An Issue Of Growing Import: How Insects Find Food

Living plants and animals produce volatile chemicals as a consequence of their normal metabolic activities. In addition, many plants produce odors as interspecific signals and animals often communicate intraspecifically by odors (pheromones). Decomposition results in the production of odors from dead organic materials. These different odors are used by many insects to locate their food and by others to find oviposition sites where the larvae will subsequently feed. Consequently, the range of insects responding to food odors is very wide. Before the late 1960s studies in these phenomena were largely descriptive, but host-locating by blood-sucking flies was an exception. Work in this area was stimulatedby the recognition that an understanding of host-finding behavior might provide effective means of monitoring populations and perhaps even controlling them. Work on phytophagous insects has subsequently been stimulated by the world food shortage and the realization that here, too, the knowledge would be of practical importance. Further impetus has been derived from the increasing interest in evolutionary biology and the concept of plant-herbivore coevolution. At the same time, developments in chemical techniques (making it possible to collect, identify, and quantify minute quantities of volatile chemicals) and in electron microscopy and electrophysiology (making it possible to understand the mechanisms of olfactory perception) have allowed research to advance in a way that previously was impossible. Most recent research has focused on two areas: identification of the chemicals involved in attraction to food, and explanation of the sensory mechanisms involved in the perception of these chemicals. A third area, the behavioral mechanisms used in locating food, has been neglected, but more thorough studies have been made on the mechanisms employed by insects that use odors for locating the opposite sex. A fourth area, the role of odors in behavior after arriving at the host, is still descriptive. Among blood-sucking flies, carbon dioxide has been known for some time as an important attractant, but other components of the breath or body odors of animals are also involved. For example, acetone and octenol attract tsetse flies, and the mosquito Aedes aegypti is attracted by lactic acid. A mixture of these chemicals is generally more effective than single chemicals in attracting the insects experimentally, and it is likely that particular mixtures of a number of chemicals contribute to host specificity and to greater attractiveness of some individuals even within a species. In like manner, green plants produce a suite of odors as a consequence of their primary metabolism, and these, too, attract insects. The "green odor" of potato comprises various isomers of hexanol, hexenol, and hexanal, which are derived in the leaf from linoleic and linolenic acids. The same compounds are produced by many plants, but proportions vary and other compounds may also occur. In addition to the common "green odor," many plants also have characteristic odors due to chemicals produced in secondary metabolic pathways. The smells of mint, cabbage, and onions are well-known examples. Their odors are due to monoterpenes, thiocyanates, and mercaptans, respectively, and these classes of compounds are widely distributed in the plant families to which these examples belong. The evolutionary significance of these odors has not been widely considered. Although it may be that their primary function is to repel potential herbivores, many insects now use these odors as cues for locating their host plants. Most flower odors are assumed to have evolved specifically to attract insects, or to facilitate flower recognition, in order to effect poilination; the insect is rewarded by the provision of nectar. Monoterpenes and sesquiterpenes are common constituents of these odors, but volatile aromatic phenols and simple alcohols, ketones, and esters may also be present. In most cases the odor is a mixture of several components. Bees, butterflies, and flies are all known to be attracted by floral odors, although vision also has a major role. Behavioral Response An odor may elicit two different types of response in an insect some distance from the source. The insect may become active (for example, it may take off), without making any directed movement relative to the origin of the odor, or it may orient itself toward the source. These two components, activation and orientation, are often not clearly distinguished and are generally confused in the single-term attraction. Strictly, use of the terms attraction and attractant should be restricted to situations in which an oriented response has been demonstrated. The mere fact of arrival at an odor source does not indicate attraction, since activation followed by random flight, or by flight directed by some other stimulus, can produce the same result. Activation by food odors has been separated from orientation in some cases. Biting flies provide an example; carbon dioxide may have this effect as well as being an attractant in the strict sense. It is generally assumed that the responses of flying insects to food odors are qualitatively similar to the responses to sex pheromones, which have been more extensively studied. However, sex pheromones usually arise from individuals, providing a much smaller odor source than many food sources. The odor plume arising from a point source is qualitatively different both spatially and temporally from that produced by a field of potatoes or, depending on the distance, even by a cow, and the insects' responses may vary accordingly. Odors emitted from a point source arrive at any downwind position as a series of pulses. As a consequence of air turbulence, there is no discrete plume in the sense of a continuous belt of odor, nor is there an odor gradient except, possibly, close to the source. Thus, an insect orienting to a point source from a distance can do so only by orienting upwind. Because of continuous changes in wind direction, the insect perceives only intermittent pulses of odor as it approaches a small source. Many species are known to maximize the chances of perceiving the odor by zigzagging from side to side as they move upwind, or even casting from side to side with no forward progression. It had been thought that this counterturning occurred whenever the insect lost contact with the pheromone plume, but it has now been demonstrated that perception of the chemical switches on the appropriate behavior pattern and the insect will continue to counterturn even in the absence of odor. The pattern of counterturning is modulated both by the concentration of odor in the pulses that reach the insect and by the frequency with which pulses are perceived; higher concentrations and higher frequencies probably reduce the interval between turns. This is consistent with the idea of the insect "homing-in" on the odor source. The distances from which insects are attracted to their food by odors are poorly documented. The only good evidence for a phytophagous insect suggests that the cabbage root fly is attracted to a small plot of cabbage from no more than 15 meters. This behavioral observation is in moderately good agreement with calculations based on the sensitivity of the antennal olfactory receptors and the release rate of volatiles by the cabbage. At The Source In addition to providing signals by which insects can locate food from a distance, odors may also be important in providing more specific information about the host at close quarters. This is most obvious with flower odors; in many flowers, odors are produced in different forms or quantities in different parts of the flower, and they provide the olfactory equivalents of nectar guides. Among phytophagous insects, host plant odors—in addition to being attractants—are known to induce landing or oviposition, or biting and feeding. The antennae are the principal olfactory organs of insects and typically bear large numbers of olfactory sensilla. The number of sensilla on an antenna is proportional to its length, as well as its complexity, and length is generally related to the size of the insect. Since sensitivity and number are also correlated, this implies that big insects are, in general, more sensitive to odors than small insects. The logic of this may rest in the greater ability of large insects to make headway against a wind and so reach an odor source from a greater distance. The greatest number of sensilla on the antennae are found in insects that respond to sex pheromones, but sensilla concerned with the perception of food odors may still be present in considerable numbers. The difference probably reflects the fact that food odors may be present in higher concentrations than sex-attractant pheromones. Whereas the, receptor cells for pheromones are highly specific, food-odor receptors generally respond to a wide range of different chemicals, although their sensitivity varies within this range. There is some evidence for specific receptors in flies. Where receptors tuned to a chemical characteristic of the host occur, it is probable that decision making by the insect is strongly influenced by the input of these labeled lines, as it is with pheromones. No case is known, however, in which the olfactory response to food is based wholly on the perception of a single chemical. It is assumed that decisions are based on the integration of inputs from a number of receptors with different ranges of sensitivity. The responsiveness of some, perhaps, all, insects to food odors fluctuates in relation to previous feeding, and blood-sucking insects are extreme examples of this phenomenon. After taking a blood meal, a mosquito ceases to respond to host odors. This is due to a reduction in sensitivity of the lactic acid receptor regulated by a blood-borne factor. The causes for the change in sensitivity by other insects have not been investigated. Future Directions One key question that remains is how insects locate a food source from a distance. While it has been assumed that this involves a behavior pattern similar to the attraction to the source of a pheromone, the few examples studied (such as the tsetse fly and Delia species) do not fit this model. Perhaps different behavior patterns depend on the flight capacities of the insect and the form of the food source. Understandably, most work on insect olfaction has focused on attraction from a distance. However, most insects are small in relation to their food sources and, having arrived at the food, may be completely immersed in its boundary layer. Here they will perceive odors that are quantitatively, and perhaps qualitatively, different from those perceived at greater distances. These odors have the potential to modify behavior in the same way as other chemicals at the surface. Work on this aspect will be limited until techniques are developed for collecting and analyzing the odors within the boundary layer, but it is an obvious and important frontier area. Reginald F Chapman is an adjunct professor of entomological sciences at the University of California, Berkeley. [Ed. note: A full version of Chapman's review, "Odors and the feeding behavior of insects," appears in the ISI Atlas of Science: Animal and Plant Sciences, 1 (2), 208-12, 1988. Copyright Institute for Scientific Information 1988.]

An Issue Of Growing Import: How Insects Find Food

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