DETECTION AND DEACTIVATION OF PHEROMONE AND PLANT ODOR COMPONENTS BY THE BEET ARMYWORM, SPODOPTERA-EXIGUA (HUBNER) (LEPIDOPTERA, NOCTUIDAE)

Detection and deactivation of pheromone and plant odor components were investigated in both sexes of the beet armyworm, Spodoptera exigua (Hubner) (Lepidoptera: Noctuidae). Electroantennograms (EAGs) enhanced by novel computer techniques which facilitated investigation of novel or seldom considered parameters, and single sensillum recordings were used. Parameters of the EAGs investigated included rise of the negative phase, and decay following stimulation, both at several time intervals. A novel parameter called level was also investigated. Responses of both sexes to 50 mug stimulus loads of 23 odorants including five components of the volatile emission of females were determined. Male moths were more responsive to plant odors than females based on normalized peaks. While none of the five components of female volatile emissions were detected by female moths, receptors in male moths responded to four of the five components previously shown to possess biological activity. Green leaf volatiles, linalool, myrcene and benzaldehyde elicited the largest normalized peaks in both sexes. Among the green leaf volatiles, the E-2 and Z-3 configurations were preferred as were the alcohol and acetate moieties. Values for rise for pheromone components in males were significantly greater than for green leaf volatiles. Decay values were the same for plant odors in both sexes. Decay values for pheromone components in males were greater than those for plant odors thus indicating specificity of deactivation processes. An analogous situation existed in female moths for benzaldehyde (a common component of male scent gland secretions in noctuid moths, see above) where decay values significantly exceeded that for any other compound. Level was considered to be a measure of the number of molecules arriving at the receptor sites (N(A)) minus the number of molecules removed (N(R)). Differential level values were found for: pheromone components (low); green leaf volatiles, benzaldehyde, pentanol and myrcene (intermediate); and linalool and heptanol (high). Low values for level were thought to indicate efficient mechanisms for transport and removal of molecules for interaction with receptors, and could be indicative of increased signal to noise ratios. Recordings from the sexually dimorphic trichoid sensilla of males revealed in most instances two receptor neurons, one which produced a large amplitude spike and was reliably stimulated by Z-9, E-12-tetradecenyl acetate, while the neuron with the small amplitude spike responded to Z-9-tetradecen-1-ol. These neurons also responded with increasing spike frequencies to a green leaf volatile and a monoterpene. These results may help to explain not only the larger EAGs recorded from male antennae, but also the observed synergism of the female pheromone by green leaf volatiles.