Vestalis

Abstract: Host–parasite systems are characterised by coevolutionary arms races between host and parasite. Parasites are often the driving force, as they replicate much faster than their hosts and have shorter generation times and larger population sizes, resulting in higher mutation rates per time interval. This scenario does not fit all host–parasite systems. Socially parasitic cuckoo bumblebees (Bombus (Psithyrus) vestalis) parasitise colonies of Bombus terrestris share most life history characteristics with their hosts. As they parasitise only a subset of all available colonies, their population size should be lower than that of their hosts. This might have strong negative effects on the genetic diversity of B. vestalis and their adaptability. Here, we study for the first time the population structure of a Bombus/Bombus (Psithyrus) system. Highly polymorphic DNA markers were used to reconstruct sibships from individuals collected in the wild. The analysis of the host and parasite populations revealed a rate of parasitism of about 42% (range 33–50%). The population size of B. vestalis was lower compared to their hosts, which was also reflected in low within-group genetic distance. An analysis of the reconstructed queen genotypes revealed more supersisters amongst the B. vestalis queens when compared to the B. terrestris host. The data suggest that B. vestalis females and males do not disperse over long distances. This shows a potential for local adaptation to their hosts.

Abstract: Understanding the factors influencing host-selection behavior of parasi- toids is essential in studies on host-parasitoid ecology and evolution, and in combin- ing sustainable strategies of pest management, such as host-plant resistance and biological control. The effects of host-plant resistance on the olfactory response and parasitism success by Cotesia vestalis, a parasitoid of diamondback moth (Plutella xylostella) larvae were examined. Here, it was demonstrated that host-plant resistance can strongly influence foraging behavior and parasitism success of the parasitoid. In olfactometer experiments, C. vestalis did not differentiate between crucifer plant types with similar levels of susceptibility or resistance to P. xylostella but showed a strong preference for susceptible compared with partially-resistant host plants. The influence of previous oviposition activity varied with the host-plant type experienced by the parasitoid. In cage experiments, C. vestalis preferred to parasitize P. xylostella larvae on a susceptible plant compared with larvae on a partially resistant host plant when exposed to hosts for 24 h. However, this preference appeared to be transitory, and was not found after 96 h exposure. The present study suggests that combining partial host-plant resistance with biological control by C. vestalis for the control of P. xylostella may in some circumstances be antagonistic and negatively affect para- sitism success.

Abstract: Two females of Psithyrus vestalis were caught when they were in search for nests of their host, and introduced in the outer compartments of the nestboxes of captive Bombus terrestris colonies. Their behaviour towards queen and workers of the host species was registered on videotape and analysed. The Psithyrus are able to prevent worker ovipositions for some time by displaying agonistic behaviour, but apparently do not produce pheromones inhibiting ovary development. Their reproductive success depends largely upon the age of the workers that survive their assaults. Workers that are three weeks old start to lay eggs, thus placing the Psithyrus in a position similar to that of the Bombus queen at the end of their dominance over the workers, when she loses her reproductivity.