Compounds that are structurally closely related to the western corn rootworm (WCR) (Diabrotica v. virgifera, Coleoptera: Chrysomelidae) pheromone were prepared and screened for biological activity in the field, presented alone or in combination with the pheromone 8-methyldecan-2-yl propanoate. None of the synthetic compounds showed attraction when presented alone. However, when presented in combination with the pheromone, catches in traps containing 8-methyldecan-2-yl acetate as a second component were dramatically reduced, suggesting strong inhibitory activity for this compound. The addition of the inhibitory acetate to the known floral WCR lure (4-methoxycinnamaldehyde plus indole) did not influence male (or female) catches suggesting that the inhibitor interferes in the perception process of the pheromone and not by exerting repellency per se. To our knowledge, this is the first report on an inhibitor of response to pheromone in WCR. 8-Methyldecan-2-yl acetate has previously been described as a sex attractant of Diabrotica cristata, so its inhibitory activity towards males of WCR may reflect a role in maintaining reproductive isolation between the two taxa.