Wasp Bembix rostrata as a Response to Parasites and Predators (Hymenoptera: Sphecidae).-Entomol. Gener. 12 (1): 07 1-075; Stuttgart 1986.-[ Article]. Solitary wasps are affected by a wide variety of organisms acting as parasites or predators. Observations in a nesting aggregation of the digger wasp Bembix rostra ta Linnaeus 1758 revealed the fly Me topia le ucocephala Rossi 1790 to be important as a nest parasite. The frequency of these parasitic flies correlated significantly with increased density of B. rostrata nests. Calculated per nest , however, the relative incidence of such animals was declining as nest density increased. The same tendency was noted for ants visiting the nesting area. These findings may support the hypothesis of gregarious nesting as a "selfish herd" mechanism against parasites and predators. Key w 0 r d s: Gregarious nesting-selfish herd-nest density-Me topia leucocephala-Bembix rostra ta. The digger wasp Bembix rostrata Linnaeus 1758 provides a good example of gregarious nesting. This wasp prefers sandy habitats, exposed to the sun, and in Europe occurs as far N' as Denmark and S' Sweden. It is one of the largest Hymenoptera in this area, with a length of 13-25 mm [Lornholdt 1975]. Living in dense and numerous aggregations,
Abstract. 1. In a 3‐year study of the solitary bee Colletes cunicularius L. in Sweden, average body size and population density fluctuated greatly between years.2. In this protandrous population, females mated just once and the sex ratio was slightly male biased. Males were smaller than females.3. Size assortative mating (homogamy), associated with an increase in population density during the central days of female emergence and mating, was observed in two out of three years. Homogamy was also observed in pairs with remating males.4. Most of the mating males had emerged the day they mated, but 42% were older. We found no support for a general large‐male mating advantage.5. Weight of emerging females and mating males were negatively correlated with ground temperature, indicating thermoregulatory influence on the process of sexual selection in this species.
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