Fitness in invasive social waspsSubmitted by editor on 9 May 2017.Get the paper!
Figure 1: Foraging common wasp worker carrying its prey.
The beech forests in Nelson Lakes National Park, New Zealand, are filled with the buzzing sound of wasps. Nests are well hidden in the ground but in this forest, they can be easily spotted by observing foraging workers returning to their colonies. Since the common wasp became established in New Zealand in the 1980s it has become an abundant pest harming native species both directly and indirectly. In our study, we investigated factors that influence fitness in the common wasp (Vespula vulgaris), in order to better understand population dynamics in this social insect species.
Figure 2: Beech forest with tree trunks covered by a black mould. Invasive wasps occur in extremely high densities in this forest and harm native bird and insect species through predation and competition for food sources.
RNA viruses have recently gained attention because of their association with Colony Collapse Disorder in honey bees, and have also been found in wasps and other arthropod species. An effective immune defence is crucial to protect the colony from pathogens transmission and infections, especially in high density populations. Polyandry in queens, which has been associated with lower levels of disease infections in social insects, leads to genetically diverse colonies. Intracolony genetic diversity could provide a mechanism to colony-level pathogen resistance allowing colonies to expend resources towards larger colonies that produce more new queens.
Our study showed that polyandry, viral load and antiviral immune defence are significant predictors of nest size in the common wasp. The Kashmir Bee Virus is highly prevalent in the Nelson Lakes population, replicates in wasps, and was found to increase immune gene expression in individual wasps. These findings indicate that wasps suffer from this virus and expend resources towards immune defence to reduce pathogen-impact on colony fitness.
Genetically diverse colonies were larger and produced larger numbers of queens. However, within colonies patrilines (genotypes) did not influence virus susceptibility or immune response. Polyandry does not benefit colonies by conferring colony-level resistance specifically to Kashmir Bee Virus or increase in antiviral immune defence. Whether colonies benefit from polyandry by resistance to other pathogens or whether they benefit from effects unrelated to disease remains to be tested.
Determining factors that affect fitness in wasps can improve the management of invasive species. The finding that wasps suffer from RNA viruses agrees with other studies that argue that RNA viruses have broader impacts on the health of arthropod communities than previously thought.
Figure 5: Combs of a nest cut out and laid on a table to analyse the size and cell number of the nest. Small cells (left and middle) are used to rear worker brood and the majority of males, whereas larger cells (right) are used to rear new queens. Annual colonies are founded by a single hibernating queen in spring and extended throughout the season, new queens and males are produced by the end of the season in autumn.