What does this strikingly black and red coloured moth say to you? Eat me? Or, don’t touch me because I am poisonous? It should say the latter! Because bright, contrasting colours like this are aposematic. They are universal warning colours which apply throughout the animal kingdom; and if you don’t know them, you will soon learn when you try to eat one! They are poisonous. Full of toxins! They might not kill you, but you will soon spit them out!
Burnet moths (Zygaena) obtain their toxins – the cyanogenic glucosides linamarin and lotaustralin – from their food plants (Fabaceae), but they are also able to make them themselves. These bitter tasting compounds release hydrogen cyanide (HCN) when brought into contact with enzymes in larval haemolymph (Pentzold et al., 2017), or in the gut of predators.
This ability, or capacity, to de novo synthesize cyanogenic glycosides (also called as cyanogens) evolved independently in both plants and insects. Cyanide is formed following the hydrolysis of cyanogens that can occur, either during crushing of plant material, or during consumption of the insects! The moths can, in effect ‘top up’ the cyanogens they obtained from their food-plants.
Cyanogens are stored in cuticular cavities in larval burnet moths and are secreted on stimulation, or irritation, as sticky droplets to deter potential predators. Linamarin may serve as a better deterrent of predators than lotaustralin since a higher amount of linamarin is present in the most vulnerable stages (adults, eggs and newly hatched larvae) of some moths (Zagrobelny et al., 2018).
The droplets secreted by the larvae glue together the mandibles and legs of potential predators and immobilise them (Pentzold et al., 2016). The content and ratio of the two poisons (linamarin:lotaustralin) are strongly regulated, with an approximate 1:1 ratio during larval stages L4–L7 of Zygaena filipendulae (below), and at an increased ratio of at least 2:1 at the adult stage (Zagrobelny et al., 2007a).
The larvae are also aposematic: but black and yellow at this stage (below). Yellow forms of some burnet moths do exist however: see here.
Burnet moths (Zygaena spp.) may transfer a nuptial gift of cyanogenic glucosides during mating (below), and females may assess males on the basis of how much hydrogen cyanide they give them! (Zagrobelny & Møller 2011). Talk about poisoned love! The BugBlog by Africa Gomez explains that ‘females will mate preferentially with those males better loaded with chemical weapons’!
Virgin female burnet moths actively call for males by extruding their abdominal tip to expose their pheromone gland (Zagrobelny et al., 2015). Females of Z. filipendulae remain motionless with the extruded tip of the abdomen exposing a pocket-like yellow pheromone gland located dorsally on the intersegmental membrane between the 8th and 9th abdominal segments. I wish I had a photograph of this behaviour!
The average hydrogen cyanide (HCN) emission from adult females is said to be 19 times higher than from males, suggesting that it is the plumes of HCN emitted from the perching females that serve to attract flying males (Zagrobelny et al., 2007).)
Both Z. filipendulae and Z. trifolii, potentially along with other European species (Hofmann and Kia-Hofmann 2010), are thought to employ two alternative mating strategies, with males relying on pheromone plumes to locate calling females in the afternoon, but using visual cues to find mates in the morning, when females are not producing pheromones (Naumann et al. 1999; Subchev, 2014; in Briolat et al., 2018).
In female Zygaena filipendulae, higher cyanogenic glucoside levels are associated with smaller and lighter red forewing markings, but in general, the red markings of this species were not ‘straightforward quantitatively honest signals of the levels of defensive compounds’ in the moths (Briolat et al., 2018). This is perhaps not too surprising, as the wing colours can fade over time (see below). Nevertheless, even if their colours do fade a bit over time, they remain highly defended and potentially poisonous. And also very beautiful!
Briolat, E. S., Zagrobelny, M., Olsen, C. E., Blount, J. D., & Stevens, M. (2018). Sex differences but no evidence of quantitative honesty in the warning signals of six‐spot burnet moths (Zygaena filipendulae L.). Evolution, 72(7), 1460-1474.
Hofmann, A. X. E. L., & Kia-Hofmann, T. A. B. A. S. S. O. M. (2010). Experiments and observations on pheromone attraction and mating in burnet moths (Zygaena Fabricius, 1777)(Lepidoptera: Zygaenidae). Entomologist”s Gazette, 61(2), 83.
Pentzold, S., Zagrobelny, M., Khakimov, B., Engelsen, S. B., Clausen, H., Petersen, B. L., … & Bak, S. (2016). Lepidopteran defence droplets-a composite physical and chemical weapon against potential predators. Scientific reports, 6(1), 1-11.
Subchev, M. (2014). Sex pheromone communication in the family Zygaenidae (Insecta: Lepidoptera): a review. Acta zoologica bulgarica, 66(2), 147-157.
Zagrobelny, M., Bak, S., Olsen, C. E., & Møller, B. L. (2007). Intimate roles for cyanogenic glucosides in the life cycle of Zygaena filipendulae (Lepidoptera, Zygaenidae). Insect biochemistry and molecular biology, 37(11), 1189-1197.
Zagrobelny, M., & Møller, B. L. (2011). Cyanogenic glucosides in the biological warfare between plants and insects: the burnet moth-birdsfoot trefoil model system. Phytochemistry, 72(13), 1585-1592.
Zagrobelny, M., Olsen, C. E., Pentzold, S., Fürstenberg-Hägg, J., Jørgensen, K., Bak, S., … & Motawia, M. S. (2014). Sequestration, tissue distribution and developmental transmission of cyanogenic glucosides in a specialist insect herbivore. Insect biochemistry and molecular biology, 44, 44-53.
Zagrobelny, M., Simonsen, H. T., Olsen, C. E., Bak, S., & Møller, B. L. (2015). Volatiles from the burnet moth Zygaena filipendulae (Lepidoptera) and associated flowers, and their involvement in mating communication. Physiological Entomology, 40(4), 284-295.
Zagrobelny, M., De Castro, É. C. P., Møller, B. L., & Bak, S. (2018). Cyanogenesis in arthropods: from chemical warfare to nuptial gifts. Insects, 9(2), 51.