Pollination Biology of Nemophila menziesii (Hydrophyllaceae) with Comments on the Evolution of Oligolectic Bees

Abstract

The flowers of Nemophila agg. are strongly protandrous. This necessitates the movement of pollen between flowers and increases the chances for out-crossing. Thus an insect to be an effective pollinator must visit flowers with receptive stigmas as well as flowers with dehiscing anthers. Although the flowers of Nemophila agg. are visited by individuals of many orders, Hymenoptera and Diptera usually compose 90% or more of the flower visitors. Throughout most of its distribution range four species of bees are the primary pollinators, viz. Apis mellifera, Andrena macrocephala, Andrena torulosa and Andrena crudeni. These bees are medium sized and have behavioral patterns which make them efficient pollinators. The three species of Andrena are oligolectic on Nemophila agg. Indeed, A. macrocephala and A. crudeni with a few exceptions, are apparently restricted to N. menziesii. Andrena torulosa is associated with N. alomaria. The three species oligolectic on Nemophila are not present, or present only in small numbers, in the northern Sierra Nevada foothills. In this region the primary pollinators of N. menziesii are various polylectic Megachilidae. A second exception to pollination by the oligolectic bees was studied at Point Reyes, Marin County, California. Here, two species of Bibio (Bibionidae; Diptera) are the primary pollinators of N. atomaria. The meager available data suggest that N. integrifolia is also pollinated by various polylectic Megachilidae. In the North Coast Ranges the flower constancy of Andrena macrocephala and A. torulosa breaks down. In this region the flowers of N. menziesii are non-reflective, whereas they are reflective throughout the rest of its distribution range. Although the two bees show a high degree of discrimination in mixed populations of N. menziesii and N. atomaria, each is also associated with populations of the "wrong" plant. Following the retreat of Miocene seas from the South Coast Ranges, N. menziesii may have migrated into the North Coast Ranges ahead of its oligolectic bees. Here A. torulosa, the oligolege of non-reflective N. atomaria, may have acted as a selective force favoring success of non-reflective flowers. When A. macrocephala migrated into this region, the non-reflective menziesii flowers provided a "steppingstone" to the utilization of N. atomaria as a host plant. This switch has not occurred in the South Coast Ranges where N. menziesii is usually highly reflective. Both Andrena macrocephala and A. torulosa visit flowers other than those of Nemophila during periods of pollen shortage. During such crises oligolectic bees visit flowers taxonomically unrelated to their host plant. Such "promiscuous" behavior may provide local populations the chance to discover appropriate alternative pollen sources. A single population of A. macrocephala was studied whose foraging activity was confined to Pholistoma auritum (Hydrophyllaceae). In contrast to seasonal shifts to unrelated taxa, evolutionary shifts, such as the one to Pholistoma, are to taxa closely related to the host plant. A model is presented that suggests how such an evolutionary shift may occur.