Japanese beetle and annual white grubs continue to actively feed throughout the state despite the falling temperatures. Japanese beetle grubs descend deeper into the soil when root-zone temperatures drop to 60°F. Masked chafer grubs remain in the root zone until the temperature drops to 50°F. If grub damage is occurring, pull back the turf to see if grubs are still present in the root zone.
Whether to treat this late in the growing season depends on several factors. If soil temperatures indicate that descent out of the root zone is imminent, feeding should cease soon. Increased irrigation may allow the turf to grow roots faster than the grubs eat them and continue to produce roots into the fall after the grubs have descended. Although the same grubs come back up in the spring to feed, rainfall and a short grub-feeding period in the spring may allow the turf to tolerate feeding with little obvious injury.
Alternatively, high grub numbers now will likely result in high numbers in the spring. A dry spring could easily result in damage that could be prevented now. In addition, warm late fall weather could allow the grubs to feed for several weeks yet. Decisions need to be made based on grub numbers, client attitudes, and other factors. Trichlorfon (Dylox) irrigated with at least 1/2 inch of water to carry the insecticide into the root zone is likely to be the most effective insecticide at this time of year.