The migratory locust, Locusta migratoria, is a crop pest of economic importance that is mentioned in the Old Testament as the eighth plague in Egypt, where it was sent to devour all plant life. While the locust is rarely found in Europe, it causes significant damage in Africa and Asia, threatening food security and livelihoods. Locusts exist in two phases, as solitary animals and in swarms, with the latter causing the most destruction to crops and vegetation.
One distinguishing feature of migratory locusts is the anatomical structure of their olfactory brain, specifically the antennal lobe, which processes olfactory information. Unlike other insects, the locust antennal lobe has over 2000 functional olfactory units called glomeruli, compared to the 20-300 found in most insects. Scientists at the Max Planck Institute have been studying how insects perceive odors and process them in their brains, aiming to understand how odor perception influences behavior and ultimately affects their ecological interactions.
A breakthrough in their research came with the introduction of the CRISPR/Cas9 method, allowing the researchers to create transgenic migratory locusts expressing a genetically encoded calcium sensor, GCaMP, in olfactory sensory neurons. This sensor fluoresces when it binds calcium, providing insights into neuronal activity. By using 2-photon calcium imaging, the scientists were able to map spatial activation patterns for a range of odors across all developmental stages of the migratory locust, revealing a unique ring-shaped organization of glomerular clusters in the antennal lobe.
The spatial coding of odors in the locust antennal lobe was found to reflect the chemical structure of the odors rather than their valence, unlike in other insects like flies where valence is represented in the antennal lobe. The researchers observed that odors of certain chemical classes evoke specific patterns in the antennal lobe, suggesting that odor valence is encoded in higher brain centers like the mushroom body and lateral horn. This unique coding mechanism is specific to the migratory locust and may not be transferable to other locust species.
While the locust is not a model organism like the vinegar fly, Drosophila melanogaster, gene transformation posed a challenge due to the species’ large brain volume. However, the researchers successfully applied the site-specific knock-in method to locusts, achieving transgenesis despite the low success rate reported in the literature. Future studies will investigate whether other locust species exhibit a similar coding pattern in their antennal lobes, shedding light on the rules of odor coding in insects.
Understanding how insects perceive and process odors, and how this influences their behavior, is crucial for ecological interactions and pest management strategies. By deepening our knowledge of the neuronal modulation underlying olfaction-mediated behaviors, such as swarm formation in migratory locusts, researchers aim to optimize control measures for crop pests. Further studies on odor coding in insect brains, including locusts, will contribute to a comprehensive understanding of insect behaviors and interactions with the environment.