Variability within sex pheromone signalling systems is generally believed to be low because of strong stabilizing selection; yet the noctuid moth Heliothis subflexa (Hs) shows significant intraspecific variation. One possible explanation is that females may alter their sex pheromone blend depending on prevailing olfactory cues in the habitat, which we termed the "experience hypothesis". This could be adaptive if Hs females experiencing the pheromone of another species, Heliothis virescens (Hv), responded to reduce the frequency of heterospecific matings. We exposed Hs females to no pheromone, Hs pheromone, or Hv pheromone in the first 3 days of their adult lives. Hs females in the latter treatment produced signficantly more of the acetate Z11-16:OAc, which inhibits the attraction of Hv males. To our knowledge, this is the first study showing adaptive phenotypic plasticity in a moth sex pheromone, and suggests that behavioral differentiation may precede genetic divergence in the sexual signals of moths.