Age-related changes in the reproductive potential of the Queensland fruit fly

Despite the known negative impacts of aging on the reproductive potential of many insects, Bactrocera tryoni populations show a rapid increase in abundance from early to late spring when the population is composed of predominantly old individuals. While some aspects of how male and female reproductive potential are influenced by age for this species are known, no study investigates lifelong reproductive potential of either sex. We conducted a whole-of-life study in the laboratory to assess the effect of age and mating-partner age on reproductive potential of B. tryoni. The fertility of 70 individual females was directly measured by the number of eggs laid and hatched; while 70 individual males' fertility was assessed indirectly by measuring the hatch rate of eggs laid by a female partner. Half of the males and females had access to a same-age virgin mating partner, while the other half received a prime-age virgin partner (17-19 days old): in both groups mating partners were replaced weekly. Results showed that independent of the age of male mating partner, increasing age significantly reduced the fecundity and fertility of female B. tryoni after a peak at approximately 20 days of age. However, females mated with prime-age males showed higher egg hatch rates during early life than did females mated with a same-age mating partner. As indirectly measured through their partner's egg hatch rate, the fertility of B. tryoni males was also affected by the age of the male and their mating-partner's age. Males mated consistently with a prime-age partner showed an increasing trend in the egg hatch rate of their partner: indirect evidence of increasing fertility in males with increasing age. No such affect was seen when males were mated with a same-age female, possible because of the age-related changes in female fecundity and fertility. While fecundity is greatly reduced in old females, the whole-of-life data shows that the very old flies present in the field at the end of winter are physiologically capable of starting the new season's F₁ generation. Beyond getting it begun, old females are unlikely to further contribute to the new season's population as their fecundity does not increase even if mated with a prime-age, new generation male. In contrast, old males, if they have subsequent access to new generation females, have the capacity to help contribute to the rapid spring population growth which is observed in the field.