Food availability affects Osmia pumila (Hymenoptera: Megachilidae) foraging, reproduction, and brood parasitism

Abstract

Food limitation can reduce reproductive success directly, as well as indirectly, if foraging imposes a risk of predation or parasitism. The solitary bee Osmia pumila suffers brood parasitism by the cleptoparasitic wasp Sapyga centrata, which enters the host nest to oviposit while the female bee is away. I studied foraging and reproduction of O. pumila nesting within cages stocked with rich or sparse floral resources, and the presence or absence of S. centrata to test (1) the response of nesting female O. pumila to food shortages, (2) the response of nesting female O. pumila to the presence of parasites, and (3) whether brood produced under scarce resources are more likely to be parasitized by S. centrata. The rate of brood cell production was significantly lower in cages with sparse floral resources, although females in sparse cages did not produce significantly fewer brood cells overall. Sapyga centrata did not influence the rate of brood cell production, but females exposed to the cleptoparasites had marginally significantly lower reproductive output. Nests in parasite cages had significantly fewer brood cells than those in parasite free cages. The mean duration of foraging bouts made by female O. pumila in sparse cages was not significantly longer than that in rich cages. O. pumila spent less time in the nest between pollen and nectar foraging bouts in sparse cages with S. centrata than those in other cages suggesting that these individuals made more frequent food foraging trips. Despite the weak effects of parasites and bloom density on foraging behavior, O. pumila brood cells experienced a 5-fold higher probability of parasitism by S. centrata in cages with sparse bloom than in those with rich bloom [corrected]. These results support the hypothesis that indirect effects of food scarcity increase O. pumila susceptibility to brood parasitism, although the exact mechanism is not entirely clear yet.

The Role of Resources and Risks in Regulating Wild Bee Populations

“…Experimental manipulation of both floral resources and parasites within enclosures surrounding nesting populations of a twig-nesting solitary bee, Osmia pumila, indicated a greater influence of resources than of parasites on overall brood cell production; 60% more brood cells were produced when floral resources were doubled versus 12% mortality caused by parasites when present versus absent (38). Importantly, parasitism rates rose to 25% of brood cells under sparse floral resources, five times higher than under rich floral resource environments.…”

Multitrophic effects of experimental changes in plant diversity on cavity-nesting bees, wasps, and their parasitoids

“…By using SEM, we elucidated underlying mechanisms of plant diversity on higher trophic levels, with direct effects of plant diversity on wasp diversity, but indirect on bee diversity (via flower diversity and brood-cell density). Open questions include the potential role of competitive effects among flower visitors with increasing diversity, changes in the behavior of bees and wasps leading to the decreased rates of parasitism (Goodell 2003) and landscape level effects.…”

“…Another possibility for interpretation is a positive host interaction, whereas a diverse and dense host community can offer a more effective protection against parasitoids (e.g. by patrol, by confusing) (Goodell 2003). The decrease in parasitism rate of bees arising from increasing brood cell density may also have been caused by egg limitation of the parasitoid species (Heimpel and Rosenheim 1998).…”

Reproduction and survival of a solitary bee along native and exotic floral resource gradients

“…Finally, plant invasion and associated changes in forb communities might indirectly alter bees by affecting the populations or behaviors of bee natural enemies. For example, Goodell (2003) found that reductions in floral availability increased rates of brood parasitism for Osmia pumila by the cleptoparasitic wasp Sapyga centrata, possibly because the nest was more frequently left undefended by females (Goodell 2003).…”

“…While these natural enemies can reduce the performance of honey bees (vanEngelsdorp et al 2009) and bumble bees (Dukas 2005; Osborne 2009), the influence of natural enemies on native solitary bee populations have rarely been examined directly (Roulston and Goodell 2011). In particular, brood parasites, which develop inside brood chambers of bees, killing eggs and consuming pollen provisioned for larval development, are often regarded as having little influence on bee dynamics (Goodell 2003;Roulston and Goodell 2011;Steffan-Dewenter and Schiele 2008;Wcislo and Cane 1996), despite often accounting for a large share of documented mortality (Steffan-Dewenter and Schiele 2008;Vicens et al 1994).…”