Sex ratio variations with temperature in an egg parasitoid: behavioural adjustment and physiological constraint

Sex allocation in haplodiploid arthropods is a central fitness-related decision that has received much attention in insect parasitoids. The effect of temperature on the reproductive strategy of female parasitoids has rarely been addressed, despite evidence of its influence on other fitness-related traits. We explored mechanisms inducing the higher production of males typically observed at low and high temperature in parasitic wasps, considering that this pattern may result from both behavioural adjustment and physiological constraint. By observing the oviposition behaviour of an egg parasitoid, Trichogramma euproctidis, we were able to distinguish the sex ratio intended by the female from the secondary sex ratio, thereby discriminating between a change in behaviour and a physiological constraint on egg fertilization. More males emerged from eggs laid at low (+45%) or high (+80%) temperature than at medium temperature, but the underlying mechanisms differed between the two conditions. We observed a behavioural change in sex allocation at high temperature, suggesting that laying of sons may be advantageous at high temperature. At low temperature, the femalesʹ intended sex ratio was similar to that at medium temperature, but physiological constraints prevented egg fertilization during oviposition, resulting in more males emerging from eggs intended to be females. To our knowledge this is the first experimental evidence that temperature modulates both sex allocation and physiological constraints in egg fertilization in parasitoids.