Host instar, body size and fitness in the koinobiotic parasitoid Aphidius nigripes

In aphidiine parasitoids, resources for growth and adult body size increase with host instar used by ovipositing females, but the fitness consequences of body size on fitness are poorly documented. We compared the fitness of male and female A. nigripes adults that varied in size as a consequence of developing in different instars of their host Macrosiphum euphorbiae. When reproductive fitness was measured without considering time, female wasps fr om small and large hosts performed similarly, contributing 125-175 foundres ses plus 100-180 sons to the next generation. However, when expressed as th e innate capacity for increase (r(m)), female fitness correlated with host- induced variation of wasp size, indicating that micropopulations initiated by large wasps would increase faster. In a wind-tunnel, a sex pheromone plu me from large female wasps induced more males to fly upwind when released a t a distance of 50 cm downwind than small females, indicating that large fe males were sexually more attractive. With respect to male body size effects on fitness, large individuals performed similar to small ones, whether fit ness was measured by lifetime mating frequency, fertile inseminations, or p roportion of daughters among progeny born from their mates. When young naiv e males of unequal size were directly competing for mating with a virgin fe male, small and large males had equal mating success, and large individuals were no more successful than small ones at displacing a competitor already positioned on a receptive female. In a wind-tunnel test where males were s cored on their ability to reach a female pheromone source, small and large males were equally affected by wind speed but reached the source located 50 cm downwind in equal proportions, suggesting similar capacity for finding mates by flying upwind. Our results indicate that despite host resources no t being fixed at the time of attack for the koinobiont A. nigripes, fitness consequences of resource limitation by the mother may be perceived to be g reater for daughters than sons, which would explain male-biased sex ratio i n early-instar hosts.