The herbivorous water moth Acentria ephemerella shows strong seasonal outbreak dynamics from June to August in many temperate lakes in Europe and North America and is thus important in shaping trophic interactions in macrophyte patches in the littoral zone. The larval and pupal stages live underwater, attached to submerged macrophytes, and the adult stages show a characteristic wing dimorphism. We have shown previously that larvae are exposed to strong predation pressure by fish, i.e. sticklebacks (Gasterosteus aculeatus). As a consequence of their wing dimorphism short-winged aquatic females may be also threatened by fish predation, whereas winged terrestrial males may escape predation due to metamorphosis. We performed an outdoor mesocosm experiment to test the hypothesis that fish predation will result in sex-specific larval life history decisions.Results/Conclusions

Fish predation reduced the size and presumably age at metamorphosis for both male and female moth larvae. For females this reduction in size resulted in a reduction of clutch size by 22 %. Additionally, also as a result of fish predation more larvae went into diapause, suggesting that diapause was an anti-predator strategy alternative to earlier metamorphosis. Sex determination of metamorphosing and diapausing larvae showed that female larvae opted for a different strategy than male larvae under fish predation. Female larvae preferred diapause over metamorphosis, whereas male larvae chose metamorphosis and diapause in equal shares. We propose, that the increased propensity of female larvae to enter diapause reflects the higher mortality risk of adult females compared to males, i.e. metamorphosis seems to be only a temporal refuge from fish predation for females (through the pupal stage) but a permanent one for males. These differences in life history decisions should have important secondary implications, e.g. for population dynamics as well as for the operational sex ratio of Acentria. Finally, our experiment suggests that Acentria larvae can anticipate adult mortality risk and include this information into their life history decisions.