Manfredini | Invasive social insects

BACKGROUND. Social insects are characterised by their double nature of beneficial organisms (e.g. pollinators or predators of agricultural pests) and environmental threats (e.g. invasive species). Much attention has been paid in the recent years on pollinator declines, while just very little is known about the invasion biology of social insects. invasive social insects have the great power to quickly adapt to new environments that pose novel different challenges.

Why social insects like the wasp Polistes dominula and the ant Solenopsis invicta are successful global invaders while other (often closely related) social insects do not become invasive or even fail to cope with changing environmental conditions?

Understanding the mechanisms behind these scenarios - particular at the molecular level - is of great importance to reduce the drastic impact of invasive species and to develop new strategies of conservation. I have started to address this issue by looking at antiparasitic response in invasive wasps in USA (Manfredini et al. 2013 Naturwissenschaften). I am planning to focus more on the molecular regulation of behavioural responses to new environmental challenges in the future, as behaviour is a highly plastic phenotypic trait and it is the first line of response of an organism to the environment.

Examining the “evolution of increased competitive ability” hypothesis in response to parasites and pathogens in the invasive paper wasp Polistes dominula

TOP: graphical representation of honey bee hygienic behaviour (from Wilson-Rich et al. Annu. Rev. Entomol. 2009). BOTTOM: Colony-level response to X. vesparum parasites. Bars indicate the average percentage ±S.E. of infected larvae and controls that were removed in colonies of Italian native wasps (ITdom) and American invasive (USdom) at three different time points post-infection in four separate trials (***=P<0.001, Pearson's chi-squared test with odds ratios, from Manfredini et al. 2013 Naturwissenschaften).

The “evolution of increased competitive ability” hypothesis predicts that invasive species are subjected to less predation and parasitization than sympatric native species, and thus can allocate resources from defence and immunity to growth and fecundity, thereby achieving higher fitness. In this study, I examined whether American invasive Polistes dominula paper wasps have reduced immunocompetence. To explore this scenario, I tested their susceptibility towards parasites and pathogens at both the individual (immune defence) and colony levels, i.e. hygienic behaviour (removal of diseased individuals by nestmates). First, I examined the response to the specific coevolved parasite Xenos vesparum (lost after invasion) in terms of individual host susceptibility and hygienic behaviour. Second, I explored the response to general pathogens by quantifying the bacterial clearance in individual wasps after a challenge with Escherichia coli and hygienic behaviour after a challenge with the fungus Beauveria bassiana. Results show that American invasive P. dominula have a higher response against X. vesparum at the colony level, but at the individual level their susceptibility is not significantly different from conspecifics of the native range. On the other hand, invasive P. dominula display lower response after a challenge with general pathogens at both the individual and colony levels. While supporting the hypothesis of a reduction of immunocompetence towards general pathogens in invasive species, these findings also suggest that the response against coevolved parasites might follow different evolutionary pathways which are not always easily predictable.

P. dominula wasp harbouring a female X. vesparum in her abdomen and walking on a nest populated by wasp larvae. Tiny, parasitic X. vesparum larvae can be seen exiting the body of their mother and infecting wasp larvae within their nest cell (video taken by the Dallai group at the University of Siena, Italy).

Clockwise from top left: 1-3) three photos showing myself and Prof Beani following a typical protocol for collecting overwintering wasps in Tuscany; 4) shipment of Italian P. dominula to the USA for lab work; 5) wasp larvae maintained in artificial sterile conditions after an immune challenge; 6) incubation of wasp larvae at 28 °C; 7) mature nest of P. dominula with larvae and pupae (white caps) clearly visible; 8) aberrant summer aggregation of female wasps parasitised by X. vesparum; 9) detail of a parasitised wasp.