The effect of insect cyanoglucosides on predation by domestic chicks

Abstract

AbstractCyanogenic insects release cyanide and other byproducts that are thought to make them unpalatable to would be predators. In fact, aposematic Heliconius butterflies and Zygaena moths are rejected by vertebrate predators. Nevertheless, there have been few studies testing the effect of cyanoglucosides on vertebrate predation. Here I report tests undertook with captive domestic chicks in order to evaluate the effect of two chemically diverse cyanoglucosides, linamarin and prunasin. In insects as well as plants, cyanoglucosides are stored in vacuoles and are enzymatically broken down when the tissue is disrupted as in the case of a predator attack. Linamarin is an aliphatic that releases cyanide and acetone upon breakdown, while prunasin is an aromatic cyanoglucoside that releases cyanide and benzaldehyde. Using concentrations that mimicked the average concentration of a Heliconius butterfly, supplemented by test with higher concentrations, I ran a series of trials with naïve chicks by offering prey laced with chemicals. I categorized prey acceptance and compared the behavior of the predators towards test and control larvae. Prey coated with cyanide and benzaldehyde were significantly rejected by the birds, while acetone did not elicit avoidance behavior. Intact cyanoglucosides apparently were not detected by the predators, presumably because of fast ingestion time or lack of enzymes to breakdown cyanoglucosides. The rejection of cyanide laced prey confirm the protective nature of cyanoglucosides against a vertebrate predator. Additionally, the rejection of the pungent but not toxic benzaldehyde suggests that some species that store aromatic cyanoglucosides could be detected via smell as well by taste. These results provide support for cyanoglucosides as defensive chemicals of aposematic lepidopterans and related arthropods.