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Chapter 2 : Pantoea and the Locust
The last 20 years have seen an emerging consensus that several types of stimuli, including tactile and visual clues, contribute to the process of gregarization. Particularly significant is chemical communication in the form of pheromones: volatile substances, effective in vanishingly tiny concentrations, which in insects other than locusts perform functions such as attracting mates and promoting social cohesion. During the 1990s, several research groups detected aromatic compounds in the atmosphere surrounding gregarious locusts and their fecal pellets and found that they played an essential role in gregarization. They first reared locusts in the conventional manner and confirmed that volatiles from adult and juvenile fecal pellets had profiles, including guaiacol and phenol, similar to those reported by previous workers. Next, in order to explore the possible involvement of the gut flora in the production of the pheromones, they sterilized eggs and reared the locusts in a sterile isolator system. The next step was to see whether the introduction of Pantoea agglomerans, a bacterium commonly isolated from locusts, restored synthesis of the two phenolics. Indeed, the most likely precursor of the phenolics is vanillic acid, derived from the lignin of plants eaten by the locusts. Fecal pellets from both conventional and axenic locusts contain large quantities of vanillic acid, which can be converted to guaiacol by simple decarboxylation.