Trapping the Asian hornet inspired by nature

By studying how carnivorous plants attract the Asian hornet, the Carnivespa project team is inventing an ecological trap against this predator of honey bees. This project was supported by the I-SITE MUSE as part of its 2017 research support program.

When Laurence Gaume learned that the carnivorous plants in Nantes' botanical gardens were capturing Asian hornets, she had an idea. If the trap set by the Sarracenia is so effective, why not draw inspiration from it to combat this invasive insect in Europe? "By studying the visual and olfactory lures that work specifically on Asian hornets, we will be able to create a trap targeted at capturing this insect", explains the specialist in carnivorous plants at the CNRS, who is delighted with the initial results of the Carnivespa project she has been leading for the past three years within the UMR AMAP.

So far, the means of combating this hymenoptera have been inconclusive. Probably arriving by boat from China in 2004, the Vespa velutina hornet has since colonized the whole of France(http://frelonasiatique.mnhn.fr). This rapid expansion is not surprising when you consider the animal's biology: nests can exceed 10,000 individuals, releasing hundreds of future queens ready to found new colonies. The hornet is a formidable predator of honeybees. Hovering in front of hives, it decimates some of the foragers who leave the hive and paralyzes others who dare not leave. In just a few years, the Asian hornet has become a pest species in Europe.

A crack team

To create her biomimetic trap, ecologist Laurence Gaume surrounded herself with a crack team. A chemist specializing in volatile organic compounds (VOCs), Jean-Marie Bessière, is in charge of identifying the substances emitted by plants. A specialist in visual communication from the Centre d'Ecologie Fonctionnelle et Evolutive (CEFE), Doris Gomez, is analyzing how visual signals from plants are perceived by hornets. Two hornet entomologists from the Museum national d'histoire naturelle (MNHN) and a bee specialist from the Institut national de la recherche agronomique et de l'environnement (Inrae) complete the team.

To complete this multi-disciplinary group, Laurence Gaume has added a beekeeper, Jean-Luc Delon, President of the Groupement de défense sanitaire apicole de l'Hérault, and a specialist in the cultivation of carnivorous plants, botanist and nurseryman Jean-Jaques Labat. They will contribute to the first experimental stage of the project: setting up six sites on which carnivorous plant peat bogs - and beehives - will be placed. Every two months, the researcher collects the urns of carnivorous plants, whose contents are analyzed by entomologist Claire Villemant. Eighteen surveys later, two species of Sarracenia stand out from the crowd, with 70% of their urns trapping hornets in autumn. This performance is confirmed by terrarium observation, which shows that these plants receive more hornet visits than expected, but that these visits do not necessarily result in capture. These plants are therefore very attractive, even if trapping is imperfect.

Hornet-specific lures

But there's a problem. The urns contain insects other than hornets. The team is therefore seeking to identify plant lures that would specifically attract hornets, to prevent their trap from harming other insects. To study visual lures, Doris Gomez measures plant reflectance (the wavelengths reflected). She then cross-references this parameter with the Hymenoptera's vision system. This modeling enables her to characterize the role of certain colors and contrasts in attracting hornets. "This may enable us to rule out insects with other vision systems", hopes Laurence Gaume.

With regard to olfactory signals, J-M Bessière identified a wide variety of VOCs in the odors emitted by plants, but the majority were terpenes. Laurence Gaume and Corentin Dupont, the PhD student recruited for the project, then tested these VOCs to identify those responsible for attracting hornets. However, some of them also attract bees, which should not be targeted. The researchers then turned their attention to the neurosensory sensors on the insects' antennae. They studied the response of these sensors to the plant's VOCs, which they measured using electroantennography (a technique based on the measurement of electrical signals from the antennae): "We found seven molecules that are specifically recognized by the hornets' antennae and not by those of the bees. We now need to check that these molecules do indeed have an attractive effect", explains the project leader, who hopes to find the most selective lures by the end of 2021 to create "an effective and ecological trap".