Variation in hatching time (phenology) might cause size differences within populations resulting in size-mediated priority effects (SMPEs) shaping intraspecific interactions. These phenology-driven effects potentially can be strengthened by seasonal time constraints caused by a short growth season, and depend on latitude. Here the single and combined effects of phenology and latitude-associated time constraints on SMPEs in larvae of an aquatic insect, the damselfly Lestes sponsa, are studied. We did so by rearing larvae in groups of 16 individuals with different phenology (hatching date) thereby imposing strong intraspecific competition, resulting in cannibalism. We thereby manipulated in a fully crossed way time constraints (combination of temperature and photoperiod: thermo-photoperiod) in larvae from low-latitude and more time constrained high-latitude populations, and examined effects on life history (survival, development, growth) and physiology (fat and protein contents, and phenoloxidase activity as a measure of immune function). Phenology, time constraints and latitude of origin had strong effects on life history, but only the time constraint affected the physiology. We detected a SMPE for survival that, however, was not stronger under time constraints and was consistent in strength between latitudes. Phenology and time constraints interacted for development and growth in a direction suggesting adaptive responses to time constraints but these life history traits did not show SMPEs. We provided important insights in the study of SMPEs thereby showing these to be trait-dependent and not more pronounced under experimentally manipulated or latitude-associated time constraints. Our study thereby makes an important addition to geographic variation in SMPEs, a largely neglected topic.

Non-consumptive predator effects experienced in early life stages of prey may result in life-history costs in later life stages. Such effects can, for example, alter the growth rate during the juvenile stage, which may carry over to size at maturity. However, we have limited knowledge of the carry-over effects starting from the egg stage through the larval stage to the adult stage. Here, we present results from a laboratory experiment in which we exposed a damselfly, Ischnura elegans, to chemical cues originating from a fish predator, perch. We used a 2 Â 2-full-factorial design in which the damselflies were exposed to predator cues during either the immobile egg or the mobile larval stage. The presence of predator cues, i.e., non-consumptive predator effects, during the egg stage caused decreased survival, but only until 2 wk after larval hatching. Predator cues during the larval stage caused decreased survival until emergence and an increase in development time until emergence. However, mass at emergence was not affected by predator cues. When fish cues were present in the egg or larval stage, there was a lower growth rate until final-instar larvae than in larvae that did not receive fish cues. Our results add to the growing number of studies showing that predation-risk cues in the egg stage can carry over to the adult stage, which ultimately could have consequences for adult life-history traits, such as survival and fecundity.

Under climate warming, temperate ectotherms are expected to hatch earlier and grow faster, increase the number of generations per season, i.e., voltinism. Here, we studied, under laboratory conditions, the impact of artificial warming and manipulated hatching dates on life history (voltinism, age and mass at emergence and growth rate) and physiological traits (phenoloxidase (PO) activity at emergence, as an indicator of investment in immune function) and larval survival rate in high-latitude populations of the damselfly Ischnura elegans. Larvae were divided into four groups based on crossing two treatments: early versus late hatching dates and warmer versus control rearing temperature. Damselflies were reared in groups over the course of one (univoltine) or two (semivoltine) growth seasons, depending on the voltinism. Warming temperature did not affect survival rate. However, warming increased the number of univoltine larvae compared to semivoltine larvae. There was no effect of hatching phenology on voltinism. Early hatched larvae reared under warming had elevated PO activity, regardless of their voltinism, indicating increased investment in immune function against pathogens. Increased PO activity was not associated with effects on age or mass at emergence or growth rate. Instead, life history traits were mainly affected by temperature and voltinism. Warming decreased development time and increased growth rate in univoltine females, yet decreased growth rate in univoltine males. This indicates a stronger direct impact of warming and voltinism compared to impacts of hatching phenology on life history traits. The results strengthen the evidence that phenological shifts in a warming world may affect physiology and life history in freshwater insects.