Olfactory and tactile behaviour of the Khapra beetle, Trogoderma granarium, with special reference to its assembling scent

Abstract

The assembling scent released by female Trogoderma granarium has attractant and arrestant functions for unmated males and females, the former being much more affected than the latter. After several copulations the males do not respond further to the scent, whereas the mated females are still attracted by it, but to a smaller extent. Orientation of male and female T. granarium in a gradient of assembling scent was studied in a circular arena at 30°C. In the vicinity of the odour-emitting zone, the behaviour of males is characterized by vibration of antennae and zigzag pattern of approach with intermittent stops. The scent current retains male beetles for variable periods and has an aphrodisiac effect in presence of the female. The odour causes temporaty immobilization of females but their response is definitely weaker than that of males. An odour threshold of about 0·0025 female equivalent (0·5 μg ether-soluble body extract) causes attraction of 60 per cent of males as well as one-sixth of the retention time of males brought about by 1 female equivalent. The scent emitted by one female is located by males after an average of 3 min and by females after 7 min. The arresting effect lasts on the average 2·5 min for males and 0·3 min for females. Percentage, velocity, and duration of attraction were found to depend on scent concentration. Secretion of the scent begins on the first day after pupal-adult ecdysis of the female. The odour of the pharate female is strongly adsorbed to the last larval cuticle which has fixative properties. The action of Trogoderma scents appears to be genus-specific. Male T. granarium were found attracted by the scents of female T. inclusum or T. glabrum, but not by those of Anthrenus vorax, Attagenus megatoma, or Dermestes maculatus. Female and male T. granarium avoid smooth surfaces and settle preferably on hairy or rough materials. The aggregations formed thereby are maintained by thigmotaxis as well as olfactory responses. It was suggested that the sensory behaviour of the Khapra beetle may be utilized to rid infested cereals of this pest.