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 when threatened. Several studies have shown that, depending on the species, the droplets may contain alarm pheromones and/or repellents against enemies. In the eusocial gall‐inducing thrips Kladothrips intermedius both larvae and adults produce such droplets. There are two castes of adults in this species, soldiers (the sub‐fertile and gall‐bound defenders) and dispersers (winged and capable of initiating a gall). We tested the proclivity of secreting anal droplets by the two castes and whether the anal droplets induce different behavioural responses in relation to the emitter–receiver's caste in a contact chemoreception bioassay. Although secretion patterns were similar between castes, exposure to anal droplets emitted by different castes elicited different behavioural responses in adults in the bioassay. When exposed to soldiers’ anal droplets, dispersers significantly reduced the distance moved while soldiers significantly increased the distance moved, compared to when they were exposed to hexane control. In contrast, no differences in the distance moved were observed for any caste when exposed to dispersers’ anal droplets versus hexane control. Increased activity in soldiers when exposed to their own anal droplets is a predicted response to enhance the overall defence of the gall when under threat, whereas dispersers should slow down their activity when exposed to such ‘warning signal’. Thus, the behavioural data indicate that the anal droplets emitted by soldiers are likely to contain an alarm pheromone.