Preference of the aphid parasitoid Monoctonus paulensis (Hymenoptera : Braconidae, Aphidiinae) for different aphid species: Female choice and offspring survival

Host choice by Monoctonus paulensis (Ashmead) (Hymenoptera: Braconidae, Aphidiinae) was examined in the laboratory. In dichotomous choice tests, females mere provided with four kinds of hosts that were similar in size and equally available. They preferred second instars of Acyrthosiphon Acyrthosiphon pisum (Harris) over third instars of Sitobion avenae (F.) and both over second instars of Macrosiphum creelii Davis and Metopolophium dirhodum (Walker) (Hemiptera: Aphidoidea: Aphididae). Host choice was influenced by differences in aphid size. First instars of M. creelii and second instars of A. pisum were accepted at the same rate; first instars are smaller, have fewer defenses, and are easier to handle than second instars. Overall, A. pisum was the best host in terms of successful oviposition (76% of encountered aphids), eclosion from mummies (91%), and offspring sex ratio (72% females among offspring). Nearly 50% of encountered M. creelii escaped or were released by females without stinging, and 26% of encountered M. dirhodum survived stinging because either eggs were not laid or did not develop. Values for S. avenae were intermediate, with over 30% of encountered individuals escaping. Attacks on M. dirhodum and S. avenae, with or without oviposition, incurred lower time costs (17 s) than attacks on A. pisum and M. creelii (23 s). Relative to A. pisum (value = 1.0), a female's fitness returns in terms of offspring produced per encountered aphid were 0.75 for S. avenae, 0.36 for M. creelii, and 0.33 for M. dirhodum; her fitness costs in terms of time invested per offspring produced were 0.99 for S. avenae, 0.44 for M. dirhodum, and 0.35 for M. creelii. The ranked order of aphid species as reflected in host choice by females of M. paulensis was qualitatively the same as host order based on estimated fitness returns. (C) 2000 Academic Press.