Oviposition behaviour and host colony size discrimination in Episyrphus balteatus (Diptera : Syrphidae)

Three bioassays were used to examine the oviposition behaviour of the hover fly Episyrphus balteatus (Degeer) in which various stimuli purported to inf luence host assessment and choice were tested. Episyrphus balteatus failed to exhibit enhanced behavioural responses, in terms of approaches and landi ngs, to artificial leaves with the highest numbers of aphids, suggesting th at females are merely responding to the colour of the artificial leaves. A density-dependent oviposition response was reported in a second bioassay us ing whole bean plants, although there was an asymptotic relationship. Syrph id responses, which were measured by oviposition over a five-day period, we re greatest towards those plants with the highest number of aphids. In a fi nal series of trials, gravid female E. balteatus showed a behavioural prefe rence for filter papers treated with the greatest honeydew concentrations ( 0.26 mg mul(-1) honeydew). More time was spent in the treated areas and syr phids demonstrated more proboscis and ovipositor extensions in these treatm ents. These bioassays provided an opportunity to isolate some of the indivi dual components of decision-making by female E. balteatus during egg-laying behaviour. A greater understanding of natural enemy behaviour is essential before enhanced control of pest populations in the field can be establishe d.