Oviposition behaviour by female Sympetrum dragonflies (Odonata) affects the conditions eggs experience during their over-winter diapause. Interestingly, females will oviposit both into water and onto dried sediments above the waterline, including females that lay in both habitats while depositing a single clutch. Terrestrial and aquatic environments differ in the benefits and challenges they impose on overwintering insects. Aquatic environments experience milder and more consistent temperatures throughout winter than terrestrial environments, but eggs laid there could be vulnerable to predation, hypoxia, or mechanical damage from ice. In comparison, terrestrial eggs may experience more variable and extreme winter temperatures, desiccation, and risk failure to hatch should the pond not flood sufficiently in spring. Females may be engaging in a risk-spreading strategy but little is known about the performance of eggs in these habitats. To assess how female oviposition choices affect egg performance and hatching phenology, eggs from two Sympetrum species, S. vicinum and S. obtrusum, were exposed to three in situ treatments encompassing the conditions eggs experience: Deep water, Shallow water, and Shore (dry).
Results/Conclusions
Hatch success was significantly higher for S. vicinum than S. obtrusum in all treatments (F1,120=341.1, P>0.001) but both species showed the same patterns across treatments. For both species, Deep and Shore treatments showed equivalent high hatching success as well as early and synchronized hatching, while in the Shallow treatment hatching was significantly lower (F2,120=60.4, p<0.001), delayed, and occurred over an extended period. Delayed hatching date in Shallow treatments may have further negative consequences for dragonfly larvae as smaller relative body size renders larvae more vulnerable to intra-guild predation. If they avoid the shallow margins, females may therefore be successfully risk-spreading in ovipositing across these habitats. However, the drawbacks of only laying eggs in deep water remain unclear. We are continuing to test the trade-offs driving female oviposition behaviour, including the effects of benthic detritivores, hypoxia, and mechanical ice damage. Knowledge of how winter conditions affect survival and phenology of organisms is an important step in gaining a cross-seasonal understanding of population dynamics and species interactions.