FITNESS CONSEQUENCES OF ALTERNATIVE LIFE-HISTORIES IN WATER-STRIDERS,AQUARIUS-REMIGIS (HETEROPTERA, GERRIDAE)

Using field and laboratory observations and experiments over 3 years, I investigated whether reproductive trade-offs shape individual life h istories in two natural populations of the water strider, Aquarius rem igis, in which univoltine and bivoltine life cycles coexist. Both late r eclosion dates and food shortages, even after adult eclosion, induce d diapause in females, thus deferring reproduction to the following sp ring. Adult body size was positively affected by food availability dur ing juvenile development. Higher food levels also increased the reprod uctive output of females, but not their longevity or oviposition perio d. When compared to spring breeders (univoltine life cycle), direct (s ummer) breeders (bivoltine life cycle) experienced reduced lifetime eg g numbers and longevity, as well as reduced survivorship of their seco nd-summer-generation offspring; these reproductive costs offset, at le ast in part, the advantage in non-decreasing populations of having two generations per year. Fecundity was correlated with body size, and am ong summer-generation females direct breeders were larger than non-bre eders. The time remaining before the onset of winter and/or the time s ince adult eclosion augmented cumulative energy uptake, and consequent ly the lipid reserves and winter survival probability of non-breeding (diapausing) summer adults approaching hibernation. Overwintered sprin g reproductives died at faster rates than non-reproductive summer indi viduals despite greater food availability in spring, indicating a mort ality cost of reproduction. Body length correlated with absolute and n ot with proportional lipid content but showed no consistent relationsh ip with survivorship in the field. These results are in agreement with current theory on the evolution of insect voltinism patterns, and fur ther indicate high degrees of life history flexibility (phenotypic pla sticity) in the study populations in response to variable environmenta l factors (notably photoperiod and food availability). This may be rel ated to their location in a geographic transition zone from uni- to bi voltine life cycles.