Top 10 invasive species: when pest control goes wrong

Kudzu vine

Native to Japan and south-east China, this climbing perennial vine was initially introduced to the USA during the Centennial International Exhibition of 1876, the first official World’s Fair in America, held to celebrate the 100th anniversary of the signing of the Declaration of Independence in Philadelphia.

Although it was first thought as a great forage and ornamental plant because of its sweet blooms and impressive leaves, it was promoted between the 1930s and 1950s by the Soil Conservation Service as a tool to prevent soil erosion and was consequently mass-​planted through the south.

Things quickly got out of hand. When established, kudzu – also known as the ‘mile-a-minute’ plant – grows up to 30cm a day, and mature vines can reach 30m long. It overtakes and grows over anything in its way, killing other flora and foliage with shading. An invasion of kudzu means there are leaf litter changes and decomposition processes alter, with a 28 per cent reduction in stocks of soil carbon, so the spread of the vine could contribute to climate change.

As well as the south-east of the USA, kudzu has been discovered in Canada, north-eastern Australia and northern Italy. The vine became a problem weed when introduced in Vanuatu and Fiji by US Armed Forces to use as camouflage during the Second World War. In 2002, kudzu was added to the Biosecurity New Zealand register as an ‘unwanted organism’.

In 2015, the United States Forest Service estimated the spreading rate of kudzu to be 2,500 acres (1,000ha) per year.

Continuous mowing and grazing by cattle and goats can weaken the plant. Some herbicides can manage kudzu.

Cane toad

In 1930s Queensland, Australia, cane beetles were destroying sugar cane fields with their voracious appetites. Instead of opting for vicious, primitive pesticides, farmers heard of a toad that had a taste for the destructive insects.

They believed that upon releasing the cane toads, which can easily reach up to 2kg in weight and grow to the size of dinner plates, they would eat the beetles and farming would continue to prosper.

So in 1935, 102 toads travelled in two suitcases from Puerto Rico to Australia. Instead of doing their job in the cane field, they wandered off and now their number stands at more than 1.5 billion, covering more than 386,000 square miles of Oz. Instead of beating the pests, they became one.

The cane toad is likely to deplete native species that eat them, as behind the toads’ eyes are glands that secrete deadly toxins. Each one has enough venom to kill a large crocodile. Pets and humans can also be poisoned.

Native plants are gobbled up and insectivorous populations are reduced thanks to the cane toad’s appetite. They adapt well to their environment and can partly live on dry land. Their body balloons and stores water to stop dehydration on long journeys, they have fat stores for energy and they have evolved longer legs, making them faster and able to spread more quickly.

Due to their reproduction rate and chemical defence system, they have become one of the world’s top 100 invasive species, spreading at a rate of about 30 miles a year.

In Australia, toad bashing is encouraged but any killing must be humane as they are covered under animal welfare laws. Over AUS$20m has been spent on trying to control the spread.

Surveys from website CaneToadsinOz have shown that if you kill 98 out of 100 toads, the remaining two can create 30,000 new spawn and even if many tadpoles die there will be 10 times as many toads in the area in a week.

As well as Australia, which has thrown in the towel trying to reduce the cane toad population, the unstoppable amphibian has shown up in the Caribbean, the Philippines, Fiji, New Guinea and the USA.

Harlequin ladybird

One of the most invasive insect species is the harlequin ladybird (Harmonia axyridis), which tends to outcompete and eat native ladybirds. Originally from central Asia, the harlequin was introduced in Europe and North America to control aphids. Its dominance soon became a problem to native ladybirds.

The Asian species also carries single-celled parasites called microsporidia. These don’t harm the harlequins themselves, but do kill other species, including native seven-spot ladybirds. The parasites are contained in the blood (or haemolymph) and live in the harlequin’s eggs and larvae, but are in a dormant state and don’t hurt the carrier. However, ladybirds often eat each others’ eggs and larvae, so the native species may contract the parasite in this way and die, leading to declining populations.

The dominant harlequin is also extremely resistant to a fungal disease that kills native ladybirds.

The ‘Four Pests’ campaign

The campaign is something a little different from invasive species, but important nonetheless. In 1958, communist revolutionary Mao Zedong, who became the founding father of the People’s Republic of China, thought it was an excellent idea to introduce a hygiene campaign to get rid of the pests responsible for the “transmission of pestilence and disease”. This was known as The Four Pests campaign.

The pests were mosquitoes (malaria), rodents (plague), flies (disease) and sparrows (the Eurasian tree sparrow, for example, eats grain, fruit and seeds). When the government declared that “birds are public animals of capitalism”, people tried to eradicate the sparrows. The birds would fall from the sky, dying of exhaustion because citizens made lots of noise and wouldn’t let the birds rest on trees. Nests, eggs and chicks were destroyed. People shot sparrows out of the sky, too. Contests were held and prizes given to those who presented the most rat tails, mosquitoes, flies and dead sparrows.

The birds were on the brink of extinction when, in April 1960, ornithologist Tso-hsin Cheng highlighted that sparrows didn’t just eat grain, they also ate insects and because of their deline rice yields were depleting. Mao stopped the campaign against the birds. However, as there were almost no sparrows remaining, locust numbers exploded and food crops were destroyed.

The Four Pests campaign is now seen as a contributing factor to the Great Chinese Famine, where 20-45 million people died of starvation.

Cannibal snail

Euglandina rosea, also known as the rosy wolfsnail or cannibal snail, is a medium-to-large carnivorous predator, which hunts and eats other snails and slugs. It is so effective because it is much faster than its counterparts – it’ll even climb trees to get its next meal and uses slime trails left by potential prey to track them.

Originating from the southern part of the US, the rosy wolfsnail was introduced into Hawaii in 1955 as a biological control for the invasive African land snail.

Unfortunately, native molluscs are now at risk of extinction due to the wolfsnail’s cannibalistic appetite and it contributed to the extinction of Partula tree snails in French Polynesia.

Paper mulberry tree

A silkworm’s favourite food is the leaves of the paper mulberry tree. US entrepreneurs thought that if they introduced the foliage, they could start their own silk industry. Unfortunately, the climate was not appropriate for the silkworm and the mulberry is a highly invasive species, so it began disrupting the natural ecosystem.

As well as the US, it has invaded Latin America and South Asia. The tree doesn’t have any native predators and has started replacing natural flora in the areas it inhabits.

The mulberry tree consumes an extremely high amount of water, which chokes the native foliage. Its root systems are also very strong and fast-growing – they tend to cause problems with drainage pipes.

Small Asian mongoose

This mongoose was originally introduced to Hawaii in 1883 after native sugar cane farmers heard about Jamaican plantations unleashing the predator to control rat populations.

It was a mistake of epic proportions. Unfortunately, the targeted rats are nocturnal and the exotic mongooses are diurnal, so they never crossed paths. Rather than rats, the mongoose began eating the native birds instead.

An opportunistic feeder, the mongoose also enjoys baby birds and turtle eggs. When food is abundant, it will cleverly avoid traps. Mongoose-proof fencing and expensive culling schemes are the only options to remove them.

The exotic predator was also introduced in the Caribbean and Japan’s Okinawa island as a kind of pest control. This was unsuccessful and the mongoose is now a prolific pest.

European starling

A native of Europe, Asia and northern Africa, this attractive breed of starling was introduced to North America, southern Africa, Australia and New Zealand as a biological pest control for insects, as a pet and by people who - strangely - wanted to get all birds mentioned by Shakespeare into North America.

These pesky birds can cause many problems. Their flock sizes often exceed 3,000 and they can decimate agricultural crops as they feed on fruit and grains. They are also aggressive, so will happily fight with native birds over food and territory. Starlings are savvy at finding places to nest (fence posts and roof linings, for example) and quickly fill up tree hollows, leaving other birds struggling to find a spot. Starlings regularly take over other species’ nests, meaning native birds find it difficult to lay eggs or raise their young. They are also seen as a public nuisance, as they damage infrastructure.

Africanised honey bee

Also more famously known as the killer bee, it was imported and bred with European honey bees to increase honey production. However, as the breed is more aggressive than its European counterparts, it has infiltrated and killed off native hives.

Africanised honey bee swarms can invade European honey hives, where they kill the queen and replace it with their own leader. This has also impacted on honey production, as they’re quite bad at making it.

The bee was made when a Brazilian beekeeper accidently released some Tanzanian queen bees which then mated with native European honey bees, creating the killer bee.

Although its sting has about the same toxicity as the European’s, the Africanised honey bee is known to repeatedly sting with some people falling victim up to 1,000 times.

Mosquitofish

This tiddler originates from southern parts of Illinois and Indiana, throughout the Mississippi River, USA. Found in shallow water away from larger fish, the little creature was intentionally introduced to areas with large populations of – you guessed it – mosquitoes to decrease the number of bugs by eating their larvae. However, native fish were already good at supplying ‘maximal control’ – introducing the mosquitofish has turned out to be more damaging to aquatic life.

Mosquitofish are aggressive and injure or kill other small fish. They are also very good at breeding, so take over the natural habitat.

In Australia, the fish are considered ‘noxious pests’ and threaten native fish and frogs. There is also no evidence that they have contributed to controlling the mosquito population there.

Yet they have been useful in history. From the 1920s to the 1950s, mosquitofish helped destroy the malaria outbreak in South America, southern Russia and Ukraine. In 2008, they were bred in some aquariums in California and Clark County, Nevada, so stagnant pools of water could be filled with the fish to reduce cases of West Nile virus.