Figure 1.
Autumn Tree Colors as Warning Signals to Aphids?
Two papers at the beginning at this decade challenged our view that the beauty of autumn leaves is only a by-product of physiological processes inside the doomed leaves [13,14]. According to the new hypothesis, trees with particularly strong coloration send an honest signal to aphids, informing them of the strength of anti-herbivore defenses of these trees. But to appropriately predict the responses of aphids to colors requires us not only to examine the physiology of their eyes (inset lower right: scanning electron micrograph of the eye of the black bean aphid Aphis fabae, courtesy of J. Hardie) but also their behavioral responses to colors under controlled laboratory conditions (inset, upper left: the foxglove aphid Aulacorthum solani probing a yellow artificial target; photo by S. Kirchner).
Figure 2.
Typical Aphid Annual Life Cycle
In spring the stem mother (fundatrix) hatches from a fertilized egg and asexually produces female offspring. During spring and summer, there is little variation between target colors, since most leaves will be green. In autumn, winged aphids migrate towards their winter hosts, and might choose between leaves based on variation in color cues. Note that many aphid species diverge considerably from this stereotypic life cycle.
Figure 3.
The Perception of Colored Leaves by an Aphid
(A) A green leaf reflects the light from the sun and is seen by an aphid with a green (G), blue (B), and ultraviolet (U) photoreceptor [29]. Evidence from behavioral studies [31] indicates that a COM processes the input from the photoreceptors. (B) Reflectance spectra of three leaves from the bird-cherry, Prunus padus. To humans, these leaves appear green, red, and yellow. For measurement methods see [26]. (C) Tentative spectral sensitivities of the green peach aphid's UV, blue, and green photoreceptors. (D) Relative excitation of the UV, blue, and green photoreceptors produced by the green, yellow, and red bird-cherry leaves shown in (B). For methods of calculation, see [48]. (E) Excitation spectrum of a COM inferred from behavioral data [26,31]. The mechanism is fed by the three photoreceptors in (C), with positive input from the green and negative input from the other two receptors. Based on behavioral data, a mechanism of this kind is presumably what many insect herbivores use as a “greenness detector.” (F) Excitation of the COM by the three bird-cherry leaves and a blue-purple flower (Ajuga genevensis) on a one-dimensional scale. Note how the yellow leaf produces an even more positive signal than the green leaf.