A brilliantly coloured day-flying moth of chalk downland and coastal grassland — the six-spot burnet has six vivid red spots on metallic blue-black forewings, a warning colouration that advertises its toxicity; burnet moths produce hydrogen cyanide from their own tissues as a chemical defence, making them poisonous to predators; conspicuous in sunshine on downland flowers, flying weakly but apparently without concern for predators.
Cyanogenic chemistry
Burnet moths are poisonous, and their warning colouration advertises this fact. They synthesise and sequester hydrogen cyanide — the same compound used in industrial fumigants and historically in executions — from their larval foodplant and from their own metabolic processes. The cyanide is stored in specialised cells throughout the body. A predator that catches and bites a burnet moth receives a dose of hydrogen cyanide and quickly releases the moth, learning the warning colours through aversive experience.
Day-flying
Most moths are cryptically coloured and nocturnal. Burnet moths have abandoned both strategies — they are brightly coloured and fly freely in sunshine. Their chemical defence removes the need for concealment or nocturnal habits. They fly weakly and slowly, tumbling through tall vegetation and flowers without the evasive aerial behaviour of undefended moths. Predators that have experienced burnet moths avoid their colours; naïve young birds may attempt an attack but quickly learn.
The papery cocoon
Burnet moth larvae spin conspicuous papery, boat-shaped cocoons on grass stems, typically 30–60 cm above ground level. The cocoons are yellow or straw-coloured, hard, and attached longitudinally to the stem. A meadow with a good burnet population can have dozens of cocoons visible on grass stems. After emergence, the empty cocoons persist for weeks, a distinctive feature of chalk downland.
Grassland indicators
Burnet moths are reliable indicators of high-quality unimproved chalk or limestone grassland, requiring bird’s-foot trefoil in abundance and the warm, sheltered conditions of south-facing slopes. Their conspicuous appearance and day-flying habit makes them one of the most visible insect indicators of grassland quality in chalk and limestone areas.