Using Odors To Find Hosts

There are many examples of insects being attracted to the odors of their host plants, both by flying and by walking or crawling. Generalists such as the moths Trichoplusia ni and Heliothis virescens and the desert locust Schistocerca gregaria fly or walk upwind in wind tunnels toward general green plant odors, and there are examples among all orders of specialist herbivores being attracted to chemicals arising specifically from their host plants (Table I).

Because of air turbulence, concentration gradients that an insect might follow do not generally exist, except within centimeters of the plant. Instead, there are pockets of odor-carrying air that are carried downwind in a rough plume, and an insect encounters and perceives an irregular series of these pockets. An insect usually responds in two stages. First, there is "arousal," preparing the insect to respond to some further stimulus. Then orientation occurs, either on the substrate or in the air. Usually the orientation response is a response to the wind, with the insect turning upwind, and this is termed an odor-induced upwind (positive) anemotaxis. Among flying moths, this has been demonstrated clearly in wind tunnels, and the same kind of response may be seen when males fly upwind toward the source of female pheromone.

TABLE I Example of Host Plant Volatiles Attracting Specific Phytophagous Insects

Insect

Chemical(s) or host odors

Cavariella aegopodii (carrot aphid)

Brevicoryne brassicae (cabbage aphid) Aphis gossypii (cotton aphid) Leptinotarsa decemlineata

(Colorado potato beetle) Ceutorhynchus assimilais

(Cabbage seedpod weevil) Psila rosae (carrot fly) Delia antiqua (onion maggot) Acrolepiopsis assectella (leek moth) Plutella xylostella (diamondback moth) Manduca sexta (tobacco hornworm) Heliothis subflexa (groundcherry moth)

Carvone (one of the host volatiles) Isothiocyanates (host volatiles) Host plant odor Host plant odor

Isothiocyanates (host volatiles)

Mixture of five host volatiles Disulfides (host volatiles) Thiosufinates (host volatiles) Host plant odor Host plant odor Host plant odor

Once airborne, the insect needs to monitor its ground speed, so that it can increase its airspeed if the wind is strong. To do this it uses visual information (i.e., image movement across the eyes from front to back). If the wind is too strong and the insect is unable to keep the images flowing, it turns and flies downwind or lands. The use of visual images by a flying or swimming insect to maintain orientation to a current flow is called an optomotor reaction. It enables the insect to maintain an orientation at any angle to the wind, not just directly up- or downwind. If the insect is unable to see the pattern of objects on the ground, it cannot orient. As well as generally flying upwind in response to a particular odor, many moths and beetles follow zigzag flight paths. This behavior, which evidently is programmed in the insect central nervous system, has the possible function of increasing the chances of encountering a pocket of odor.

Walking insects also show odor-induced anemotaxis. This has been demonstrated in locust nymphs, certain beetles, and aphids, for example, where individuals walk upwind in response to host odors.

In a number of smaller insects such as phytophagous flies, the odor-induced anemotaxis is slightly different; this is well studied are the onion maggot, Delia antiqua, and the cabbage maggot, D. radicum. In these species, after perception of the host odor, an individual fly turns into the wind and makes short flights. After landing, and again detecting the odor, it reorients into the wind and takes off. This tactic is particularly effective for host finding in vegetation, where the path to the food plant may be rather devious and the odor plume very broken.

A different response to host odor after the initial arousal is to move toward or land on a relevant visual target. This odor-induced visual orientation is believed to occur, for example, in the cabbage seed weevil, which uses odor-conditioned anemotaxis from a distance and then odor-conditioned landing responses on yellow targets close to the source. A number of insect species may be readily trapped by means of a yellow water trap combined with a host odor source, and it is probably generally true that landing responses induced by the host odor are responsible.

Insects living in soil use odors alone to find hosts. Since, the air moves little in soil, steep gradients of volatile chemicals can be achieved and maintained. Carbon dioxide is commonly used by such larvae, but for specialists, host-specific compounds are also used. Root-feeding larvae, such as that of the carrot fly, Psila rosae, and the corn rootworm, respond by moving directly up a concentration gradient. Larvae of the carrot fly respond to a mixture of five compounds found in carrot odor.

For insects that fly or walk, the distances from which olfactory cues elicit responses vary from less than a meter as in the Colorado potato beetle, Leptinotarsa decemlineata, to about 30 m in some bark beetles and 100 m in some flies such as the onion maggot. Those that crawl in soil respond from just a few centimeters.

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