Tribe Pogonieae - The Pollination Biology of North American Orchids: North of Florida and Mexico - page 179

Calopogon tuberosus , Arethusa bulbosa , P. ophioglossoides , and Calypso bulbosa
(Heinrich 1970, 1975, 1979a ; Thien and Marcks 1972 ; Ackerman 1981 ; Boyden
1982 ; Gregg 1989 ). However, in addition to Cleistesiopsis tetrads on the head and
dorsal thorax, Gregg ( 1991b ) found them in the corbicular loads (pollen baskets) of
22 worker bumblebees. Thousands of Cleistesiopsis tetrads representing one-third to
nearly 100% of the corbicular masses were found on eight bumblebees from North
Carolina and one from West Virginia. Several hundred were found in the corbiculae
of four others from North Carolina and one from West Virginia. In all, 42% of the
bees captured that were carrying pollen of Cleistesiopsis had deliberately stored sig-
nificant amounts of it in their corbiculae. Pollen was removed from the frons and
scutum and transferred to the hind tibial corbiculae by grooming. Although most
retained some tetrads on their dorsal thorax, five of the bumblebees in North Carolina
and one in West Virginia that carried tetrads in corbicular masses had none on their
dorsal thorax or head. Apparently, all the pollen is therefore sometimes lost to col-
lecting, but such loss may be offset by an increase in the number of visiting bees
seeking a genuine reward with a consequent overall increase in capsule set.
The type of bees carrying and actively collecting pollen differed in North Carolina
and West Virginia. In North Carolina, Cleistesiopsis pollen was found on the dorsal
thorax of three species of Bombus , but only B. pennsylvanicus had large numbers of
tetrads in its corbiculae. Of 21 bees captured carrying Cleistesiopsis pollen, 16 were
B. pennsylvanicus ; nine of these had just visited C. divaricata and four, C. bifaria . The
remaining three had most recently entered other flowers, including those of P. ophio-
glossoides and Calopogon pallidus . Smaller numbers of Cleistesiopsis tetrads, one to
27, were also present in the corbiculae of B. griseocollis , B. impatiens , and four other
B. pennsylvanicus and may have been accidentally transferred while grooming.
In West Virginia, Cleistesiopsis pollen was observed on the dorsal thorax in four
species of Bombus and at least one Megachile , but only single individuals of B.
fervidus and B. vagans carried large numbers of tetrads in their corbiculae. In con-
trast to North Carolina, only one of the five examined individuals of B. pennsylvani-
cus carried any Cleistesiopsis tetrads and none were present in its corbicular load.
If large corbicular loads consisting of hundreds or thousands of Cleistesiopsis
tetrads are considered, the frequency of bees collecting pollen was significantly
higher at the North Carolina than at the West Virginia study site. Following attraction
by long-distance cues, bees were more likely to enter a flower at the North Carolina
than at the West Virginia location. Inspected flowers of C. bifaria in West Virginia
had no detectable odor, but flowers of both C. bifaria and C. divaricata produced
scents at Big Island Savannah in North Carolina (Gregg 1991b ; Catling and Gregg
1992 ). The odors apparently differ in the two species. Flowers of C. bifaria are said
to smell like vanilla and those of C. divaricata , like Easter lilies or daffodils (Catling
and Gregg 1992 ). There was, however, no difference in the pollinator response to the
floral odors of these orchids at the savannah site in North Carolina (Gregg 1991b ).
Floral fragrances may be associated with reward and probably provide a close
range recognition signal that stimulates the bee to enter the flower (Heinrich 1975,
1979b ; Nilsson 1979 ; Boyden 1982 ). Competition for pollinator service among a
variety of pink flowered species at the North Carolina site, not present at the West