Cues used to locate flowers

Although most pollinating insects rely on visual and/or olfactory cues to locate their flowers, the relative importance of these stimuli varies within and across orders (Proctor et al. 1996). Many beetle taxa depend on odor, in the absence of visual cues, to reach their flowers (Young 1986; Eriksson 1994), while for others, color cues alone may suffice (Dafni et al. 1990; Steiner 1998). In some cases, floral odor attracts beetles towards an inflorescence, and then releases searching behavior for a particular color or visual attractant at close range (Pellmyr & Patt 1986). Flies approaching non-deceptive flowers seem to depend on visual cues from a distance and olfactory cues at close range (Knoll 1921; Kugler 1951; Dobson 1994), but their approach to deceptive flowers is generally based on scent (Dafni 1984). Crepuscular or nocturnal hawkmoths and settling moths generally rely on scent to locate their flowers from a distance (Dobson 1994; Raguso, this volume), and vision plays a role at close range. Diurnally foraging butterflies, on the other hand, tend to use long-distance visual cues to locate their nectar sources, though odor may be important for releasing color searching behavior (Tinbergen 1968; Proctor et al. 1996) or for direct attraction to flowers (DeVries & Stiles 1990).

These different sensory weightings affect the suites of characters (floral "syndromes") that sometimes occur in flowers predominately pollinated by particular groups (Faegri & van der Pijl 1979). In general, because moths and beetles are likely to be guided by olfactory cues, flowers that depend on them as primary pollinators will tend to be strongly scented, while flowers that rely on the more visually oriented butterflies and bees will tend to emphasize colors over odors. Within these broad categories, some groups of pollinators pay particular attention to subsets of visual and olfactory space. Raguso (this volume) identifies scents characteristic of many moth-pollinated flowers; the putrid odors of carrion-mimic flowers attract only some taxa of flies and beetles (Dafni 1984).

That many beetles, flies, lepidopterans, and bees use an overlapping set of cues to locate their flowers has implications for generalization of pollination systems as well as for opportunities for pollinator shifts. Generalist foragers in all four orders can opportunistically visit flowers of a given species because they all respond to more or less the same cues. If each insect group depended on an entirely different sensory modality to locate its flowers, specialization would be the norm. Shifts from one set of pollinators to another (e.g., Steiner 1998) also depend on overlap in cues used by different taxa.

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