Effect of introduced social wasps on the standing crop of honeydew in New Zealand beech forests

Abstract

Honeydew drops produced by beech scale insects (Ultracoelostoma assimile) on beech trees (Nothofagus) were measured on three areas of trunk: an unprotected control area, an area screened to allow insects but not birds to feed on the honeydew, and an area screened to exclude all insects and birds. The experiments showed honeyeater birds (tui, bellbirds, and silvereyes) had little effect on the honeydew, but that cropping by German wasps (Vespula germanica), and particularly by common wasps (V. vulgaris), in the summer and autumn months markedly reduced the number, size, and sugar concentration of the honeydew drops. The energy in drops per unit area of tree trunk was reduced by at least 99.1%, 98.4%, and 91.3% in January, March, and May, respectively. Wasps reduced the standing crop (number, size, and sugar concentration) of the honeydew drops by cropping them before they could be fully recharged and before evaporation could concentrate the sugars. Cropping by feral honey bees (Apis mellifera) may have reduced the standing crop of honeydew slightly. Removal of the honeydew by the introduced social wasps threatens the existence of some New Zealand native animals. The wasps reduce honeydew abundance and quality to the extent that honey bees do not feed on it for 3–4 months of the year.