Plague of Locusts

Behold, the locust.

Alone? A simple and usually solitary short-horned grasshopper.

But when the conditions are right?

Well, first let's discuss what those conditions are. Following a phase of drought and rapid vegetation growth, locusts begin to breed rapidly, with the groups of nymphs developing into wandering swarms of winged adults - and it is in both forms that they become a Plague of Locusts.

During "swarming" behavior, both nymph and adult populations are known to cause serious damage to crops and fields by stripping them thoroughly. Adult locusts in particular are powerful fliers capable of traveling great distances - not only is the phenomenon unsurprisingly widespread, it's also Older Than Dirt. The earliest instances are dated back to the ancient Egyptians and prehistory, while relatively later works such as The Iliad and The Qur'an also make mention of locust swarms as well.

The devastation of crops are a major factor in famines and human migrations as well as outbreaks of disease, to the point that locusts themselves are considered harbingers, if not carriers, of actual plagues. In fiction and literature, this connection has led to them being portrayed or symbolized as Walking Wastelands, almost always including making the insects inherently poisonous themselves. Funnily enough, locusts themselves have been used as a source of food - "bald locusts" are listed among acceptable food for Israelites as far back as the Book of Leviticus 11:22; in the New Testament, Matthew 3:4 and and Mark 1:6 assert that John the Baptist ate locusts and wild honey during his stay in the wilderness; and many modern cultures across Africa, West Asia and East Asia consider locust a delicacy.

A subtrope of The Swarm, and the phenomenon that its sister trope, Horde of Alien Locusts, was named for. Compare Termite Trouble, a similar depiction of insect pests as a troublesome devouring swarm.

Film

 * As part of the ten plagues revisited upon Egypt with the release of Imhotep in 1999's The Mummy, we get to see a horde of locusts descend upon "modern" Cairo.
 * The Prince of Egypt showed a swarm of locusts during "The Plagues" musical number.

Oral Tradition, Folklore, Myths and Legends

 * This trope has a special prevalence as one of many well-established Biblical Motifs.
 * Among the most notable, Exodus depicts a swarm of locusts as the eighth of the Ten Plagues that struck Egypt in Chapter 10:4.
 * The Book of Revelation mentions a special kind of hybrid "locust" that more closely resembles armored horses with human heads and leonine teeth in Chapter 9; led by Abaddon/Apollyon, the "angel of the abyss", these locusts were sent to torment those who had not yet accepted God and Christ, as signified by the marks on the chosen's foreheads.

Tabletop Games

 * The Tyranids of Warhammer 40,000 fame are often likened to a terrifying plague of alien locusts. They travel the galaxy in Hive Fleets that are so incomprehensibly massive that at their biggest, are made of lightyears worth of ships. They descend upon planets and devour everything they can with a crazed, hungry fervor. And by "everything", we mean everything, because their rampages will end with the targeted planet being stripped of all life and left a barren rock. You can kill millions, if not billions of the damned things and still lose because they always have millions and billions more to throw at you. Furthering the comparison to an apocalyptic swarm of insects is that there's no properly defined leader: all Tyranids are governed by a ravenous hive mind that is so overwhelmingly savage and powerful that it's impossible for the setting's Chaos Gods to control or corrupt.

Video Games

 * If you donate a migratory locust to the museum in the Animal Crossing series, bug-hating curator Blathers will never miss an opportunity to freak out about their monstrous appetites and the famines swarms of them have been known to cause throughout history. During seasons where they're active, they're among the most common bugs you can find in your town. Thankfully, they aren't a threat to your crops whatsoever and are content to harmlessly hop around all day.
 * Several NetHack variants - such as SporkHack, UnNetHack, EvilHack, and SlashTHEM - have the locust as an enemy monster which often appears in swarms and is capable of inflicting terminal illness on players, bringing down even the most seasoned of combat veterans unless they can cure it and defeat the swarm quickly.

Real Life

 * Some types of cicada are referred to as "locusts" due to their swarming behavior - or in the case of a specific genus, Magicicada, their synchronous emergence from underground dwellings in tremendous numbers - upon finally reaching their adult states after 13 or 17 whole years. However, they generally avert this trope, and in fact tend to have their own set of associated motifs.