It's tough for plants – they are sitting targets for hundreds of hungry insect mouths that can munch their way through vast amounts of vegetation daily. To help withstand the siege, some plants fight back: they store non-toxic chemicals in their cells that only become toxic once the insect has started to digest its meal, possibly its last. However, some insects have developed a clever strategy. Instead of trying to beat the plants at their own game, they have developed ways of using these chemicals to their advantage. They can store the chemicals in their bodies to warn off or poison hungry predators. Tiger moth caterpillars have a more peaceful strategy. They store these plant alkaloids for use by the females in later life: making pheromones to attract males. So the moths' reproduction and survival depends on chemicals that were meant to cause them serious harm.
However, the moth caterpillars face a problem. The toxic alkaloids that they depend on are only found in three of the 60 plant species that they regularly dine on, so it is important that caterpillars can eat and store as much of the chemicals as they can while they have the chance; once they spread their wings, the adult moths cannot feed again. Elizabeth Bernays and her colleagues have investigated how two species of tiger moth caterpillar recognise the toxic alkaloids in the plants they consume and know when to start devouring as much as they can. They wondered if the insects have specific neurons that respond to the individual chemicals and trigger feeding.
The first part of the study was carried out using behavioural experiments. The team fed the caterpillars two artificial diets: one containing the non-toxic alkaloid senecionine N-oxide and one without. The insects always ate more of the diet containing the chemicals, which showed that the alkaloid drives the insect to eat more. They then tested whether the caterpillars could be tempted to eat glass fibres that had been impregnated with the alkaloid or water. The insects munched the alkaloid-soaked fibres and ignored the others,showing that the alkaloids on their own stimulate the caterpillars to feed.
The next question the team wanted to answer was whether there are specific neurons in the caterpillars' nervous system that respond to individual chemicals. By feeding alkaloids to the caterpillars and recording the neuronal responses from the taste receptors, Bernays and her colleagues identified specific neurons in the caterpillars' nervous systems that respond to the alkaloids. They also found that the neural signals depended on the substance tested, as well as its concentration. This explains how a hungry caterpillar will know what chemical it has encountered in a plant and how much it needs to eat to pick up enough of the toxic alkaloids to ensure the future of the next generation.