Role of Aphids in the Ecology of Plant Viruses

Abstract

Ecology attempts, among other things, to account for the numbers and spatial distribution of organisms. The ecology of viruses has usually been considered in terms of the number and spatial distribution of infected hosts (22, 114, 122). Although viruses cannot be regarded as small cells or micro­ organisms (85) , like other parasites, they must maintain their populations by a dependable means of transfer among susceptible hosts. The continued sur­ vival of a virus requires that the number of sources of virus shall never fall so low that transfer to other susceptible hosts becomes unlikely (22). The characteristics of a virus that permit or enhance natural transfer are thus undoubtedly subject to selective pressures. The relationships of a virus with its vectors must ensure the necessary frequency of transfers. Vectors must have a consistent relationship with host plants, although it may be only a matter of alighting by one or another noncolonizing species during migration. It is not surprising, therefore, that so many viruses are aphid-borne. Most species of angiosperms and gymno­ sperms, and some lower plants such as ferns, in the Northern Temperate Zone are hosts for at least one aphid species. The enormous reproductive potential and the behavioral patterns of aphids ensure their wide dispersal among populations of virus host plants. The numbers and distribution of plants infected with aphid-borne viruses reflect the numbers and activity of the aphid vectors. There is the added condition that the extent to which aphid populations may be important vectors depends on the sources of virus which are available. Under natural conditions and in many agricultural situations, these virus sources are plants infected by aphid transmission in previous years.