Algal blooms

In the biblical Book of Exodus, the ten plagues of Egypt begin with the transformation of water into blood. This fish-killing plague could have been an algal bloom.

When microscopic plankton receive a boost of nutrients, their population explodes. The algae's pigments make the surface of the water appear red, so these algal blooms are sometimes known as "red tides".

An enormous algal bloom has been recorded stretching from the southern coast of California to the Aleutian Islands in Alaska

Some algal blooms are destructive, killing wildlife and ruining fisheries. Inventively, they are known as harmful algal blooms (HABs). There can be millions of microorganisms per litre of water, blocking the Sun from reaching any plant and animal life beneath.

As well as blanketing the water, blooms can be toxic. Some species of algae contain neurotoxins that, once consumed by small fish, pass through the food chain with devastating consequences.

In the summer of 2015, an enormous algal bloom has been recorded stretching from the southern coast of California to the Aleutian Islands in Alaska. It has produced large amounts of a neurotoxin called domoic acid. Fisheries have been closed along the west coast of the US and multiple whale deaths have been reported in Alaska.

Mosquitoes

Many theologians interpret the Bible's fourth plague of Egypt as a swarm of flies. There are plenty of flies that adopt a "strength in numbers" strategy, and arguably the most notorious are the blood-sucking mosquitoes.

The insects are so abundant, they can alter one of the world's largest animal migrations

It is only the females that feed on blood, as part of their reproductive cycle. Thousands of newly-emerged males rise in dense swarms looking for love, and will mate on the wing with any female that enters the cloud. As well as sperm, the males pass on secretions that trigger egg production. Each mated female can then lay 300 eggs every few days.

Some of the world's largest swarms of mosquitoes have been observed in Alaska during the brief Arctic summer. There are 35 species living in Alaska, where the mosquito is jokingly known as the unofficial state bird.

The insects are so abundant, they can alter one of the world's largest animal migrations. Caribou change course to avoid them.

Mayflies

Mayflies are named for their seasonal emergence. Thousands of adult flies hatch at once in the spring.

Much like mosquitoes, the mayflies have a one-track mind. They rise in large swarms to mate and lay eggs, before crashing to the ground with exhaustion.

Mayflies in general are useful indicators of good water quality

In recent years, the mating swarms of mayflies have been large enough to be picked up on radar. In part, this is a testament to the sensitivity of modern radar, but the swarms really are dense: in 2014, the swarms along the Mississippi River in La Crosse, Wisconsin were blamed for car accidents.

"During the swarms, the mayflies near La Crosse can be considerable nuisances," says Tom Klubertanz of the University of Wisconsin in Janesville. "People do complain of the smell of dead mayflies covering the ground, and of having to sweep or shovel them away from sidewalks, store fronts, and off their vehicles."

Apart from making driving conditions hazardous, mayflies are harmless and are considered a sign of a healthy environment.

"Mayflies in general are useful indicators of good water quality," says Klubertanz.  "The return of them in large numbers to the Mississippi River and Lake Erie, for example, is something to celebrate."

Locusts

There is one group of animals whose reputation for plagues persists even today: locusts.

The issue lies with their split personalities. Locusts start off as harmless solitary insects, similar to their grasshopper cousins. But under the right conditions they can transform into colourful, muscular extroverts with an appetite for destruction.

In Nebraska in 1875, Child witnessed a swarm 110 miles wide and up to half a mile deep

In 2009, scientists discovered that a brain chemical called serotonin was responsible for the switch. In the dry season when vegetation is sparse, locusts jostle together and this triggers the release of serotonin, urging them to swarm to new pastures.  

The beefed-up locusts feed on crops and breed wherever the prevailing wind takes them. Plagues of locusts threaten farms in hotspots across Asia, Africa, the Middle East and Australia.

The largest one observed is known as "Albert's Swarm", after weather enthusiast Albert Child. In Nebraska in 1875, Child witnessed a swarm 110 miles wide and up to half a mile deep.

There may have been 3.5 trillion Rocky Mountain locusts in Albert's Swarm, according to Jeffrey Lockwood at the University of Wyoming in Laramie.

But this particular locust species has been consigned to history. It suddenly disappeared at the end of the 19th Century, when its eggs were disturbed by large-scale farming.

Armyworms

An army of caterpillars march regularly across sub-Saharan Africa, determined to eat all the crops in their path. These are the larvae of moths called African armyworms and are one of the most devastating agricultural pests in the world.

Researchers are developing a biological pesticide based on a natural disease of the caterpillar

"Determining where one outbreak starts and another begins is a tricky business," says Ken Wilson of Lancaster University in the UK and head of the Armyworm Network.

"We know that there are many outbreaks during every armyworm season, so the total area infested could amount to several hundreds of thousands of hectares in any given year. Multiply that by the number of countries infested in a bad year, and you are probably talking about millions of hectares."

The caterpillars feed on cereal plants such as maize, wheat and rice. Wilson says outbreaks correlate with rainfall, the first armyworms appearing with the short rains in October. Once the worms transform into moths they migrate on the prevailing winds, so the outbreaks spread. 

The armyworm problem is not a new phenomenon, but researchers are developing a biological pesticide based on a natural disease of the caterpillar that could potentially put a stop to the armyworms once and for all.

Queleas

Africa's plague problems aren't limited to ground assaults. The continent is also home to birds with big appetites.

Desperate farmers try everything from drumming to poisonous pesticides and even dynamite

Red-billed quelea fill the skies in enormous gatherings in sub-Saharan countries. With a total population estimated at 1.5 billion pairs, they may be the world's most abundant bird species.

They are small, finch-like birds with distinctive red bills. They feed on seeds and grains, which can be a problem when a million-strong flock targets crops and devours tonnes of grain in one sitting. As a result of this behaviour, they have been dubbed "the feathered locust".

Desperate farmers try everything from drumming to poisonous pesticides and even dynamite to get rid of the birds. Roosts are targeted under cover of darkness with exploding barrels of petrol.

Robert Cheke of the University of Greenwich in Chatham, UK, has investigated safe alternatives for controlling the birds. He suggests that they could be harvested as food.

In spring 2015, two red-billed quelea were spotted in Singapore. The birds may have been released as symbolic gestures during religious festivals. Whether their appearance will be a bad omen for Asia's farmland remains to be seen.

Giant African land snails

Sometimes, modern-day plagues begin when an animal is transported to a place where it has never lived before, and becomes wildly successful. The giant African land snail is an example: it has been listed as one of the 100 worst invasive species in the world.

Adult snails can grow up to 20cm long, making them popular novelty pets or snacks. Their slime is even believed by some to have cosmetic benefits.

The snails also eat rat faeces and can contract parasitic worms that threaten human health

That means people often ship them around. Add in the accidental transport of snail eggs on plants, containers and suitcases, and the species has had ample opportunity to invade new territories.

You might think that slow-moving snails would be nothing to worry about, but these giants are prolific breeders. They are hermaphrodites, with both male and female sex organs, so each snail can produce thousands of eggs per year. Just four years after their introduction to Fiji, 20 tonnes of snails were reportedly collected in a single day.

To fuel this reproductive orgy, the snails munch their way through the local vegetation. They have a particular taste for garden crops including cabbages, pumpkins and peas.

The snails also eat rat faeces and can contract parasitic worms that threaten human health.

Jellies

Depending on which side of the argument you listen to, we may or may not be under attack from a global invasion of jelly-like creatures.

From moon jellyfish inundating power plants to giant Nomura's jellyfish capsizing Japanese fishing boats, massive assemblies of these creatures have certainly been making headlines in recent years. But marine biologists are divided over whether jellyfish blooms are really on the rise.

They overwhelmed the Black Sea, peaking at 400 per cubic metre of water in 1989

In a bid to end the argument, a database called Jellywatch has been established to record sightings, but it turns out to be hard to find transparent animals in the ocean.

The idea of a plague of jellies might make you chuckle, but the damage done by warty comb jellies in the Black Sea is no laughing matter.

Comb jellies are not jellyfish but belong to a related group. Similarly transparent and hard to spot, they hitchhiked to the Black Sea in the 1980s in the ballast water of boats from their home on the east coasts of North and South America.

They overwhelmed the Black Sea, peaking at 400 per cubic metre of water in 1989, and devoured the tiny animals that form the basis of the food chain: zooplankton, mollusc larvae and fish eggs. Despite control measures, the fisheries have still not fully recovered.

Ants

They might be tiny but ants can wreak havoc. Thought to originate from west Africa, yellow crazy ants first invaded Christmas Island in the Indian Ocean in the 1930s, after hitching a ride on trade ships.

The average spreading speed of the super-colonies was calculated at 3m per day

Christmas Island is best known for its native red crabs, which cause their own swarming spectacle every year when up to 50 million of them march to the sea to breed. But up to a third of the population has been wiped out in recent years by the invasive ants.

By 2002, the ants had invaded 30% of the island's rainforest. A 2005 study recorded the highest-ever known density of foraging ants, with 2000 ants per square metre within super-colonies.

The long yellow legs of these ants can cover a lot of ground. The average spreading speed of the super-colonies was calculated at 3m per day, the equivalent of 1.1km per year.

To rid the island of this plague, extreme measures have been taken. In 2002 and 2009, helicopters dropped poisoned bait. To the relief of conservationists, these missions slashed the populations of yellow crazy ants. But they are only one of the front lines of the worldwide ant invasion.

Mice

When Europeans settled in Australia in the 18th Century, they brought the house mouse (Mus domesticus) with them. With few predators to bother it, the furry invader settled into its new home very well, so nowadays southern and eastern Australia are regularly overrun with mice.

In cereal-growing regions, a mouse plague occurs roughly one year in four. Densities can peak at 2,700 mice per hectare in grain stores, ruining crops and livelihoods.

Plague proportions can be reached within nine months

Since 1980, mouse plagues have been on the rise in Australia and are linked to more intensive farming practices. The worst plague on record occurred in 1993: it may have cost grain growers in Victoria and South Australia A$64.5 million.

These population explosions are linked to drought-breaking rains, but no single cause has been identified as the trigger. With favourable conditions, a good food supply and little disease or predation, mouse numbers soar.

Female mice can produce an average litter of six offspring every month, and the young will start breeding five weeks later. Plague proportions can be reached within nine months.

In April 2015, scientists recruited farmers to report sightings via a smartphone app in Australia's first Mouse Census Week. The aim is to predict future outbreaks.