What causes male-biased sex ratios in mature damselfly populations?

1. Several hypotheses to account for biased sex ratios in mature insect populations were tested by monitoring two field populations of the damselfly Lestes sponsa and by pet-forming experiments in field cages. The population sex ratios are heavily male biased in this species. 2. The observed sex ratio at emergence:was even and both sexes emerged synchronously. Females had longer maturation times but these were insufficient to explain the observed sex ratio shift. 3. Mass increases during maturation were consistently larger in females. In agreement with this, immature females made more flights per unit of time, which should make the more vulnerable to predation, however maturation probabilities were lower in females only in one field cage experiment. This inconsistency may be due to long bad weather conditions. Interestingly, predators reduced mass increase and this reduction was larger in females than in males. 4. Calculations:based on the sex specific maturation times show that only slightly lower daily survival probabilities during maturation in females are enough to generate the observed sex ratio shift. 5. Mature survival was higher in males than in females in one field population but not in another, indicating that this cannot be a general mechanism causing the sex ratio. A higher maturation probability in males is therefore the most plausible mechanism causing the sex ratio shift in damselfly populations.