Influence of repellency on the efficacy of blatticides. I. Learned modification of behavior of the German cockroach.

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Devices were developed for determining the behavior of the German cockroach, Blattella germanica (L.), when it is given the ‘choice’ of entering or avoiding areas that would normally be attractive, such as dark voids, but which have been contaminated by residues of liquid insecticides or by deposits of insecticide dusts, dust diluents, or other powders. Using such devices, repellency was found to be the most important factor affecting the insecticidal efficiency of blatticides. When using ‘choice boxes’ or other similar devices, it is important that observations be made only when the cockroaches are not in a condition of stress, otherwise the ratings given to potential blatticides may not be in accord with what is found to be their comparative value under conditions of normal behavior. The German cockroach was able to learn to avoid the repellent types of insecticide deposits applied in dark areas, thereby resisting its strong impulse to enter such areas. The period required to learn and the effectiveness of the learning process were indicated by the time-mortality curves, which were characteristically hyperbolic rather than sigmoid. The effectiveness and duration of the modified behavior pattern were increased by the ease with which the insects could habituate themselves to a light (and insecticide-free) environment. The complete insecticide-avoidance behavior pattern involved associative learning augmented and extended by a more elementary type of learning known as ‘habituation.’ Boric acid powder possessed so little repellency that German cockroaches repeatedly entered dusted areas until they succumbed to its relatively feeble contact toxic action. In both the ‘choice boxes’ and ‘wall-void devices,’ a given number of cockroaches were exterminated more rapidly with boric acid powder than with deposits of various currently used toxicants or dust desiccants, which were far more insecticidal than it, when the insects were confined continuously upon the deposits. The addition of flour or powdered sugar to boric acid, as possible attractants, was of no value; both were more repellent than boric acid to German cockroaches. In addition, sugar decreases ‘pick-up’ by the cockroaches. It becomes increasingly detrimental with passing time. An important characteristic of a boric acid deposit is that it is readily picked up by insects after it has become damp and subsequently dry. (This characteristic is not possessed by borax and many other powders.) In addition, the deposit left by an aqueous solution of boric acid is insecticidal.