetle grubs, and are therefore most likely to damage lawns in November.

A healthy lawn with a good root system is tolerant of grubs feeding. In fact, we rarely see any grub damage in lawns with a good irrigation system. This is because a healthy lawn has a large, actively growing root system. Lawns with a large root system are tolerant of grubs because even a fairly heavy infestation of white grubs will never consume more than 50 percent of the roots. In a research test at Michigan State University where we studied grub damage, no symptoms were observed until two things happened: more than 75 percent of the roots were consumed, and the soil became very dry due to a lack of rain.

If you think about new sod, it has almost no root system when it is purchased and laid. If the new sod is kept moist, it develops roots and establishes. However, if new sod is allowed to dry out, it dies quickly. This is the same situation with lawns heavily infested with grubs. Because some of the root system has been consumed, these lawns are sensitive to drought stress and may die in patches, starting with where the most grubs are found. These lawns are also susceptible to skunk and raccoon damage. Skunks and raccoons love to eat white grubs. A lawn infested with grubs becomes even more inviting to skunks and raccoons if it has a poor system because the turf becomes easy for the skunks and raccoons to dig and turn over.

Unfortunately, late October and November insecticide treatments may or may not work for grub control. While soil temperatures remain unusually high for this time of year, two insecticide products may still work: Dylox (trichlorfon) and Sevin (cvarbaryl). However, lawns should be moist when the insecticide is applied, and the application followed immediately with 0.5 inches of irrigation to soak the insecticide down into the soil where the grubs are. For homeowners treating for grubs themselves, you can determine when you have applied 0.5 inches of irrigation by putting several coffee mugs out in your lawn and running the irrigation until they fill to a level 0.5 inches above the bottom of the mug.

We are expecting colder weather next week. When the soil temperature cools in November, the grubs will stop feeding and move deeper down into the soil. Insecticides will not work for grub control under those conditions. To give you an idea of how poorly insecticides work for grub control in late October or November of a typical year, I have included the results of a grub test we did in late October 2011 (see table). None of the insecticides applied in late October worked for grub control.

Lawns heavily infested with European chafer may suffer more feeding damage this November and next year in March and April. In order to prevent grub damage next fall, adopt a good fertility program, mow at the highest setting on your lawn mower (3-4 inches) and irrigate during dry periods. Also, when seeding new lawns, Kentucky bluegrass tends to have the largest root mass of any of the available species of turfgrass. This makes Kentucky bluegrass lawns more tolerant of grubs. For more information, see the Michigan State University Extension articles “Mow high for weed and grub control” and “How to choose and when to apply grub control products for your lawn.”

*Indicates treatment mean is different from control mean at P = 0.05 by LSD test.
¹Mean number of grubs per 1.5 square feet ± standard deviation.