Insect Invasion: Locust Swarm

A Plague of Locusts

Nebraska, June 1875. The calm of the afternoon is broken by a rushing sound. A farmer runs out of the barn looking for the fire he hears. But, it isn’t a fire. It is the sound of millions of tiny jaws chewing his crop and the rustle of insect legs and wings on corn stalks. It is Albert’s Swarm—a plague of locusts invading the Midwest.

The locust swarm started in the north part of the United States and moved south through Nebraska, Minnesota, Iowa, Kansas, and Missouri.  There were so many flying grasshoppers that people thought there were storm clouds on the horizon. One eyewitness described the swarm. “It is a vast cloud of animated specks, glittering against the sun. 0n the horizon they often appear as a dust tornado, riding upon the wind like an ominous hail-storm, eddying and whirling about like the wild, dead leaves in an autumn storm, and finally sweeping up to and past you, with a power that is irresistible.”

The locusts ate every green thing in their path. They ate the crops and the leaves and shoots of trees.  They stripped off bark and went after anything that could be chewed: fence posts, axe handles, leather harness, and gloves.  They even ate the green stripes out of a shirt!  Trains couldn’t move because the crushed insects acted like oil on the tracks.

What are locusts?  They are grasshoppers that band together in a swarm and migrate in search of food. When the spring rains and temperatures are just right, millions of grasshopper eggs hatch in the soft soil and tiny, wingless grasshoppers emerge. As they climb over one another in search for food, they change color, grow wings and then start to swarm and fly together.  When this change occurs, they are called locusts. In normal years, when the numbers are smaller, the hoppers stay hoppers and do not swarm.

Until recently, no one knew what caused grasshoppers to transform into locusts. Entomologists studying grasshoppers tried to get them to change into locusts by giving them various sight, sound, taste and smell signals.  None of these senses changed their color or behavior. What was the trigger?

One experiment finally answered the question. A grasshopper was placed in a box with small paper balls. The box was tipped back and forth so the balls would hit the hopper in the legs as they rolled by. After a short time, the hopper changed color! It was the constant bumping on the hind legs that triggered the change.  Scientists trying to prevent or lessen the impact of locusts around the world wonder how they can use this information.

Albert’s swarm was just one of the hundreds of locust eruptions that have plagued the planet. In the present day, locusts affect 1/5 of the world’s land mass and impact 1/10 of the world’s population. In some continents, like Africa, locust swarms are common, causing food crisis and famine.

The 1875 swarm was named for Albert Child, a Nebraska physician and amateur meteorologist who determined the size of the swarm and the number of insects using a mathematical model. He first estimated the number of locusts by counting sample sizes and then computed the number of insects per acre. He timed their movement and calculated a rate of speed of 90 miles per day. He kept careful records of their range by telegraphing to towns across the plains. Based on the speed and time of travel, he estimated the size of the swarm to be 1800 miles long and 110 miles wide—3.5 trillion grasshoppers!

The mathematical thinking in this type of problem requires techniques such as counting insects per square yard and then determining the number of insects per acre. Rate and range allow calculation of the square area that the swarm covered. Using the total surface area of the swarm and the number of insects per acre, the total number of insects is calculate. How could you use this technique to determine how many automobiles travel on a freeway or how many people are in a crowd?