The control of migratory urge in aphis fabae scopoli (hemiptera: aphididae)

Some flightworthy alate exules of Aphis fabae Scopoli reproduce before taking off while others do not. This difference in behaviour is influenced by crowding and has previously been taken as an indication of the urge to migrate, although it has never been conclusively demonstrated. In this study, the flight performance of those aphids depositing some nymphs before take-off (‘flyers’) was compared with those which did not (‘migrants’). In fact, no significant difference was found between migrants and flyers in any of the aspects of post-take-off flight or settling behaviour tested. In tethered flight, large alatae flew for longer than small ones, irrespective of their pre-flight reproductive behaviour. In free flight, however, large alatae flew for a shorter period before actively landing on a plant. Large alatae were also subsequently more reluctant to leave the plant than small alatae, spending more time in sedentary activities such as probing and less time preparing to take off. It is concluded, therefore, that migratory urge is related more to aphid size, a product of plant nutritive quality, than to take-off urge, as measured by pre-flight reproductive behaviour. However, as high population densities tend to occur when the nutritional quality of the host is declining, take-off urge and migratory urge are likely to be synchronized in the field.