The effect of exposure to conspecifics on restlessness in the parasitoid wasp Nasonia vitripennis (Hymenoptera: Pteromalidae)

When habitat quality is variable, there should be strong selection for the ability to detect and respond to the variation. Adult females of the parasitoid wasp Nasonia vitripennis (Walker) are known to increase their restlessness (the proportion of time in locomotion) both during and after exposure to a poor quality host. Doing so provides a mechanism for leaving a poor host and potentially finding a better host. This study examined whether restlessness also changes in response to competition as indicated by the presence of adult conspecifics. Both restlessness and the probability of dispersing across an inhospitable environment were greater when a female was with another female than when she was alone. However, restlessness did not remain elevated after the other female was removed. In contrast with females, restlessness of males did not increase either during or after exposure to other males, and the probability of dispersing across an inhospitable environment was unaffected by the presence of another male. The difference between females and males may be related to differences in dispersal ability and in the abundance and distribution of hosts versus mates. Résumé—Lorsque la qualité de l’habitat est variable, il devrait y avoir une forte sélection de la capacité de déceler cette variation et d’y réagir. Les femelles adultes de la guêpe parasitoïde Nasonia vitripennis (Walker) sont connues pour augmenter leur agitation (le pourcentage du temps consacré aux déplacements) au moment où elles sont mises en présence d’un hôte de faible qualité et durant la période qui suit. Cela leur procure un mécanisme pour abandonner un hôte peu intéressant et pour potentiellement en trouver un meilleur. La présente étude examine si l’agitation change aussi en réaction à la compétition marquée par la présence d’adultes de la même espèce. Lorsqu’une femelle est en présence d’une autre femelle, son agitation ainsi que la probabilité qu’elle traverse un environnement hostile sont plus grandes que lorsqu’elle est seule. Cependant, l’agitation accrue ne se prolonge pas lorsque l’autre femelle est retirée. Contrairement aux femelles, les mâles ne s’agitent pas plus durant ou après une rencontre avec d’autres mâles; leur probabilité de se déplacer vers un environnement hostile n’est pas affectée par la présence d’un autre mâle. La différence entre les femelles et les mâles peut être reliée à des différences dans leur pouvoir de dispersion et dans l’abondance et la répartition des hôtes plutôt que celles des partenaires. [Traduit par la Rédaction] 684 King Introduction Both habitats and locations within habitats may vary considerably in quality depending on variables such as resource availability, number of predators, and number of competitors. One of several mechanisms available for leaving poor habitats or locations is to increase restlessness (the proportion of time in locomotion) after exposure to poor conditions (King and Ellison 2006). This phenomenon has been documented in both migratory and nonmigratory species (e.g., Caldwell 1974; Rankin and Riddiford 1977; Saks et al. 1988; Skalicki et al. 1988; Ashworth and Wall 1995; Keaser et al. 1996; Brzek and Konarzewski 2001). Increased restlessness can lead to dispersal on either a very large scale or a very small scale, depending on its duration. Poor conditions may affect walking and winged dispersal separately or may trigger just a general agitation. The present study examines restlessness, primarily in the form of walking, and dispersal across an inhospitable environment, using adults of a nonmigratory species, Can. Entomol. 139: 678–684 (2007) © 2007 Entomological Society of Canada 678 Received 19 December 2006. Accepted 12 April 2007. the parasitoid wasp Nasonia vitripennis (Walker) (Hymenoptera: Pteromalidae). Adults of N. vitripennis are a few millimetres long. Males have short wings and do not fly, whereas females are macropterous and fly. Females oviposit more than one egg per host, typically 20–40. Hosts are the pupal stage of certain fly species that are found in association with carrion, refuse, or birds’ nests (Whiting 1967; Rueda and Axtell 1985). The spatial distribution of hosts varies from highly clumped to just one or two hosts per patch (Werren 1983). The patches can be very far apart because the hosts are associated with carcasses, nests, and refuse piles. The offspring emerge as free-living adults in about 2 weeks, with males emerging before females. The quality of hosts that a female encounters is known to affect her restlessness (King and Ellison 2006). For example, females are more restless after ovipositing in parasitized hosts than after ovipositing in unparasitized hosts. A parasitized host represents more competition: offspring from superparasitized hosts are smaller, have lower survivorship, and produce fewer offspring of their own (Wylie 1965, 1966). The present study tests, for both females and males, whether restlessness also changes in response to the direct presence of adult conspecific competitors and whether any changes in restlessness remain after the competitors are gone. Female parasitoid wasps are frequent subjects of optimal foraging studies, particularly studies of patch residence times, although relatively few studies have looked at the effects of the physical presence of conspecifics (reviewed in Wajnberg 2006). Both female and male N. vitripennis sometimes exhibit intrasexual agonistic behavior (e.g., King et al. 1995). Females compete with other females for hosts as oviposition sites and as a source of fluids on which they themselves feed. Males compete with other males for females. Males stay near and defend hosts from which females may emerge and sites where they mated successfully (Altson 1920; King et al. 1969; van den Assem et al. 1980). Males may assess competition through responses to total density or the ratio of males to females (Saeki et al. 2005).