What is a locust?

Photo by Brandon Woo

A locust is a unique type of grasshopper (Orthoptera: Acrididae) that can form dense migrating groups as juvenile marching bands or adult flying swarms. They have strikingly different phenotypic forms, or "phases". A solitarious phase occurs in low population densities, where the locust is shy and well camouflaged, behaving like a "typical" grasshopper. The gregarious phase happens under crowded conditions and is characterized by color changes, aggregation, and swarming behavior. Scientists call this ability of locusts to change phases “phase polyphenism”, an extreme type of phenotypic plasticity. It is in the gregarious phase that locusts become a food security concern, with their ability to migrate long distances and eat their own body weight in food each day, cumulatively these many millions of hungry insects can decimate agricultural and pasture land with very little warning. Of approximately 12,000 grasshopper species, fewer than 20 are considered locusts. Swarming behavior in locusts has independently evolved multiple times around the world, indicating that ecological factors have repeatedly favored the evolution of locusts from their grasshopper ancestors.

Relevant articles

• Glossary
• Evolution, behavior, and physiology of locust phase polyphenism
• Locust ecology
• Primer on locust and grasshopper management
• Biopesticides
• Edible insects and locusts as more than pests

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  Desert locust swarm in Kenya ©FAO/Sven Torfinn
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  A swarm of desert locusts in Somaliland ©FAO/Isak Amin
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  Sean McCann desert locust 2011
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  Desert locust outbreak 2020 ©FAO/Sven Torfinn

Why are locusts an agricultural problem?

A locust outbreak could consist of nearly 100 billion ravenous insects. Such a swarm could cover around 500 square miles or 258 hectares. Additionally, an adult locust has the capacity to consume its own weight in food every day. Compare that to a person trying to do the same to get a perspective of this remarkable feat. The voracious appetite of locusts poses a formidable threat to vegetation, potentially resulting in the devastation of crops and pasture land. In a single day, an amassed swarm of locusts can consume a staggering 423 million pounds of plant matter. Once all the plants are gone, the locusts proceed en masse to the next location, leaving a devastated landscape in their wake.

During outbreak years, locusts have been known to blanket up to one-fifth of the Earth's total land area. In such circumstances, approximately one out of every ten individuals worldwide is adversely affected.

What causes a locust outbreak?

Abundant rainfall is usually the key component that drives a locust outbreak. Precipitation provides the necessary moisture for egg-laying, while subsequent heat facilitates hatching and thriving of young locusts. Concurrently, vegetation grows and offers an ample food supply for the burgeoning locust population. This surge in plant life supports rapid reproduction, further exacerbating the situation. Additionally, locusts exist in two distinct phases: the solitarious phase, characterized by solitary behavior, and the gregarious phase, when they form swarms. Crowding and competition for food can trigger a transition from the solitarious to the gregarious phase, culminating in the formation of large, migratory swarms.

Wind patterns can also serve as a pivotal factor in outbreaks. Strong winds carry locusts over extensive distances, facilitating their migration to new regions in search of food. This sudden influx of swarms in previously unaffected areas is a hallmark of locust outbreaks. Climate change and climate anomalies, such as El Niño and La Niña events, can also impact precipitation levels and vegetation growth, influencing locust dynamics.

Human activities like heavy livestock grazing, deforestation, soil degradation, and changes in land use can create environments more conducive to locust breeding and survival.

Read more about locust ecology here.

Learn from select educational media articles

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  Learn more about locusts and what makes them swarm
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  A swarm of desert locusts in Somaliland ©FAO/Isak Amin
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  The Strange Thing That Turns Grasshoppers Into Locusts, Bizarre Beasts 2022
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  A locust plague hit East Africa. The pesticide solution may have dire consequences, National Geographic 2021

Learn about locusts from select media articles
Learn about locust pop culture

Frequently asked questions

The Commission for Controlling the Desert Locust in the Central Region (CRC)
FAO Desert Locust Watch Frequently Asked Questions (FAQs) about locusts
FAO Desert Locust Q&A
Q&A: The impact of desert locusts in the Horn and Eastern Africa
The World Bank
National Geographic FAQs

FAQs to the GLI adapted from media interviews

Q: Do the grasshopper species causing problems in the western United States include any with locust-like behavior?
A: The migratory grasshopper (Melanoplus sanguinipes) is a major US rangeland pest and has some locust-like tendencies, though it is not considered a locust.

Q: Does the fact that some locusts are now displaying behavior they didn’t in the past make it more difficult for researchers to understand and manage them?
A: Certainly, it complicates things for plant protection organizations. In terms of research, I could see challenges in tracking them if they start swarming. In terms of biological research, the locusts likely didn’t evolve the capacity to be a locust in recent times but rather always had the hidden capacity but the environment changed and revealed or highlighted the phenotypic plasticity.

Q: Would it be accurate to say that the desert locust is the most concerning species globally?
A: Yes

Q: Will locust outbreaks become worse with climate change?
A: They will definitely become more uncertain, which will make them more challenging to predict and manage. In general, outbreaks are predicted to be worse, but there are cases of species or areas for specific species where the opposite is predicted.

Q: Is it correct that abundant rain and plant growth can lead to locust population increases and also trigger swarming behavior?
A: Locust species tend to live in arid environments and are really good at capitalizing on sudden plant availability. Therefore, rains in arid areas tend to promote locust outbreaks if the locust egg beds are there because it provides a good food source to support rapid growth of hatchlings to reach the adult stage.

Q: But you also said outbreaks of both grasshoppers and swarming locusts were facilitated by drought, too, possibly because their predators hadn't survived periods of dryness, yes? Does their ability to proliferate both when it's really wet and really dry increase challenges for management?

A: Drought can facilitate outbreaks by concentrating locusts on remaining vegetation. That aggregation results in shifting locusts to the gregarious phase where they get more notice than when they’re spread out. Predominantly, though drought-breaking rains are what really spur locust outbreaks. They can generally capitalize on the new vegetation quickly, before competitors and before their predator, and potentially pathogen, populations build up to suppress them.

This pattern is a particular challenge because it can exacerbate an already challenging situation for agriculture in the region that might have been navigating years of drought.

Q: Which locust species do you use in your flight experiments?
A: They all do at some point! But the one we use most commonly in the wind tunnels is the migratory locust (Locusta migratoria)

Q: Is your wind tunnel made of PVC? And was that locust flying into the wind?
A: Yes and yes.

Q: Are the experimental diets used to study locust flight and marching behavior based on specific crops or plants they prefer in the wild, such as cotton, wheat, or rangeland grasses?
A: Yes! Grasses, including cereal crops, tend to have higher carbohydrate to protein ratios and are favored by many locusts. Grasses and crops grown in nutrient-poor soils promote locust outbreaks by providing an even higher carb and lower protein diet (a donut diet!) that supports their active lifestyle and migration. I had the opportunity to do my dissertation research in Inner Mongolia, China and we had breakthrough study that first linked land use to locust outbreaks via this mechanism. Heavy livestock grazing promotes locust outbreaks by lower plant nitrogen. We’ve gone on to show this pattern in Senegal (here and here) and in Australia where we showed a continental-scale negative correlation between locust outbreaks and soil nitrogen (!) To our knowledge, this is the first study to show any consistent relationship between soil nitrogen and an animal population at a continental scale, which was really exciting. This leads to sustainable management opportunities, including increasing soil fertility (see our work in Senegal) and incorporating soil nitrogen into locust forecasts (forthcoming collaborations with FAO).

Q: Is the idea to understand what they like to eat to fuel up for a long swarm? And is it correct that they might fly thousands of miles?
A: Yes - we are studying nutrient requirements and use for migration. Migration in a single bout is probably between 50 to a few hundred miles (using wind), but over several weeks a swarm can travel thousands of miles.

Q: Is it correct that one of the studies involves testing what might cause juvenile locusts to break formation while marching in an arena, and that diet is also a factor being explored in these experiments?
A: Yes! Neema John, PhD Candidate is doing this work as part of her dissertation. She tested the nutrient demands of marching locusts for the first time in a controlled setting. She also tested how diet affects marching behavior. Indeed, when marching, locusts increase carbohydrate but not protein consumption, and locusts eating high carb and low protein diets march more than locusts eating lower carb and higher protein diets. These results corroborate lab flight experiments on adults (high carb diets promote longer flight durations) as well as field experiments showing that marching locusts are hungry for carbohydrate, not protein.

Q: Is Metarhizium approved for use in the U.S., and is it specifically authorized for controlling locusts?
A: The fungal genus Metarhizium has many species, most of which tend to infect many insect taxa. Metarhizium acridum is the species that specifically targets the order Orthoptera (crickets, katydids, and grasshoppers), mainly the family Acridids (grasshoppers and locusts). Metarhizium acridum is used in many other countries, typically at low levels, to manage locusts. It is produced under the commercial names Novacrid, Green Guard, and Green Muscle. However, it’s tough to make it economically viable for companies to produce because it’s not used consistently enough and production is intensive. Metarhizium acridum is not approved for use in the US, but there are other Metarhizium species that are used to manage other insect taxa, which I am less familiar with. There have been some grasshopper trials using Metarhizium robertsii, which is what we’ve used for experiments in our lab. That species was found and isolated in the US and is moderately effective against grasshoppers, but it will also infect other insects and there is no commercial production of it or plans to use it on a commercial scale. Check out HopperWiki's page on biopesticides.