Bioassay of the response of tribolium castaneum to repellent substance excreted by trogoderma granarium

It was found that ether extract of males and females of T. granarium contains a repellent substance active on T. castaneum. Statistical analysis of the response to concentrations varying from 10⁻⁷ to 100 beetles per cc, indicated a threshold response to female extract in the range of 10⁻⁴ to 10⁻³ beetles per cc. The response threshold to male extract was higher by two log units. The concentration that produced the maximal response to male extract was 3.1 beetles per cc and to female extract, 0.7 beetles per cc. It seemed that a greater quantity of the substance was excreted by females than by males. Comparing the two types of extracts (male or female), or extracts (either male or female) and solvent (control), or comparing the response of T. castaneum to live beetles (males and females of T. granarium), we found that repulsion was stronger from the female than from the male extract. With regard to the response and its intensity, the amount of pheromone excreted by live beetles is less than the quantity of pheromone extracted from the same number. The olfactory response of T. castaneum to the active substance varies as a function of time. For both sexes it ceases after 15 min, but the rate of response is more rapid to female (50% response after 0.8 min) than to male extract (3.3 min), and the level of response to female extract is higher for the whole duration of the experiment (up to 1.5 hrs). The behaviour of different-sized groups indicates topothigmotaxis as well as somatothigmotaxis in T. castaneum, both of which influence the intensity of the olfactory response. The fact that T. castaneum preyed on specimens of T. granarium only when the active substance had evaporated from their bodies (following death), suggests the possibility that the substance acts as a defensive one. The discussion shows that this material is not specific; nor is it a conventional sex-attractant. There is strong support for the hypothesis that it is a substance with a defensive rôle.