Environmental Influences on Floral Nectar Constituents

Although laboratory feeding experiments have been largely confined to nectar intake rates on pure sucrose solutions (but see [95-97]), floral nectars in nature are often composed of a suite of sugars in various proportions along with small concentrations of amino acids and other compounds [98]. These chemical constituents influence both the physical properties of nectar [26] and its energetic value to a given pollinator [96,98]. Fructose and glucose, for instance, which are found in moderate concentrations in insect flowers, are both less viscous than sucrose at the same concentration [26]. However, in choice tests, pure sucrose is preferred over either of these sugars

TABLE 9.4

Optimal Nectar Sugar Concentrations (% w/w) Reported for Some Nectar Feeders

TABLE 9.4

Optimal Nectar Sugar Concentrations (% w/w) Reported for Some Nectar Feeders

Common Name

Genus

Feeding Mode

Optimal %

Ref.

Ponerine ant

Pachycondyla

Lapping

50

93

Ponerine ant

Rhytidoponera

Lapping

50

93

Bumblebee

Bombus

Lapping

55

49

Honeybee

Apis

Lapping

55

101

Stingless bee

Melipona

Lapping

60

101

Leaf-nosed bat

Glossophaga

Lapping

60

118

Rufous hummingbird

Selasphorus

Lapping

50

123, 138

Honeyeater bird

Various"

Lapping

40

115

Leafcutter ant

Atta

Suction

30

93

Carpenter ant

Camponotus

Suction

40

93

Orchid bee

Euglossa

Suction

35

31

Fritillary butterfly

Agraulis

Suction

40

92

Sulphur butterfly

Phoebis

Suction

35

92

Fritillary butterfly

Speyeria

Suction

35

139

Skipper butterfly

Thymelicus

Suction

40

126

Painted lady butterfly

Vanessa

Suction

40

140

Armyworm noctuid moth

Pseudaletia

Suction

40

126

Hummingbird hawkmoth

Macroglossum

Suction

35

97

Tobacco hawkmoth

Manduca

Suction

30

141

Human

Homo

Suction

40

126

Blowfly

Phormia

Sponging

35

95

Note: In general, animals were timed while feeding from large volumes of aqueous sucrose solution and the volume or mass change of the solution was recorded upon completion of the feeding bout.

a Anthochaera (45%), Phylidonyris (45%), and Acanthorhynchus (35%) b Not including humans.

[96,99,100], perhaps because of its ease of assimilation. Nectar viscosity increases with the addition of amino acids [26], and although they are ubiquitous at low concentrations in floral nectars [98], their significance to nectar-feeding insects has yet to be convincingly demonstrated [99,101,102].

In general, insect-pollinated flowers tend to be sucrose dominant [98], and nectar intake rates observed in the laboratory provide a window to understanding the mechanics of nectar ingestion at real flowers. Sucrose concentrations of nectars in insect-pollinated flowers vary widely, ranging from just a few percent to a high of 88% in the crystallized nectar of one Mediterranean shrub [98]. High concentrations are typically diluted with saliva prior to ingestion, and under these conditions salivation rate may even be a limiting factor in foraging efficiency. Diurnally pollinated flowers normally exhibit a single peak in nectar production in the midmorning, whereas flowers that are pollinated at night exhibit this peak shortly after dusk [11,103,104]. Although available nectar volumes change over the course of the day, nectar sugar concentrations are relatively stable in most flower species [11,12, 105-107]. In fact, flowers with long corollas and concealed nectaries are less affected by evaporation or dilution by rain than flowers with open nectaries [103].

Studies isolating the effects of environmental factors such as temperature, humidity, water stress, and atmospheric carbon dioxide on nectar sugar concentrations have produced mixed results [108-111]. Similarly, the heritability of nectar sugar concentration appears to vary by species and environment, making generalizations difficult at the present time [108,112]. It is important to note, however, that whereas interindividual variation in nectar volume can be quite large, variation in nectar sugar concentration tends to be rather low [113]. Patterns of high interspecific and low intraspecific variation in sugar concentration are at least suggestive of strong stabilizing selection.

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