A tomato that has optimised its defence mechanism against insects will not only suffer less damage from caterpillars, but will also incite caterpillars to eat one another. This is a defence tactic that, until it was brought to light by a recent study conducted at the University & Wisconsin, has been hitherto unknown.

When tomato plants are attacked by insects, they produce antibodies in order to protect themselves. They actually start doing this as soon as other plants in their direct vicinity are attacked. This mechanism is activated when they detect volatile substances emitted by plants that are being eaten by predators. Researchers from the University of Wisconsin, led by John Orrock, sprayed tomatoes with volatile substances like these with a view to activating what is referred to as the jasmonic acid route. This defence route is activated to keep insects at bay.

Remarkable results

The American scientists wanted to investigate the details of this defence mechanism. They wanted to discover if the plants only ward off the insects (because they are unappetizing or toxic) or produce another detrimental effect on them (e.g. by inhibiting reproduction). The study was conducted with caterpillars of the small mottled willow moth (Spodoptera exigua). The results were remarkable: not only did the plants sprayed with these substances produce five times as much biomass, because they had barely been eaten by insects at all, but the caterpillars began to feed on one another As a result, their population shrunk notably.

Finding the right balance

It is known that caterpillars have a tendency towards cannibalism when faced with food scarcity. The researchers believe that the low nutrient value of plants in full combat mode triggered the increased instances of cannibalism. The effect of the altered composition of the plants has, until now, never been investigated. These findings have been published in Nature Ecology & Evolution. In an accompanying interview, John Orrock points out that the cost to the plant of activating its defences is very high: the plant has to invest heavily at the expense of its growth. He believes that plants will always strike a balance between maximum defence and accepting minor damage from predators.

Text: Tijs Kierkels.