“You shall sow much seed, but harvest little, for the locusts will devour it.”

Deuteronomy 28:38

This post is written by Gary Jahn and Kiersten Johnson, USAID Bureau for Resilience and Food Security

The Ongoing Locust Plague

Locusts swarms have probably devastated crops and caused starvation since the beginning of agriculture some 12,000 years ago. The current, ongoing locust plague affecting East Africa, parts of the Arabian Peninsula, Iran, Iraq, Pakistan, and India began over a year ago. Cyclones in 2018 and 2019 brought extensive rains to the Arabian Peninsula, ending a drought and creating patches of vegetation that ultimately led to the formation of desert locust swarms (Figure 1). Large swarms moved across the Red Sea to Ethiopia and Somalia by June 2019. Aided by heavy winds and rains, the locusts spread to Kenya, Uganda, and Tanzania from October to December 2019. This is the worst locust plague in 25 years for Ethiopia and in 70 years for Kenya. According to FAO, as of mid-April 2020, Ethiopia alone had already lost 356,000 tons of cereal and 1.35 million hectares of pastureland to the desert locust. Across 10 countries, FAO estimates that locust control efforts have saved over 720,000 tons of crops so far, but forecasts another wave of new locust swarms by June if the plague is not controlled. 

What Are Locusts?

Like other grasshoppers, locusts have a broad diet that includes most of the 200 crop species consumed by humans – with a special preference for pasture grasses and cereals. Unlike other grasshoppers, the 19 species of locusts have the ability to change their behavior, physiology, morphology, and color from one generation to the next.

Under normal conditions, the locusts are ordinary solitary grasshoppers (Figure 2). They lay eggs in the soil, encased in foamy masses called egg pods. The immature forms that emerge from the eggs are flightless nymphs or “hoppers.” They avoid physical contact with each other. As they grow larger they molt several times until a final adult stage where they have fully developed wings and are able to reproduce. Generation after generation continues in this solitary phase indefinitely for years until crowding triggers the gregarious phase. This crowding is usually the result of patchy vegetation following a prolonged dry spell.

How the transition from solitary to gregarious phase plays out varies from one locust species to another. In the case of the desert locust (Schistocerca gregaria), if physical contact is unavoidable, their serotonin levels rise and their behavior changes over a matter of hours as they become attracted to being in groups. If crowding continues, the next generation will take on different colors that change with life stage as they mature. While the solitary individuals are typically green as nymphs and brown as adults, the gregarious locusts are black as nymphs, pink in intermediate stages, and bright yellow when sexually mature (Figure 3). If crowding still continues, then another generation or more later the locusts change further––shorter bodies and legs, longer wings, and 30 percent larger brains with more developed centers for integrating sensory information. In addition, they produce a pheromone that attracts them to one another. 

Staying together, they move in massive swarms, landing only to feed continuously and rest when the temperature drops, then moving again, up to 200 km in a day, following wind currents. Consuming their own weight daily, a swarm covering a square kilometer with 40 to 150 million locusts can eat 80 to 250 tons of food per day. Swarms can grow to over 3000 square kilometers in size, about 3 times the area of New York City. Sexually mature swarms will seek breeding grounds where the soil is sufficiently soft, moist, and warm for egg laying. If the offspring emerge under similar crowded conditions to their parents, the cycle continues with each generation potentially growing 400 times larger, but in reality growing about 20 times larger due to high egg and nymph mortality before swarming occurs. Each generation takes 2-6 months to mature depending on the conditions. If the gregarious cycle of swarming continues for more than a year, the outbreak is referred to as a plague. If gregarious locusts are in uncrowded conditions for several generations they again revert to the solitary phase. In these recession periods, solitary locusts live in desert areas away from agricultural zones and generally do not cause significant crop damage.

The Threat of Food Insecurity and the Hope of New Technology 

Left unchecked, the second wave of desert locust swarms beginning in June could destroy more crops (Figure 4) and have a greater impact on food security than the last wave. Not only will the swarms be larger, but the food security environment could be more fragile as millions of people lose access to income and food due to measures to stop the spread of COVID-19.

However, new types of information derived from analysis of satellite remote sensing data may help governments and farmers better manage the current locust plague. These data may help prevent, control, and assess locust outbreaks by indicating which areas are suitable for locust breeding at a given time, tracking the weather patterns that can be used to forecast swarm movements, and estimating the levels of crop loss from pest attacks. 

Upcoming articles in Agrilinks will explore in more detail how these new types of Earth observations data, integrated with information gathered on the ground by citizens and scientists alike, are being used to forecast how locusts and COVID-19 may jointly impact food security and how satellite technology is helping to mitigate this crisis.