A white grub is the immature (larval) form of a scarab beetle. In NY, these include four native species (black turfgrass ataenius, green June beetle, May or June beetles, northern masked chafer) and four introduced species (Asiatic garden beetle, European chafer, Japanese beetle, Oriental beetle).
Grubs live in the soil, feeding on plant roots, so you may not be aware of them until you see damage. While the adults of some species feed on and are damaging to ornamental plants (e.g., Japanese beetle, Asiatic garden beetle), the adults of other species may not feed at all (e.g., European chafer). No adults feed directly on turfgrass.
A Grub’s Life: Egg to Beetle
Knowing the life cycle of grubs is the key to determining whether you have a problem, what to do about it, and when to do it. By considering a grub’s life cycle, you can anticipate problems before your lawn is ruined—not just by root damage, but by hungry birds and rodents as well. The biology of the Japanese beetle is typical of most grubs encountered in New York State and is illustrated below.
What Do White Grubs Look like?
White grubs are immature scarab beetles. Although all grubs cause similar damage, the treatment you choose should correspond to the species, which is why treatments should be determined after the grub has been identified by close examination.
Larvae are “C”-shaped, with six legs, and well-developed mandibles attached to a defined head capsule.
The eight species that occur in NY can be differentiated based on two characters of the abdomen: the raster pattern and the anal slit. Learn how to identify the different species by using the Cornell University Grub ID website.
Additional ID resources
Should I Worry About White Grubs?
Heavily damaged turf will feel spongy underfoot. It will peel back from the soil like a carpet because the root system has been chewed away from the leaf. Above ground, there will be thin, stressed turf. This stress decreases drought tolerance and, ultimately, increases the chance of weed invasion.
Even when grub populations do not cause visible damage, their predators might. The grubbing activities of vertebrates like raccoons, skunks and crows can be highly problematic. It is common for this feeding damage to be more troublesome than direct grub damage.
Whether the damage is directly caused by the grubs or predators, on sports field it results in an increased risk of sports injuries.
How Do I Know If I Have White Grubs?
To see if you have grubs and how bad the infestation is, you have to dig. Some turf insects, such as caterpillars, can be forced to the surface with salt or soapy water solutions. This won't work with white grubs. Use a bulb planter, shovel, or golf course cup cutter to examine soil cores for grubs in the root zone. Depending on the species, eggs and first instars (first developmental stage of the larvae) are relatively difficult to see, while the larger second and third instars are relatively easy to pick out of the soil.
The best time to sample is early August in southeastern NY and mid-August upstate. Egg hatch and grub development, however, may be delayed by cool or dry weather and may also vary from species to species.
Where you sample should be based on identified problem areas, susceptible areas, and areas that otherwise require better protection (e.g., front lawns, fairways). Increase the number of samples in high priority and high-risk areas to reduce the chances of overlooking a damaging infestation.
Watch our Using IPM to Assess Your Lawn for White Grubs video to find out if your lawn needs treatment.
How many is too many?
The question of how many is too many depends on the grub species. Typically, the larger the grub, the fewer grubs per square foot can be tolerated by the turf before damage is visible. Threshold levels have been established for each species as outlined in the Cornell Guide for Commercial Turfgrass Management (pdf).
|Species||Number of grubs per sq. ft.||Number of grubs per|
|Asiatic garden beetle||18-20||2|
|Black turfgrass ataenius||30-50||3-5|
|Green June beetle||5||Any|
|Northern masked chafer||8-12||Any|
|May and June beetle||3-4||Any|
If several sampled areas are at or above the threshold, intervention may be warranted. Remember, turfgrass with a healthy root system will tolerate higher numbers of grubs than the suggested thresholds. Extensive research in upstate NY shows that insecticide treatments are needed only 20% of the time on home lawns and golf course fairways.
Should you treat?
If your grub counts exceed the damage thresholds, you might want to consider treating for them. Your decisions will depend on when you find the grubs. If treating, target only the areas where grub populations exceed the suggested thresholds, or where you’ve had a bad history of grubs.
Grubs found in May-July
If you’ve had a history of high grub populations and damage on your lawn, you may want to protect your lawn for the upcoming generation by applying a preventative insecticide. These can be applied most any time from May-July, and for New Yorkers include imidacloprid (e.g., Merit) and chlorantraniliprole (Acelepryn). Long Islanders must note that imidacloprid may only be applied by a certified pesticide applicator and they may not use chlorantraniliprole or clothianidin.
Grubs found in August
The grubs are still small and have not caused a lot of damage yet, and are more susceptible to insecticides. In New York State you could apply beneficial nematodes, imidacloprid (Merit), or trichlorfon (e.g., Dylox), listed from least to most toxic. Note that on Long Island, imidacloprid may only be used if applied by a certified pesticide applicator. In other states, additional products may also be available.
Grubs found in September
It’s too late to use most products. Beneficial nematodes or trichlorfon may be used.
Grubs found in October
It’s really too late to treat this year’s population. Treating them is a waste of time and money. Focus on compensating for grub damage.
Compensating for grub damage
You can help your lawn to compensate for loss of roots by watering it regularly, providing good fertility, and reseeding damaged areas—actions that will improve any lawn’s looks, too.
Cornell research shows that fall fertilization (0.25-5 lb. of N per 1,000 square feet) enhances turf recovery from grub damage. Check to see if there are regional fertilizer restrictions for your region.
Disclaimer: For the most up-to-date information on insecticides approved for use in New York State, see the Cornell Guide for Commercial Turfgrass Management. To calculate the environmental impact of different pesticides, visit A Method to Measure the Environmental Impact of Pesticides.
Read pesticide labels prior to use. The information contained here is not a substitute for a pesticide label. Trade names used herein are for convenience only; no endorsement of products is intended, nor is criticism of unnamed products implied. Laws and labels change. It is your responsibility to use pesticides legally.