Trees call for help from birds and predatory insects
Forest trees emit scents when attacked by caterpillars and other herbivores. They use these to attract predatory insects and even birds, thus getting rid of their pests. This had only been demonstrated in laboratory or garden experiments so far. A team of researchers led by Martin Volf from the Biology Centre, Czech Academy of Sciences, could now demonstrate this phenomenon for the first time in a natural habitat – in the 40-metre-high canopy of the Leipzig floodplain forest. The chemical calls for help are so effective that they significantly determine the composition of the insect community in the canopy. This knowledge could be used in future for natural pest control in agriculture and forestry, the researchers write in the journal Ecology Letters.
Yes, trees can talk too. However, not acoustically like us, but via scents. Like a human fingerprint, each tree species emits its very own bouquet of volatile organic compounds (VOC). Animals learned to read this pattern in the course of evolution; leaf-eating insects find their host trees this way. But the trees are not helplessly at their mercy; they defend themselves: E.g., they produce bitter substances, which the herbivores do not like. In addition, the trees release particular VOCs to put different parts of the plant on the alert. In this way, however, they also attract other animals such as birds and predatory insects, which have also learned to read the VOC's meaning. They come flying to devour the tree's "pests". From the trees' perspective, evolution has provided them with a kind of "emergency call".
"The fact that plants can chemically attract parasitic wasps, predatory bugs and even birds when attacked by pests had been known for some time," says first author Dr Martin Volf, who led the study. "However, this defence mechanism has never been tested for adult trees in a realistic environment so far. This was made possible by a combination of research methods, from animal behaviour experiments at the height of 40 metres on iDiv's Leipzig Canopy Crane to chemical analyses of plant scents through metabolomics," says the biologist. Metabolomics systematically study the unique chemical fingerprints that specific cellular processes leave behind.
Caterpillar of the brown-tail moth (Euproctis chrysorrhoea) is one of the many species of herbivores that commonly occur on oaks. (Picture: T. Volfová)
Plastic caterpillars revealed the attackers in the branches of massive oaks
The ecological experiment took place in a floodplain forest in Leipzig on an area of 1.65 hectares for about one month during the leaf sprouting period. The researchers selected eight sturdy oaks. To test the effect of induced defence on predators, the researchers chemically simulated herbivore feeding on leaves by spraying selected branches with methyl jasmonate, a plant hormone involved in triggering plant defences. They also placed plastic caterpillar dummies on these leaves. Then, they observed a cascade of ecological processes - how the chemical composition of the leaves changed, what VOCs the trees began to emit into the air, which predators responded to these signals and attacked the caterpillars, and finally, how the abundance and composition of insects in the treetops changed. In the laboratory, they then offered leaves from induced and non-induced branches to moth caterpillars to verify that the caterpillars really did not like them.
The researchers found that predators such as birds, parasitoid wasps, predatory bugs and ants visited experimentally induced branches more often than untreated ones. The number of oak caterpillars was also significantly lower there. The most common attackers were predatory insects. In the feeding test, the caterpillars of the gypsy moth (Lymantria dispar) avoided induced leaves, indicating that the trees produced repellent substances.
Scientists conducting the experiment at the height of 40 metres on iDiv's Leipzig Canopy Crane. (Picture: T. Volfová)
Martin Volf sticks a plastic caterpillar dummy on the branch. (Picture: T. Volfová)
Chemical defence is a major driver of insect species composition
The feeding-induced chemical defence turned out to be the most critical control mechanism of the species composition of insects in tree canopies due to the new study. At the same time, the results reveal the complexity and interdependence of ecological processes between plants and animals. The study is also an excellent example of successful integrative research, as it brings together very different disciplines, such as ecology, entomology, plant physiology, and analytical chemistry.
Researchers from the German Center for Integrative Biodiversity Research (iDiv), the Friedrich Schiller University Jena and Leipzig University also participated in the research. "These findings can help us find alternative, natural strategies for pest control in agriculture and forestry and thus implement the plan to save pesticides," adds senior author Prof Nicole van Dam, head of the Molecular Interaction Ecology research group at iDiv and the University of Jena.
The caterpillars of the gypsy moth (Lymantria dispar) avoided induced leaves in the laboratory experiment. (Picture: M. Volf)
Volf, M., Volfová, T., Seifert, C. L., Ludwig, A., Engelmann, R. A., Jorge, L. R., Richter, R., Schedl, A., Weinhold, A., Wirth, C. & van Dam, N. M. (2021): A mosaic of induced and non-induced branches promotes variation in leaf traits, predation and insect herbivore assemblages in canopy trees. Ecology Letters, DOI: 10.1111/ele.13943