A “sticky”-trap bioassay system for the pheromones of the western corn rootworm (WCR), Diabrotica virgifera LeConte, and northern corn rootworm (NCR), Diabrotica longicornis (Say), is described. Living female beetles of both species and WCR pheromone extract were tested. Males of both species were strongly attracted to virgin females of either species, as well as to this extract. Male WCR and NCR responded to pheromone sources at different times of day. The NCR were most active around midnight, with virtually none coming to traps during daylight hours. Male WCR had a bimodal pattern, being more active before sunset and again after sunrise, but the pattern was less rigid than that of the NCR. The study could not demonstrate comparable emission patterns in female beetles. Relationships between trap catch and temperature, wind speed, dew point depression, solar radiation intensity, and male population are discussed. The virgin female beetles were, on the average, 3 or 4 days old before becoming attractive but then retained the ability to attract males for at least a month unless mating occurred. Mated females were not generally attractive. Males of both species appeared most active within a meter of the ground, and neither species was responsive to traps baited with WCR pheromone extract set at 2 m. Several considerations for using pheromone traps to survey populations are discussed.