Influences of Timing and Population Density on the Sex Ratio of the Regal Fritillary, <i>Speyeria idalia</i> (Lepidoptera: Nymphalidae).

Abstract

In summer during 1988-99, we conducted 735 hrs and 1416 km of transect butterfly surveys at 116 grasslands in the range of the regal fritillary, Speyeria idalia (Drury), grouped into four subregions: (1) Wisconsin, (2) northwestern Iowa and adjacent Minnesota, (3) northwestern Minnesota and adjacent North Dakota, and (4) western Missouri. We recorded 22,685 regal fritillaries at 84 sites; 92% of the 20,968 sexed individuals were males. First female emergence typically followed first male emergence by about 1-3 weeks in subregion 1. In each subregion, male density (individuals/hr) showed a single mid-flight peak, while females increased gradually and linearly. The strongest and most consistent correlation for percent unsexed individuals was negative with year. During most of the flight period, the observed "sex ratio" (percent males of sexed individuals) was strongly male-biased. Sex ratio correlated significantly and negatively with date, consistent with eclosure by males earlier than females. But in all subregions, the strongest correlation for sex ratio was negative with female density, both in the entire survey period and prime (main) flight, which by definition had positive male and female densities that did not vary significantly by date. In most subregions, sex ratio covaried significantly with male density, both over the entire survey period and during prime flight. Observed sex ratios relate to relative densities of males and females actually present but also suggest a tendency for females to become less detectable at higher male densities. Since the observed range of densities was much greater for males than females in all subregions, females appeared to adjust their detectability (i.e., behavior) based on male density but not vice versa. In addition, more females may actually be present in a site earlier in the flight than when highest observed female densities occur.