Host selection and oviposition behaviour of the solitary aphid parasitoid, Monoctonus paulensis (Hymenoptera: Braconidae, Aphidiiane)

Host selection and oviposition behaviour of the solitary aphid parasitoid Monoctonus paulensis was studied in the laboratory. In dichotomous choice tests, females ‘preferred’ the relatively younger and smaller instars of the pea aphid, Acyrthosiphon pisum, over equally available older and larger instars: instar 1 > instar 2 > instar 3 > instar 4. This order was independent of the host instar and was not influenced by female size, female age, or host mobility. The probability of an attacked aphid escaping or avoiding parasitism increased with aphid instar. Total time needed by a female to capture and oviposit into an aphid varied with instar, with fourth instars requiring twice as much time as other instars. Fourth instars were also most likely to drop from the feeding site after a successful attack.; Females frequently laid clutches of two eggs during a single oviposition bout. Oviposition time was proportional to clutch size, but handling time was influenced by host instar and/or size. Eggs were laid one at a time rather than in clumps, which suggests that egg number is controlled by the female. Contrary to expectations, offspring mortality was not influenced by clutch size. The size of male and female wasps was a positive function of host size. Female fecundity at eclosion increased with adult size. Immature mortality was highest among offspring developing in fourth-instar pea aphids.; Females were also provided with a choice between four different aphid species that were presented two at a time. Aphids were similar in size and equally available; they were ranked in the following order: pea aphid ( Acyrthosiphon pisum), English grain aphid (Sitobion avenae ), alfalfa aphid (Macrosiphum creelii), and rose-grain aphid (Metopolophium dirhodum).; The results suggest that host choice by M. paulensis is determined by the female's ability to capture and parasitize aphids differing in size and behaviour, rather than to maximize larval performance.