So what does a tree like this do all day long, anyway? It stands there, rooted to the spot. The wind blows through its needles and birds alight on its branches. “You might think a tree like this is completely passive,” Beyaert says, “but that isn’t true.” In fact, a great many processes take place within the tree itself. The tree is able to respond to its environment and even has a finely tuned sense of its natural enemies. The tree “notices” hungry insects and takes action to defend itself.
Within the Applied Zoology / Animal Ecology division at the Institute of Biology of Freie Universität Berlin, Professor Monika Hilker and her research team are studying the effects of insects’ egg laying on pine trees, elms, and members of the cabbage family. What they have found surprises laypeople, at least: Trees can even defend themselves against insect eggs, preventing the larvae that hatch from the eggs from damaging the tree later on.
Natural Arsenal of Weapons
Trees possess a natural arsenal of weapons to fight their enemies. Beyaert studies Scots pines in particular. “Plants seldom have just a Plan A; there’s always a Plan B, too,” he says. Their first line of defense: toxins. “The pine tree has terpenes, which can be toxic to insects at high enough concentrations,” Beyaert explains. “Pine resin also consists of terpenes. Resin can quickly kill insects that try to attack the tree by simply swallowing them up or gluing them in place.”
Despite these sticky, resinous weapons, many insects, such as bark beetles, some types of caterpillar, and sawflies have adapted to cope with pine terpenes in various ways. Faced with these pests the pine tree counters with Plan B, its next line of defense. When sawflies, for example, lay eggs on the tree’s needles, the pine begins to defend itself against them. The eggs represent a huge danger to the tree, since the larvae hatching from the eggs would devour large amounts of pine needles. In response to the insects’ egg laying, the pine tree starts to change its scent to attract the sawflies’ natural enemies: chalcid wasps.
Communication by Fragrance
Chalcid wasps are much smaller than sawflies and are among the parasitic members of the order Hymenoptera. They lay their eggs within the eggs of sawflies – an act that seems perfidious by human standards, but is a wise choice from a biological standpoint. Since the young of the chalcid wasps are stronger than the nascent sawflies, the sawfly larvae are usually displaced and never even hatch. “The pine tree and chalcid wasp don’t act in concert out of a sense of solidarity, but instead because they can benefit each other. The tree defends itself against the sawflies by attracting parasites that prey on them, and the parasitic chalcid wasps have a breeding ground,” Beyaert says. It’s a simple win-win situation from a business perspective.
But how does this communication by fragrance work? Once the sawflies have laid their eggs in the pine needles, the tree detects the damage, which triggers an alarm system that ultimately – via a whole cascade of chain reactions – leads to the change in the tree’s scent. The tree detects the sawfly eggs by means of a secretion that sticks to them.
An Intelligent Tree
The question of a tree’s intelligence is at least as complicated as the question of human intelligence. Beyaert squirms a bit, but then formulates his own definition of intelligence: “I think intelligence in plants would mean that they can react in a complex and effective manner to unforeseen events,” he says, “so a pine tree might not be intelligent, but it is at least smart for a tree.”