Restructuring of Lepidoptera communities by introduced Vespula wasps in a New Zealand beech forest

Abstract

Introduced social wasps (Vespula vulgaris) reach high densities in some New Zealand beech forests, because honeydew provides an abundant high-energy food source. We manipulated wasp density to estimate an "ecological damage threshold" for large, free-living Lepidoptera larvae. There will be a continuum of ecological damage thresholds for wasp density depending on the prey species or habitat. Experimentally placed small caterpillars had a significantly higher survival rate than large caterpillars, and the survival rate of both groups decreased with increasing wasp density. Spring-occurring caterpillars have a probability of surviving of 0.90-0.95, assuming wasps are the only source of mortality. However, at the peak of the wasp season we predict caterpillars would have virtually no chance (probability of 10 to 10) of surviving to adults. Wasp abundance must be reduced by at least 88% to conserve the more vulnerable species of free-living caterpillars at wasp densities similar to those observed in our study sites. This equates to a damage threshold of 2.7 wasps per Malaise trap per day. It was exceeded for about 5 months of the year in non-poisoned sites. There are currently no biological or chemical control techniques available in New Zealand that will reduce wasp abundance below this damage threshold throughout the year. Our models show that most Lepidoptera with spring caterpillars will be able to persist, but species with caterpillars occurring in the peak wasp season will be eliminated.

Traumatic Ventriculitis Following Consumption of Introduced Insect Prey (Hymenoptera) in Nestling Hihi (Notiomystis Cincta)

“…Accidentally introduced German (Vespula germanica) and common (Vespula vulgaris) wasps have established across a wide range of habitats and altitudes in urban and rural landscapes (Harris et al, 1991). Formidable competitors across much of the forest habitat, both species of wasps compete with native and endemic avian species for nectar and insects (Thomas et al, 1990;Beggs and Rees, 1999) and there is growing concern about the role of these species as conservation pests (Beggs, 2000). Honey bees (Somerville, 2000) and vespulid wasps (Spurr, 1996) …”

No evidence for establishment of the wasp parasitoid,Sphecophaga vesparum burra(Cresson) (Hymenoptera: Ichneumonidae) at two sites in New Zealand

“…Wasps reach very high densities in about 1 million ha of this native beech forest, (Thomas et al 1990) and are a conservation threat to native fauna (Beggs * 2001). It is estimated that wasp abundance needs to be reduced by 80-90% to mitigate the impact on some native invertebrate species in beech forest Beggs & Rees 1999). Vespula spp.…”

Can the wasp parasitoidSphecophaga vesparumsignificantly reduce the density ofVespulawasps?

“…Experiments to measure the impact of wasps on native invertebrates suggest that wasp abundance needs to be reduced by 80-90% of current levels (Toft & Rees 1998;Beggs & Rees 1999). Even if the parasitoid achieved a 25% suppression, further control tools are required to achieve conservation gains.…”