Difficulty in assigning a role to secretions of the cornices was in part due to the inability to analyzes the droplet contents. It was not until after the invention of GC-MS (gas chromatography – mass spectroscopy) that secretion components were identified and misconceptions put to rest.
A.F.G. Dixon was one of the first to propose a defensive function in 1958. A 1967 paper published in the journal Nature, “Defense by Smear” by John S Edwards more fully described a defensive function of cornice secretions. Edwards had collected aphids from the field with parasitoids stuck to the cornices. In laboratory observations, he noted that poking and aphid would induce it to release a droplet form the cornicle on the side that the poke was received. He found that droplets collected from aphids quickly crystallized. The droplets were capable of immobilizing predators.
Edward’s paper stirred a flurry of research into aphid secretions. These investigations found no evidence of honeydew secretion. Instead they uncovered a variety of lipids that differed among aphid species and led to the identification of aphid alarm pheromones, chemicals that cause aphids to cease feeding and drop from the plant. Today we understand cornicles as primarily defensive organs that produce lipids and alarm pheromones.
*J.S. EDWARDS. 1966. Defense by smear: supercooling in the cornicle wax of aphids. Nature, Lond. 211, 73-74.