Pollination and Reproductive Ecology of Acuña Cactus, Echinomastus erectrocentrus var. Acunensis (Cactaceae)

Abstract

Echinomastus erectrocentrus var. acunensis initiates flowering in early March and is the first cactus species to bloom in Organ Pipe Cactus National Monument, Arizona. Flowering is correlated with plant size; the smallest plant to flower in the study area was 24 mm tall, and thereafter the percentage of plants that produced flowers increased to near 100%, at ≥ 90 mm in height. For flower-bearing plants, the number of flowers produced increased with plant height but was better correlated with plant volume. Number of flowers produced varied among years when controlling for plant volume and was positively associated with annual rainfall. Echinomastus is self-incompatible and thus requires insect vectors for pollination. Flower visitors are mostly solitary, polylectic bee species, largely Andrenidae, Anthophoridae, Halictidae, and Megachilidae, but the primary pollinators in each of 2 yr were Megachile palmensis (Megachilidae) and Diadasia rinconis (Anthophoridae). These same two species also accounted for ca...