In addition to cold temperature mortality, Japanese beetle larvae require approximately 11 inches of water from egg hatch in late July into the fall. Although we received abundant rain in spring of 2013, much less rain fell from July through October, averaging 9.5 inches during that time in most of the state. Although that was made up in irrigated turf, many grubs probably died in other areas. This will reduce the number of adult beetles in southern Illinois and cause even further reductions in the northern half of the state.
Japanese beetle larva in root zone.
Northern and southern masked chafer grubs require less water and tunnel deeper than Japanese beetle grubs. In areas of central Illinois and other areas where these grubs are numerous, neither drought nor cold is likely to have reduced their numbers. Where Japanese beetle adults are few, masked chafers are likely to invade those turf areas. However, that is likely to be short-lived as it appears that where Japanese beetle is numerous, masked chafers almost disappear. (Phil Nixon)Source: Home, Yard, & Garden News