Treatment for grubs is usually not necessary if the lawn is healthy; there is no evidence of lawn or turf damage and grub populations are below threshold levels. Grubs commonly referred to as white grubs are in the Scarabaeidae family and include thousands of beetles worldwide. Larval stages of Japanese beetles, green June beetles, Oriental beetles, Asiatic beetles or European chafers may be encountered in the Midwest. Grubs are typically white, opaque or grayish in color; C- shaped; ¼ to ¾ inch in size depending on their stage of development and have three sets of relatively small legs near their usually brown to dark colored head. Grubs are common and if you have ever dug in the top soil you have likely encountered them. Typically grubs co-exist in healthy lawns, gardens and flower beds with no apparent noticeable damage to the lawn or turf.
Unfortunately it’s not uncommon for homeowners to treat their lawns with pesticides for grubs even when there isn’t any visible evidence that grubs are causing damage, are at or above threshold levels or even present. The pesticides may also result in collateral damage to other non-targeted organisms including beneficial soil organisms that play an important role in organic lawn care.
Evidence of Grubs and Damage
Damage from grubs may appear as irregular patches of wilting, thinning, yellowing, browning, dying or dead grass in the lawn that otherwise is receiving adequate moisture and sunlight. Damage is most likely to appear during hot dry periods. The beetles commonly lay eggs in areas of the lawn that receive adequate sun and moisture. Areas may feel spongy under foot from grub activity.
In the suspect areas of the lawn, firmly take hold of the grass with your hand and lift. If the sod lifts free of the soil and rolls back like a carpet of synthetic grass it may be evidence that grubs are chewing off the grass roots near the soil surface. Under the sod grubs may be visible at or just below the surface of the soil if animals haven’t already eaten them. Moles, raccoons, skunks, armadillos and birds foraging in a lawn may also be a sign grubs are present. Most birds won’t leave evidence that they are foraging for grubs, but crows and starlings may. The best times to scout for grubs will vary by region, weather, grass and grub type. In the Midwest, scout cool season grasses for evidence of grubs in August to September.
The number of grubs per square foot threshold for tolerance varies by season, grub and grass type and other factors. “The NOFA Organic Lawn and Turf Handbook” suggests a threshold of 6 – 10 grubs per square foot warrants concern. To determine the number of grubs per square foot use a spade, shovel, sod cutter, golf cup hole cutter or other tool to cut and lift a small area of lawn in a suspect area to determine the presence, type and number of grubs. It may be helpful to lay the sod on a piece of plastic or other material to facilitate returning the sod and soil to the area it was taken from. Turn the piece of sod over to expose the roots and look for and count any grubs in the roots or the soil where the sod was removed from. If the sod is difficult to lift it may be evidence that grubs are not occurring at or above threshold levels that warrant treatment. If you suspect grubs check suspect areas and compare your findings to the recommended thresholds and consider if further treatment is needed.
*Manage lawns to keep them healthy and vigorous to increase tolerance to damage from grubs.
*Organic fertilizers, compost, irrigation and other soil amendments may attract egg laying females.
*Irrigating in the summer when surrounding lawns are dry may attract egg laying females and aid in their survival. Irrigating in late summer and fall may improve the tolerance of lawns to grub damage and help lawns recover and survive.
*Cool and warm season grasses are susceptible to damage from grubs with varying degrees of tolerance by species. To the extent possible, use more grub tolerant grasses that are adapted to your area if grubs are a problem. Cool season grasses listed in order of most to least grub tolerant are tall fescue, Kentucky bluegrass, fine fescue and perennial ryegrass.
*Floral or pheromone traps or lures may not yield the desired results. Beetles are attracted to traps and lures from outside the area and may not end up in the traps and result in more damage to vegetation and more eggs deposited in surrounding lawns than would have occurred otherwise.
*Milky disease, available commercially as Milky Spore, has been effective in helping manage Japanese beetles. Before treating, determine that the grubs in the lawn are Japanese beetles because the spores have not been as effective in managing grubs of other beetles. Grubs are typically identified by examining the raster on the tail end. The link below provides information that will assist in identifying the raster type. Milky spore is readily available by mail and easy to apply. The application is facilitated by the relatively inexpensive dispenser tube.
*Nematodes such as Heterorhabditis bacteriophora offer grub control options. Effectiveness of control may be determined by grub and nematode variety; time of application; larval stage of development; soil type and temperature and other factors. Nematodes have been effective in managing Japanese beetle larval. Effectiveness on other grubs varies. Efforts are underway to make other nematode varieties available in the US.
Resources and References:
Klass, Caroline. et al. “Lawn Insects – White Grub.” Cornell Cooperative Extension website.
http://counties.cce.cornell.edu/chemung/agriculture/publications/white-grubs-may08.pdf. Accessed September 2013.
The Northeast Organic Farming Association. “The NOFA Organic Lawn and Turf Handbook”, Connecticut and Massachusetts. First Edition, March 2007.
The Northeast Organic Farming Association. “NOFA Standards for Organic Land Care”, Stevenson, CT. 5th Edition – January 2011, www.organiclandcare.net.
Tukey, Paul. “The Organic Lawn Care Manual.” Library of Congress. 2007.
University of Massachusetts Amherst. Turf Program website. http://extension.umass.edu/turf/fact-sheets/white-grub-identification. Accessed September, 2013.
Williams, Priscilla., et al. “Lawns: An Organic Approach to Grubs.” Northeast Organic Farming Association website. http://www.organiclandcare.net/green-room/olc-articles/lawns-organic-approach-grubs. Accessed September, 2013.