A 2-yr study was conducted to evaluate the effect of ethephon (2-chloroethylphosphonic acid) on efficacy of pheromone traps in monitoring fall and spring populations of adult boll weevils. The experimental procedure consisted of a grandlure pheromone strip placed in a boll weevil trap together with a 7.5-ml aqueous solution of ethephon at 0.1, 1, 10, and 100 ppm concentrations. Treatments also included traps with nothing, ethephon alone at each of the four concentrations, and grandlure alone. Traps were monitored twice a week, and ethephon solutions were replenished during each trap inspection. Trap catch was significantly affected by year, season, and treatments. Overall, significantly higher numbers of boll weevils were captured in 1996–1997 compared with 1997–1998. Average numbers of boll weevils captured in fall season were approximately six times higher than the numbers captured in the spring season. Ethephon alone did not elicit a significant attraction to fall or spring populations at any concentration evaluated. However, the combination of ethephon and grandlure pheromone showed a distinct synergistic effect, enhancing the efficacy of pheromone-baited traps in monitoring fall migration of boll weevil populations. Ethephon at 1 and 10 ppm combined with grandlure were most attractive to fall-migrating weevils, capturing 20–35% more weevils than the grandlure alone. The combination of ethephon at 10 ppm and grandlure also had a significant effect on weevil attractancy for spring populations, but the results were not consistent between years. Data from this study clearly suggest that the addition of ethephon to grandlure can significantly improve trap attractancy and will aid in boll weevil monitoring and eradication strategies that aim at attacking the weevils during the fall, with some potential of improved attractancy in monitoring spring populations.