Insect feeding leaves a detectable footprint in the soil that causes plants that grow in it to hide belowground

Robin Heinen, Madhav P. Thakur, Jetske R. Hiddes De Fries, Katja Steinauer, Simon Vandenbrande, Renske Jongen and T. Martijn Bezemer

This is a plain language summary of a Functional Ecology research article. Read the article in full here.

A caterpillar feeding on one of the experimental plants; yarrow (Achillea millefolium)

Plant communities growing in cages in the greenhouse during the experiment.

Plants and soil organisms live in close relationships and strongly depend on each other. Plants release sugars and dead material into the soil, which serve as an important resource for microscopic soil organisms. In fact, the soil organisms found in the soil may differ from plant species to plant species. These living soil footprints also influence the performance of any plant that grows in that soil afterwards, a process we call plant-soil feedback. Insect herbivores – like caterpillars – are abundant in nature, and most plants experience insect feeding over the course of their lives. When insects feed on a plant, this changes the way plants behave both above- and belowground. As a result, different soil organisms accumulate around their roots. Little is known about how these belowground consequences of insect feeding change the process of plant-soil feedback. To investigate this, we grew ten common grass and forb species in cages, and either released caterpillars, or left caterpillars out, so we could assess the effect of insect feeding. After several weeks of insect feeding, we used the soils that the plants had grown in, and used them to grow a new plant community in it. We also introduced new caterpillars to the new plant communities. With this approach, we could test how plants – and caterpillars feeding on them – responded to soils with insect footprints, compared to those growing in soils without an insect footprint. It turns out that plants recognize insect footprints in the soil, and may use it as a warning sign. On soils with an insect footprint, plants invest their energy more towards roots than towards leaves. This way, plants may partially escape the damaging effects of aboveground insect herbivores. Although such a strategy may effectively protect the plant, the insects do not seem to mind. In fact, in some cases caterpillars grow bigger on soils with an insect footprint. You might call that a win-win? The early recognition of different soil footprints by plants helps them to test local conditions, and allows them to grow in the most favorable way possible, given the circumstances.