Local population dynamics of two co-existing birch aphid species: competion or intrinsic cycles of abundance?

Populations of four aphid species coexisting on their Betula pendula host-plant were followed in Brussels for two years. The population dynamics of Euceraphis betulae and Symydobius oblongus exhibited consistent patterns in 2001 and 2002, but Callipterinella tuberculata and Betulaphis brevipilosa populations exploded in 2001 and in 2002, respectively. When C. tuberculata was abundant, the population of B. brevipilosa was low. The reverse was true in 2002 as C. tuberculata became abundant only when the B. brevipilosa population declined, leading us to suspect competition between the two species. However, instead of avoiding each other, they were positively associated on shoots, probably as a result of aggregation at suitable feeding sites. Colonies of both species were similar or larger in mixed aggregates than in monospecific ones. Moreover, the colony size of each species in mixed aggregates was independent from that of the other species. Since there is no direct evidence of competition between those two species, the alternate changes in abundance could rather result from the intraspecific properties of aphid life cycles that self-induce consecutive years of high and low population growth in relation to fluctuations in the environmental conditions. These two co-existing species thus deserve further close study to elucidate their relationships.