Crop Production With Pheromones
Pheromones are on of the best ways to manage insect infestation on crops. In this article, we will discuss how that works.
Within a few cm of the odor source, where a relatively steep concentration gradient might persist, an insect may use chemoklinotaxis or chemotropotaxis in determining the direction of the origin of the chemical. In employing chemoklinotaxis, the insect swings its antennae from side to side and makes successive comparisons of the chemical concentration on each side of its body (Shorey 1973). In using chemotropotaxis, the individual simultaneously compares the pheromone concentration on each side of its body with its two antennae (Shorey 1973). In either tactic reaction the insect turns toward the side of highest stimulation (if the pheromone chemical is an attractant) and, in this way, becomes oriented toward the stimulus source. Tactic reactions only imply steering per se and a chemo-orthokinetic reaction speed of locomotion dependent on chemical concentration (Fraenkel and Gunn 1961) maintains forward progress toward the source (Janders 1963). This relates to crop production with pheromones.
At our present level of knowledge it appears that two different male pheromone components of orientation may function simultaneously to enable the insect to approach a distant odor source. The first component allows the insect to determine which direction to follow along the trail so as to arrive at, as opposed to move away from, the odor source. The second component is superimposed on the rst and enables the insect to make lateral correcting movements so as to remain near the central longitudinal axis of the trail.
Several pheromone mechanisms have been proposed by which an insect can determine the first component the actual direction of an odor source. These include: 1) anemotaxis; 2) use of an elongate gradient of odor filaments; and 3) use of an elongate gradient of molecular density. To what extent these mechanisms are actually used is a matter of conjecture. Therefore, the discussion that follows is largely hypothetical.
An anemotactic response occurs when the insect orients its body axis parallel with the direction of movement of an air current (Fraenkel and Gunn 1961). If the insect were in physical contact with the substratum, the direction of an air current could, in principle, be determined by the direction of deection of mechanoreceptors on the body (Weis-Fogh 1948, 1949; Camhj 1969a, b, 1970). However, ying insects cannot rely entirely upon mechanoreception in obtaining this directional information. The following example will illustrate this point. Suppose that a weightless, blind insect is free-oating in a current of air. When the insect is not ying, both the direction in which it is carried and its velocity are equal to the direction and velocity of the air current. Its pheromone mechanoreceptors can only detect that it is motionless with respect to the air mass within which it is carried.
Male Pheromone Response
Therefore, contact with the substratum or other xed objects in the male pheromone environment must be maintained in order to sense the true direction of a owing medium. Kennedy (1939) reported that the yellow-fever mosquito, Aedes aegypti (L.), uses visual contact with the substratum to maintain an upwind ight direction and he hypothesized that the steering mechanism involved both the speed and the direction at which objects passed below the eye. The apparent direction and velocity of these images are dependent upon the air velocity, the ight speed of the insect, and the angle at which the insect is ying with respect to the wind. Upwind ight must produce a lower apparent velocity of images passing beneath the eyes than downwind ight, and the apparent direction of the substrate objects must be from fore to aft, parallel to the body axis. Crosswind ight causes a side slippage of the insect’s body with relation to the substrate that results in substrate objects moving at an apparent oblique angle to the eye.
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ABOUT THE AUTHOR
Alexander Pommett is a blogger in Los Angeles who studies pheromones.