Feature pests| PPC88 September 2017

Simon Forrester investigates whether or not complete eradication of a species is the only way to preserve public health, and the hidden impacts of wiping out a pest organism.

Speed view

• The only way to be sure?
• Where has eradication has been successful?
• Where has eradication has failed?
• Top contenders for UK-wide eradication

Our constant battle against pests is, on balance, probably unwinnable. Nature will always find a way, and evolution generates new variants of pests better adapted to deal with whatever poisons, traps and exclusion measures we throw at them. So is the answer to completely remove the species in question?

Before we start killing off an entire population, let’s get our facts straight, and think hard about the role these species play in their ecosystem, and our planet’s. The law of unintended consequences applies. For example, by banning the use of DDT to prevent harm to people who might be consuming trace quantities, we effectively crippled the fight against malaria leading directly to millions of avoidable deaths around the developing world while, perhaps, having limited impact on human health.

Eradication programmes have met with mixed success through the years, and some methods that at the time seemed innovative, have resulted in different and, in some cases, worse problems than before.

According to tradition, Saint Patrick, a fifth-century Christian missionary, chased Ireland’s snakes into the sea. And it’s easy to see why this story has survived. Islands are possibly the best places from which to eradicate a pest. You are surrounded by an impenetrable barrier (water) so the pests can’t quickly retake the ground, you can control movement in and out (via border patrols), and it’s a finite area of land to manage. So is the UK ripe for the removal of a pest species?

The success of an eradication programme is limited by the scale and scope of the problems, the terrain and, of course, the prevailing conditions whether climatic, economic or regulatory. It’s clear that a range of pest species cause problems for UK plc. But how much do we really want to spend to deal with them? Perhaps the recent influx of Aedes albopictus (Asian tiger mosquitoes) will make our government wake up and realise the impact of pest species on public health. But I wouldn’t bet on it in the current climate.

Ethically, we may also be on dodgy ground. Do we have a right to destroy a species? Humans have been responsible for the extinction of a wide range of creatures. Scientists at the Center for Health and the Global Environment estimate we’re now losing species at 1,000 to 10,000 times the background rate of extinctions, with literally dozens becoming extinct every day. Their data predicts we will lose around 30-50% of all species by mid-century, mostly because of human activities; primarily those driving habitat loss, the introduction of invasive species, and global warming. The loss of one species can cause a chain reaction in a delicate ecosystem. If we carry out a planned extinction of a pest species, what will be the larger scale effect? As can be seen from the example of failed eradication projects, removing a species can have unwanted and significant effects – as can the introduction of an invasive species.

Effective control of a spreading or established pest species requires a coordinated approach across borders. Public education programmes and trade restrictions can have a positive effect to prevent new introductions of non-native or eradicated species.

Improved checks of incoming items can close borders to these immigrants and prevent a reoccurrence.

However, the spectre of public opinion will affect the success of any eradication programme. The UK is a nation of animal lovers, and there is always someone who’s a fan of a particular pest, no matter how crazy it might seem.

One final thought: the more cynical reader will say, “That’s all very well, but what about the impact on my business model if we kill off the rats or roaches?” Well, the response is that Mother Nature will bring a different set of pest problems to our door, whether from adaptation or new invasives. Either way, pest control is still here to stay.

Where eradication has been successful

Hogging all the crabs: Clipperton Island

The tiny island of Clipperton sits about 800 miles out to sea from Acapulco Mexico. It was once the home to millions of crabs and seabirds as well as a stopping point for sailors as far back as the 1700s, and at some point, people tried to settle there.

While the settlers left, their livestock remained and, unfortunately, their drove of pigs developed a taste for the crabs and bird eggs. In the 1950s American ornithologist Ken Stager came to the island to collect bird specimens for the National History Museum of Los Angeles County.

Seeing first hand what the pigs were doing to the birds, Ken took out his shotgun and killed all 58 feral pigs on the island in a single visit. Clipperton’s wildlife has since recovered with thousands of crabs and seabirds reclaiming the island.

Tightening the screw on The Screwworm

Eliminating a pest species in an island environment is significantly easier than tackling a mainland problem.

The southern United States and Mexico had serious issues with screwworm – the larvae form of Cochliomyia hominivorax (blowflies). The larvae feed on living tissue and cause a parasitic infestation within live mammals – hominivorax translates roughly as ‘man-eating’. The agricultural industry was losing millions of dollars annually due to treating fly-struck cattle.

In 1958, the United States Department of Agriculture approved the use of the ‘sterile insect technique’ to eradicate the pest. The female screwworm fly generally can only mate once in a lifetime and retains the sperm for fertilisation of all the egg batches it will ever produce. Because of this, screwworm populations could be suppressed by releasing factory-reared sterile screwworm flies.
At the height of the programme, 155 million flies were being released weekly. The screwworm was completely eradicated from the southern United States in 1966 and from Mexico in 1991.

Invasive goats removed from Darwin’s garden

Clipperton isn’t the only island to have had it’s wildlife saved through eradication.

The Galapagos Islands have some of the most diverse wildlife in the world. Each island has unique wildlife, which has made them a place of pilgrimage for many biologists and naturalists. An unlikely pest, (again brought to the shores by humans) ravaged the land and wildlife: goats. Conservationists struggled to control the goat populations as they’d hide from hunters, climbing further into the hills, only to reproduce and start to ravage the landscape again.

In the end, Karl Campbell led a huge effort to rid the island from goats once and for all. 160,000 goats were killed with guns, dogs and helicopters. Conservationists even used radio-collared, super-sexed female goats to lure hiding males out into the open.

Reclaiming South Georgia from the rats

This sub-Antarctic British Overseas Territory was host to probably the largest ever island eradication mission. The five-year project, led by South Georgia Heritage Trust, saw over 1,000 square kilometres treated in three phases between 2011 and 2015.

Rats, brought to the island 200 years ago, had destroyed around 90% of the seabirds. Many of these seabirds have no natural defence against rats, and therefore chicks and eggs were consistently getting eaten.

The Trust used bait dropping helicopters to ensure that it was distributed right across the island. Although the bait would inevitably kill some seabirds, experts carefully worked out that those losses could be sustainable.

It will take more than a decade to be sure that all the rats on South Georgia are gone, but experts can say with some confidence that the numbers of seabirds on the island are starting to recover.

Where eradication has failed

Cane Toads blight Australia

Cane Toads were introduced by the Australian Government from Hawaii in June 1935 in an attempt to control the grey-backed cane beetle (Dermolepida albohirtum) – a native species also detrimental to sugarcane crops. Adult cane beetles have heavy exoskeletons, and their eggs and larvae are often buried underground making them difficult to exterminate. Cane toads were to replace the use of pesticides like arsenic, pitch and copper.

Once released the cane toads spread quickly, and now number over 200 million. Cane toads deplete native species by spreading disease; they poison pets and humans; deplete native fauna; and reduce prey populations for native insectivores, such as skinks. Unfortunately, the introduction of the toads has not only caused large environmental detriment, but there is no evidence that they have affected the cane beetles they were introduced to predate. The toads have steadily expanded their range, evolving larger legs. In 2014, Professor Shine, from the University of Sydney indicated that the migration rate had increased to 60km per year, and is seemingly unstoppable.

There is a current battle over whether to introduce another species to predate or infect cane toads.

Borneo’s Rats and Parachuting Cats

In the 1950s, a malaria outbreak occurred among Borneo’s Dayak people. The World Health Organization (WHO) tried to alleviate the problem by spraying their thatch-roofed huts with dichlorodiphenyltrichloroethane (DDT). The DDT killed the malaria-bearing mosquitoes but also killed the parasitic wasp that kept thatch-eating caterpillars under control. At night the buzz of the malarial mosquitoes was silenced, but creaks and then screams followed as people’s roofs collapsed.

But this was hardly the end of the problem – geckos ate the toxic mosquitoes, slowing them down. They became prey for feral cats, which then died, allowing the rat population to grow ten-fold.
Rats were everywhere, scurrying over and through the Dayak’s roofless huts, spreading diseases like bubonic plague – a condition that’s even more serious than malaria.

The WHO was rightly afraid of additional disasters that might occur if they poisoned the rats, and decided to re-introduce the rats’ natural predator right back into the remotest parts of Borneo. So one morning the locals were woken by a plane flying overhead, dropping parachuting cats! Operation Cat Drop saw 14,000 felines fall onto Borneo, reducing the rat population and restoring some balance to the country.

I vant to kill your bats: South America’s ongoing battle

Vampire bats (Desmodus rotundus) have a scary reputation. In Latin America they destroy lives and livelihoods, feeding on pigs, cats, even children. Their hearing is sensitive to low-frequency sound such as large animals breathing, their teeth are so sharp their bite can’t be felt, and their noses have infra-red heat detection meaning they can detect veins beneath the skin of mammals. According to Wired online magazine, rabies costs farmers in South America $30m per year and kills dozens of people. Once the symptoms appear, it’s too late to save the patient.

Cattle ranchers in Panama want the government to eradicate the bats as a species. The Panamanian government has tried to eradicate the bats by spreading a toxic paste onto the backs of the bats, which is then spread throughout the colony. However this seems ineffective as it’s only a matter of time before more bats take their place, actually increasing the spread of rabies.

Another common practice is the use of flamethrowers to exterminate bats, and the dynamiting of bat caves. Sadly, the exterminators often have little to no formal training in identifying a vampire bat, and so non-target insectivorous bat species are threatened. This has a knock-on effect of increasing the local bug population, and thus the number of disease vectors harming local people and their livestock.

Hawaii’s rat problem becomes a mongoose problem

In 1883, a sugar mill in Hawaii decided to import 72 mongooses in an attempt to stop rats eating their sugar cane. As it turns out, the mongoose is diurnal, and the rats are nocturnal. The result was that with virtually no natural predator the mongoose population increased exponentially and wound up devastating the native (much easier to catch) bird population instead.

Top contenders for UK-wide eradication?

Grey Squirrel (Scuirus carolinensis)

The grey squirrel was introduced to the UK in 1876 and across Europe just after World War II and has swiftly taken advantage of the lack of natural predators to establish themselves. Both reds and greys have similar diets, habitats and activity patterns. The greys displace reds, eating food stores and spreading squirrelpox, a disease that kills the reds in around a week. The invasion of the grey squirrel in the United Kingdom can be classified as a Disease Mediated Invasion (DMI), and some observers have classed grey squirrels as biological weapons.

Many programmes have sprung up to trap, poison, shoot or otherwise control greys, and this has helped the re-establishment of pockets of reds across the UK. Selectively-timed culls can also have an effect of pushing back greys from an area. Alternatively, non-lethal methods of population control, such as fertility treatments, are being developed, although the effectiveness of these treatments is unknown.

The European pine marten (Martes martes) is a natural predator of grey squirrels, and its reintroduction in areas where greys are dominant may help reduce populations, as the pine marten and red squirrel can co-exist. Non-lethal treatments are being developed such as fertility drugs, but their effectiveness is as yet unknown.

Mosquito (Aedes)

Potentially the most dangerous animal on the planet, the UK has over 30 native species of mosquito, many of which don’t bite people or pose a significant risk to public health. However, with confirmed reports of yellow fever mosquitoes (Aedes aegypti) and tiger mosquitoes (Aedes albopictus), the UK could see the zika virus, Chikungunya and a whole host of other nasty diseases spread to our shores.

Monitoring projects are being carried out by Public Health England at major ports and airports across the UK and, currently, it is believed that neither species has taken root or spread further afield.
However, the UK may have a role to play in the global eradication of the pest.
Genetically modified ‘sterile’ mosquitoes, created by the Oxford-based biotech firm Oxitec Ltd, are already being trialled around the world. Male mosquitoes have been genetically engineered to have a ‘kill switch’ so that their offspring die before reaching maturity. Male mosquitoes don’t bite or spread disease meaning these new GM mosquitoes have no significant adverse effect on human health.

Trials in Brazil, Panama and the Cayman Islands have seen fantastic results, reducing the number of Aedes aegypti by more than 90%. In comparison, control methods such as using insecticides have seen an efficiency rate of around 30-50%.

Cockroach (Blattella germanica, Blattella asahinai)

Although cockroaches pose a significant threat to food safety in the UK, eradication would be no easy task. It’s often said that cockroaches would be the only species to survive a nuclear holocaust. Mythbusters even put this to the test by subjecting German cockroaches to varying levels of radiation. An impressive 10% of roaches survived 10,000 radon units of exposure (as a comparison, the Hiroshima bomb emitted gamma rays at around 10,000 rads). Cockroaches are nature’s perfect survivors for numerous reasons. Firstly, their cells divide far more slowly than other organisms cells, giving them more time to fix the problems caused by radiation, like broken strands of DNA.

Secondly, they adapt incredibly quickly. In the 1980s, sugary roach-bait was an effective pest-control strategy, at least at the time. By 1993 the toxins stopped working as their internal chemistry changed so glucose tastes bitter to them. This trait was passed through the generations. These beasties can even reproduce without the need for males. In extreme situations, cockroaches are capable of what scientists call parthenogenetic reproduction – or virgin births. Oh, and they can continue to live without a head.

A matchbox-sized robot cockroach has been developed that can infiltrate a group of cockroaches and influence their collective behaviour. The robot smells and acts like the real thing, fooling the insects into accepting it. The robot can use its programmed behaviour to persuade the group to, for example, venture out into the light despite a preference for the dark. In theory, you could program a robotic cockroach army to lead the real thing out into the open to be destroyed like some sort of sci-fi version of the Pied Piper.

In reality, it’s unlikely that we’ll see the cockroach disappearing from the UK anytime soon.

Brown Rat (Rattus norvegicus)

We’ve already shared a project that’s well on the way to eradicating rats on a small island, so why not apply the theory to the UK mainland? Well, first of all, nobody would ever seriously recommend dropping millions of tons of bait from helicopters around Britain due to the environmental cost alone – never mind the enormous expense.

It’s potentially impossible to rid the UK of rats completely. Throughout human history, wherever there have been people, rats have followed, poisoning our food, causing fires, decimating crops and spreading disease. When a single pair of rats can create up to 15,000 descendants in a lifetime, and there are potentially over 10 million rats in the UK – it seems total eradication is unlikely.
Additionally, what else do we have to fight them with? Over the last hundred years, we’ve poisoned them, invented elaborate traps, trained cats, dogs and ferrets to catch them, made ultrasonic machines to drive them away – Rikers Island, New York even tried to use mustard gas to get rid of them once and for all. But still, rats remain.

A new weapon in our arsenal might soon make it at least plausible that we could one day be rat free.

In 2015, New York City invested around $3million on rat control after it became increasingly apparent that the city was losing the war on rats. Now officials are trying a new liquid bait which supposedly makes rats infertile but is otherwise non-toxic to the rest of the environment. The key chemical 4-vinylcyclohexene diepoxide (VCD) destroys female rats’ ovarian follicles and impairs the sperm production in males.

The trial supposedly begins this year and, if successful, then maybe it will work within the UK.

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Source: PPC88