Tribe Neottieae - The Pollination Biology of North American Orchids: North of Florida and Mexico - page 61

availability, and reliability, need to be considered, and the success of L. cordata , as
reflected in its circumboreal distribution, may be related to a broad pollinator
spectrum.
Additional Species of Listera
Some workers have reported that L. ovata ISAPOMICTIC0ROCHAZKAAND6ELISEK
1983 ) or autogamous with incoherent pollen falling directly onto the surface of the
stigma (e.g., Hooker 1854 ; von Kirchner 1922 ; Kugler 1970 ). Others assert that pol-
len in the latter case is dislodged by the activity of thrips and that the process should,
therefore, not be considered autogamy but a form of entomogamy (Darwin 1862,
1869 ). Nilsson ( 1981a, b ), on the other hand, found no evidence for either autogamy
or thrip-mediated entomogamy in extensive and detailed studies of L. ovata on
Oland and in Uppland, Sweden, and bagging experiments have now demonstrated
that a vector is needed for pollination (Brys et al. 2008 ).
Artificial self-pollination produced fruit, and since L. ovata is clonal some level
of inbreeding probably occurs. However, selfing led to a reduction in embryo num-
ber and size. Like L. cordata , cross-pollination is promoted by the positioning of the
rostellum.
Natural fruit set varied from up to 80% in one population from Gotland to
13-70% (mean value, 37%) on Oland. Marked differences were evident among
populations and years and also among individual plants within populations.
In a study in eastern Belgium, Brys et al. ( 2008 ) examined the effects of popula-
tion size, local plant density, and floral display on pollination success and reproduc-
tion. They found that pollination efficiency, pollen removal, and fruit set were
positively correlated with population size up to a threshold value of 30-40 plants.
Beyond this number, pollination efficiency and reproductive output decreased.
0OLLINIAREMOVALANDFRUITSETVALUESWEREHIGHERINPLANTSWITHLARGERmORALDISPLAYS
Fruit production was also positively correlated with local plant density, and this
relationship was equally valid in emasculated plants, implying that female function
was unaffected by geitonogamous pollination in this population. These results differ
from Melendez-Ackerman and Ackerman's ( 2001 ) observations on L. cordata in
Colorado, where reproductive success was found to be independent of plant density,
except in highly dispersed plants.
0OLLINATORSOF L. ovata in North America are unknown, but the flowers are larger
than those of L. cordata (Table 3.1 ) and the pollinators are probably also larger.
Nilsson ( 1981a, b ) considers that the flowers are adapted for strong, 8-12-mm long
insects with nearly hairless heads and short, fairly small mouthparts. In studies con-
ducted in Europe, sawflies (Hymenoptera, Symphyta) and more than 70 species of
ichneumons (Hymenoptera, Ichneumonidae) were attracted to the flowers in large
numbers (Darwin 1862 ; Kunth 1898-1905 0ORSCH 1958 ; Nilsson 1981b ). Also
attracted were beetles, primitive flies, and other hymenoptera (Sprengel 1793 ; Muller
1883 ; Kunth 1898-1905 ; Heimans and Thijsse 1907 ; Godfery 1931 0ORSCH 1958 ;
Schremmer 1961 VANDER0IJLAND$ODSON 1966 ; Nilsson 1981b ), including wasps