Aggregation of Rhynchophorus palmarum weevils on host plants is mediated by a male pheromone (rhynchophorol: R) and host-plant volatiles (PVs) acting in synergy. Synthetic PV blends synergizing pheromone contain acetoin (A) and ethyl acetate (EtAc). R, A, and EtAc are detected by specialized olfactory receptor neurons (ORNs). In addition, particular types of ORNs are tuned to both A and R. To specify the role played by acetoin in pheromone perception, we recorded the responses of ORNs to 100 ng of A or R presented either separately or mixed. Behavioral responses to R, A, and EtAc were studied in a four-armed olfactometer and by field trapping. We screened 59 R-, A-, and AR-tuned ORNs by recording specific responses to odors presented either separately or mixed. Stimulations by blends elicited complex response profiles from the three ORN types: some gave synergistic responses, others were inhibited, and the remainder responded as though both odors were detected independently. Several gave either a weak or no response to a first stimulation by R, but responded clearly to a second stimulation after an intercalary stimulation by A. In the olfactometer, both sexes were more attracted to a blend of A + R (1 + 0.01 ng/sec) than to pure compounds, whereas EtAc did not enhance response to R. Pheromone-baited traps (1 mg/day) containing PV blends (650 mg/day) based on an ethanol/EtAc blend (1:1), plus either 5 or 10% A, or a more complex reference blend, or sugarcane (natural pheromone synergist), caught similar numbers of weevils and about twice as many insects as a control ethanol/EtAc blend. Traps with only pheromone caught about 10 times fewer insects. Behavioral results support the role of acetoin as a pheromone synergist for R. palmarum, and electrophysiological data provide evidence of modulation of peripheral sensory responses to pheromone by acetoin. Sexual dimorphism was observed neither at the ORN nor at the behavioral levels.