Volatile spider‐mite pheromone and host‐plant kairomone, involved in spaced‐out gregariousness in the spider mite Tetranychus urticae

. From the host plant‐spider mite complex Phaseolus lunatus—Tetranychus urticae Koch a volatile chemical is emitted that acts as a kairomone for the predatory mite Phytoseiulus persimilis Athias‐Henriot (Sabelis et al. , 1984a). This kairomone is apparently a byproduct of a vital physiological process and/or it has a function in the biology of the spider mite as well. The spider mite—host plant complex also emits a volatile spider‐mite dispersing pheromone. This is shown in the present study where spider mites were introduced into an odour patch on a horizontal screen in a vertical airflow olfactometer. When spider‐mite infested leaves of Lima bean are offered, the spider mites walk mainly straight and soon reach the edge of the screen. On the other hand, when clean Lima bean leaves are offered, the mites walk tortuously most of the time and reach the edge of the screen much later. Artificially damaged plants elicit the same response as undamaged plants. Differences in spider‐mite behaviour are observed in the vertical airflow olfactometer when odour of either clean or spider‐mite infested leaves is offered. A comparison of the behaviour in these two situations with that when no odour was offered suggests that Lima bean leaves emit a volatile kairomone that activates T. urticae and makes them return after losing the stimulus. A Y‐tube olfactometer experiment confirms the existence of this kairomone. At a low ratio of dispersing pheromone to plant kairomone, the spider mites behave as if only kairomone is present, walking mainly tortuously. At a high ratio they disperse. No aggregation‐pheromonal effect is observed. The possibility that the spider‐mite dispersing pheromone acts as a kairomone for P. persimilis is discussed.