The nesting ecology, habitat preference, abundance and impacts of Polistes dominula in New Zealand
Introduced wasps can have significant impacts on communities within their newly invaded range. A recent wasp introduction to New Zealand is the European paper wasp, Polistes dominula Christ, which was first observed in the Nelson-Tasman region in 2016. The ecology and behaviour of P. dominula was investigated, as well as the direct and indirect impacts of this wasp on local communities in Nelson. Aspects of nesting behaviour, habitat preference and abundance were studied with the intent to inform management decisions. The abundance and colony survival of P. dominula was greatest in suburban areas, where nests were predominantly built within anthropogenic structures. Colony survival of P. dominula was comparatively higher than for colonies of the longer established exotic congener Polistes chinensis Pérez. Neither Polistes species were found to nest within native or exotic forest sites, with translocated colonies of P. dominula unsuccessful within forest habitats. As a species that originates from a warmer climate, the lower temperatures in forests are likely a contributor to the inability of P. dominula to thrive in these areas.
The impacts that newly invasive predators have on communities within their invaded range are often poorly understood. Following the arrival of P. dominula, I show data indicating that there have been substantial declines in the abundance of several local butterfly species in Nelson. The monarch butterfly, Danaus plexippus Linnaeus was one of the species reported by residents to be in population decline. Field experiments found P. dominula to be a major cause of monarch caterpillar mortality. Caterpillar survival was the lowest in suburban areas, where the abundance of P. dominula was highest. Early instar caterpillars had comparatively higher mortality rates than late instar caterpillars, as the defensive behaviour of larger caterpillars had greater success against these wasps. Whilst P. dominula was the main driver behind caterpillar mortality throughout my experiments, the discovery of caterpillars by wasps appeared to happen by chance through direct contact between the wasp and their prey. There was also no evidence of nestmate recruitment by foraging wasps to sites where caterpillars were found.
Predation has the capacity to not only impact prey populations, but through the suppression of herbivory, plants can indirectly benefit from the presence of predators. The presence of P. dominula was found to have strong, cascading effects on the milkweed host and food plant of D. plexippus. Milkweed fitness in areas populated by P. dominula was substantially greater than the fitness of plants in areas without this invasive predator. Plant growth and reproduction of milkweeds placed in areas where P. dominula were present, and able to predate upon caterpillars, were found to exhibit a similar amount of growth and flower production as netted control plants which excluded monarch caterpillars completely. The outcomes of this experiment suggest that P. dominula is strongly suppressing these herbivorous caterpillars.
Overall, this thesis provides insight into the behaviour and ecology of P. dominula in New Zealand. Findings of this research suggest that this newly invasive wasp is having detrimental effects on local communities and that P. dominula should be controlled to reduce the impacts of this wasp on New Zealand’s ecosystems. Knowledge obtained in this study can direct conservation management strategies and aid in the development of effective control methods.