Seasonal Adult Sex Ratio Shift in the Praying Mantid Iris oratoria (Mantodea: Mantidae)

Abstract

The adult sex ratio of the praying mantid Iris aratoria L. becomes increasingly female-biased as the season progresses. This 3-yr field study examines 3 possible causes of this shift in sex ratio: differences between the sexes in the date of adult eclosion, movement to and from the census area, and adult lifespan. In the field, male and female eclosion dates did not differ significantly. Movement to and from the census area was low for females and males. For each year, the number of females that moved into the area (immigrants) exceeded the number that moved out (emigrants), although this difference was not statistically significant in any year. Fewer males were recaptured; those that were did not show either immigration or emigration. Minimum lifespan for adults in the field was greater for females than males. Female lifespan also was greater among adults maintained in individual field containers. Furthermore, the eclosion dates of these captive adults did not significantly differ between the sexes. Thus, the sexes evidently eclosed at the same time of year, and the females most likely outlived the males in the field. This difference in adult lifespan can lead to an increasingly female-biased sex ratio over the course of the season, with different movement patterns between the sexes remaining a possible factor.