COLONIZATION, AMONG SHOOT MOVEMENT, AND LOCAL-POPULATION NEIGHBORHOODS OF 2 APHID SPECIES

We demonstrate differences in the local population neighborhoods of two aphid species, Aphis varians and Macrosiphum valeriani, and how these neighborhoods correspond to the patchiness of their host plant, fireweed (Epilobium angustifolium). We define a "neighborhood" as the area in a patchy environment that the aphids use for a particular process during a specific period of time - a neighborhood may cover only part of a patch, or may encompass several patches. In a colonization experiment we measured initial settlement of fireweed from winter hosts at three levels of patchiness: areas of fireweed, plots within areas, and individual fireweed shoots within plots. Both aphid species colonized taller shoots that were exposed above their background vegetation, but M. valeriani colonization differed among areas and plots of fireweed as well. On the other hand, A. varians did not discriminate among areas or plots, but discriminated only among individual shoots. Settlement onto fireweed shoots was not affected by the numbers of aphids already on the shoots. In a movement experiment, we measured colony persistence and dispersal from the colony onto surrounding fireweed. A varians colonies persisted longer than M. valeriani colonies, because A. varians seldom walk from the central shoot to surrounding fireweed. M. valeriani regularly walked to surrounding shoots and 95% of movements were within 75 cm of the central colony. The restricted movement by A. varians means that single fireweed shoots represent the neighborhood of local populations of A. varians, whereas a local population neighborhood of M. valeriani includes all the shoots within 75 cm of an initially colonized shoot. We hypothesize that the differences in local neighborhoods of the two aphid species may represent different means of minimizing the impacts of the many natural in this system. A. varians escapes by predator satisation, whereas M. valeriani escapes by regular dispersal.