A poison as an indicator of food

The results. published in the journal Ecology Letters, open up new avenues for the control of this herbivorous pest.

The western corn rootworm is a real problem for farmers. In the USA, this pest causes about one billion dollars in damages and control measures per year. To this amount a half billion euros yearly can be added on the European continent, which was only recently invaded by the pest. The researchers in Neuchâtel, in collaboration with six other laboratories across the world, have found an explanation for the voracious behaviour of the rootworm on maize. "The plant places its defence priorities on tissues that are valuable to it," states Christelle Robert, PhD student in the FARCE laboratory under the supervision of Ted Turlings.

"These are the most nutrient rich plant organs that are also coveted by herbivorous insects. Logically, it is in these organs that the plant produces the highest concentrations of toxins used to repel or poison pests that feed off them." This strategy can also be seen in maize roots. The most precious of these are called crown roots, which form a crown in the soil that contain valuable nutrients for larvae in search of food.

"Compared to primary and secondary roots, crown roots are richer in sucrose, a substance known stimulate the appetites of insects. They also contain more proteins and free amino acids, of which certain are essential to insects," explains Christelle Robert.

To protect its tissues, maize relies on a fleet of toxic substances that are in most cases very effective. "We showed this in tests with generalists herbivorous insects that can also feed on other plants, specifies the young biologist. However, when we tested the effect of these toxins on corn rootworm larvae, we found something quite different."

The experiments, which were developed by Christelle Robert and Matthias Erb, who is currently at the Max Planck Institute for Chemical Ecology in Jena (Germany), revealed that the larvae not only tolerate the poison, but they even use it to locate the zones rich in nutritional substances, in this case crown roots. Hence, the insects use the toxins, which are supposed to deter them, to their benefit. It is the first time that such a phenomenon has been observed at the plant root level.

To arrive at this surprising conclusion, the researchers made use of a mutant maize plant that is incapable of producing the toxins in question. They offered the rootworm larvae a choice between roots from plants that can produce the toxic substances and those that cannot. Against all expectations, the larvae clearly preferred the plants with an active defence mechanism. Moreover, they observed the larvae on the toxin producing plants would settle on a plant within three hours and would fix themselves to the crown roots permanently. On the other hand, on non-toxic plants, the larvae seemed disoriented and wandered continuously from one type of root to the other.

"We have therefore concluded that the toxins are an indispensable indicator for rootworm in search of food. We also showed that the larvae that fed on crown roots developed better than larvae feeding on primary or secondary roots," states Christelle Robert.

The researchers' next step is to uncover the mechanism that enables the larvae to tolerate the poison. The most promising option are enzymes that the insect produces to detoxify the plant's defence compounds.