[LUM#6] Asian hornet vs. carnivorous plant
The Asian hornet is invading France and Europe, threatening our already struggling bee populations. How can we combat it? The pitcher plant, a carnivorous plant, knows how to attract and capture it... and provides an inspiring model for making an Asian hornet trap.
A new threat looms over our bees... After pesticides and parasites, here comes the Asian hornet. This cousin of our European hornet, which arrived in France less than fifteen years ago, is wreaking havoc on bee colonies. It hovers in mid-air, attacks them, and then dismembers them on the spot to feed its larvae. Our precious pollinators are helpless against them. Having never encountered this Asian predator before, they have not been able to develop appropriate defense behaviors during their evolution. As a result, already in decline and yet crucial to our ecosystems, they are easy prey for this hornet, which is new to our region.
Against him, an exotic plant offers hope. Its name: the pitcher plant. Native to North America, this plant is carnivorous. It feeds on insects thanks to its brightly colored, urn-shaped leaves. Some species of pitcher plants are particularly fond of Asian hornets, as the Nantes Botanical Garden discovered by chance in 2015. Up to 50% of these insects were found in the pitchers of certain plants.
Leaves that mimic flowers
How do pitcher plants attract and capture the famous insect from the Far East? Unraveling this secret would enable us to create a trap inspired by the carnivorous plant. And thus, catch voracious hornets en masse.
"To imagine such an artificial trap, you have to understand that carnivorous plants attract their prey by making their leaves look like flowers," explains Laurence Gaume-Vial, a specialist in plant-insect interactions at the AMAP laboratory (botany and modeling of plant and vegetation architecture). The leaves of carnivorous plants take on the colors and scents of flowers, or even fruits, and even produce nectar. Pitcher plants, more specifically, have urn-shaped leaves. Once insects are attracted and trapped in these urns, the plant simply digests them. An effective biomimetic trap would therefore replicate the plant's lures: its smell and appearance.
The first task, therefore, is to identify the molecules in pitcher plant scents that attract Asian hornets. "First, we test whether the insect is attracted to the plant's scent. We place it in a Y-shaped tube. In one branch, we blow clean air. In the other, we blow the plant's scent. The hornet then has to choose. We carry out around thirty tests, using a different individual each time," explains Laurence Gaume-Vial (The mimicry of the scent of masks masks a deadly trap in the carnivorous plant Nepenthse rafflesiana, in Journal of Ecology, 2010).
Once a scent has been identified as attractive, what can be done with it? Identify the molecules that trigger a reaction in Asian hornets! To do this, the ecologist separates the different volatile molecules that make up the scent. She then measures the insects' reaction to each molecule at their antennae. "In the bouquet of odors, we can see which specific molecules will trigger a reaction in the hornet," says Laurence Gaume-Vial enthusiastically.
Do not harm other insects!
The second task in designing the famous trap was to reproduce the visual appearance of pitcher plants. These flower-like pitchers can vary in length and width. "We chose to study four species of pitcher plants that produce pitchers from spring, when hornet queens emerge, until fall, when hornet workers attack bees. Of the four species, two capture insects: those with long, narrow pitchers" (Different pitcher shapes and trapping syndromes explain resource allocation in Nepenthes species, in Ecology and Evolution, 2016).
Beyond their shape, other parameters are important: their color, how red they are, how much UV light they reflect, and whether they have white spots. But also the number of guard hairs.
Once the elements most attractive to Asian hornets have been identified, a challenge will arise. "We must be sure not to catch bees, which are the main pollinators of our cultivated and wild plants. Nor should we catch other pollinators, such as flies. This is in order to protect ecosystems," explains Laurence Gaume-Vial. The various components of the trap, particularly the scent molecules, will therefore also need to be tested on other insects. This is a promising avenue of research that could soon enable the mass neutralization of Asian hornets, and only Asian hornets.
The Asian hornet, a rapid invasion
The first Asian hornets arrived in France in 2004 in pottery imported from China in the Lot-et-Garonne department. It took just one female to lay eggs for the species to begin spreading and invade France within a few years. Today, the Chinese insect is present in three-quarters of the country and in certain regions of neighboring countries and Portugal. It is officially classified as an invasive and harmful exotic species.
How can you spot it? Its legs are yellow, and it is dark with a few yellow stripes, unlike its European cousin, which is yellow, brown, and red. Although its sting is no more dangerous, it is more painful. Weakened colonies are its favorite prey, which is the case for declining bee colonies. To combat this insect, the only solution is to trap as many female founders as possible, those that lay eggs and create new nests in the spring. Most beekeepers have so far used traditional techniques. A classic method involves mixing syrup and beer in plastic bottles. Unfortunately, this technique only attracts a tiny proportion of Asian hornets and kills other insects, particularly flies, which poses a threat to biodiversity.
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