Turf drought stress and white grub damage

Justin Quetone Moss and Eric Rebek

During late summer, irregular brown patches may show up in the lawn due to a combination of stress factors, chief among them a lack of sufficient water to the turfgrass plants. Such drought stress may be induced by inadequate management factors such as lack of irrigation water, uneven distribution of lawn irrigation, or excessive thatch layer. Biotic factors such as insect damage and diseases can also contribute or exacerbate summer stress symptoms in the lawn.

The combination of drought stress and insect damage from white grubs can cause turf loss and unsightly brown patches in the lawn during August. White grubs refer to the larvae of certain species of scarab beetles (Figure 1). In Oklahoma, we see white grubs of May/June beetle (many species), southern masked chafer, and Japanese beetle. These species damage turfgrass by feeding directly on grass roots below the soil surface. If white grub populations are large enough, the turfgrass root system can be completely severed from the grass plant. Thus, turfgrasses with significant white grub damage can be easily pulled up from the ground, almost like a piece of new sod. Above ground, turfgrass leaf blades will turn brown and appear wilted. Severe white grub damage may be masked or confused with turfgrass drought stress during summer months. Certain species of white grub larvae become very active in August, especially those of southern masked chafer and Japanese beetle. In addition, August is typically an extremely dry month in Oklahoma. Therefore, initial damage to turfgrass roots from white grub larvae can be exacerbated by drought conditions, which can quickly lead to plant death.

During August in Oklahoma, you may want to double check those brown patches in your yard for signs of drought stress and white grub damage. White grub populations should be monitored in areas where damage is suspected or has occurred historically by cutting and rolling back several blocks of turf measuring one square foot and counting the number of grubs encountered. For vigorously growing turfgrass, treatment may be warranted if an average of five or more May/June beetle white grubs are found per square foot. This treatment threshold increases to 20 grubs per square foot for southern masked chafer and Japanese beetle, which typically are smaller than May/June beetle white grubs in August. So, how do you tell the difference among these types of white grubs? You have to check the setal pattern (arrangement of hairs) on the bottom of the last abdominal segment (what we affectionately refer to as “counting butt hairs”). May/June beetles have two central, parallel rows of spine-like hairs that resemble a zipper, masked chafers have a random distribution of hairs, and Japanese beetles have a distinct V-shaped pattern of hairs (Figure 2). If you believe you have a white grub problem in your yard, contact your local OSU County Extension Educator for treatment and management options.

Figure 1. White grubs in soil. Photo courtesy of Dr. Tom Royer, Professor, OSU Department of Entomology and Plant Pathology.

Figure 2. Arrangement of setae (hairs) on bottom of last abdominal segment for white grubs of (A) May/June beetle, (B) masked chafer, and (C) Japanese beetle. Note the two central, parallel rows of hairs in (A), the random distribution of hairs in (C), and the V-shaped arrangement of hairs in (C).