Tribe Neottieae - The Pollination Biology of North American Orchids: North of Florida and Mexico - page 60

recorded in Finland (Silen 1906 ) and a species of Limnophila Macquart (Tipulidae)
in Great Britain (Willis and Burkhill 1908 ). Keenan ( 1998 ) mentioned that mosqui-
toes might also be involved, but even though present in immoderate numbers, they
played no apparent role in the pollination of this orchid at a site in Quebec.
Although Ackerman and Mesler ( 1979 ) consider nectar to be the primary attrac-
tant, a fetid odor is also mentioned, implying adaptation to the attraction of sapro-
myophilous or mycetophilous insects. A similar scent is found in some other species
of Listera (Ramsey 1950 ; Schremmer 1961 VANDER0IJLAND$ODSON 1966 ;
Ackerman and Mesler 1979 ). Nevertheless, the flowers of L. cordata , at least, do not
appear to be pollinated by egg-laying insects normally attracted by the smell of
feces or decaying plant or animal bodies nor do they chemically mimic fungi to
ATTRACTOVIPOSITINGFUNGUSGNATS&AEGRIANDVANDER0IJL 1971 ; Ackerman and Mesler
1979 ). Ackerman and Mesler ( 1979 ) reported no evidence of oviposition or of eggs
or larvae on the flowers. Moreover, they found that both male and female fungus
gnats were attracted and observed no evidence of sexual activity, which might sug-
gest that the males were attracted only by the presence of females.
Fruiting Success and Limiting Factors
Floral rewards are widely scattered in the cool, damp coastal redwood forests.
Efficient pollinators, such as bumblebees, require ample and relatively concentrated
energy sources and are not common in this habitat (Moldenke 1976 0LANTSSUCHAS
L. cordata , may of necessity depend on small, less-efficient pollinators that require
less-abundant nectar supplies (Stebbins 1974 ). The behavior of most flies and wasps
is often considered erratic (Sprengel 1793 ; Kunth 1898 nVANDER0IJLAND
Dodson 1966 0ROCTORAND9EO 1972 ; Moldenke 1976 ; Leppik 1977 ), yet 61-78%
of the flowers monitored in three northern California populations of L. cordata pro-
duced capsules (Ackerman and Mesler 1979 ). This percentage is higher than that
recorded for some temperate, terrestrial, nectariferous orchids pollinated by butter-
flies, bumblebees, moths, and syrphids (Ackerman 1975 ; Smith and Snow 1976 ;
Ivri and Dafni 1977 ; Ackerman and Mesler 1979 ). Although Mesler et al. ( 1980 )
attributed this rate of success to the large number of gnats present at their study site
rather than to their efficiency as pollinators, they found that the gnats were effective
vectors not only in terms of overall fruit set, but also in their ability to bring about
cross-pollination. The potential of a single pollinium to pollinate several flowers
may also compensate for pollinator inefficiency (Mesler et al. 1980 ).
Listera , thought by Darwin ( 1862 ) to have one of the most refined flower types
among the orchids, provides an example of a genus that has evolved specialized
flowers but includes members which are, nevertheless, effectively pollinated by
relatively unspecialized insects. The significant feature here is the exposed nectar,
which presumably allows the utilization of the flower by a wide variety of insects
with primitive mouthparts. Although trends leading to a reduction in the number of
pollinators is thought to result in more efficient pollination and a lowering of the cost
of pollen transfer (e.g., Tremblay 1992 ), other factors, such as climate, pollinator