Tribe Triphoreae - The Pollination Biology of North American Orchids: North of Florida and Mexico - page 89

Fig. 4.2 Augochlora pura with pollinia of Triphora trianthophora attached to its thorax, scale
bar = 1 mm
afternoon, A. pura and unidentified species of Lasioglossum may resort to thievery,
robbing nectar at the base of the sepals. Medley ( 1979 ) thought that this activity was
prompted by an absence of nectar at this time of day, and in his opinion it had no
effect on pollination efficiency.
Fruiting Success and Limiting Factors
Medley ( 1979 ) found that capsule enlargement at his site in Berrien County,
Michigan, approached 100% if the weather was favorable for bee activity and plants
were grouped so that three or more flowers were available. Pollination was uncom-
mon, however, in isolated groups of only one or two plants.
Willems ( 1994 ), on the other hand, reported very low levels of pollination and
capsule set in a western Massachusetts population of this orchid over a 6-year period.
Ninety percent or more of all emergent stems flowered, but less than 10% of the
flower buds produced mature capsules. As only 5% of developing capsules were
destroyed by predators, she attributed low fruit production to an abbreviated bloom-
ing period combined with a shortage of pollinators and considered that reproduction
was primarily vegetative. Similarly, Lownes ( 1920 ) found that pollination rarely
occurred at his study site in central New Hampshire and that seeds usually failed to
ripen. This was true despite the fact that 20-40 plants were clustered within an area
of a square foot. Like Willems, he thought that reproduction was chiefly vegetative.
In a more recent 11-year investigation at the site where Lownes conducted his
1920 study, Keenan ( 1996 ) found that slightly less than 50% of the flowers formed
capsules, a level lower than that reported by Medley but far higher than that
reported by Willems and Lownes. Although further work is needed, the available