The white grubs that routinely damage lawns and turfgrass in Iowa are the annual white grubs. These grubs take just one year to complete each life cycle of egg - larva - pupa - adult. The adult beetles of our annual white grubs are called masked chafers. They are slightly smaller than Junebugs and tan or straw brown in color. As the name implies, they have a black stripe across the eyes and face. The masked chafers have been flying and laying eggs for most of the month of July. Like other things, they were about one week behind normal in their emergence and activity this year.
The eggs hatch in 2 to 3 weeks meaning early to mid-August is the earliest the white grub larvae will be in the soil to begin feeding on grass roots. The damage symptoms that grass plants express because of the root feeding going on in the soil usually do not appear until late August, September or October. Early symptoms include wilting and tan discoloration. Later symptoms can vary from small discolored patches to large, irregular dead areas. It normally takes at least 10 annual white grubs per square foot to cause damage to healthy, vigorous (i.e, watered) turfgrass, fewer if grass is non-irrigated and under stress. With this year~s abundance of July rain in most areas of the state, turf is generally in much better than average condition and the higher threshold will be needed to cause damage and justify treatment. Return to typical, rainless conditions could change the health of the turfgrass very quickly.
It is possible to check turfgrass for white grub damage before it becomes severe and noticeable. First, check your records or memory for the location of damage in previous years. Then consider areas where masked chafers are most frequently seen such as along sidewalks, driveways and streets and at the outside edge of the light thrown by a street light or yard light. Beginning in mid August look for grubs in the soil in these ares by slicing a square from the turf with a knife or spade and pulling it up. Grub- infested turf will pull up easily and the white, C-shaped larvae will be between the plant crown and the soil.
Several insecticides are available for white grub control. All must be carefully applied according to label directions and thoroughly watered in. Liquid insecticide sprays for white grubs must be watered in immediately before the spray can dry on the grass. Within 30 minutes, less if it is hot, dry day, water the sprayed area with at least 1/2 inch of irrigation. It may help if sprays can be applied to dampened turf (either wet with dew or lightly sprinkled before spraying). Granule insecticides must also be watered in to be effective against grubs, but the demand for immediate irrigation is lessened.
The insecticides labeled for white grubs in turfgrass include:
for homeowners: diazinon, trichlorfon (Dylox and Proxol) Sevin and Oftanol;
for commercial applicators: homeowner products plus Turcam, Triumph, Mocap and Crusade.
There are several restrictions with each product, so read and follow label directions very carefully.
This article originally appeared in the July 29, 1992 issue, p. 135.