Swarming animals can be dangerous and give us the creeps and even threaten food supplies. But overall, they are mostly just particular species behaving naturally and of no concern to humans.
Here are some of the more spectacular examples, some of which you might want to avoid.
10 deadliest swarms
African honeybees
In the unintentionally hilarious 1970s B (and bee) movie The Swarm, starring among others Michael Caine, Katharine Ross and even Henry Fonda, a swarm of killer bees threatens the American mid-west. “It is more than speculation,” warned the trailer ominously. “It is a prediction. The swarm is coming.”
Ridiculous as it sounds, it wasn’t based on a complete fantasy – African honeybees, which have become widespread in both North and South America, have a reputation for being killers, aggressively defending their colonies against potential assailants.
- Does the world's deadliest bee deserve its killer reputation?
- ’s bigger than your thumb, has immense jaws and is nicknamed the flying bulldog – Meet the largest bee on Earth
According the Natural History Museum, swarms can number up to 800,000 animals, and the stings from just 1,000 could kill an adult human. And as this report from USA Today makes clear, people are killed by them every year. But do they threaten civilisation as we know it? Probably not.
- Bee vs wasp: what's the difference between these two stinging insects?
- Meet the deadliest and biggest wasp in the world - complete with its scary 6mm stinger - and learn why it's called the murder hornet
Fat-tailed scorpions
In the 10 plagues that befell the Egyptians in the Book of Exodus, there are a suite of natural disasters involving unnatural swarms of animals, including flies and frogs, but not fat-tailed scorpions. But this is what happened in the province of Aswan in 2021 after severe flooding. According to the Smithsonian, the scorpions – whose sting can be fatal if not treated – were forced out of underground burrows by rising water levels, and in a single night, stung more than 500 of people. Thankfully, nobody died.
Locusts
Locusts were one of the plagues sent to Egypt, and with good reason. According to this paper published in Sustainable Agriculture, desert locusts are “one of the most destructive migratory pests in the world”.
A single swarm of this species can cover an area of 1,200km2 and contain 80 million individuals per km2. A locust outbreak in East Africa in 2020 put more than 20 million people at risk of food shortages and caused $US 8.5 billion worth of damage. Climate change will only increase the threat they pose. Biblical indeed.
Fire ants
Capable of causing kidney damage, seizures and death through anaphylactic shock, fire ants are well worth steering clear of. Though native to South America, they can now be found in Australia, New Zealand, North America, parts of the Caribbean, China and Taiwan.
- How fire ants can shift their shape
- It floats, it's larger than a dinner plate and it's made up of more than 100,000 individuals
Fire ant stings, according to the National Library of Medicine, cause an immediate burning sensation (hence their name) and thereafter wheals, pustules and intense itching. Attacks often take place while someone is working in their backyard, but it’s not clear how many deaths occur a year in any of the countries they have become established in.
Piranhas
Like the killer bees in The Swarm, this infamous South American freshwater fish also got its own piece of celluloid history thanks to the imaginatively titled Piranha (coincidentally, also released in 1978). And in much the same way, this movie also hugely overegged the ecological pudding with its depictions of mass slaughter being committed in hitherto safe bathing lakes.
- It barks like a dog, has razor-sharp teeth and is a horror movie favourite – but is this scary-looking fish just misunderstood?
- Can piranhas actually kill a human?
In reality, they rarely go for humans, though some piranhas attacked a group of bathers in a tourist resort in the Brazilian state of Amazonas in 2023, leaving eight people injured. Scientists told Live Science that such behaviour was exceptional – the fish had become accustomed to free food and it was a case of mistaken identity. Nevertheless, for most people, the word piranha conjures up images of stricken swimmers and a sudden upwelling of dark, crimson blood in the water.
- Pound for pound, this little fish has the most savage, brutal bite ever measured – way stronger than a great white
- "The 200kg beast ripped off a large part of his calf and nearly severed his leg" – 10 deadliest river animals that can deliver a lethal punch
Water-line isopods
Water-line whats? They don’t exactly sound like killers, do they – and they’re not – but swarms of these marine relatives of the wood louse can reach 1,000 or more and they bite. Rather painfully, by the sound of it.
Beachgoers in southern California found this out for themselves in 2022 when they were assailed by tiny beasts less than 1cm long. “It was like small piranhas had bit me,” reported one victim, who had perhaps watched the film and didn’t realise that fins weren’t always what they were in the movies.
Mice
We don’t usually think of nice furry mammals as swarming – insects and fish yes – but in Australia, a good rain year brings huge wheat, barley and hay harvests and consequential mass outbreaks of non-native mice. According to Australia’s national science agency CSIRO, for reasons not understood, mouse plagues only occur in China and Australia.
In 1993, Australia’s worst ever mouse plague cost an estimated $AU96 million (£50 million). “They can give birth to a litter of up to 10 offspring every 20 days, and they can fall pregnant as soon as they have given birth,” it says. It used to be that a high population one year was followed by a crash in subsequent ones, but plagues are becoming a more persistent problem, CSIRO warns.
Fruit bats
In 2020, a huge population of fruit bats or flying foxes, animals with a wingspan of up to 1m and weighing on average 700g, invaded the small Queensland, Australia, town of Ingham. Footage from BBC News shows tens of thousands of these behemoths darkening the sky.
“The stench is just horrific,” mayor Ramon Jayo said, “and they’re peeing on you. People should not have to put up with that.” The authorities announced they were planning on persuading the bats to move back to a more rural existence – it’s not clear whether they were successful or not.
Army ants
Army ants patrol the rainforest in colonies millions strong in search of prey. They are often described as relentless and unstoppable as they take tarantulas, scorpions, beetles and other ant species, with even the occasional frog, snake or lizard.
Hundreds of other species (such as the aptly named antbirds) follow them to snap up what the ants leave behind. Entomologist Frank Nischk describes how he defended – just – his research station in Ecuador against one of these fearsome hordes. Though he knew he wasn’t at risk, his food supplies could have been.
Red crabs
There’s nothing deadly about the Christmas Island red crab migration, but when listing animals that swarm in huge numbers, it would be wrong not to include it.
That’s because more than 100 million of these crabs leave their burrows in the island’s forests at exactly the same time (with the first rains in October or November) and march towards the sea where they will mate, and the females will release their fertilised eggs into the sea. What’s more, numbers are on the rise since conservationists began to tackle the non-native yellow ants that decimated numbers at the start of the 21st century.
