Using Vision To Find Hosts

Visual attraction can result from responding to the color or form of the host plant. Because these vary so greatly within a species, and because there is relatively little specificity of shape among plant species, visual responses often occur only in the presence of an appropriate olfactory signal.

In a few examples, visual responses to host features have been demonstrated without the presence of odors. Walking insects of several species are attracted to narrow vertical targets in a plain arena, but the precise significance of this attraction is unknown. Perhaps it is a response to potential vegetation or shelter. Several species of butterflies, however, have been shown to land preferentially on leaves of particular shapes, with further discrimination occurring only after landing. Shape may interact with color as in the apple maggot, Rhagoletis pomonella. Host odors play a role here, but when colored rectangles are offered, the only color to attract flies is yellow, perhaps representing vegetation. If colored spheres are presented, the red and black shapes attract flies, perhaps representing the host fruit.

With respect to color, both wavelength and intensity are important. D. radicum lands preferentially on leaves with a leaf reflectance pattern characteristic of its host, whereas the western flower thrips, Frankliniella occidentalis, land most on yellows and whites, and more at highest intensities of reflected light. Patterns can also matter. For example, females of Heliconius butterflies lay their eggs on Passiflora leaves but tend not to oviposit on leaves that already have eggs on them. This is known to be a visual response to the yellow eggs, because if the eggs are painted green to match the leaf, butterflies do not discriminate against them.

A response to color is often coupled with a chemical cue. Pieris rapae require the presence of glucosinolates to oviposit but still responds to these chemicals only if they are on blue, yellow, green, or white substrates. Females reject red or black substrates.

Visual cues are usually important only at close range, though occasionally they attract specific herbivores from 10 m or so. This is true for the apple maggot, which has a very clear signal in the bright red fruits of its host substrate.

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