Later studies using actual carnivorous pitcher plants and artificial pitcher models
did not support the attraction function of red pitcher traps, with the redness of the
traps having no effect on the quantity of ants trapped by Sarracenia alata [45] or
Nepenthes gracilis [56]. Bennett and Ellison [55] also showed that the attraction of
ants as prey in Sarracenia purpurea was not affected by the redness of the trap leaves
but rather the presence of nectar [55]. These results are reasonable considering that
ants have dichromatic color vision based on two spectrally distinct photoreceptors
([57, 58] but see [59, 60]), which will make it difficult for them to forage based on
color. In addition, experiments using artificial adhesive traps showed that red traps
caught fewer prey individuals (mainly Diptera) than green traps [19, 61, 62],
suggesting that prey species can recognize the difference between red and green
traps and avoid landing on red traps. Since many insects, including Diptera, do not
possess red color receptors (only possessing blue, green, and ultraviolet receptors
[57]), they may identify and avoid red traps based on other color-related features,
such as the intensity of reflected light [16, 63].
These findings indicate that the production of red traps is likely to have associated
costs, including a reduction in the number of trapped prey and the costs associated
with anthocyanin biosynthesis [64]. So, what are the benefits of having red trap
leaves? In general, anthocyanin provides plants with protection from light stress
under intense light conditions [65], direct protection from herbivores and pathogens
[66, 67], and honest signals indicating the degree of chemical protection [68, 69].
Since many carnivorous plants grow in sunlit habitats [8, 70] and an in situ
experiment showed that the trap leaves of Drosera rotundifolia became redder
when the canopy light transmission was increased through the removal of vegetation
[71], it may be reasonable to consider that red trap leaves function to reduce light
stress. Red trap leaves may also provide defense against herbivores since herbivorous insects of carnivorous plants exist [72, 73]; indeed, it has been shown that the
red pitchers of Nepenthes gracilis experience less herbivory than the green pitchers
[56].
In addition to these general functions, Jürgens et al. (2015) proposed the hypothesis that the red coloration of the trap leaves reduces the risk of trapping pollinators
[19], demonstrating that half the number of prey individuals was captured in artificial
red traps than in green traps, and 25% of the number of pollinator individuals was
captured in artificial red traps compared with green traps. These results suggest that
the red color may function as an honest signal to pollinators, and that pollinators
avoid landing on red trap leaves.
This raises the question of why insects avoid landing on red trap leaves.
First, insects may learn the danger of trap leaves in relation to the red color.
Large insects that are trapped by chance will be able to escape from the trap
leaves (see Sect. 2) and learn to avoid landing on other trap leaves with similar
characteristics. Second, insects may have an inherent tendency to avoid landing
and foraging on red leaves because the red color that is produced by anthocyanin signals the existence of direct and/or indirect chemical defense – consequently, insects also avoid landing on the red leaves of noncarnivorous plant
species [69].
30 Pollinator Trapping in Carnivorous Plants
783
did not support the attraction function of red pitcher traps, with the redness of the
traps having no effect on the quantity of ants trapped by Sarracenia alata [45] or
Nepenthes gracilis [56]. Bennett and Ellison [55] also showed that the attraction of
ants as prey in Sarracenia purpurea was not affected by the redness of the trap leaves
but rather the presence of nectar [55]. These results are reasonable considering that
ants have dichromatic color vision based on two spectrally distinct photoreceptors
([57, 58] but see [59, 60]), which will make it difficult for them to forage based on
color. In addition, experiments using artificial adhesive traps showed that red traps
caught fewer prey individuals (mainly Diptera) than green traps [19, 61, 62],
suggesting that prey species can recognize the difference between red and green
traps and avoid landing on red traps. Since many insects, including Diptera, do not
possess red color receptors (only possessing blue, green, and ultraviolet receptors
[57]), they may identify and avoid red traps based on other color-related features,
such as the intensity of reflected light [16, 63].
These findings indicate that the production of red traps is likely to have associated
costs, including a reduction in the number of trapped prey and the costs associated
with anthocyanin biosynthesis [64]. So, what are the benefits of having red trap
leaves? In general, anthocyanin provides plants with protection from light stress
under intense light conditions [65], direct protection from herbivores and pathogens
[66, 67], and honest signals indicating the degree of chemical protection [68, 69].
Since many carnivorous plants grow in sunlit habitats [8, 70] and an in situ
experiment showed that the trap leaves of Drosera rotundifolia became redder
when the canopy light transmission was increased through the removal of vegetation
[71], it may be reasonable to consider that red trap leaves function to reduce light
stress. Red trap leaves may also provide defense against herbivores since herbivorous insects of carnivorous plants exist [72, 73]; indeed, it has been shown that the
red pitchers of Nepenthes gracilis experience less herbivory than the green pitchers
[56].
In addition to these general functions, Jürgens et al. (2015) proposed the hypothesis that the red coloration of the trap leaves reduces the risk of trapping pollinators
[19], demonstrating that half the number of prey individuals was captured in artificial
red traps than in green traps, and 25% of the number of pollinator individuals was
captured in artificial red traps compared with green traps. These results suggest that
the red color may function as an honest signal to pollinators, and that pollinators
avoid landing on red trap leaves.
This raises the question of why insects avoid landing on red trap leaves.
First, insects may learn the danger of trap leaves in relation to the red color.
Large insects that are trapped by chance will be able to escape from the trap
leaves (see Sect. 2) and learn to avoid landing on other trap leaves with similar
characteristics. Second, insects may have an inherent tendency to avoid landing
and foraging on red leaves because the red color that is produced by anthocyanin signals the existence of direct and/or indirect chemical defense – consequently, insects also avoid landing on the red leaves of noncarnivorous plant
species [69].
30 Pollinator Trapping in Carnivorous Plants
783