Yellow color provides more efficient mating in locust swarm

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 Jozef Vanden Broeck
Jozef Vanden Broeck

An international team of researchers led by Professor Jozef Vanden Broeck (KU Leuven) discovered why adult male desert locusts in a swarm have a yellow color, in contrast to their brownish, female counterparts. This color difference ensures that male locusts can recognize fertile females during mating in a large swarm and avoid ending up with another male or with an already formed pair. The research results are published today in the scientific journal PNAS.

Chameleons, frogs, octopuses, polar foxes, spiders, seahorses.... all can adapt the color of their skin or fur to the environment. In this way, they are perfectly camouflaged and are not noticed by other animals. Young desert locusts also appear in different colors, although there is a different explanation for this. When they are born in a small population, they spread out in the landscape and - like many other animals - develop a camouflaging color that makes them unnoticeable in their environment. Young grasshoppers that end up in a large group, however, become very distinctive yellow and black, similar to the colors of a wasp. This color pattern has previously been shown to keep natural enemies at bay.

Strikingly, young locusts that grow up scattered in the landscape develop into inconspicuous brownish adult males and females. However, among their conspecifics that occur in large groups and eventually form immense swarms, a clear distinction can be made between brownish, fertile females and bright yellow, fertile males. This observation occurred as early as 1921, when it was discovered that desert locusts usually occur solitarily, but will form huge groups when habitat conditions are favorable. In contrast, the underlying reason for the striking yellow coloration of adult males living in large groups remained unknown for another 100 years.

Color change is often used by animals as a protective mechanism to hide from predators. The fact that adult males with their yellow color just make themselves stand out in groups is an unusual phenomenon. So there had to be an important reason for it.

Professor of animal physiology Jozef Vanden Broeck

Recognition during mating in swarm

The results show that male locusts have a hard time distinguishing between female conspecifics and males during mating in which the production of the protein responsible for the yellow color is blocked. With the yellow color as a visual signal, causing adult males to avoid each other, this process does appear to be very efficient. After all, in nature this happens in a giant swarm that can contain about 50 million grasshoppers per square kilometer, so the choice of a mate should be able to happen quickly and without major conflicts.

"Thus, the yellow color serves as a necessary feature of recognition between male conspecifics in order for mating in a large swarm to proceed properly and thus maintain the population. For the scattered, solitary grasshoppers, this yellow color is not necessary and may even be detrimental to their survival because they will then reveal their camouflage effect," explains Professor Vanden Broeck.

Control of pests

Swarms of desert locusts are not only known from the Bible as one of the plagues of Egypt (Exodus 10), but even today they are still considered by the Food and Agriculture Organization (FAO) of the United Nations as one of the most damaging pests that pose a major threat to agricultural production around the world. "Control of large, migrating locust swarms often involves the massive use of neurotoxic insecticides that by no means affect only locusts. Consequently, there is an urgent need for more selective and environmentally friendly means. The knowledge we are acquiring about important insect life processes can lead to new strategies for pest control and thus help farmers to protect their crops," concludes Professor Vanden Broeck.