A sex pheromone from females of the western corn rootworm (WCR), Diabrotica virgifera LeConte, was collected by a semi-continuous, nondestructive method; and an improved laboratory bioassay was developed. Field studies with the unfractionated pheromone showed that males of the WCR and northern corn rootworm (NCR), Diabrotica longicornus (Say), were attracted to traps baited with WCR pheromone. The NCR males displayed a distinct quotidian variation in their response to the baited traps. The field traps remained attractive for at least 22 days after a single application of pheromone providing a triglyceride extender was incorporated into the pheromone solution.