Entomologia Experimentalis et Applicata 97: 301–308, 2000. © 2000 Kluwer Academic Publishers. Printed in the Netherlands. 301 Oviposition behaviour, influence of experience on host size selection, and niche overlap of the solitary Leptomastix epona and the gregarious Pseudaphycus flavidulus, two endoparasitoids of the mealybug Pseudococcus viburni Filitsa Karamaouna & Michael J. W. Copland ∗ Biology Department, Imperial College at Wye, University of London, Wye, Ashford, Kent TN25 5AH, UK ∗ Author for correspondence (E-mail: m.copland@ic.ac.uk) Accepted: June 8, 2000 Key words: Leptomastix epona, Pseudaphycus flavidulus, Pseudococcus viburni, oviposition behaviour, host size selection, experience, developmental refuge, biological control Abstract Oviposition behaviour and host size selection of the solitary parasitoid Leptomastix epona (Walker) and the gregar- ious Pseudaphycus flavidulus (Brèthes) [both Hymenoptera: Encyrtidae] were examined on five size classes of the mealybug Pseudococcus viburni (Signoret) [Hemiptera: Pseudococcidae]. The host size classes mostly consisted of one stage (first, second, third instar nymph, young adult and preovipositing adult) and were presented together to wasps of either parasitoid species. Both parasitoid species locate the host by drumming the surface of the patch with the antennae. Leptomastix epona seems to use mainly the antennae to examine the host but P. flavidulus may accept or reject a host for oviposition after antennation or insertion of the ovipositor. Leptomastix epona attempts oviposition in all the host stages from second instar nymphs but P. flavidulus includes first instar. Both parasitoid species select mainly larger hosts (>1 mm, third instar nymphs) to oviposit but P. flavidulus is able to parasitize more second instar nymphs compared to L. epona. Female wasps of L. epona may host feed on small mealybugs (second and third instar nymphs) that they do not use for oviposition. Oviposition experience of either parasitoid species for 24 hours does not influence host size selection on patches with hosts of similar mixed sizes. Oviposition decisions are independent of the host sizes of the preceding ovipositions. Implications about stability of a single parasitoid – host system and the success of biological control of the mealybug were discussed in respect of the developmental refugia of the two parasitoid species. Niche overlap of the two parasitoid species was discussed with a view to giving an insight into a single or multiple introduction. Introduction Hymenopterous parasitoids have to find and accept a host to complete their reproductive cycle. Once a parasitoid has physically contacted a potential host, evaluation follows to determine whether the host is the appropriate species and growth stage to use for its offspring development (van Baaren & Nenon, 1996). Selection by one parasitoid between hosts of different age (stages) or of different species is defined as host selection. Because the relative abundance of the most suit- able stages may vary both spatially and temporally, parasitoids have evolved behavioural, ecological and physiological adaptations to discriminate among, and utilize their hosts (Nechols & Kikuchi, 1985). Host ex- amination and attack may include different steps such as host encounter, antennation (drumming), probing, oviposition and marking (Van Driesche & Bellows, 1996) and different stimuli may be necessary to elicit this chain of behaviour. Thus parasitoids may use movements and vibrations by the host, chemical cues, both on the outside and inside of hosts, as well as phys-