Chiral Flexibility in Insect Pheromones: Why Do Some Species Tolerate the Wrong Stereoisomer?

Many insect pheromones are chiral, and receivers respond to non-natural stereoisomers in strikingly different ways: some are completely inhibited by a few per cent of the antipode, some ignore it, some require both enantiomers, and at least one responds more strongly to stereoisomers it does not produce. These response types were catalogued two decades ago, and the most common of them, a non-natural enantiomer that neither attracts nor inhibits, was recognised as allowing racemic lures in practice. What remains unclear is why a given species falls into one response type rather than another. We define chiral flexibility as behavioural tolerance of, or dependence on, stereoisomers other than the natural one, distinguish it from failure to detect the antipode, and reassemble the evidence across Coleoptera, Hemiptera and Lepidoptera. We then evaluate three classes of explanation. First, receptor architecture: broadly and narrowly tuned receptors coexist in the same antenna, and behavioural fidelity is weighted centrally rather than fixed at the periphery. Second, biosynthesis: in stink bugs the terpene synthase is stereospecific while the natural stereoisomeric blend is set by later steps, and in termites a single amino acid substitution shifts the enantiomeric composition of an enzyme's product. Third, community context: fidelity is often, though not always, tight where a synchronously active species uses the antipode. We examine methodological artefacts that can mimic flexibility, show that the cost of enantiopure lures depends on whether plants supply the insect's configuration, and propose comparative tests within clades.