Humans have been seeking to control and limit the spread of creatures they regard as pests for thousands of years.
Ancient Sumerians – in what is now Iraq – were recorded as having used substances containing sulphur to eliminate insects in 2500 BCE.
Similar actions continue today, of course, and on a large scale. A recent market research report suggested that the pest control industry was worth $24.6 billion in 2022.
But nature fights back, with animals ranging from insects to rodents often evolving resistance to poisons or diseases used to control them.
A recent study of houseflies in Abu Dhabi has demonstrated this, with many populations of the creatures found to have developed resistance to insecticides.
Here we look at this and other ways in which creatures have fought back against human efforts to control them and consider why, in some instances, the advice is to live and let live rather than to kill.
Resilient houseflies
In the study, researchers from UAE University and Tadweer, formerly the Abu Dhabi Waste Management Company, collected houseflies from 19 locations in Abu Dhabi and analysed them genetically.
They wanted to find out whether the insects had “knock-down resistance” (kdr) mutations, which confer resistance to insecticides containing pyrethrin, a substance found naturally in some chrysanthemum flowers.
These kdr mutations were detected in houseflies from 18 out of the 19 locations in Abu Dhabi where they were collected, at a prevalence from around 9 per cent to about 47 per cent.
In their paper, published in Parasites and Vectors, the scientists said that they identified two particular kdr mutations that had not previously been recorded in the UAE.
They also noted that the UAE is one of the few countries in the Middle East and Asia where kdr mutations have been identified.
If there is repeated use of pyrethroid insecticides, it creates what biologists call a selection pressure in favour of gene mutations that confer resistance.
It is a familiar story in evolution: A genetic mutation that confers resistance arises by chance, and individuals with this mutation survive and reproduce more successfully than those without it. In this way, using the insecticide causes resistance in the population to increase.
“As resistance alleles [forms of genes] spread, insecticides targeting susceptible alleles lose effectiveness, necessitating alternative control methods,” the researchers wrote.
“Controlling house fly populations through the sole use of insecticides can drive not only the development of even higher levels of resistance but also environmental chemical pollution, both of which are detrimental to human health.”
Antibiotic resistance in bacteria
One of the most notorious examples of the spread of resistance concerns bacteria that cannot be killed by antibiotics, as they are very dangerous to human health.
According to the World Health Organisation, antimicrobial resistance (AMR)– which covers bacteria, viruses, fungi and other pathogens that are resistant to medicines designed to control them – was directly responsible for 1.27 million deaths worldwide in 2019.
As the WHO has noted, AMR makes it harder to treat many infections and increases the risks of surgery, because infections that develop after surgery may prove fatal if the microorganisms causing them are resistant.
It also puts in jeopardy cancer chemotherapy, because this suppresses the immune system and makes infections common.
AMR could become a much bigger problem than it is now, with forecasts suggesting that it could cause 10 million deaths a year by 2050.
The mechanism by which it develops is similar to that associated with insecticide resistance in houseflies: Individual microorganisms with genes that confer resistance multiply, while that those that lack these genes are wiped out.
The overuse of antibiotics in medicine and agriculture is often blamed for the increasing prevalence of resistant forms of bacteria.
According to Compassion in World Farming, a campaign organisation, 73 per cent of antibiotic use is in farm animals, often poultry and pigs, where it allows the animals to be reared in more intensive conditions that are associated with poorer welfare standards and the spread of disease.
Only this month, UAE researchers reported the country’s first case of a drug-resistant fungus called Trichophyton indotineae, which causes itchy skin lesions. This pathogen evolved from fungi that were susceptible to drugs used to kill them.
Anticoagulants used to poison rodents
It is not only insects, bacteria, fungi and viruses that develop resistance to poisons used to control their numbers – rodents and other larger animals do too.
This is seen in rat populations against which people have put down anticoagulant poisons, which are blood thinners that cause the animals to bleed to death, typically about a week after exposure.
Anticoagulants were used in parts of western Europe from the 1950s and initially resulted in mortality rates as high as 100 per cent.
However, it took less than a decade for evolution to produce rats and mice that were resistant to anticoagulants. These animals are described as warfarin-resistant after one of the key anticoagulants used.
“Warfarin started to be used against brown rats in 1950 in Scotland and by 1958 the first case of resistance was reported,” said Dan Eatherley, an environmental consultant and author of Invasive Aliens, a book about non-native species.
“A gene for resistance had evolved and, by 1972, there were three well-established resistant populations in Britain.”
As a result, scientists developed “second-generation” anticoagulants that were released in the 1970s, although rats developed resistance to some of these too.
Instead of trying to kill rats and mice with poisons, animal welfare organisations such as the RSPCA in the UK suggest using more humane methods to reduce the problems that they cause.
For example, they advise people to ensure that bins or food containers are airtight and able to withstand rodents, and suggest cleaning up after feeding garden birds or other wildlife.
Rats and mice are “neophobic” – uncomfortable around anything new – so the organisation recommends moving around garden furniture and other objects, as this confuses and alarms the animals and may keep them away.
Myxomatosis in rabbits
Myxomatosis is a disease that occurs naturally in rabbit populations in South, Central and South America, where it leads to relatively mild illness.
Caused by a virus that is spread by insects and directly between rabbits, the disease was deliberately introduced into Australia more than once, most notably in 1950.
Rabbits were introduced into Australia in 1859 and their numbers grew dramatically because the animals had few natural threats. Farmers said that they competed with livestock for food and there were concerns about environmental damage.
The 1950 release of rabbits infected with myxomatosis at several locations in Australia had a huge effect on rabbit numbers, with the population estimated to have dropped by 95 per cent.
This was welcomed by farmers, but it was at the cost of significant animal suffering, as the disease causes symptoms such as redness, swelling and ulcers around the eyes and nose, as well as breathing difficulties.
However, the rabbit population in Australia eventually evolved some resistance and numbers partially recovered, albeit not to the levels before the disease was introduced.
A similar pattern was seen after deliberate introductions of myxomatosis in Europe.
“It was introduced in France in 1952 and spread to Britain in 1953, where 99 per cent of rabbits died, but then they evolved resistance,” Mr Eatherley said.
“They are now in equilibrium with the disease and today, in Britain, the rabbit population is half the pre-myxomatosis levels.”
A 2019 study involving the University of Oxford found that resistance to myxomatosis probably developed because of changes to several genes, each of which had a small effect on the rabbits’ ability to fight the disease.
An additional factor to have affected rabbit numbers is another viral condition, rabbit haemorrhagic disease, which was first detected in China in the mid-1980s and has since spread widely.
