Scientists have discovered a new function of this substance that can induce locusts to gather and form disasters

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.

Speaking of locusts, everyone should be familiar with them. They are insects that jump around in city lawns or grass in country fields in summer and autumn, and sometimes fly quickly and low. They are commonly known as "grasshoppers" and their scientific name is Locust migratoria (there are many kinds of grasshoppers, and Locust migratoria is just one of them).

However, in the early years, when our parents and grandparents lived, people were terrified at the mention of the word "locust". This can be seen from the creation of the character. The character "locust" is a combination of "insect" and "emperor". The ancients were helpless in the face of the overwhelming locust plague. They believed that locusts were the king of insects and even built locust temples to worship them and pray that there would be no locust plague.

locust

(Photo source: veer photo gallery)

Have you ever wondered: How can a tiny locust become a disaster? Why have we never heard of a serious locust plague in China in decades? Is the rumor of a locust plague an exaggeration?

The answer is that locust plagues are not alarmist, but real. The reason why China has not heard of a large-scale locust plague is that after the founding of the People's Republic of China, locust plagues have been basically eliminated through scientific management. So it may be difficult for everyone to understand the scenes of locust plagues and people's miserable lives described in film and television dramas. However, locust plagues in history are known as the three major natural disasters along with floods and droughts. In ancient times, locusts flying all over the sky would eat up the people's harvest for a year, and even all green leafy plants were not spared. China has been plagued by locust plagues for thousands of years and is one of the countries with the most serious disasters in the world!

Therefore, in order to prevent problems before they occur, our country has been conducting research on locust control, and has long gone beyond statistical classification and morphological observation, but has gone deep into the molecular field and even is at the forefront of the world.

What attracts locust swarms?

After observation and research, scientists have found that these scattered green locusts in the grass do not constitute a natural disaster, but are an important part of the ecosystem. However, when they gather together, they become a disaster. So, what exactly makes the originally harmless locusts gather together and become a disaster?

This problem has puzzled scientists for nearly a hundred years since 1921 when the father of locust science, Igor Yuvalov, discovered that locusts exist in two forms: solitary (not a plague) and gregarious (a plague).

Solitary and gregarious locusts

(Image source: Reference 1)

It was not until 2020 that Kang Le, an academician of the Chinese Academy of Sciences, led a team from the Institute of Zoology of the Chinese Academy of Sciences, and finally discovered the chemical substance that attracts locusts to gather in disasters through rigorous and meticulous research: 4-vinylanisole (hereinafter referred to as 4VA). The compound 4VA acts as an attractant to attract locusts to gather in disasters through the locusts' olfactory receptors.

This research is of great significance. For the first time, the chemical signals that induce locust plagues have been found at the microscopic level , solving a problem that has lingered in the insect field for a century. After modifying the 4VA receptors of locusts' sense of smell, locusts can no longer be attracted and gathered by 4VA, and locust plagues will not form.

Another identity of 4VA that caused locust swarms

At this point in the research, the mystery of locust aggregation has been revealed. However, continued research on 4VA has revealed its other identity.

First, a large number of studies have shown that social animals often exhibit a high degree of reproductive synchronization , that is, individuals in the group have the same or similar reproductive cycles.

This characteristic has many benefits for animals to survive in nature. Synchronous sexual maturity and reproductive cycles are conducive to the maintenance of groups and collective migration. Synchronous breeding can greatly improve the individual mating success rate and offspring survival rate within the animal group, reduce the risk of individual predation, and enhance the environmental adaptability of group animals. It is one of the core biological characteristics of animal group behavior.

However, how does this reproductive synchronization occur? This question has always been of concern to scientists.

At present, in addition to the influence of external climate and biological rhythms, more and more studies have found that information exchange between individuals also plays a role in reproductive synchronization.

For example, biologist Martha McClintock first proposed the "White" effect in 1971. Her research focused on observing and analyzing the synchronization of physiological cycles among female mice. She proposed that chemical signals released by males may affect the physiological cycles of females and induce synchronization of physiological cycles of females in the group. This theory was later extended to other mammals, but the underlying mechanism is still unclear.

Locust laying eggs

(Photo source: University of Chinese Academy of Sciences News Network)

Based on the above research background, the reproductive synchronization of gregarious locusts is a good research direction.

Therefore, the team of researcher Wang Xianhui and the team of Academician Kang Le cooperated and discovered that the locust aggregation pheromone (4-VA) also has the function of promoting the synchronous sexual maturation of female individuals.

First, scientists analyzed the oocyte development and first egg-laying time of gregarious and solitary females, and found that gregarious females matured earlier and more consistently than solitary individuals, and laid eggs more centrally. By controlling for the variable of sex, they further found that the synchronization of sexual maturity of female locusts was induced by chemical volatiles on the surface of gregarious males, while volatiles on the surface of gregarious females and solitary males could not induce synchronous reproduction in females.

When female locusts that have lost their sense of smell are stimulated with the odor emitted by gregarious males, the phenomenon of synchronous sexual maturity does not occur. These experiments prove that an olfactory signal released by gregarious male locusts induces synchronous sexual maturity in female locusts.

So what exactly is this olfactory signal?

By analyzing and comparing the composition and content of volatiles on the body surface of gregarious males, gregarious females, and solitary male adults, five volatiles with high release levels in gregarious male adults were identified. After testing the effects of the five volatile odors on the synchronous sexual maturation of females one by one, it was found that only 4-vinyl anisole (4-VA) had a significant effect.

Surprisingly, testing female individuals of different ages found that 4-VA could promote sexual maturity in young females, but had no significant effect on older females (5-6 days after eclosion).

However, the results of the antennal nerve response experiment showed that there was no difference in the olfactory perception of 4-VA among female locusts of different ages, indicating that the age difference in this sexual maturation-promoting effect was not caused by differences in olfaction.

Locust on plant

(Photo source: veer photo gallery)

So what mechanism affects the age difference in the effect of 4-VA on sexual maturation?

Further studies found that the genes for juvenile hormone (JH) metabolism in the lateral pharyngeal bodies (similar to neuroendocrine glands) of young females treated with 4-VA underwent significant changes, and the content of JH in the body increased, but there was no change in older females.

Experiments were conducted using mutant locusts that had lost their sense of smell. They were artificially injected with JH 3-4 days after emergence, and the synchronization of sexual maturity of mutant females was improved. Because JH is a key endocrine hormone in regulating locust reproduction, it is juvenile hormone (JH) that causes the difference in sensitivity to 4-VA in female individuals of different ages.

Mechanism of high volatile 4-VA in social male adults promoting synchronization of female sexual maturation

(Image source: Reference 2)

Conclusion

In the world of migratory locusts, at certain times, the smell of "4-VA" emitted by males activates the JH system of female adults, accelerating the sexual maturity of young females. Through this "catch-up" strategy, the sexual maturity synchronization of the entire group is achieved. Obviously, the functional diversity of 4-VA shows that this compound plays a very important role in the formation and maintenance of large-scale migratory locust groups. To eliminate locust plagues, we must treat the symptoms and the root causes. The magical chemical 4VA brings us hope, because by inhibiting it, we can not only prevent the contemporary locusts from gathering and causing disasters, but also destroy the consistency of locust growth and development, making the next generation of locusts scattered and less likely to cause disasters.

This research result also tells us that in the process of exploration and discovery, we must dare to imagine and draw inferences from one example to another, and that old friends may have new faces.

References:

1. Guo, X., Yu, Q., Chen, D. et al. 4-Vinylanisole is an aggregation pheromone in locusts. Nature 584, 584–588 (2020).

2. Dafeng Chen, Li Hou, Jianing Wei, Siyuan Guo, Weichan Cui, Pengcheng Yang, Le Kang, Xianhui Wang (2022) Aggregation pheromone 4-vinylanisole promotes the synchrony of sexual maturation in female locusts eLife 11:e74581