Lawn grubs — the larval stage of certain adult beetles — are common in yards across the country and do play a role in the natural order of things, but only in the right numbers. But when the populations grow out of control, you will want to know how and when to kill the grubs in your lawn.
“They’re sort of nature’s ‘de-thatchers,’” says David Shetlar is a professor emeritus with a doctoral degree in entomology at The Ohio State University. “But the problem is, [if] they’re eating that thatch, they do eat the roots and the crowns [of turfgrass], which kills the plant.”
Short on time and just want to kill some grubs? Here are our picks for the best grub killers.
Signs of a Lawn Grub Problem
All grass can tolerate some grub feeding, but be aware of the signs of when it gets out of control. “In many cases, grubs aren’t even discovered until the skunks and raccoons start digging them up,” said Shetlar.
- Patches of thinning turf: Dead patches appear and grow larger each week.
- Grass will pull out very easily at the roots.
- Animals start digging in the lawn, especially moles, skunks, crows and racoons.
- Beetles or moths start flying around, usually low, looking for a place to lay their eggs.
- A bouncy feeling as you walk on the grass.
- Signs of drought in the grass, even though there is no drought, and there is plenty of water.
How to Scout for Lawn Grubs
The only way to be sure that you have grubs is to scout for them in your lawn.
- Cut in a one-foot section of your lawn. You can use a shovel or a lawn edger, in a place where you suspect grub activity.
- Peel up the soil from that cut. If there is grub damage, the grass will peel up easily.
- Sift around the soil, counting the grubs you find.
- Tally the count. If there are 10 or more grubs per square foot, you have a serious infestation.
- Replace the grass you’ve cut. You don’t want to damage the lawn.
- Repeat the process. Scout in other places on the lawn, just to be sure.
Getting Rid of Lawn Grubs
- Keep down the thatch. “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, usually in late October, early November.”
- Think before you mow. Keep your lawn properly mowed. For grub control, use a mower’s highest setting, which can be 3.5 to 4 inches. This will produce a deeper, therefore strong, root system. If you mow high and use fertilizer in the spring, you might not need a pesticide.
- Choose the right turfgrass. Daniel. A. Potter, an entomologist at the University of Kentucky, “has conclusively shown that the tall [fescue grasses] are much more tolerant of grub populations,” says Shetlar. “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.
- Deny moisture to the grubs. Keep your lawn dry during July and August, something that will cause beetle eggs to dry and die. If your lawn starts to brown, you can water more; there will be no residual damage.
- Turn to chemical insecticides. Effective preventive grub treatments include imidacloprid, thiamethoxam and clothianidin. The best time to apply these insecticides runs from the middle of April to the middle of June, depending on where you live. If you wait until brown patches appear, it is too late.
Note: Any use of insecticides poses a risk of harm not only to 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.
- Curative chemical insecticides. Carbaryl and trichlorfon are considered curative treatments. You apply them once you’ve noticed grubs in the grass. They are short-lived products effective immediately. Treated grubs turn yellow or brown within a week, so reevaluate the turf the week after that to see if the chemicals have been successful.
- Milky spore disease. “The commercial milky spore disease is only for Japanese beetles,” Shetlar says. “Even then, milky spore disease is a weak pathogen that results in 20% to 25% infection at best.”
- Turn to natural enemies. Naturally occurring pathogens (fungi, bacteria, protozoa) kill or weaken grubs; they already exist in the soil, but chemicals you apply will kill them, so you might want to hold off and let the pathogens do their work. Ground beetles and ants are beneficial in that they feed on eggs and young grubs. Parasitic wasps and flies go after older grubs.
- Consider beneficial nematodes. Shetlar says, “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 beneficial nematodes 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.”
When to Call Pest Control Pros
If in doing the tally, you find more than six grubs per square foot, it’s a good time to take action. Don’t wait until you have a serious infestation.
“I always chuckle about [checking the threshold], in that skunks and raccoons haven’t read about this threshold,” says Shetlar. “If there are four to five grubs per square foot, that’s usually enough for them to be digging around.”
It might also be enough for you to call in your local pest control professionals to avoid further damage to your lawn.
Life Cycle of Lawn Grubs
- In summer, eggs are laid. European chafers do this in late June, Japanese beetles in July and August.
- One to two weeks later, the eggs hatch and the young grubs begin feeding on grass roots.
- In July and August, dry soils cause many eggs to die from moisture stress.
- Until fall, grubs feed and then burrow deep into the soil.
- In winter, grubs are burrowed deep, overwintering in the lawn.
- In spring, grubs burrow upwards to grass roots and resume feeding.
Lawn Grubs: What They Look Like
- White grubs are similar in appearance but may differ in size.
- Size varies with the species, but when fully developed, they can range from 3/8 inch to nearly 2 inches in length.
- Grubs are cream-colored with a brown head. They have three pairs of short legs.
- Soft bodies that are usually curled into a C shape.
- Adult beetles are oval in shape. They can be green, tan, brown, or black and can range from 3/16 of an inch to 1 inch in length.
- Most species aren’t noticed because they are active at night. Often they are seen only when attracted to outdoor lights.
Kinds of Lawn Grubs: They All Do the Same Damage
“It’s irrelevant which species it is,” says Shetlar. “They cause the same kind of damage.”
But here are the lawn grubs you are likely to find right outside your door:
- Japanese beetle grubs are widespread invasive insects, and their white grub larvae are found in lawns all over the country. Japanese beetles can be seen mating at their feeding sites during the day.
- Asiatic garden beetles are light brown as adults and white with a brown head as grubs (similar to other white grubs). There are signs they are overtaking the Japanese beetle in some areas of the eastern U.S. The Asiatic garden beetles are nocturnal and will come to lights at night.
- Chafer species, led by the masked chafer beetles (such as the European chafer) also lead to white grub infestations. European chafers conduct mating flights at dusk near trees or chimneys.
- May beetles: Also known as June bugs, they are members of a large family of beetles called scarabs.
- Black turfgrass ataenius: This shiny black adult beetle can be found in turfgrass but is mostly found on golf courses.
“Everybody says grubs eat the grass roots,” says Shetlar, professor emeritus at The Ohio State University, 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.
Many lawn issues that resemble grub damage can be caused by something else:
• Soil compaction
• Drought stress
• Hairy chinch bugs
• Sod webworms
In the early spring, many homeowners suspect grub damage because of the condition of their lawn after snow melt.
Lots of adult beetles on the lawn in July is one indication. Watch lawns closely starting in the middle of August and continuing into September for wilting and browning areas.
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.
If strange brown spots show up on your lawn with something even stranger, a bright green grass ring, you might check for dogs. Dog urine does just this kind of damage.
Conclusion: No Time to Wait
Don’t wait for lawn grubs to destroy your lawn. Take action before you see them by taking care in selecting your grass and caring for it. If you still get them, don’t wait. Be willing to call in professionals, especially if you are considering chemical treatments.
For year-round lawn care maintenance, call a LawnStarter pro to keep your grass strong and healthy (and better able to resist grub damage) all growing season long.
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