Can a Swarm of Locusts Really Block Out the Sun?

Editor’s Note: This article originally appeared in Entomology Today, a project of the Entomological Society of America with the goal of reporting interesting discoveries in the world of insect science and news from various entomological societies. To learn more, visit www.entomologytoday.org.

We know the moon can do it, but what about a swarm of locusts?

On Monday, Aug. 21, 2017, denizens of North America looked skyward to witness a solar eclipse — the first in which the path of totality would cross the contiguous United States since 1979. Those who have witnessed a full solar eclipse say it defies description.

Likewise, those who have witnessed a swarm of locusts say it is also a bewildering sight — deemed a “plague” since biblical times — and popular descriptions often allude to the darkening of the sun in the midst of such a swarm.

So, is it true, or just hyperbole? We turned to an expert entomologist to try to find out.

Hojun Song, Ph.D., is an associate professor of entomology, specializing in arthropod systematics and biodiversity, at Texas A&M University. Among his current research portfolio is a National Science Foundation-funded project examining the evolution of locust species to understand why some form large swarms and others do not. Locusts are a type of grasshopper species that, under the right conditions, shift from a typically solitary lifestyle to a “gregarious” one, in which they gather, breed prolifically and migrate en masse. Two widely studied species are the desert locust (Schistocerca gregaria) and the migratory locust (Locusta migratoria).

In the course of that work, Song has seen one locust swarm with his own eyes, on Socorro Island in Mexico.

“It was a rather small swarm, but enough to make it a surreal experience,” he says. “You have millions of locusts flying above you. All you hear is their wing flaps. Because it was a small swarm, it was not like a cloud and it certainly did not block the sun.”

But what about the big swarms? Song offers some back-of-the-napkin calculations to estimate whether locusts could genuinely block out the sun:

“A very large swarm can contain up to about 80 million locusts in a square kilometer, and the size of swarm can reach up to several hundred square kilometers. Let’s do some calculation. Each locust is about 6-8 centimeters (cm) in body length, with a wing span of 8-10 cm. For simplicity, let’s just assume that each locust in flight can occupy about 64 square cm (8 cm body x 8 cm wing).

“If there can be as many as 80 million locusts in a square kilometer, that means there can be 8,944 locusts in a linear kilometer (calculation: square root of 80,000,000 = 8944.27191). If we line up all locusts wing to wing, the total length would be 0.71552 kilometers (km) (8,944 x 8 cm = 71552 cm = 715.52 m = 0.71552 km). Likewise, if we line up all locusts from head and abdomen, the calculation will be the same, 0.71552 km.

“This means that the physical area occupied by 80 million locusts, if they are all next to each other horizontally and vertically, is just a little bit over 51 percent of a square kilometer (0.71552 km x 0.71552 km = 0.5119688704 square km).

“Of course, locusts do not fly side by side touching each other’s wings, and the swarm is three dimensional, meaning that some locusts fly above others. Eighty million locusts per square meter is a lot of locusts, but they are not dense enough to block the sun.

“I think that, even with a very large swarm, with the size reaching several hundred square kilometers, you should still be able to see the sun without any issue.”

So, there you have it. A swarm of locusts is no solar eclipse.

But locusts do eat, so the impact that 80 million of them (again, that’s just per one square kilometer) have on vegetation and crops can be hugely destructive. Song’s research seeks to understand the roots of their swarming behavior, in hopes of better informing potential control efforts.

“There are more than a dozen locust species around the world, but we really don’t know much about how and why they swarm, except for two well-studied species, the desert locust and the migratory locust. Swarming locusts have evolved multiple times and different locust species have different mechanisms for forming swarms. And what we have learned from these two species does not necessarily apply to other locust species,” Song says.

“Locusts are the only mass-migrating pests that can have a very quick devastating result to people. In this day and age, we still do not have a very good way to predict and manage these pests. This intrigues me quite a bit,” he added.