The study, recently published in Ecological Monographs, outlines the results of considerable data gathered on the physiology of South American locusts, and demonstrates that species distribution models that consider physiology in addition to temperature may reshape what we can expect to see as climate change continues.
The international team included Youngblood and researchers from ASU's Global Locust Initiative: assistant professor Arianne Cease, President's Professor Michael Angilletta and professor Jon Harrison from the School of Life Sciences, and postdoc Stav Talal from the Global Institute of Sustainability and Innovation, as well as innovators and collaborators in South America.
When in their gregarious phase, these South American locusts can form swarms of millions, capable of migrating 90 miles in a day and consuming as much food as 35,000 people.
Since at least the days of the pharaohs of ancient Egypt in 3200 B.C., locusts have been erupting into massive swarms that descend upon crops and plant life, causing almost total devastation.
Just like people, locusts can be either shy or gregarious. For the most part, locust populations can spend several seasons in a low-density population, called a solitarious phase. The locusts are a cryptic brown or green—shy, solitary and relatively harmless on a global economic scale. However, when circumstances are just right, the locust numbers swell to overcrowding, triggering a drastic switch to a gregarious phase—social, brightly-colored and capable of forming migratory swarms of 80 million locusts per square kilometer.
With each locust consuming up to 2 grams of vegetation every day, a swarm of this size can travel up to 90 miles a day, consuming the same amount of food as 35,000 people. It's no wonder they are considered the world's most devastating pest.
To help unravel the driving forces behind swarms, the team studied the physiology of the South American locust (Schistocerca cancellata).
Most research on locusts has been performed on colonies that have been raised in the lab for years, so our research was a rare opportunity to study outbreaking locusts in their natural environment
To try to predict where swarms will migrate, and where crops will be threatened, scientists use species distribution models—computer algorithms that predict the distributions of a species across a geographic area using environmental data. The most common modeling technique has been correlative models. However, given the unknown variables inherent in a changing global climate, this method has lost its efficacy.
The research team employed a mechanistic modeling approach, gathering data about locust physiology to inform their model. In this case, the researchers measured how quickly locusts digest food in different environments.
A key factor of environmental data used for traditional correlative models is temperature, which has major impacts on locusts' eating habits.
However, this environmental data alone cannot adequately predict the effects of climate change on locust populations. First, locusts can exist and eat in a variety of temperatures. And, as generalist herbivores who can travel long distances for readily-available food, locusts can fill their stomachs with food faster than they can digest it.
While locusts can and will eat in a wide range of temperatures, the optimal temperature for digestion is much more specific.
The team measured how thermal conditions affected the rates of eating and digesting for field-captured locusts, and used this data to model energy gain in both current and future climate scenarios. Next, they established this new data as a predictive variable for a new species distribution model that predicted the spread of locust outbreaks across multiple scenarios.
Their predictions show that locusts will be able to assimilate far more energy in future climates than current climates, between 8-17% more energy per wet season than currently, proportional to how much warmer it is.
Migratory populations of South American locusts are also expected to expand their range away from the equator due to climate change. Models that consider locust physiology actually predict a smaller range of expansion than typical correlative models, but the physiology-based models also predict an increase in population growth rate, resulting in even greater crop damage.
Previous models predicted crop loss from insect pests to increase by 10-25% under climate change, but scientists did not know if these predictions were relevant to the South American locust. The new model created by Younglood matched the earlier models, predicting a 17% increase in crop losses from South American locusts.
This research is part of an ongoing partnership between ASU's Global Locust Initiative (GLI) and national plant protection organizations, farmers' groups, and universities in Argentina, Bolivia, and Paraguay, that started at the beginning of the South American Locust upsurge.
In 2020, GLI led a stakeholder workshop in Argentina to bring diverse participants together to formalize what they experience on a daily basis as locust governance.
A Guest Editorial