Adults of masked chafers (annual white grubs) and Japanese beetles are slow in emerging this year, probably due to the long, cool spring. Many insects have emerged about a week later than normal.
Weather conditions are ripe for a large grub infestation. After adequate rains in the spring, many areas had little rainfall recently. Lack of rain and presence of sunny skies is causing nonirrigated turf to become dormant. This will cause the adult Japanese beetles and masked chafers to concentrate their egg-laying in green, irrigated turf, which should result in damaging numbers of grubs from mid-August into October.
Applying imidacloprid (Merit) or halofenozide (Mach 2) during July is recommended for irrigated turf to prevent damage later. Both insecticides take about 3 weeks to kill grubs but last for months. They are most effective on small, newly hatched grubs.
Watering in the insecticide with at least 1/2 inch of water is recommended. Mach 2 is quite water-soluble, and dried residues readily wash off the grass and into the root zone with rainfall. 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. Also, insecticide on the grass blades and thatch is not in the soil controlling grubs. Because the grubs do 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 would be a good idea.
You will want to cut through and pull back treated turf in August to make sure that the insecticide was effective. Wait at least 3 weeks after the application. You will also want to check lightly irrigated or nonirrigated, untreated turf in early August to determine whether a spot-rescue treatment with trichlorfon (Dylox, Proxol) is needed.