Is there anything more damaging (or, in some peoples’ eyes, revolting) than grubs in your lawn?
Few admire their looks. But, before dismissing them as nothing but gross menaces, grubs — the larval stage of certain adult beetles — are common in lawns and do play a role in the natural order of things (well, at least in the right numbers).
David Shetlar is a professor emeritus with a doctoral degree in entomology at The Ohio State University. He’s also co-author of the comprehensive guide “Garden Insects of North America,” plus other pest management tomes including “Managing Turfgrass Pests.”
Shetlar chuckled brightly when asked if lawn grubs, commonly called “white grubs,” play a positive part in a lawn’s “ecosystem.” “Everybody says grubs eat the grass roots,” he said, but there’s more to them than that. “White grubs are eating the accumulated thatch and organic matter that’s in the top inch of the soil profile.
“They’re sort of nature’s ‘de-thatchers,’” he added. “But the problem is, [if] they’re eating that thatch, they do eat the roots and the crowns [of turfgrass], which kills the plant.”
It’s when these “de-thatcher” populations grow out of control that the dreaded lawn damage takes place.
Lawn Grubs’ Life Cycle
While there are variations depending on which species of beetle they come from, the larvae tend to be stout, white-to-grayish with brown heads, about 3/4 of an inch to 2 inches in length. These lawn pests tend to curl up in a “C” shape at rest.
The grubs get their start when the adult beetles lay eggs in your turf, usually in spring. After the eggs hatch, the grubs evolve in three stages, with grub damage greatest in late summer, when you will see areas of your lawn thinning and yellowing. Later, irregular brown patches appear in your turf.
There are “two major white grub species that cause problems in Ohio. That’s the Japanese beetle and the masked chafer,” said Shetlar about his home turf.
Japanese beetle grubs are widespread invasive insects, and their white grub larvae are bound to be found in lawns all over the country too, not just Ohio. Other chafer species besides the masked chafer beetles (like the European chafer) also lead to white grub infestations and can establish in lawns beyond the state’s borders.
Other types of common lawn grubs around the United States include the larvae of the May and June beetles, green June beetle and the black turfgrass ataenius.
“It’s irrelevant which species it is,” advised Shetlar. “They cause the same kind of damage.”
Some of it manifests as dry and deadened looking patches in your lawn. “The typical damage usually seen here is something that looks like drought stress usually,” Shetlar said. This is the result of grub activity: specifically, the consequences of their nibbling on your tender, tasty turfgrass roots.
Grub-Hunters Cause Damage, Too
But it can get worse: Large urban and suburban critters, such as skunks and raccoons, are apt to claw up your entire lawn looking for them, considering them hard-to-come-by delicacies. In fact, the damage from these animals alone may be the bigger issue to prevent. “In many cases, grubs aren’t even discovered until the skunks and raccoons start digging them up,” said Shetlar.
Like anything else in lawns and gardens, white grubs are a force of nature. We must either tolerate them, battle them head-on, or strike a compromise with them — or, ideally, prevent their spread beforehand to reduce the risk of eventual over-infestation.
Do I Have a Lawn Grub Problem?
Lawn grubs can be present in your lawn but cause absolutely no problems. But how do you know if you have a real lawn grub problem?
Since grubs eat grass roots, the lawn will be easy to pull up in chunks. If it’s still firmly rooted, then you have another problem, such as brown patch or dog urine damage. Here’s a good test, from the University of California Integrated Pest Management Program: Pull up about 1 square foot of the earth around the base of your sod (where the stems meet the roots). If you notice more than 6 grubs in this space, it’s a good time to consider preventative methods or treatments to reduce overpopulation.
Shetlar, however, recommended that lawn lovers should be even more vigilant, especially in order to prevent large critters from eating them. When talking about the maximum threshold of tolerance for grubs, “I always chuckle about [checking the threshold], in that skunks and raccoons haven’t read about this threshold.”
As such, it may be worthwhile for homeowners to be just a little warier on account of skunks and raccoons. Shetlar added, “If there are four to five grubs per square foot, that’s usually enough for them to be digging around.”
Once grubs are established, what can you do about it? The most common (and, sadly, most effective) ways to kill grubs are chemical insecticides. Effective insecticide control products against grubs include imidacloprid, thiamethoxam and clothianidin. However, they only work if used in the lawn in June and July. If you wait until August when the brown patches appear, it’s too late. The chemicals carbaryl and trichlorfon that are considered curative treatments — they are short-lived products effective immediately.
Always follow label directions carefully when using any chemical pesticide. Any use poses a risk of harm to not just people, but other insects, including lawn-beneficial pollinators. One way to mitigate the damage: Mow your lawn just before applying so there are no weed flower heads to attract the pollinators to your temporarily poisoned yard.
To Deter Lawn Grubs, Dethatch
But what are the most effective options for sustainable-minded lawn owners?
Preventive methods top Shetlar’s list for fighting grubs nonchemically. “I emphasize a lot — especially to lawn care and sport field managers — [to try] to keep the thatch down to a minimum,” Shetlar said. His key to this: fertilizing one’s lawn with nitrogen only once per year, and doing it very strategically “and judiciously,” to quote Shetlar, “usually in late October, early November.”
He also mentions research from a University of Kentucky entomologist suggesting you should choose one species of turfgrass over another.
Professor Daniel. A. Potter “has conclusively shown that the tall [fescue grasses] are much more tolerant of grub populations,” said Shetlar, who then added, “Where Kentucky bluegrass and perennial ryegrass will be damaged with … eight to 10 grubs per square foot, the tall fescues usually require somewhere around 12 to 14 grubs per square foot” before showing damage.
What about those common organic recommendations for killing lawn grubs online, such as milky spore disease or beneficial nematodes? “The commercial milky spore disease is only for Japanese beetles,” Shetlar warned. “Even then, milky spore disease is a weak pathogen that results in 20% to 25% infection at best.”
As for beneficial nematodes, Shetlar said, “There’s a very steep learning curve to using the insect parasitic nematodes.” They can only be effective when used curatively, and even then, only on small affected areas.
However, Shetlar did let on that the most experienced organic lawn care professionals may have the knack for using them effectively against white grubs. “In that case, they work quite well,” he said. “You can typically get 60% to 100% control with the nematodes when they’re used at the right time in the right way.”