Effects Of Body Size And Instar Of Aphis Fabae Scopoli On The Host Selection And Fitness Returns Of Lysiphlebus Ambiguus Haliday (Braconidae, Hymenoptera)

The black bean aphid, Aphis fabae (Scopoli) (Aphididae, Homoptera), is a world-wide insect of ca 200 host plants. It not only inflicts feeding damages, but also transmits plant viruses, to agricultural crops. The aphidiid Lysiphlebus ambiguus (Haliday) is one of common solitary endoparasitoids attacking A. fabae. Until now studies on host selection and parasitism of L ambiguus employed host aphids reared at optimal temperature. Therefore, host size is confounded with instar or age, as these attributes of host aphid nymphs are inevitably related with each other, which made testing of the hypothesis difficult. We manipulate rearing temperatures (high and low) to obtain host aphids of different sizes at the same instar, which were subjected to L. ambiguus for parasitism. We intended by this approach to evaluate whether this generalist aphidiine wasp would show a consistent response to host size for aphid cohorts raised at low and high temperatures, and how well the response to aphid size matched the suitability of the aphids for subsequent parasitoid fitness-related performance.1. Parasitism and fitness-related performances of L. ambiguus for different A. fabae instars reared at the high temperaturePrevalent experiments to investigate the host-stage selection of parasitoids manipulate host cohorts at the optimal temperature (^- 25℃). But these hosts only represented a part of natural populations exposed to parasitoids. To explore the parasitization strategy and fitness-related performances of parasitoids on parasitizing different host instars, the black bean aphid, Aphis fabae, Scopli was cultured at 30℃to obtain cohorts of different instars and adults and then exposed to parasitism of Lysiphlebus ambiguus (Haliday). The parasitism and fitness-related performances were recorded including parasitization rate, emergence rate, sex ratio, body size, and development time. The aphid parasitoid attacked more young aphids than the old even though the first through the forth instar nymphs and eclosing adult were susceptible to parasitism. The parasitism rate on the adult aphid (28.0%) was significantly lower than on the first (40.0%) and the second instar (42.8%). The proportion of female progeny and emerging adult size of the offspring parasitoid declined with aphid instars (and body sizes) at parasitization but the development time remained constant, which contrasted with fitness performances when attacking the host reared at the optimal temperature. It is argued that L ambiguus may not or may not always base the host body size and instar on the host value but other clues (e.g. behavior, chemical, et.al.) associated with dynamic of endosymbiotic bacteria in the host aphid.2. Parasitism of L. ambiguous for A. fabae instars reared at the low temperatureHost aphids at low temperature (15℃) grow slowly but with larger body sizescompared to aphids at high (30℃). When each host instar or adult reared at 15℃was subjected to parasitism, younger aphids were more susceptible to parasitism. The parasitism rate on the 4th instar and adult was lower than the other instars. The adult was least parasitized (23.0%), which was only half of the 1st instar (44.0%). The proportion of female progeny, emerging adult size and the development time increased with aphid instars (and also body sizes).3. Host-instar preference of L. ambiguous and fitness-related performances of offspring in relation to host instars reared at different temperaturesPair-choice experiments were exercised to study preferences for host sizes versus host instar instars of apterous Aphis fabae. Cohorts of same instars of different body sizes were created by rearing host aphids at high (30℃) and low (15℃) temperatures. Host aphids of each instar from both high and low temperatures were subjected to parasitism. The fitness-related parameters, development time, body size and sex ratio of offspring parasitoids were measured. The aphid parasitoid preferred host aphids reared at the high temperature than at the low temperature. The proportion of female progeny was higher for the 1st instar host reared at the high temperature than at the low temperature. However female progeny seemed to account for less proportion for the other instar hosts reared at the high temperature (without statistically significant differences). Parasitoids that attacked hosts reared at the high temperature have longer development time. Body sizes of offspring parasitoids, as measured by hind tibia length, varied with host instars. Parasitoids attacked the 1^st and 2^nd hosts reared at the high temperature were larger than counterparts reared at the low temperature, whereas the reverse came about to parasitism for the 4^th and adult hosts. There was no significant difference in body size of offspring parasitoids attacking the 3^rd instar between the high and low temperatures.