We have several reports of spotty, high infestations of white grubs, with resulting heavy damage to turf. Much of Illinois has had lighter infestations than one would expect, considering the size of grub beetle flight and growing conditions for eggs and larvae from August through October.
Grubs start moving down out of the turfgrass root zone when soil temperatures drop below 50°F, below 60°F for Japanese beetle grubs. In the northern two-thirds of the state, grubs will soon move down. They may have already descended in some areas. Once they burrow below the root zone, turf damage will cease, but so will the chances at effective control. As long as the grubs are in the root zone, insecticidal or nematode control should be effective.
At this time of year, treatment is usually done in response to grub damage, which means that quick control is important to ease the client's mind. Trichlorfon, sold as Dylox and Proxol, kills the grubs in three days and is probably the insecticide of choice. Insecticidal nematodes vary in ability under cooler conditions, but Heterorhabditis bacteriophora, sold as Cruiser and Hb nematodes, should still be effective when the grubs are in the root zone. Nematodes usually kill grubs in less than a week, but dead grubs may still be found because bacteria in the nematodes produce antibiotics that keep the grub from rotting.
If damaging grub populations are not controlled before descending for the winter, be ready to irrigate during dry spells next spring. When soil temperatures rise above 50°F in the turf root zone, grubs will rise and start feeding. However, they will be very difficult to control with insecticides. Irrigation during dry spells allows the grass to grow more roots than the grubs can eat, resulting in attractive turf.