Issue 13, August 9, 2013

White Grubs

Now is the time to be scouting for white grubs throughout the state. The eggs have hatched so there are grubs to find, but they will be too small until mid to late August to eat enough roots to cause dieback.

Cut through the turf with a sturdy knife, I like to use a folding outdoor knife. Select an area where white grubs have been previously, areas that are damper, or the edges of brownish areas. Cut about a one-foot square on three sides, and pull back the sod. In moist soil, the white grubs will be obvious on the soil surface where they have been feeding on the roots of the grass. Look for grubs hanging onto the underside of the sod, and till the upper three inches or so of the soil with the knife blade to flip any deeper grubs onto the soil surface. In dry soil, the grubs will descend downward to where the moisture is located, so they are likely to be four to six inches deep.

White grub scouting.

Ten to twelve grubs per square foot are enough to cause damage. Stressed turf, such as a heavily used soccer field, is damaged by fewer grubs, about eight grubs per square foot. Lightly used turf can have fourteen or more grubs per square foot without showing dieback.

Even one to three grubs per square foot are attractive enough to predators to result in digging damage. Raccoons peal back several inch wide sections of turf to feed on the grubs. A single skunk in one night will create 100 or so three inch diameter shallow holes in the turf seeking grubs. Insectivorous birds, such as cowbirds, blackbirds, starlings, and robins will "chicken-scratch" through the turf to expose and feed on white grubs. In the southern third of the Illinois, where armadillos occur, they will dig holes several inches deep while searching for grubs. None of these animals fill in their holes or replace their divots.

Japanese beetle larva in root zone cross section.

High grub numbers can still be reduced to non-damaging levels with an application of trichlorfon (Dylox) or chlorantroniliprole (Acelepryn). Imidacloprid (Merit), thiamethoxam (Meridian), and clothianidin (Arena) are also effective, but the grubs may not die for two to three weeks. All grub insecticides provide better control if watered in with at least one-half inch of water. (Phil Nixon)

Phil Nixon

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