Chemical composition of anal droplets of the eusocial gall-inducing thrips <i>Kladothrips intermedius</i>

Abstract:
The eusocial Acacia gall-inducing thrips Kladothrips intermedius (Thysanoptera: Phlaeothripinae) secretes defensive anal droplets when threatened. In addition to harmful substances, droplets of other thysanopteran species are known to contain semiochemicals such as alarm pheromones and/or repellents against enemies. Both Kl. intermedius larval stages and adult castes (soldiers and dispersers) produce anal droplets. In addition, a recent study has shown that soldier droplets may contain an alarm pheromone. Using multiple sampling methods (extraction in solvent, solid phase microextraction and direct injection), we analyzed the content of adults and second instar larvae anal droplets by gas chromatography coupled to mass spectrometry. Anal droplets comprised of a set of saturated and monounsaturated short-chain fatty acids, including three compounds with a terminal double bond (7-octenoic acid, 8-nonenoic acid and 9-decenoic acid). The latter are here reported for the first time in an insect species. In addition, putative unsaturated wax esters similar to those found in the secretion of other social insects were also detected in droplets. In order to investigate the physiological activity of the acids, single sensillum recordings were performed on adult Kl. intermedius as well as on adults of its kleptoparasite, Koptothrips dyskritus, targeting olfactory receptor neurons in the antennal sensilla basiconica. While olfactory perception of the acids in these species cannot be completely excluded, our analyses indicate that sensilla basiconica are not strongly sensitive to these droplet compounds.