Factors affecting reproduction and the importance of adult size on reproductive output of the midge Chironomus tentans

We conducted two separate tests to evaluate the influence of several factor s, that could affect estimation and interpretation of effects on reproducti ve output of Chironomus tentans in sediment toxicity tests. Specifically, t he influence of adult size, mating frequency in males (male), and age of bo th males and females (female) at first mating on mating success (number of successful matings), fecundity (number of eggs/female), percentage hatch, a nd number of offspring (number of hatched eggs) was assessed. In the first experiment, the influence of adult size on reproductive output was determin ed by growing midges fed a low (0.29 mg/individual/d) and a high (0.42 mg/i ndividual/d) a-mount of food to produce small (S) and large (L) adults, res pectively. The adults were then mated in one of four scenarios: S male X S female, S male X L female, L male X S female, and L male X L female. An inc rease in male weight at a constant female weight had no significant effect on fecundity, whereas an increase in female weight yielded 49.5 and 60.7% i ncreases in fecundity when mated with low- and high-fed males, respectively . Similarly, mean number of offspring (fecundity X percent hatch) increased by 19.8 and 48.3% when male weight was increased and female weight was hel d constant and by 141.9 and 180.4% when female weight was increased and mal e weight was held constant. In the second experiment, conducted at a single feeding rate, fecundity increased significantly with an increase in female weight but not male weight. The number of offspring was not significantly influenced by adult weight. The age of males or females at first mating, th e frequency with which males were mated (up to seven consecutive days), and the proportion of males successfully mating had no effect on fecundity or number of offspring; however, large increases in variability about mean est imates were observed in some endpoints for older males. These results sugge st that adult female size is the most important factor affecting reproducti ve output in C. tentans, and that enhanced performance of the life-cycle te st with respect to all reproductive endpoints might be achieved if males ar e not mated for more than five consecutive days.