Which Pheromones Work the Best?

The second proposed component of pheromone orientation behavior, that which enables the insect to remain within the aerial trail, involves the correction of lateral deviation from the central axis of the trail. Farkas and Shorey (1972) reported that males of Pectinophora gossypiella, as well as males or females of many other insect species, exhibit a lateral zigzag flight pattern in approaching a sex pheromone source.

Visual and photographic analysis of the flight tracks of males of P. gossypiella indicated that the lateral amplitude of the zigzag approximates the lateral dimension of the pheromone trail. Analysis of the nature of the stimuli that are responsible for turns made during this zigzag flight behavior has been attempted only superficially. Such analysis may be of utmost importance in further understanding the pheromone mechanism by which lateral corrections are made.

Working with Aedes aegypti, Daykin et al. (1965) found that the tendency to execute a turn was greater when the mosquitoes left the active pheromone space than when they entered it. Using photographic techniques, Kellogg et al. (1962) found that a similar turning response for Drosophila melanogaster occurred 0.1~0.2 sec after losing the odor. Perttunen et al. (1968) also found a greater tendency to turn when individuals of the scolytid beetle Blastophagus piniperda L. walked from an area containing an odorous attractant to an area without the chemical. Farkas et al. (1974) found that male moths of Pectinophora gossypiella executed their turns at slightly sharper angles as they encountered higher concentrations of female sex pheromone.

Correlating the typical zigzag flight pattern with the physical nature of the chemical trail, Farkas and Shorey (1972) speculated that these turns are probably initiated near the boundaries of the active space by either a drop in molecular concentration or a decreased frequency at which odor filaments are perceived. In this way the insect is able to reorient continually toward the longitudinal axis of the trail.

The direction in which the insect turns near the boundaries of the active pheromone space may possibly be regulated by: 1) detection of the direction of visual sideslippage during its anemotactic response (Kennedy 1939); 2) chemotaxis mechanisms (Martin 1965); 3) a pre-programmed zigzag flight pattern in which right and left turns are alternately executed; 4) detection of an elongate concentration gradient of odor molecules or filaments (Wright 1958); or 5) a combination of these four.

Certain aspects of the male pheromone behavior displayed by an insect in flying toward a chemic- al source along an aerial trail appear similar to those exhibited by insects that follow chemical terrestrial trails. Many species of ants characteristically follow a terrestrial trail in a zigzag pattern (Carthy 1951; Wilson 1962; Wilson and Bossert 1963; Hangartner 1967). Hangartner (1967) presents strong evidence that the zig- zag turning exhibited by workers of Lasius fuliginosus Latr. in following terrestrial pheromone trails is the result of the ant using chemotropotaxis. As the ant moves pheromone from the queen’s tergite glands (pages 18 and 20) might’ be important in the mating process because the odour of it is very noticeable in virgin queens of one to two weeks old.

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