Where did the warning color of locusts come from?

Produced by: Science Popularization China

Author: Insect intern (PhD in Biology, Chinese Academy of Sciences)

Producer: China Science Expo

When it comes to animals that change their body color according to the environment, one animal that comes to mind is the chameleon.

chameleon

(Photo source: veer photo gallery)

It adapts to the surrounding environment and avoids predation by quickly changing its body color. This protective color is a way for many small animals to protect themselves.

I believe you must have noticed that the locusts and mantises we often see in the wild are green. This is also a protective color that allows them to blend into the surrounding green plants and avoid being discovered by natural enemies.

Can locusts change color?

However, to our surprise, when locusts gather together and the population density is high, their body color will slowly develop a strange pattern of black on the back and brown on the belly .

Most of the time, this seemingly insignificant change foreshadows the coming of a locust plague. Therefore, scientists have long studied how the green solitary migratory locust transforms into the black and brown gregarious migratory locust. The answer to this question not only satisfies the curiosity of scientific research, but also provides a multi-angle analysis of the formation mechanism of locust plagues, which is of great significance for the control of locust plagues.

The fifth instar gregarious and solitary migratory locust

(Image source: References)

The change in locusts' body color from scattered green to gregarious black and brown reflects their wonderful wisdom in adapting to the environment.

Because when natural enemies such as birds encounter locusts of these two colors, they tend to prey on green locusts and avoid the conspicuous black-brown locusts, because the contrasting black and brown colors are warning colors in the eyes of predators.

When the tiny locusts are alone and weak, they use green as a protective color, while when they are numerous and powerful, the gregarious locusts will change into black and brown warning colors.

Comparison between gregarious and solitary locusts

(Photo source: Voice of Chinese Academy of Sciences)

Seeing this, I believe that you have the same doubts as scientists: What exactly caused this change? How did the black color on the back of the locust come about?

Why can gregarious locusts change color?

In fact, as early as a few years ago, the team of Academician Kang Le discovered that the black color on the back of the gregarious locust is not ordinary melanin deposition, but a red substance formed by β-carotene binding protein (βCBP) and β-carotene superimposed on the green skin of the solitary locust.

This color change from green to black is completely in line with the law of the three primary colors in physics. It seems that there is a "palette" in the locust's body. The beta-carotene binding protein acts as a "brush" and dips red "paint" to paint on the green locusts, and their color turns black. The color matching rules of the three primary colors in physics are perfectly interpreted in the locust's body color change.

Three primary colors

(Photo source: veer photo gallery)

However, in addition to the black back, the ventral side of the gregarious locusts is brown, and the formation and mechanism of this obvious black-brown warning color pattern were not understood at the time.

In order to explore the formation mechanism of the black-brown warning body color of locusts, the team led by Academician Kang Le of the Chinese Academy of Sciences searched for genes related to the formation of warning body color and analyzed the composition of black and brown cuticular proteins of gregarious locusts. They found that it was still this old friend: βCBP that plays a key role in the formation of the black-brown body color of locusts.

Specifically, it is the different distribution contents of βCBP-β-carotene complexes that determine the difference in body color between the black back and brown belly of the locust, among which the distribution amount of βCBP-β-carotene in the brown epidermis is higher than that in the black epidermis.

Migratory locust

(Image source: Chinese Academy of Sciences)

What influences the color change of gregarious locusts?

Many people would stop their research at this point, but the scientists' thirst for knowledge and spirit of exploration prompted them to ask: What causes the difference in the distribution of βCBP in locusts? Through step-by-step screening, they found a regulator, ATF2.

The scientists also found that ATF2 is mainly distributed in the cytoplasm of black epidermis, while it is mainly distributed in the nucleus of brown epidermis. This difference in localization is caused by phosphorylation of ATF2 serine sites.

Through various experimental studies, scientists have found that the PKC signaling pathway can phosphorylate an important serine in ATF2 and promote the transcription of βCBP. When they reduce PKC through experiments, the production of βCBP is inhibited, and the result is that the body color of the gregarious locusts changes from black-brown warning color to green protective color .

This regulatory mechanism is closely related to the population density of locusts. As the population density of locusts increases, the body color of locusts will also change. Among them, the intrinsic regulatory process is that PKC activates phosphorylated ATF2 Ser327 after sensing the high population density, allowing ATF2 to enter the cell nucleus, bind to the βCBP promoter and activate transcription.

The difference in phosphorylation levels of ATF2 in the black and brown epidermis leads to different distribution amounts of βCBP on the dorsal and ventral surfaces, ultimately causing the gregarious locusts to present a warning body color of black back and brown ventral surface.

The regulation mechanism of black-brown body color in migratory locusts

(Image source: References)

Do you find it amazing when you see this?

Although locusts are small, the study of their body color has important biological significance.

Many insects have green body coloration, which is similar to the locust, which is formed by a combination of yellow and blue. However, the formation of black body coloration varies, but is mostly due to the deposition of melanin and eye pigment.

The way locusts form a black color by combining the red complex formed by βCBP and β-carotene with other pigments is truly refreshing.

The differences in the location distribution of βCBP expression and ATF2 phosphorylation in gregarious locusts form signal instructions, thus producing the contrasting warning colors of black dorsal surface and brown ventral surface.

Nature is so magical. Not only can we use the three primary colors to "paint" on locusts, but we can also control the amount of "pigment" to produce different contrast effects. This is the survival strategy of locusts to adapt to the environment and defend against natural enemies. Not only that, the striking black and brown body color of gregarious locusts is also a signal for them to identify "friendly" species. After all, only by distinguishing between the enemy and us can we avoid riots and maintain these huge locust swarms of tens of thousands.

Conclusion

In the insect world, it is a common phenomenon that insects' body color darkens as population density increases, but this is almost always caused by black pigments produced directly in the body. Therefore, this first-time discovery of the "three-primary color palette" law is of great significance in revealing the changes in body color under different population densities, and has important practical value for pest prediction.

References:

【1】Xinle Kang et al. ,Spatially differential regulation of ATF2 phosphorylation contributes to warning coloration of gregarious locusts.Sci. Adv.9,eadi5168(2023).