Facultative sex avoids many hypothesized costs of sexual and asexual reproduction, while gaining benefits of both. However, this strategy brings with it a suite of additional life history traits to optimize, not least the frequency, timing and duration of sexual periods. These have flow-on effects on traits such as offspring sex allocation, with consequences for sex ratio dynamics that can feed back to affect the relative costs of sexual and asexual reproduction. So when should facultative sexuals invest in sex? I will present results from a field study documenting the dynamics of sexual reproduction and sex ratios in natural populations of Daphnia magna over their growing season, which suggest that sex is more common when its costs relative to asexuality are lowest: when population density is high, mortality rates increase, and asexual fecundity is low. Not unexpectedly, as males are necessary for sexual reproduction, this pattern of sex exerts a strong influence on the production of sons vs. daughters. However, simple experimental manipulations show that sex allocation also responds to the current population sex ratio. We find that beyond seasonal ecological determinants, both investment in sex and sex allocation in Daphnia are shaped by their relative costs.