Interactions between cactus-specialist solitary bees and their host cacti

Abstract

Although bees are herbivorous insects, they are seldom studied as such. My dissertation research characterizes the interactions between a guild of solitary cactus-specialist bees and the cacti they visit, examining these bees both as herbivores and as pollinators. I first examined the reproductive biology of two species of Ferocactus as background for the pollination study. Both F. cylindraceus and F. wislizeni were obligate outcrossers, and neither species was pollen-limited. Fruit set was high in both species, and reproductive output was limited by architectural constraints. Whereas most other herbivorous insects are antagonists of the plants they feed on, bees are usually regarded as mutualists (pollinators) of their host plants. However, not all floral visitors are pollinators, and pollen-foraging specialists have been postulated by some to be particularly antagonistic to the plants they visit. To test whether cactus-specialist bees are actually mutualists of the cacti they visit, I examined both the quantity and quality components of pollinator effectiveness of bees visiting F. cylindraceus and F. wislizeni. Despite the generalized morphology of the flowers of these plants, there were very few floral visitors other than three species of cactus-specialist bees. Flowers of both species were pollinated almost exclusively by cactus-specialist bees, primarily Diadasia rinconis. I also tested the pollen preferences of four species of cactus bees. Bees were offered flowers from their normal host cactus in which the stamens had been removed and replaced with novel pollens. Novel cactus pollens were accepted to some degree by all four species, but Diadasia rinconis and D. opuntiae accepted more novel non-cactus pollens than did either Lithurge apicalis or Idiomelissodes duplocincta. Both species of Diadasia showed significant acceptance of Sphaeralcea (Malvaceae) pollen, thought to be the ancestral host plant for the genus Diadasia. Caged, naive D. rinconis bees did not initiate nesting when presented with Sphaeralcea flowers alone; nesting began immediately when cactus flowers were offered. Once nesting was underway, most D. rinconis bees switched to Sphaeralcea if cactus flowers were removed. In both D. rinconis and D. opuntiae, there were striking differences in pollen preferences among individuals.