Context‐dependent chemical communication: Alarm pheromones of thrips larvae

Thrips have several advantages that make them particularly suitable for the study of the evolution of alarm signalling. When in danger, thrips larvae defend themselves by the excretion of ‘anal droplets’: a predator touched by such a droplet interrupts the attack and switches to cleaning. These droplets may contain an alarm pheromone, consisting of two compounds: decyl acetate and dodecyl actetate. The presence of alarm pheromone evokes anti‐predator behaviour in thrips, such as elevated alertness and moving away from the scene, and this behaviour potentially improves the chances of survival of the signal receivers. Thrips larvae may encounter a range of predators, the one more dangerous than the other. If a thrips larva survives the presence of a predator, this larva may become a signal sender on a next occasion. Thrips live in groups, comprising both related and unrelated individuals. A practical advantage of the alarm pheromone of thrips is that synthetic mimics of its two components are available. The anal droplets can be observed, counted, collected and analysed for the presence and composition of pheromone. Furthermore, thrips larvae can be stimulated to produce droplets by prodding them with a fine brush. The combination of these advantages enables the manipulation of pheromone production as well as the determination of quality and quantity of the alarm pheromone in presence of various types of predator.
Three main questions are central to this thesis. First, does the alarm pheromone of thrips larvae indeed improve the defensive capacities of conspecific thrips? Second, is thrips alarm pheromone production context‐dependent? And third, how does relatedness influence alarm communication?