Adults of masked chafers, also known as annual white grubs, appear to be low in number this year, at least in central Illinois. However, Japanese beetles appear to be making up for the shortage of masked chafers, with their numbers appearing to be higher than usual in many areas of the state.
Weather conditions are ripe for a large grub infestation. After adequate rains across the state in early July, follow-up rainfall has not occurred. The forecast for the middle of July is for no rain throughout Illinois. The lack of rain and the presence of sunny skies will likely cause nonirrigated turf to return to dormancy. If so, the adult Japanese beetles and masked chafers will concentrate their egg-laying in green, irrigated turf. This should result in damaging numbers of grubs from mid-August into October.
Application of imidacloprid (Merit) or halofen-ozide (Mach 2) to irrigated turf during July is recommended to prevent turf damage later. Both insecticides take about 3 weeks to kill grubs but last for several months. They are most effective on small, newly hatched grubs. Both insecticides are also available to homeowners, with halofenozide also available as Grub-ex, Ortho Bug-B-Gone, and Scotts grub control.
Watering in the insecticide application with at least 1/2 inch of water is recommended. Mach 2 is quite water-soluble, and dried residue readily washes off the grass and into the root zone with rainfall. Imidacloprid is more water-soluble than older grub insecticides and washes off with rainfall almost as easily as halofenozide. However, ultraviolet light (sunlight) is a major factor in the breakdown of many pesticides, and leaving the insecticide residue in strong sunlight on the grass waiting for a rain may result in some breakdown of product. In addition, insecticide on the grass blades and thatch is not in the soil controlling grubs. As the grubs will not hatch until late July, applications at that time should be watered in so that activity against the grubs starts as soon as possible. In any case, if rainfall has not occurred within 3 or 4 days after application, irrigating the insecticide into the soil is a good idea.
You will want to cut through and pull back treated turf in August to make sure that the insecticide application was effective. Wait until at least 3 weeks have elapsed since the insecticide application. You will also want to check lightly irrigated and nonirrigated, untreated turf in early August to determine if a spot rescue treatment with trichlorfon (Dylox or Proxol) is needed.