Tribe Calypsoeae - The Pollination Biology of North American Orchids: North of Florida and Mexico - page 110

queen bumblebees, and pollination rates are positively correlated with bumblebee
abundance (Mosquin 1970 ; Stoutamire 1971 ; Luer 1975 ; Wollin 1975 ; Krell 1977 ;
Ackerman 1981 ). Boyden ( 1982 ) in a study near Banff, Alberta, found that 57% of
168 captured bumblebees carried at least one pollinarium or viscidium of variety
americana while 43% carried none. Thus, over half of newly emerged bees appar-
ently made an exploratory visit to the flowers of Calypso. They visited flowers more
or less indiscriminately on their first foraging trips, but as indicated by a low polli-
nation rate (see below), they soon learned, usually by the second to sixth trips, to
select only species with the most rewarding flowers.
Boyden's data support Mosquin ( 1970 ), Stoutamire ( 1971 ), and Ackerman
( 1981 ) in their view that naïve bumblebee queens quickly learn to avoid the flowers
of Calypso after visiting them only a few times. Bumblebee queens in temperate
climates live only 1 year (Heinrich 1979 ), and selection may favor pollinators that
quickly learn to recognize unrewarding flowers. Although pollination levels are low,
a compensatory effect in this and other unrewarding orchids, possibly related to the
evolution of deception, is the production of an enormous number of seeds in those
few plants that are successfully pollinated.
Mimicry in various forms has been invoked as a possible strategy for luring pol-
LINATORSTOUNREWARDINGmOWERS6OGEL 1978 ) suggested a role for deceptive struc-
tures that simulate the presence of pollen. In Calopogon , Pogonia , Arethusa , and
other genera yellow hairs are present on the labella that appear to imitate the appear-
ance of anthers and pollen (e.g., Stoutamire 1971 ; Thien and Marcks 1972 ; see
below). The yellow hairs on the labellum of variety americana , also present in the
Old World variety bulbosa but absent in variety occidentalis , may likewise function
as pollen dummies to attract pollinators by deception (Boyden 1982 ). Comparative
STUDIESOF56ABSORPTIONORVISITATIONRATESFORTHEmOWERSOFVARIETIES americana
and occidentalis are not yet available.
Mosquin ( 1970 ), in his study of variety americana in Alberta, suggested that the
flowers might mimic those of Dodecatheon radicatum Greene, a plant with simi-
larly colored flowers offering a rich pollen reward. He captured bees carrying pol-
linaria of C. bulbosa var. americana on the flowers of this plant. However, since he
found flowers of C. bulbosa to be very plentiful and those of D. radicatum to be
relatively uncommon, the latter would seem to be a poor model for a mimic based
on relative abundance alone. Moreover, Boyden ( 1982 ), studying plants in the same
area, found no significant difference in the number of pollinaria or viscidia of C.
bulbosa var. americana on bumblebees visiting flowers of D. radicatum versus
those captured on flowers of other species or flying free. Boyden's observations
therefore fail to support Mosquin's ( 1970 ) mimicry hypothesis. The bees appear to
be well capable of distinguishing between the flowers of these two species. In addi-
tion to a difference in floral odor, pollen is extracted from Dodecatheon flowers by
buzz pollination, an entirely different type of bee behavior than exhibited by bees
during visits to Calypso (Boyden 1982 ). In like manner, Ackerman ( 1981 ) found no
evidence that variety occidentalis mimics other spring-blooming plants in northern
California.