Zygaena filipendulae, the day-flying Six-spotted Burnet Moth, is one of seven Burnet moth species to be found in the British Isles, and is the commonest of them. In most years at St Nicholas Fields we see the adult moths in good numbers, feeding mainly on the nectar of thistles and knapweed, sometimes several on one flower head and doing a good job of pollinating the plants.
They fly during June and July, and sometimes into August, but have been present on the reserve as larvae since the previous summer. When the adults emerge from June onwards they mate, and clusters of eggs are laid on trefoil plants, mostly Common Birdsfoot Trefoil Lotus corniculatus (‘Eggs and Bacon’) which is present in many places on the reserve. When the eggs hatch the striking black-spotted yellow larvae feed on this plant and slowly grow until day temperature falls to the point where they stop feeding and prepare to hibernate for the winter in dying plant material near the ground.
They wake up again the following spring, and as the trefoil plants grow the increasingly plump caterpillars continue feeding until fully grown in late May or June. They now climb up an exposed plant stem, usually a grass, and spin a long scythe-shaped cocoon with their spinnerets, attaching it at both ends to the stem. Within the cocoon (which is a straw-yellow colour and looks rather papery) the larva’s tissues break down and reform into an adult moth, which then emerges in June and July
to continue the life cycle. Adults feed on nectar using a curled-up proboscis which is unfurled and inserted like a drinking straw into the many individual flowers of the composite flower heads of the thistles and knapweed plants. They need the sustenance to bring them into breeding condition.
Trefoil plants contain small quantities of poisonous alkaloids, mainly hydrogen cyanide (prussic acid). These are ingested by the caterpillars as they feed, and are retained in the body of the adult moth. This makes both larvae and adults poisonous or distasteful to predators, particularly birds, and both have developed warning colouration to deter attack. The yellow-and-black of the caterpillars and the red-and-black of the adults are known to be effective warnings. The yellow cocoons on their grass stems are surprisingly obvious and look rather exposed, but predators have presumably learned that these do not taste nice!
Areas of St Nicholas Fields where Six-spotted Burnets are often seen in the summer are near the Dragon Stones, and on knapweed and thistle plants in the meadow areas. The adult moths, which are sometimes mistaken for butterflies, have a rather heavy-looking slow buzzing flight. It is good to know that in most years there is a strong population of these strikingly attractive moths at St Nicholas Fields. They are signs of a healthy environment.
All photos were taken at St Nicks