Butterflies are the exhibitionists of the insect world. For many invertebrates, camouflage is the best defence against predators, so how do brightly coloured butterflies protect themselves?
The monarch has 2 defence mechanisms
The monarch (Danaus plexippus) uses two methods of self-defence – warning colouration and toxicity. The monarch butterfly’s bright colours are warning signs for vertebrates to stay away. Scientists call this aposematic colouration. Just as we humans learn that high-visibility vests and orange cones mean danger, birds and other predators learn that brightly coloured monarchs are harmful to eat. This is due to the monarch’s other method of self-defence – toxicity.
Monarchs lay their eggs on milkweed (swan plants), a member of the genus Asclepias. As the caterpillars eat the milkweed leaves, they ingest chemicals called cardiac glycosides. Birds or other animals that eat the caterpillars (or milkweed itself) become sick and vomit. The caterpillars sequester (hold on to) this toxin as they pupate, and the toxins are transferred to the adult butterflies. Birds or other creatures that eat the monarchs become sick, so they learn to leave both the butterflies and larvae alone. Not all monarch butterflies are equally poisonous. The concentration of cardiac glycosides decreases as individual butterflies age, and male monarchs have lower concentrations of toxins than females. Predators recognise the aposematic colouration and tend to avoid all monarchs.
There are exceptions. In New Zealand, shining cuckoos feed on monarch caterpillars. In North America, some birds have learned to eat only the parts of the butterfly containing the least amount of poison.
Red admiral butterflies and stinging nettle
The red admiral (Bassaris gonerilla gonerilla) uses toxin in a different way. It does not sequester toxins like the monarch but uses ongaonga or stinging nettle (Urtica ferox) as its larval food plant. Most animals will not approach ongaonga because of its vicious needles. The red admiral lays its eggs on the nettle leaves, even on the sides of the stinging hairs. The larvae bend down the tips of the leaves and fasten them with strands of silk. They feed inside these little tents and move outside of their protection to feed at night.
Ongaonga is called stinging nettle for good reason. Its needle-like stinging hairs give off a poison when you brush against them. There are cases of horses and dogs dying after coming into contact with the plant.
An evolutionary breakthrough
Although milkweed and stinging nettle are poisonous to many creatures, the butterfly larvae associated with them are immune to their toxins. Their immunity is regarded as an evolutionary breakthrough. It allows them to feed on plants that are out of bounds to so many other animals.
Swan plants in the classroom
Swan plants are toxic. The New Zealand National Poisons Centre website states: “Ingestion of the plant material can affect the heart, breathing, central nervous system and the stomach.” The website goes on to say that children do not regard the plant as attractive to eat. Sometimes, however, children might eat the chrysalis, mistaking it for a lolly. Though this sounds alarming, there is no poisoning concern.
Due to its toxicity, there is some confusion as to whether swan plants should be allowed in classrooms with young children. Landcare Research publishes a list of plants that should not be grown or tolerated in preschool centres. It does not include swan plants on this list due to the educational value of learning about the monarch’s life cycle. The New Zealand National Poisons Centre information sheet on swan plants gives sensible advice on how to minimise the risk of poisoning when using swan plants with children.
Alternatively, use white butterfly larvae to teach about the butterfly’s life cycle. They eat cabbage, cauliflower and other brassicas, making them safer plants to keep in the classroom!
Nature of science
Scientists often provide information on matters of public concern. Public agencies then use that information to create policies.