Edge effect in the distribution of some conifer aphids across the forest border
The distribution of some aphid species (Homoptera: Adelgidae, Aphididae Mindarinae, Aphididae: Lachninae) across the forest border was studied in conifer forests of the Alps (North Italy, France), Central Siberia (Krasnoyarskaya Oblast) and Kamchatka. The survey concerned 11 conifer species, which are taxonomically or ecologically related: <i>Abies alba </i>Mill., 1768, <i>A. sibirica </i>Ledeb., 1833; <i>Larix decidua </i>Mill., 1768, <i>L. gmelini </i>(Rupr.) Litv., 1854, <i>L. sibirica </i>Ledeb., 1833; <i>Picea abies </i>(L.) Karst., 1881, <i>P. jezoensis </i>(Sieb. et Zucc.) Carr., 1855, <i>P. obovata </i>Ledeb., 1833; <i>Pinus cembra </i>L., 1753, <i>P. pumila </i>Regel., 1858, <i>P. sibirica </i>Du Tour, 1803. The observations were performed along transects crossing the boundary between a stand and a neighbouring opening disseminated with isolated trees. A total of 13 aphid species (6 Adelgidae and 7 Aphididae) were surveyed and classified into three types of edge effect (present and positive, i.e., showing a greater abundance of individuals on border trees; present and negative, i.e. showing a lower abundance of individuals on border trees; absent). The surveyed Adelgidae species exhibited an edge effect, either positive or negative, with the only exception of <i>Pineus cembrae </i>(Cholodkovsky, 1888). In Aphididae, an edge effect was observed for two species of the subfamily Mindarinae but only for 1 out of 5 species of Lachninae. Border trees seem to act as a barrier affecting the distribution of aphid species, especially those presenting a regular flight between the primary and the secondary host. After the colonisation, additional factors can affect the population density, and the final result in term of distribution across the forest border may also depend on the mobility of different aphid species.