Frequency of female morphs is related to an index of male density in the damselfly, Nehalennia irene (Hagen)

Researchers have questioned whether variation in population density influences the maintenance of two or more morphs within and between insect populations. Two distinct morphs of female Nehalennia irene (Hagen) (Odonata: Coenagrionidae) occurred at each of eight sites in eastern Ontario, Canada; one morph (i.e. andromorph) was coloured and patterned like the conspecific male. We found significant differences in relative frequencies of andromorphs and in male density between sites; frequency of andromorphs ranged from 2–28% and was positively correlated with male density. Sites with high densities of males were located closer to low-density sites than to other high-density sites, indicating that high-density sites were somewhat independent of one another. We also found spatial variation in density of males within sites; males were present almost exclusively at edges of ponds or marshes. Proportionally more andromorphs than heteromorphs were found at the edges of sites, where male-female interactions were likely frequent. Sex ratios at the edges of sites were most strongly male-biased at two of three high-density sites, indicating that males may force non-receptive females away from the edge of high-density sites. Spatial and temporal variation in male density of N. irene may be an important factor influencing morph coexistence.