The European oil beetle’s parasitic larvae emit a floral scent that attracts wild bees. This allows the larvae to sneak into the bees’ nests
A cluster of European oil beetle (Meloe proscarabaeus) larvae on a blade of grass: Not only could they be mistaken for a flower at first glance, but to wild bees, their smell cannot be distinguished from the scent of many flowers.
© MPI f. chemische Ökologie/ Danny Kessler
To the Point
- A new form of chemical mimicry: Researchers have shown for the first time that insects can mimic floral scents to attract and trick pollinators. The attracted bees carry the larvae into their nests, where the larvae exploit the bee eggs and food reserves intended for the bees’ offspring for their own development.
- Beetle larvae with floral scent: The larvae produce the monoterpenoids themselves, rather than sequestering them from the environment.
- Enzymes at the heart of the deception: Two cytochrome P450 enzymes are crucial for the biosynthesis of the flower-like scents.
The female European oil beetle (Meloe proscarabaeus): These flightless, highly poisonous beetles are hard to miss during spring walks through fields and meadows. In Germany, they are commonly known as “Maiwurm” (May worm) because they appear in the spring and the females, which can grow up to 35 mm long, have a swollen abdomen.
© Danny Kessler, Max Planck Institute for Chemical Ecology
The European oil beetle’s life cycle is truly spectacular. The tiny larvae of this species of blister beetles must find their way into the nests of solitary bee species to survive. There, they feed on the bee’s eggs and food stores, which are intended for the bee’s offspring. Since the survival rate of the individual larvae is extremely low, a female oil beetle lays several thousand eggs to ensure the survival of the species.
To gain access to a bee nest, the larvae rely on deception. Clustered together on a blade of grass, they could be mistaken for a grass flower. Researchers at the Max Planck Institute for Chemical Ecology have discovered that chemical mimicry also plays a role in this hitchhiking strategy to reach a bee’s nest.
From hypothesis to chemical detective work
Grey-backed mining bee (Andrena vaga) on the tip of a cotton swab to which a drop of the monoterpenoid mixture emitted by the larvae of the European oil beetle was added. Field experiments using this odor and a control group confirmed the attractant effect of the larval scent.
© Danny Kessler, Max Planck Institute for Chemical Ecology
For the research project on the black-blue oil beetle, the team led by Ryan Alam and Tobias Köllner from the Department of Natural Product Biosynthesis was inspired by earlier research showing that a related oil beetle species of the same genus from North America, Meloe franciscanus, attracts a specific host bee species by mimicking the bee’s sex pheromones. “We wondered whether its European relative Meloe proscarabaeus -whose larvae accumulate on plants and apparently use a wide range of wild bee species as hosts-employs a similar strategy or a completely different form of chemical deception,” says study leader Tobias Köllner.
To solve the mystery, the researchers collected oil beetles in the Jena area in the early spring. They allowed the beetles to mate and lay eggs. After about three weeks, the larvae hatched, and the team studied the volatile compounds they emitted. Chemical analyses revealed that the larvae release 17 different monoterpenoids. These are well-known floral aromas, similar to those emitted by plum and cherry blossoms. The flowers bloom when the tiny oil beetle larvae are waiting for the bees to arrive. Behavioral experiments in an olfactometer confirmed that bees prefer the larvae’s scent to that of the control samples, even when the larvae were not visible. This proves that attraction is primarily based on chemistry. Field experiments in which the research team applied isolated monoterpenoids to cotton swabs to test their effect on wild bees revealed that Andrena sand bees frequently flew toward the tip of the cotton swab. This observation confirms that the volatile monoterpenes are sufficient to attract the larvae’s potential hosts.
Oil beetle larvae produce the floral scent themselves
The team then wondered how the larvae acquired the volatile substances that smell like flowers and are thought to provide them with a ticket to the bee nest. Was this the result of sequestering plant compounds, or did the mother beetle transfer them during egg-laying? Or do the larvae produce the substances themselves?
The scientists were unable to detect any flower-like monoterpenoids in the eggs or beetles. Through genetic analyses, the team finally succeeded in demonstrating that two specific cytochrome P450 enzymes in the larvae-CYP347BT1 and CYP345BZ1-produce the biosynthetic precursors for the formation of the flower-like substances. Therefore, the larvae produce the odor compounds de novo, and the identified enzymes are the key to this chemical deception.
A new paradigm of mimicry
Andrena bee (Andrena sp.) on a flower. Several orange-colored oil beetle larvae have clung to its body, hoping to make their way into the bee nest. There, they intend to feed on bee eggs and food stores meant for the bee’s brood and continue their development into an adult beetle.
© Danny Kessler, Max Planck Institute for Chemical Ecology
“This discovery describes a previously unknown form of mimicry and expands our understanding of how parasites use chemical signals to manipulate other organisms,” said lead author Ryan Alam. The study is the first to demonstrate how an animal produces chemical signals to mimic a plant’s identity.
The scientists believe that the oil beetle larvae exploit the chemical attractiveness of flowers to parasitize various bee species. “By mimicking a resource that attracts many different species of wild bees, the larvae are able to reach a broader range of potential hosts,” says Ryan Alam.
This discovery illustrates the complexity of the relationships between living organisms: Animals that mimic plant signals to deceive other animals not only influence food webs but also shed light on the complex communication structures in natural ecosystems. “Our next goal is to elucidate the entire biosynthetic pathway of these volatile compounds,” said Sarah O’Connor, head of the Department of Natural Product Biosynthesis. “To that end, we also plan to study related beetle species to understand how widespread this strategy is and how it has evolved.”
