Plants continuously emit organic volatiles into the air, but upon herbivory the amounts increase dramatically. Emission of herbivore-induced plant volatiles, i.e. especially of terpenoids, aromatic compounds and C-6 based ‘green leaf volatiles’ (GLVs), leads to the attraction of foraging natural enemies of herbivores and therefore is referred to as indirect plant defence. We recently found that the increased emission of (E)-isomers of GLVs, induced in wild tobacco (Nicotiana attenuata) by tobacco hornworm (Manduca sexta) feeding, tripled the foraging efficiency of its natural predators in nature. Surprisingly, we discovered that the conversion of the plant’s (Z)-aldehydes to (E)-aldehydes was solely and directly due to isomerase activity in the herbivore’s oral secretions (Allmann and Baldwin (2010) Science 329). The fact that the insect thus betrays its whereabouts to its enemies raises an intriguing question: why did evolution not select against the presence of this enzyme? Here I propose to characterize the enzyme responsible for GLV isomerisation and to elucidate its function in the caterpillar’s physiology, life history and ecology. First I will purify the enzyme, clone the corresponding gene and cDNA, and characterize the enzymatic properties; second I will describe the expression and activity throughout the hornworm’s life cycle; thirdly I will manipulate its expression to investigate the consequences of increased or decreased emission of GLV (E)-isomers for the caterpillar’s growth, development and immune response. Finally, I will investigate the costs and benefits of GLV-isomerisation by tobacco hornworms under natural conditions in the field. Doing so I expect to learn which physiological or ecological benefits have allowed the insect to maintain this treacherous enzyme.