Behavioural experiments on alarm pheromone effect when puffed just after the Cimex lectularius male mounted a female. The male mating behaviour is regulated by nymph-emitted pheromone cues only. (a) Frequency of male mounting on females with covered methathoracic glands did not differ between the five experimental groups (n = 27-30; Kruskal-Wallis test: χ2 = 5.322, P = 0.256) before puffing. (b) Stimulation with a male [5:4] (n = 19) or a female [1:1] (n = 20) ratio of (E)-2-hexenal:(E)-2-octenal did not significantly alter the percentage of male mating with these females, compared to control. However, stimulation with either the nymph-specific ratio of these aldehydes [2:5] (n = 23)) or 4-oxo-(E)-2-hexenal (n = 22) caused a significant decrease in percentage of mating (Kruskal-Wallis test: χ2 = 35.602, P < 0.001; followed by Mann Whitney U-tests).