Can locust plagues be avoided by pinching the locusts' "nose"?

Produced by: Science Popularization China

Author: Insect intern (Chinese Academy of Sciences)

Producer: China Science Expo

Editor's note: In order to decode the latest mysteries of life science, the China Science Popularization Frontier Science Project has launched a series of articles called "New Knowledge of Life" to interpret life phenomena and reveal biological mysteries from a unique perspective. Let us delve into the world of life and explore infinite possibilities.

For most animals, the sense of smell is essential and closely related to their life activities.

Take us humans as an example. Once we lose our sense of smell, not only will our appetite be greatly reduced due to the lack of fragrance, but we will also lose a major early warning mechanism, because many unpleasant gases are often toxic, and inhaling too much will seriously harm our health.

Blow your nose

(Photo source: veer photo gallery)

For many other animals, especially insects , their vision is far less developed than that of higher animals. Instead, they need to rely on a complex olfactory system to identify and classify various odors in the environment in order to complete activities such as hunting and reproduction. Therefore, the sense of smell is extremely important to them.

Studying the olfactory mechanism of insects can not only give us a deeper understanding of the physiology and habits of insects, but also help us better deal with the disasters caused by these insects. For example, locusts often form locust plagues because they gather too much together.

Recent studies have found that the compound 4VA acts as an attractant to attract locusts to gather and form a plague through the locusts' olfactory receptors. This research is of great significance. For the first time, the chemical signal that induces locust plagues has been found at the microscopic level, solving a century-old problem in the field of insects. After modifying the locusts' olfactory receptors of 4VA, locusts cannot be attracted to gather by 4VA, and locust plagues will not form.

locust

(Photo source: veer photo gallery)

In addition to the above discoveries, there has been another "top discovery" in biology recently: the unique circular coding pattern of locusts' sense of smell.

The human nose is the organ of smell, but generally speaking, the antennae on the head of insects are the most important organs of smell . The surface of the antennae is covered with various types of receptors, and the receptors contain different numbers of extremely sensitive olfactory neurons. The receptors together with the olfactory neurons constitute the basic unit of insect smell.

Nerve structure of locust head

(Photo source: provided by the author)

On the dendrite surface of olfactory neurons, there are transmembrane olfactory receptors (OR) and olfactory co-receptors (Orco). Olfactory receptors OR and ionotropic receptors IR constitute the two most important types of olfactory receptors, among which the importance of olfactory receptors OR lies in the recognition of different odors.

Subsequently, the olfactory nerves with ORs transmit their axons to the antennal lobe region of the central nervous system, where they converge into a ball-shaped structure called the olfactory glomerulus. In addition, one olfactory glomerulus receives signals transmitted by one olfactory receptor neuron, thus ensuring that one odor corresponds to one olfactory glomerulus.

The spherical structure of the primary olfactory center (olfactory glomerulus) in different species

(Image source: Reference 1)

It is a consensus that these rules exist in the antennae of both fruit flies and moths.

The migratory locust has the largest known sequenced insect genome, with more than 140 olfactory receptors. According to previous rules, the migratory locust should have 140 olfactory glomeruli, which means it can only recognize 140 odors. However, the migratory locust has more than 2,000 olfactory glomeruli in the antennal lobe area , which is inconsistent with the previous rules!

Because the axon of a single olfactory sensory neuron of the migratory locust can transmit to multiple olfactory glomeruli, breaking the traditional one-to-one model, this complex neural conduction pattern makes it very difficult to understand the olfactory coding mechanism of the migratory locust, and this problem has been shelved for decades.

It was not until recently that an international collaborative project led by scientists from the Max Planck Institute for Chemical Ecology in Germany and the Institute of Zoology of the Chinese Academy of Sciences solved this problem!

Over a five-year period, they started from scratch and first used the latest gene knock-in technology to insert the calcium ion indicator GCaMP6s into the locust genome (because signals of neural activity will produce fluctuations in calcium ions, by indicating changes in calcium ions, the neural activity of organisms can be studied intuitively).

Subsequently, the researchers used two-photon microscopy to perform calcium imaging analysis on locusts at different developmental stages, recording the neural responses to various odor stimuli in the antennal lobe. They found that different types of odors activated different groups of olfactory glomeruli in the antennal lobe, which were arranged in a ring. Therefore, the form of olfactory perception of locusts is very stable during their development from small to large.

The ring-shaped distribution of olfactory glomeruli and the olfactory glomeruli of locusts at different growth stages

(Image source: drawn by the author based on references)

Further research has found that the olfactory system of migratory locusts can not only distinguish the chemical characteristics of odors, but also classify and encode them according to the ecological significance of odors. For example, locust-related odors mainly activate the peripheral areas of the antennal lobes, while plant-derived odors mainly activate the central areas. This makes us marvel at the wonders of biology. Migratory locusts actually "classify" odors with similar components in the antennal lobes and put similar odors on a ring.

In addition, one olfactory sensory neuron of the migratory locust can transmit to multiple olfactory glomeruli at the same time. This multiple transmission method further strengthens the function of the ring structure, which is equivalent to leaving multiple insurances for each smell. It is like installing multiple cameras in an area, which must be easier to capture the images people need than a single camera. Therefore, it is difficult for the migratory locust to miss every smell. This helps the migratory locust to efficiently identify important smells, such as food and natural enemies, in complex environments, thus playing a key role in foraging and hiding.

Schematic diagram of the olfactory response of locust antennal lobes to different odors

(Image source: drawn by the author based on references)

Conclusion

The locust's olfactory system is not only of great significance in biological research, but also has potential application value in the fields of agriculture and ecological protection. The swarming behavior of locusts often leads to devastating damage to crops. Understanding their olfactory mechanism can help us develop new control methods and reduce the harm of locusts to agriculture. For example, to some extent, pinching the locust's "nose" and suppressing its sense of smell can help prevent locust plagues.

This research has also promoted the development of the entire field of insect neurobiology. Although it seems that we only understand the olfactory system of locusts, in the future, with the continuous advancement of technology, we are expected to uncover the mysteries of the sensory systems of more organisms (including humans) and promote the continuous advancement of scientific research and application.

References

1. Jiang X, Dimitriou E, Grabe V, et al. Ring-shaped odor coding in the antennal lobe of migratory locusts. Cell. Published online June 13, 2024. doi:10.1016/j.cell.2024.05.036