Positive Effects of Endophyte Infection on Sycamore Aphids

Abstract

In this study, I examined the effects of infection of leaves by an endophytic fungus (Rhytisma acerinum) on the populations and individual performance of two species of aphid (Drepanosiphum platanoidis and Periphyllus acericola), living on Acer pseudoplatanus. Populations of aphids were followed through one season and numbers compared on infected and uninfected leaves. Both species of aphid aestivated during the summer. D. platanoidis aestivated as non-reproducing adults, while P. acericola did so as nymphs. Fungal abundance, as measured by stromata number, was very low compared with previous studies in the UK. Nevertheless, in contrast to other studies of arboreal endophytes and insects, fungal presence affected aphid numbers in a positive manner, and in late summer, aphid numbers were higher on infected leaves. In D. platanoidis, higher aphid numbers resulted from an increased nymphal production by adults, but in P. acericola there was an accumulation of dispersing nymphs on infected leaves. Field rearing experiments with both aphid species demonstrated an increase in adult weight and potential fecundity in two autumnal generations. The infected leaves had higher soluble and total nitrogen and total carbon contents and the increases in aphid performance are attributed to the increase in soluble nitrogen. The numbers of sycamore aphids in one year can influence the dynamics of the population in the following season. It is suggested that increased numbers of aphids, resulting from endophytic infection, may therefore alter the between- as well as the within-year population dynamics of the species. In addition, the data appear to provide support for the hypothesis that endophyte presence may determine seasonal patterns of herbivory. The hypothesis may be interpreted with an endophyte-induced change in food quality, but other possibilities are discussed.