Pesticides: farming chemicals make insects sick at non-deadly doses – especially in hot weather

The various regulatory systems for approving pesticides in operation around the world are crude and flawed. This has long been clear to scientists and it is deeply worrying, as this regulation is supposed to protect people and the environment from harm.

The EU regulatory system for pesticides is arguably the most rigorous in the world, yet it has repeatedly approved the use of pesticides that have subsequently been found to cause harm to humans or wildlife, leading to eventual bans. It often takes decades for the harm to accumulate before it is recognised.

The history of pesticide use is littered with such examples: DDT, parathion, paraquat, chlorpyrifos, neonicotinoids, chlorothalonil and many more. Most pesticides that were once deemed safe for humans and wildlife that aren’t the target, like bees, have since been banned. This ought to tell us that the regulatory system is not working.

A new study offers yet more evidence. Research by the European Molecular Biology Laboratory shows how pesticide tests focus on the death of an animal and ignore any important “sublethal” effects.

If a creature, such as a honeybee, is alive 48 hours after exposure, then it is deemed that all is well, and the chemical may be approved for use. The bee may be unable to fly or navigate, or its immune system may no longer function, but that is not recorded.

Multiple regulatory failings

There are many other failings in UK and EU pesticide regulation.

Regulatory tests assess the “active substance” in a pesticide, but farmers use products with lots of extra ingredients that can amplify its toxicity. Strangely, the product used by farmers is not evaluated.

Tests to ascertain how deadly new pesticides are for wildlife are often done in-house by the companies seeking approval. This research is rarely made public as it is considered commercially sensitive.

Tests focus on the short-term (often 48-hour) effects of exposure in healthy test animals, such as honeybees, predatory beetles or zebra fish. In reality, exposure may last for weeks, months or years, and its effects may be cumulative.

Tests also focus on exposing subjects to a single pesticide, when wild organisms – and humans – are exposed to complex mixtures of pesticides, some of which act synergistically (meaning the harm they do is more than the sum of the effects of each chemical in isolation).

In the new study, the researchers used the larvae of fruit flies (Drosophila melanogaster) as a model species. This provided the scientists with huge numbers of test insects with which to study the lethal and sublethal effects of 1,024 different pesticides (almost all of the chemicals available to farmers worldwide, predominantly comprising insecticides, fungicides and herbicides).

The researchers exposed these larvae to a range of concentrations of pesticides, spanning what insects are likely to encounter in cropland, and subsequently measured aspects of their behaviour, physiology, fitness and survival over time.

Herbicides and fungicides harm insects too

Several of this study’s findings highlight inadequacies in pesticide regulation.

First, many non-insecticides kill insects. Farmers often avoid spraying insecticides when beneficial insects such as bees are active and instead spray late in the evening. They don’t usually worry about when they spray chemicals designed to target weeds and fungi. The new study suggests that it would be safer to assume that all pesticides can harm insects.

Second, many non-insecticides killed few if any insects during the 16 hours for which they were exposed to them in this study, but many died in the following ten days. Clearly, only assessing short-term effects misses the total impact.

Third, 57% of the pesticides tested affected the behaviour of insect larvae, including 382 non-insecticides, demonstrating that sublethal effects are widespread.

Fourth, the researchers found that the effects of pesticides on insect survival were often much higher at elevated temperatures, something not examined by any regulatory system in the world.

Exposure to a concentration of less than one part per million of the insecticide lindane, for example, killed no insects at 25°C but killed 79% of them at 29°C. This is obviously relevant to climate change, and particularly to the increasing frequency of heatwaves. We should perhaps not be surprised that organisms struggle to cope when faced with multiple sources of stress at the same time.

There have been attempts to introduce more rigorous regulations that include assessing the sublethal and chronic effects of pesticides. In 2013, the European Food Standards Agency published a revised protocol for safety testing of the effects of new pesticides on bees with a group of independent scientists. Eleven years on and the protocol has not been adopted due to stiff opposition from the pesticide industry, which argues that it would be more expensive to implement.

We are in the midst of a biodiversity crisis. A recent study estimated that wild populations of vertebrates have declined by 73% since 1970. Insects are less thoroughly monitored, but recent reviews estimate that their populations have fallen dramatically and continue to decline at an average rate of 1-2% a year.

There is lots of evidence that pesticides are contributing to these declines, and that the regulatory system has failed us. Ian Boyd, the UK Department for Environment, Food and Rural Affairs chief scientist, wrote in 2017 that pesticides passing a battery of tests in a lab or field trial are assumed to be benign even when used at industrial scales. “The effects of dosing whole landscapes with chemicals have been largely ignored by regulatory systems,” he said.

Despite this admission by a senior government scientist, the system remains unchanged in both the UK and EU. While this remains the case, insect populations will continue to decline, with consequences for all of us.

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