I have noticed sporadic holes being dug in a section of my yard and a neighbor suggested that I have grubs in the lawn. Is this something to worry about?
— Alfredo Ruiz, Highwood
Holes being dug in the lawn at this time of year is an indication that grubs are present in the lawn. This is not necessarily a call to action on your part. Late summer into early fall is the time grub damage is most likely to show up in lawns. Grubs are the larval stage of beetles. The grubs feed on roots of grasses, so there can be wilting and browning of irregular-shaped areas. Skunks and raccoons dig up the lawn while searching for the grubs, resulting in the holes that you are seeing. I have seen lawns with extensive damage from skunks and raccoons that look like a rototiller ran through the lawn. Carefully pull back the sod in any suspect areas and look for the white, c-shaped grubs just under the soil. Turf damage is likely when grub populations reach more than 10 to 12 per square foot of lawn.
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Adult beetles generally lay their eggs in full-sun lawns that are moist in July. If your lawn has been irrigated and is surrounded by dry lawns and you see adult beetles flying about, there is a greater chance of having grub problems. I do not water my lawn at home and have only had minor grub damage once in 12 years. This is a lawn pest that I do not worry much about. Monitor lawns for signs of grubs starting in mid-August and continuing into September. Damage from grubs can continue into October. As temperatures cool in the fall, the grubs tunnel down to overwinter. Make sure that grubs are the problem before applying any controls.
Lawns showing damage from grubs later in the season can be treated with insecticide if the extent of damage to the lawn warrants control. Read and follow all label instructions. When treating your lawn after grub damage is observed, treat in and around the affected areas only, since grubs may only be damaging a small part of the lawn. Insecticides such as Trichlorfon that are used for controlling grubs after damage is present work by contact, so it is important to promptly water in the product. Grub control products using a systemic insecticide work more slowly over the course of the season and are not appropriate for use at this time of year for grub control.
For more plant advice, contact the Plant Information Service at the Chicago Botanic Garden at plantinfo@chicagobotanic.org. Tim Johnson is senior director of horticulture at the Chicago Botanic Garden.