The Psocid Fauna of Larch Trees in Northern England--An Ecological Study of Mixed Species Populations Exploiting a Common Resource

Abstract

Nine psocid species make up over 95% of the psocid population on larch trees (Larix decidua). Their food consists of lichen (Lecanora conizaeoides Cromb.), Pleurococcus sp., and fungal spores. Seven species prefer Pleurococcus to lichen. When lichen only is presented, oviposition rates fall and they gradually starve. Reuterella helvimacula eats the whole lichen. Mortality and oviposition rates on Pleurococcus are the same as on lichen. Elipsocus mclachlani prefers lichen but feeds only on the apothecia; its oviposition-rate on Pleurococcus is greater than that of lichen. Each species has the same habitat in all stages of its life history, and the nymphs eat the same foods as the adults. The 2 Mesopsocus species overwinter as eggs, become adult in June, and have only one generation a year. The others also overwinter as eggs but become adult in Aug. and Sept. They also have 1 generation a year. Nymphs and adults of Reuterella helvimacula and Elipsocus hyalinus can survive the winter in the lowlands, but not at higher altitudes. Misopsocus unipunctatus occurs at all altitudes, M. immunis only below 800 ft., Elipsocus mclachlani occurs only above 600 ft., and the remaining 6 species are present at all altitudes. The late summer species show denser populations levels with increasing altitude. Species feeding on Pleurococcus and fungal spores often occur at moderate or high densities on the same trees at the same time. Amphigerontia contaminata predominates on living twigs, A. bifasciata on dead ones. Elipsocus westwoodi lives mainly on living branches, E. mclachlani is almost restricted to dead ones, and E. hyalinus is intermediate. The 2 Mesopsocus species differ less and their populations overlap spatially. At the highland localities the distribution of the various species closely followed that of their foods. The populations of the 3 Pleurococcus-feeding species were here highly interspersed at high densities. At the lowland locality the distributions of the 4 common Pleurococcus -feeders differ and show no close correspondence with that of their food; their populations overlap considerably. In one lowland plantation 2 Mesopsocus species were present in almost equal numbers. On twigs of any one type, these 2 species appear randomly interspersed. The problems presented by the coexistence of these abundant species exploiting the same food at the same time are briefly discussed.