Evidence for caste differences in anal droplet alarm pheromone production and responses in the eusocial thrips Kladothrips intermedius

Thrips (Thysanoptera) are tiny insects that produce anal secretions whenthreatened. Several studies have shown that, depending on the species,the droplets may contain alarm pheromones and/or repellents againstenemies. In the eusocial gall-inducing thrips Kladothrips intermedius bothlarvae and adults produce such droplets. There are two castes of adults inthis species, soldiers (the sub-fertile and gall-bound defenders) and dispersers(winged and capable of initiating a gall). We tested the proclivityof secreting anal droplets by the two castes and whether the anal dropletsinduce different behavioural responses in relation to the emitter–receiver’scaste in a contact chemoreception bioassay. Although secretionpatterns were similar between castes, exposure to anal droplets emitted bydifferent castes elicited different behavioural responses in adults in thebioassay. When exposed to soldiers’ anal droplets, dispersers significantlyreduced the distance moved while soldiers significantly increased thedistance moved, compared to when they were exposed to hexane control.In contrast, no differences in the distance moved were observed for anycaste when exposed to dispersers’ anal droplets versus hexane control.Increased activity in soldiers when exposed to their own anal droplets is apredicted response to enhance the overall defence of the gall when underthreat, whereas dispersers should slow down their activity when exposedto such ‘warning signal’. Thus, the behavioural data indicate that the analdroplets emitted by soldiers are likely to contain an alarm pheromone