Olfaction as an important sensory modality in insects is essential for identification of hosts, mates, oviposition sites, and food resources in nature. In the cockroach, both olfactory sensitivity in the antennae and the formation of shortand long-term olfactory memories exhibit daily fluctuations that are regulated by the circadian system. An important problem is to characterize the signalling systems and molecules that are involved in this regulation of olfactory reception and olfactory behaviour. Recent results suggest that insect olfactory systems are modulated by both biogenic amines and neuropeptides. However, it remains elusive how these molecules modulate olfactory system in the peripheral systems. In the present study, our aim was to characterize the structure and organization of these signalling systems in the peripheral olfactory system of the American cockroach, Periplaneta americana. This work illuminated that tachykinin and its receptors regulate olfactory sensitivity in the antennae of the cockroach. Injections of tachykinin peptides caused decreases in the amplitude of the electroantennoogram (EAG), cells that produce tachykinin were localized in the antennae, and olfactory receptor neurons expressed tachykinin receptors. Interestingly, the tachykinin expressing cells also express receptors for the biogenic amine, octopamine and injections of octopamine also cause reductions in EAG amplitude. These results suggest that both octopaminergic and tachykinin peptide signalling pathways are important regulators of olfactory reception in the cockroach. We propose the hypothesis that octopamine regulates the release of tachykinin from cells in the antennae that, in turn, modulate the sensitivity of olfactory receptor neurons.

Origanum essential oil [Origanum acutidens (Hand.-Mazz.)] showing insecticidal activity and repellency against red flour beetle, Tribolium castaneum (Coleoptera: Tenebrionidae) adults was analysed by GC-MS. All constituents of the oil were identified, and the main components were carvacrol (67.2%), p-cymene (16.2%), γ-terpinene (5.5%), thymol (4.9%),and linalool(2.1%). In vapor phase fumigant assay, the origanum oil was more effective in closed condition (LD50 = 0.555 mg/cm3) than in open one (LD50 = 0.353 mg/cm3).This result suggests that the toxicity of the oil to red flour beetle is exerted largely by vapor phase. Based on 24-h LD50 values, the toxicity of caryophyllene oxide (0.00018 mg/cm3) was comparable with that of dichlorvos (0.00007mg/cm3) as a positive insecticide. In addition, thymol, camphene, α-pinene, p-cymene, and γ-terpinene gave good insecticidal activity (LD50 = 0.012 - 0.195 mg/cm3). In repellent test using 9 constituents of origanum oil, caryophyllene oxide showed 100% repellent activity at 0.0158 and 0.0032 mg/cm2. These results indicate that the origanum oil and its some components could be potential candidates as a fumigant and repellent for managing T. castaneum adults. Potential molecular target of repellents will be discussed.