Invasive paper wasp (Polistes chinensis) ecology and invertebrate conservation on Onetahua (Farewell Spit)
Social insects are among the most ecologically and economically damaging, and abundant invasive species worldwide. Invasive paper wasps in Aotearoa have been shown to cause significant declines in native insect populations though predation, with cascading ecological impacts. No effective control methods for paper wasps are currently available. One such wasp, Polistes chinensis, can become especially abundant in coastal scrub and low-elevation grassland habitats. Together these habitats are estimated to house 60% of Aotearoa’s threatened native Lepidopterans alone.
The Onetahua (Farewell Spit) Nature Reserve is a highly protected coastal site encompassing nationally rare habitat types and several at-risk plant and invertebrate species. During a 2019 BioBlitz, paper wasp abundance at this site was noted as likely threatening these diverse invertebrate communities. To inform decision-making on the potential need for local management of these wasps, I aimed to assess their distribution and population dynamics and identify species in their prey community. Over two austral summers, 2020 – 2021 and 2021 – 2022, I surveyed paper wasp colonies on Onetahua and tracked colony survival and development rates. Using wasp larvae gut samples, I conducted a CO1 metabarcoding analysis to identify prey species over a temporal gradient. During data and sample collection, I observed two species of entomopathogenic fungi fatally infecting P. chinensis wasps at their nests. Taking samples of these fungi, I identified each species using morphological and molecular methods, attempted to quantify their prevalence in the studied wasp population, and conducted infection bioassays with caged wasp nests to test fungal pathogenicity.
From colony surveying and monitoring, I found that approximately 20% of all spring colonies survived through each summer. Colony development rates indicated that wasp predation pressure likely peaked in February of each year, and I estimated that average colony density in February 2022 was approximately 23 colonies ha-1. The most ubiquitous prey species identified were several endemic cicadas and numerous Lepidopteran species, including common copper butterflies (Lycaena spp.). However, ~60% of unique taxa could not be identified using this barcoding method, likely due to the absence of barcodes for these species in reference libraries. I conclude from this result that metabarcoding analysis is probably not suitable for the identification of many species of conservation significance, as these taxa are likely to be rare, understudied, or undescribed species. A more comprehensive unification of taxonomy and genetic sequencing of invertebrates would be beneficial to improve the useability of such analyses.
Fungal pathogens were estimated to have infected wasps in ~3% of all surveyed colonies. I identified the fungal species observed as Beauveria malawiensis and Ophiocordyceps humbertii. In bioassays I successfully infected healthy P. chinensis colonies with B. malawiensis, with significantly increased wasp mortality rates and conidiogenesis in treatment nests. These fungal species may have potential for development as biological control agents for Polistes wasps in Aotearoa and elsewhere around the world.
I did not find sufficient evidence to warrant diversion of current conservation resources to high-intensity manual control of P. chinensis on Onetahua. However, I do recommend that repeated monitoring of paper wasp and prey species’ relative abundances be carried out at Onetahua and similar sites. Determination of population trends in these species could be invaluable for future management of Polistes wasps, if and when efficient control methods are eventually developed.