Competition between the introduced wasps Vespula germanica and V. vulgaris in honeydew beech forest, north-western South Island, New Zealand.
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
New Zealand was colonised by the German wasp, Vespula germanica (F.) in the 1940s and it became established throughout the country, The common wasp, V. vulgaris (L) colonised in the late 1970s and is still expanding its range. Wasps are now common in honeydew beech forests of the South Island and can reach high densities. The behaviour and ecology of V. vulgaris and V. germanica were studied from 1988 to 1991 to investigate competitive interactions in honeydew beech forests of the north-western South Island. v: germanica disappeared from areas of honeydew beech forest within 3-5 years of the first arrival of v: vulgaris and has not returned to them. A change in the relative abundance of the two species occurred in spring following the arrival of V. vulgaris, and was consistent with the predictions of Archer's (1985) model of wasp population fluctuation. In north-western South Island honeydew beech forests where both species occur, the more abundant species dominates honeydew trunks. Aggressive interactions may take place on this high quality, potentially defensible sugar resource. The two wasp species show different foraging patterns that provide the potential for local coexistence. Although both are generalist feeders, v: germanica is more commonly found foraging for protein amongst the forest litter, whereas v: vulgaris forages more on shrubs and tree saplings. Approximately 15% of foragers returning to sampled nests carried animal prey. The proportion of protein in the diets of wasps decreased sharply after rain. V. germanica collected more Orthoptera and large Hymenoptera, whereas V. VUlgaris collected more Hemiptera and Lepidoptera. An estimated 0.8 and 4.8 million prey loads, or 1.4 and 8.1 kg per hectare per year was taken into nests in western and northern South Island honeydew beech forests, respectively. Wasp foraging and rainfall reduced the honeydew standing crop. As standing crop decreased, more wasps occurred on honeydew trees, they became less active, spent more time lapping the tree surface, and fed at a slower rate. v: vulgaris was more active and fed more efficiently than v: germanica. Variation in queen size was found between wasp colonies and sites. Small queens were present in juvenile queen populations but were under represented in the reproductive population suggesting that small queens have a lower probability of survival. A conceptual model of population regulation and species replacement involving an overcompensating control mechanism is presented. Variation in queen quality is a consequence of differential larval nutrition in autumn when the food resource fluctuates with changes in wasp density and environmental conditions. The competitive advantage that V. vulgaris has in harvesting honeydew means that foragers spend less time obtaining carbohydrate and more time meeting larval food demands. This increases the relative fitness of V. vulgaris queens, and results in V. vulgaris densities increasing relative to those of V. gennanica. Eventually, V. gennanica is eliminated from patches of honeydew beech forest.