ETD

• Polistes sulcifer
• social parasite
• wasps

Insect social parasites exploit host’s social structure (in particular the cooperative brood
care) to rear their own brood and thus boost their own fitness. Parasitisation, however, is
not an easy task: the social parasites must locate a suitable host colony, overcome host
defence and integrate in its social system. Typically, larger and more productive colonies
represent ideal targets, but they are also better defended, imposing a trade-off between
costs and benefits for the parasite. I studied these dynamics in the social parasite-host
couple of paper wasps Polistes sulcifer (the obligate and workerless parasite) and P.
dominula (the host). To reproduce, P. sulcifer must usurp the nests of P. dominula, dominate
the foundresses, and integrate into the host colony to exploit its parental care. Therefore,
the parasite’s reproductive success strongly depends on the characteristics of the host
colony.
I investigated the trade-off between costs and benefits faced by P. sulcifer when selecting a
host colony to usurp, aiming at understanding which factors influence the nest choice and
the success of usurpation. In particular, I hypothesised that the parasite prefers nests with
higher productivity (e.g. brood maturity and number of foundresses) but with a group size
(number of foundresses) that does not hinder successful usurpation. Additionally, I
considered the parasite’s body size relative to that of the foundresses as a potential
determinant of success.
To test these hypotheses, laboratory experiments were conducted under controlled
conditions. The foundresses were marked, and during the preparatory phase of the trials,
hierarchies were recorded in poligynic nests. For each nest, were collected the number of
cells, larvae and pupae.
In each trial, the parasite was presented with a host nest for 30 minutes, during which scan
observation were conducted at one-minute intervals, along with all-occurrence sampling of
interactions between the parasite and the foundresses. For each trial, we recorded the
parasite’s interest (measured as approach within 1 cm of the nest) and the outcome of the
usurpation. Following the trials, the wasps were frozen for morphometric measurements
using ImageJ software. The maximum head width was used as a proxy for body size.
The preliminary results suggest that the number of foundresses alone does not dramatically
influence the choice of the social parasite, which might be more affected by the colony
productivity and intrinsic factor of the social parasite.
This study may contribute to a better understanding of usurpation dynamics and the role of
selective pressures on specific parasite and host traits, thus shedding light on this complex
coevolutionary arms race.