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By: Efraim Harari

Grasshoppers are primarily solitary creatures throughout their lives and can stay in the same habitat for long periods of time. Grasshoppers never or rarely swarm. Locusts, on the other hand, are actually types of grasshoppers that have entered into a migratory phase(where they migrate from one location to another, in search of food) and are able to swarm.

These locusts first undergo a solitary phase, in which they act like grasshoppers and keep to themselves. Then they enter a gregarious phase, in which they become very social, and mass together in large groups.

Locusts develop their gregarious characteristics in certain environmental conditions. Drought drives locusts to crowd in small areas where there is vegetation. This crowding triggers a dramatic and rapid change in the locusts’ behavior. They become very mobile and actively seek the company of other locusts. They also change physically, becoming darker in color, and stronger, and they begin multiplying rapidly. Their increased strength enables them to fly for prolonged flights in swarms.

In their swarms, locusts move in a single direction, making stopovers on any green area they notice. This causes extensive damage to crops. Locusts are known to cover long distances in short time periods, leaving behind a trail of damage.

Make Way!

A desert locust swarm consists of up to eighty million locusts in less than half a square mile. These swarms can be 460 square miles in size!

Why Locusts Swarm

Scientists have discovered that a chemical in the brain, called serotonin, is what is responsible for turning solitary locusts into gregarious, swarming insects, and physical touch is what triggers the production of serotonin in these creatures. When the locusts crowd together, physical touch is unavoidable, and this causes the locusts to produce serotonin.

Scientists performed studies on the desert locust, one of the most devastating pests in the world. (This species is infamous for wreaking havoc in both Africa and Asia.) In the laboratory, solitary desert locusts turned into gregarious ones in just two hours, simply by the scientists tickling their hind legs to imitate the jostling that locusts experience in a crowd. During this period, there was an increase in the chemical serotonin in specific parts of the insects’ nervous system.

Experiments were then designed to show that serotonin is indeed the reason behind the drastic change of behavior. When scientists injected the desert locusts with a serotonin blocker (chemicals that block the action of serotonin), the locusts stayed solitary even in swarm-inducing conditions.

Knowing what causes this swift change of behavior in locusts may help governments and farmers develop methods to control future locust outbreaks with chemicals that would suppress the offending serotonin.


The locust is mentioned in the Torah by a variety of names, with arbehbeing the most common one. The word arbehis derived from the root ravah, which means “to increase and multiply.” Some trace it to the root arav, which means “to attack and devastate.” The locust possesses both of these characteristics. During its gregarious phase, the locust multiplies at an alarming rate, and it is a potential destroyer of crops; it devours plant growth with an insatiable appetite.

The locust’s destructive nature is described in various places throughout Tanach. For example, when the Midyanim attacked Bnei Yisrael in the days of Gideon, destroying the produce of the earth and leaving no sustenance for the Jewish people, the ruination is described in this way: …And they came as numerous as locusts; both they and their camels without number, and they came into the land to destroy it (Shoftim6:5).

It is also interesting to note that during the plague of locusts, Pharaoh hurried to call Moshe and Aharon; he seemed to want to repent.
Then Pharaoh changed his mind. What happened? Rav Yehoshua Leib Diskin, zt”l, explains that each of the previous plagues had lasted seven days. Therefore, Pharaoh thought that if he “repented” before the seven days of this plague were up, he could still save some of his crops. However, once Pharaoh saw how much damage had already been done by the locusts, he decided that there was nothing to gain from repentance. So, he changed his mind and would not repent, after all.