THE RELATIVE IMPORTANCE OF SHORT‐ AND LONG‐RANGE MOVEMENT OF FLYING APHIDS

Abstract

Summary1. Aphids are notorious pests of world agriculture. Even so, uncertainty persists as to their capacity for successful aerial dispersal. Evidence exists that, under some conditions, aphids can be wind‐borne over long distances, i.e. hundreds of kilometers over desert or sea. It has been argued, in the recent past, that this phenomenon may be part of a strategy to locate fresh host plants in new distant areas. However, the proportion of these insects successfully colonizing new hosts is unknown.2. Other work using meteorological backtracking has also indicated long‐distance movement, but the accuracy of such predictions is dubious unless the altitude of transport is known. Mark‐releaseecapture experiments with such small insects have limited potential due to large dilution effects. Static ‘snap‐shots’ of demographic population densities, using suction traps, cannot accurately distinguish local aerial density fluxes and population movements from a distance. However, genetic and physiological markers may provide more direct information on population mixing; for example, some allozyme studies have shown a limited level of inter‐population gene flow.3. Under suitable conditions, aphids take off, maintain flight and alight in response to the appropriate visual and olfactory cues. Undoubtedly successful long‐distance movement occurs from time to time, but its ecological relevance may have been overstated in the past. It may be selectively disadvantageous for aphids to move from areas containing their host plants. In contrast, it is advantageous for aphids to maximise their chances of survival and reproductive success by landing on suitable plant hosts at the earliest opportunity.4. The clonal nature of aphids (a single genotype may comprise vast numbers of individuals) means that there may be advantages to phenotypic variation between individuals in the readiness to move. Recent evidence indicates that such a variation exists in the duration of the behavioural migratory phase, the initial period of maiden flight when host‐plant cues are ignored and when landing is inhibited.5. The relative biological importance of short‐ us. long‐distance movements is reassessed with reference to plant virus epidemiology and the spread of new genotypes, e.g. insecticide resistance. It is concluded that the biological relevance of short‐distance movements have a much greater impact on population and genotype distribution than long‐distance movements, which may be comparatively infrequent.