Insect chemosensory responses: a chemical legacy hypothesis

Abstract. 1. Changes in the chemosensory responses and physiological host tolerance of phytophagous insects and parasitoids may result in new host associations. This paper considers the origin of those changes in ecological and evolutionary time. 2. The chemical legacy hypothesis postulates that effects of larval chemosensory environment on adult chemosensory responsiveness depend not (or not only) on persistent neural changes (‘memory’), but on traces of chemical cues, inside or outside the insect's body, which influence adult behaviour, perhaps particularly during a ‘sensitive period’ associated with adult emergence or hatching of the egg. 3. Chemical cues bequeathed by earlier stages may affect adults or larvae by reducing peripheral sensitivity. Depending on the form of the dose–response curve, this change could decrease aversion or increase a positive response to a previously deterrent stimulus. 4. Attention is drawn to a possible link between this behavioural induction and the induction of detoxification enzymes. This link might be of practical importance if exposure to host allelochemicals were to influence insects' responses to insecticides, and vice versa. 5. An influence on the chemosensory responsiveness of an individual by chemical cues derived from its parents would be hard to distinguish from a genetic effect. 6. Some testable postulates of the chemical legacy hypothesis are identified.