Technical Abstract: Activity patterns of Phyllophaga crinita (Burmeister), P. congrua (LeConte), P. crassissima (Blanchard), and Cyclocephala lurida (Bland) grubs were acoustically monitored in small pots of bluegrass, Poa arachnifera Torr, at varying and constant temperatures over multiple-day periods. Distinctive temporal and frequency patterns were heard that listeners readily separated into three groups of different durations and intensities. Loud, short (<10 ms) snaps were reminiscent of cutting, chewing, or feeding activity. Frequently occurring weaker, (>100 ms) rustling sounds were identified as general movement activity. A group of repeated-pulse sounds was suggestive of scraping activity. Grubs of all four species produced detectable sounds at rates that increased with temperature (0.45 sounds/((min)(°C))) and weight (6.3 sounds/((min)(g))). Mean sound rates were independent of grub species and time of day. At 25-26 °C, the average sound rate of 3rd-instar P. crinita was 12.1/min, increasing to 13.6 between 30-32 °C. Below 9 °C, however, the average rate of activity was only 0.4 sounds/min and 67% of 3-min recording intervals contained no detectable sounds. In field studies previously, activity rates below 2/min would not usually be considered predictive of an insect infestation, which suggests that acoustic monitoring to detect field infestations may not be reliable below 10 °C. The consistency of results from tests with small numbers of larvae suggests that acoustic systems have potential as tools for nondestructive, short- or long-term monitoring of the efficacy of control treatments or the effects of changing environmental conditions.