Locusts have brains built for complex odor processing 

The migratory locust, Locusta migratoria, a notorious crop pest, has long been known for its devastating swarms that threaten agriculture and food security, particularly in Africa and Asia. 

Recent research by scientists at the Max Planck Institute for Chemical Ecology has revealed the unique olfactory brain structure of these locusts. 

Unlike other insects, migratory locusts have over 2,000 glomeruli in their antennal lobe, compared to 20-300 in most insects. This distinctive feature enables complex odor processing in the locusts, crucial for their behavior and swarming.

Complex odor processing

Migratory locusts’ antennal lobes, essential for odor processing, exhibit a complex neuronal architecture. This complex structure, with more than 2,000 spherical functional units known as glomeruli, differs significantly from the much lower number of glomeruli found in other insects. 

This unique configuration has puzzled scientists for decades. Now, the experts aimed to decode how odors are encoded within this extensive network of glomeruli.

Mapping brain activation in locusts

The introduction of CRISPR/Cas9 was a game-changer for the researchers, enabling the development of the first transgenic migratory locusts expressing GCaMP, a genetically encoded calcium sensor. This sensor fluoresces upon binding calcium, released when cells are active. 

Using functional 2-photon calcium imaging, the team could map spatial activation patterns in the locusts’ antennal lobes for a variety of ecologically relevant odors across all six developmental stages.

“Our results reveal an unusual functional ring-shaped organization of the antennal lobe,” explained Silke Sachse, head of the Olfactory Coding Research Group at Max Planck. 

This structure consists of specific glomerular clusters consistently present throughout the locust’s development, from nymph stages to adulthood.

Challenging research 

Achieving gene transformation in migratory locusts, which are not standard model organisms like Drosophila melanogaster, was particularly challenging. 

The large brain volume of locusts further complicated imaging and data analysis. “We are the first group in the world to successfully apply the site-specific knock-in method to locusts,” Jiang said.

Spatial odor coding in locusts 

Interestingly, the locust’s spatial odor coding reflects the chemical structure of odors rather than their valence. This is unlike flies, where the valence (pleasant or repulsive) is represented in the antennal lobe. 

“We have observed that odors of certain chemical classes evoke a certain pattern: For example, aromatic compounds with similar chemical structure but opposite behavioral significance evoke stronger responses in the peripheral regions of the antennal lobe,” noted co-lead author Bill Hansson, Director of the Department of Evolutionary Neuroethology at Max Planck.

“We conclude that the representation of odor valence is not encoded in the antennal lobe, but in higher brain centers such as the mushroom body and the lateral horn.”

Understanding odor perception in locusts 

The unique ring structure of the olfactory code in migratory locusts raises questions about its advantages or disadvantages compared to other insects. Future studies aim to investigate if other locust species exhibit similar coding patterns. 

Understanding odor perception and processing in locusts is crucial for comprehending their ecological interactions and could aid in developing better pest control strategies. 

“We believe that a better understanding of the odor coding mechanisms in the primary olfactory center of the locust brain will significantly deepen our knowledge of the neuronal modulation underlying olfaction-mediated behaviors, such as the formation of locust swarms,” Hanssen concluded.

Sense of smell in migratory locusts 

Migratory locusts possess a highly developed sense of smell, which plays a crucial role in their survival and behavior. Their olfactory system allows them to detect and respond to a variety of chemical cues in their environment. 

Their sophisticated sense of smell helps the locusts locate food sources, identify suitable habitats, and communicate with other locusts. 

The antennae of migratory locusts are equipped with numerous sensory receptors that can detect specific chemical compounds, enabling them to navigate and make decisions critical for their migration and swarming behavior. 

Their ability to sense pheromones also facilitates the coordination of mass movements and the formation of large swarms, which are characteristic of migratory locusts.

The study is published in the journal Cell.

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