[LUM#6] Asian Giant 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 them… and offers an inspiring model for building a trap for Asian hornets.
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. While hovering, it attacks them and then tears them apart on the spot to feed its larvae. Faced with this threat, our precious pollinators are defenseless. Having never encountered this Asian predator, they have not been able to develop appropriate defensive behaviors over the course of 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.
However, one 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, pitcher-shaped leaves. Some species of pitcher plants readily devour Asian hornets; the Nantes Botanical Garden discovered this by chance in 2015. In the pitchers of certain plants, up to 50% of the contents were these insects.
Leaves that look like flowers
How do pitcher plants attract and capture that famous insect from the Far East? Unraveling this secret could lead to the development of a trap modeled after the carnivorous plant’s mechanism—and thus enable the mass capture of voracious hornets.
“To conceive of such an artificial trap, one must 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 go so far as to produce nectar. The pitcher plant, more specifically, has urn-shaped leaves. Once insects are lured and trapped inside these urns, the plant simply digests them. An effective biomimetic trap would therefore replicate the plant’s decoys: its scent and appearance.
The first task, then, is to focus on scents: identifying the molecules in the scent of the pitcherplantthat attract Asian hornets.“First, we test whether the insect is attracted by the plant’s scent. We place it in a Y-shaped tube. Into one branch, we blow clean air; into the other, the plant’s scent. The hornet simply has to choose. We conduct about thirty tests, using a different individual each time,” explains Laurence Gaume-Vial (The mimicry of the scent of the traps conceals a deadly trap in the carnivorous plant Nepenthes rafflesiana, in Journal of Ecology, 2010).
Once a scent has been identified as attractive, what’s next? Identifying the molecules that trigger a response in Asian hornets! To do this, the ecologist separates the various volatile molecules that make up the scent. She then measures the insects’ reaction to each molecule at the level of their antennae. “In the mix of scents, we can thus see which specific molecules will trigger a reaction in the hornet,” says Laurence Gaume-Vial enthusiastically.
Don't harm other insects!
The second step in designing the famous trap was to replicate the visual appearance of pitcher plants. These flower-like pitchers can vary in length and width.“We chose to study four species of Sarracenia that produce pitchers starting in the spring—when hornet queens emerge—and continuing through the fall, when hornet workers attack bees. Of the four species, two capture the insect: those with elongated, narrow pitchers” (Different pitcher shapes and trapping syndromes explain resource allocation in Nepenthes species, in Ecology and Evolution, 2016).
Beyond their shape, other factors are important: their color—which can vary in redness—the extent to which they reflect UV rays, and the presence of white spots. Another factor is the number of guide hairs.
Once the elements most attractive to the Asian hornet have been identified, a challenge will arise.“We must ensure that we do not trap bees, which are the primary pollinators of our cultivated and wild plants. Nor, for that matter, other pollinators, such as flies. This is to protect ecosystems,” explains Laurence Gaume-Vial. We will therefore also need to test the various components of the trap—and in particular the scent molecules—on other insects. This is a promising line of research that could soon enable us to massively neutralize Asian hornets, and only them.
The Asian giant 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 only a single founding female to lay her eggs for the species to begin spreading and invading France within a few years. Today, the Chinese hornet is present across 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. While its sting is no more dangerous, it is more painful. Weakened colonies are its favorite prey, which is the case with declining bee colonies. To combat this insect, the only solution is to trap as many queen bees as possible—those that lay eggs and create new nests in the spring. Most beekeepers have so far relied on traditional methods. A classic example: a mixture of cherry syrup and beer in plastic bottles. Unfortunately, this method attracts only a tiny fraction of Asian hornets and kills other insects, particularly flies, posing a threat to biodiversity.
UM podcasts are now available on your favorite platform (Spotify, Deezer, Apple Podcasts, Amazon Music, etc.).