Issue 13, August 8, 2011

White Grubs

Scouting at this time will allow control decisions to be made in areas where preventative insecticides have not been applied. Even in areas with large numbers of grubs, damage is unlikely to become evident until the second half of August.

Scout for grubs by cutting through the turf with a stout knife and prying it up. A few grubs are likely to be in the sod, but most will be exposed on the soil surface. White grubs are C-shaped, white scarab larvae with six legs, and brown heads. White grubs in early August are usually about one-quarter inch long. Till the soil with the knife to reveal any grubs below the main root zone. In moist turf, the grubs will typically be within two inches below the sod; in dry soils, the grubs are likely to be four to six inches deep. Once the grubs are counted, replace the sod and tamp it down with your foot. Watering the area reduces the likelihood of brown patches where the sod was disturbed.

At this time of year, it is easy to mistake white grubs for other species. Japanese beetle grubs and masked chafer grubs will be one-eighth to one-quarter inch long, so will billbug and black turfgrass ataenius grubs. Billbug larvae are chunky, thicker than white grubs, and will not have any legs. Black turfgrass ataenius larvae will be C-shaped like white grubs but will be more slender for their size. One can verify that the larvae are ataenius by looking for a raster pattern. Ataenius grubs will have two large pads instead of a raster pattern of spines or thick setae at the posterior underside of the abdomen.

Japanese Beetle larva

If ten to twelve or more white grubs are present per square foot, treatment is recommended. The quickest result will be an application of trichlorfon (Dylox) watered in with at least one-half inch of water. Because Dylox will not move very deep into the soil and only lasts about five days, the larvae need to be near the soil surface at application. If the soil is dry, irrigating one or two days before treatment will draw the larvae up close to the surface. Insecticides usually used as grub preventatives will still be effective in preventing damage, particularly if applied early in August. (Phil Nixon)

Phil Nixon

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