We tested the hypotheses that reproductive interference between 2 congeneric damselfly species influences their local population densities and the female morph ratios in one of the species. Nehalennia irene has 2 female types (andromorph and gynomorph), whereas N. gracilis exhibits only one female type. Andromorphic N. irene females not only resemble conspecific males in body coloration, but also resemble heterospecific females of N. gracilis. We predicted male N. irene to be most attracted to gynomorphs of N. irene and male N. gracilis to be least attracted to them. Further, if N. gracilis males harass andromorphic N. irene females excessively, then they may reduce andromorph frequencies of N. irene locally. Our results indicate hybridization to be prevented by a “lock-and-key” mechanism, but male N. irene often attempt mating with female N. gracilis. Contrary to prediction, andromorph frequency in N. irene did not depend on whether N. irene populations were in sympatry or allopatry with N.gracilis. As predicted, N. irene males attempted tandem formation most frequently with conspecific gynomorphs, while N. gracilis males made most heterospecific tandem attempts on N. irene andromorphs. Collectively, our results suggest that N. gracilis females may be frequently harassed by N. irene males, and that this may help explain the relative rarity of N. gracilis.

Nomenclature: Walker, 1953.