Abstract:
Explosive pollen release is documented in many plant families, including the Fabaceae. Desmodium
setigerum E. Mey (Fabaceae) is a perennial herb with single trip explosive pollen release found in eastern Africa, and
the unique ability to reverse floral colour change if insufficient pollination has occurred. However, little else is
known about the pollination ecology of this species, what visitors can trigger explosive pollen release, and whether
bee body size is related to pollination efficiency. We investigated: 1) the breeding system of D. setigerum, and
whether it is pollen limited; 2) whether flowers are visited early in the day allowing sufficient time for a second
opportunity for pollination; and 3) what insect species visit D. setigerum and the relative efficacy of different flower
visitors in relation to visitor size and pollination success. We found that although self-compatible, D. setigerum
requires insect visitation to set seed as explosive pollen release is needed even for selfing. Most flowers are initially
visited before 1400h, and by 1800h nearly all flowers have been tripped. Flowers were not pollen limited in this
study, and were visited primarily by bees. We observed 16 visiting species, and there was a wide variation (0-404
grains) in the amount of pollen deposited on stigmas. Although almost all bees deposited some pollen, the mean
number of pollen grains deposited in a single visit per species was negatively related to body size. However, one
particular megachilid species deposited significantly more pollen grains than any other visitor and so is likely an
important pollinator of this species. This provides insights into the pollination biology of this unique plant species,
and adds to increasing literature on the relationships between bee body size, explosive pollen release and pollination
effectiveness.
