Here's how we know that the UK's insects are dying out .

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In a corner of Hertfordshire, in one of the oldest agricultural research institutes in the world – the Rothamsted Research Institute, founded in 1843 – entomologist Chris Shortall spends his days counting and categorising mounds of moths, aphids and beetles.

In fact, that is exactly what he has been doing ever since he joined the Rothamsted Insect Survey (RIS) in 2003 – a branch of the Rothamsted Institute that has been dedicated to the tracking and study of all types of bugs in the UK since 1964. The RIS has existed for almost 55 years, making it the longest-standing insect monitoring facility of its kind in the world. It has over 80 traps spread around the country to sample insect populations every day – over the years, this has come to a total of up to 250,000 samples available, containing about 120 million insects.

For entomologists like Shortall, this is a goldmine. And it has become all the more relevant in recent years, as successive scientific papers have warned about the catastrophically declining insect numbers across the planet. Earlier this week, a scientific paper reviewing 73 of the most comprehensive studies into insect decline concluded that 40 per cent of insect species are declining with the total mass of insects falling by 2.5 per cent every year.

Some of the most recent research was conducted in the Puerto Rican rainforest and found that 98 per cent of ground insects had disappeared in the last 35 years. That came on top of a study published in 2017, which concluded that flying insect populations in nature protection areas around Germany had declined by up to 82 per cent in the past 27 years.

For Henk Siepel, professor of animal ecology at Radboud University and co-author of the study, there are a number of causes that can explain such a severe decline – but the most likely culprit is the rise of pesticide use. “There is no evidence yet that this is the main reason,” he says. “But the consequences of intensive agriculture on the environment are already well known. And we can easily hypothesise that it has also affected insect population.”

That is reason to worry, because insects are the base of the food chain. As Shortall explain, they are fundamental to our ecosystem, as without them, birds and mammals couldn’t feed themselves. In that context, keeping an eye on insect population trends is crucial – and the RIS does that through two networks of traps.

The first one is a network of suction traps. At 12.2 metres high, these traps function like a giant upside-down hoover with a motor and an exhaust pipe. They can capture any insect flying within a small range of the air column. Those suction traps are made to capture aphids, which tend to fly relatively high above the ground and are infamous among farmers because of the damage they can cause to their crops.

There is a network of these suction traps around the country to monitor aphid populations. Its purpose goes beyond research: one way to reduce pesticides is to adapt their usage to the presence (or not) of crop pests. This network has existed since the sixties, established with government funding by insect ecologist Roy Taylor shortly after the publication of Rachel Carson's Silent Spring, an environmental science book documenting the danger of using pesticides.

Samples from the suction traps are changed every day and sent back to Rothamsted for professional entomologists to analyse and present to farmers in the form of a weekly bulletin that helps them manage pest control.

Without the suction trap network, the RIS couldn’t afford to run the second network of traps it has developed – light traps. These emit light to attract moths and other nocturnal insects, to sample a wider diversity of species in a given area. The samples from light traps are analysed by volunteer entomologists, and the data they produce exists to satisfy curious minds and fuel academic research alike.

“In that respect, the history of the RIS started as early as 1933,” says Shortall, “when entomologist C. B. Williams joined the Rothamsted Institute. He had previously studied butterfly movements in the Caribbean Islands, and was interested in insect migration.”

C.B. Williams is believed to have designed a first version of the modern light trap to track moths during his time in Egypt, where he was working with the Ministry of Agriculture. Back in the UK, he kept trapping moths around the time of the Second World War.

Roy Taylor, who would go on to found the RIS, started working with Williams in the early 1960s, and, in parallel to monitoring aphids, he carried on the research that his associate had started on moths.

Fast-forward to the present day, and the RIS is sitting on half a century of data on the UK’s moth population as well as other nocturnal species. And the verdict seems to confirm the trends revealed by other entomologists: the number of moths has declined by about two thirds since 1968.

The data, however, needs to be interpreted with caution, explains Shortall. While some moth species have almost disappeared – the V-moth, for one, has declined by 99 per cent – others have actually grown. Populations of the common footman moth, for example, have increased by 1,500 per cent.

What’s more, he continues, numbers don’t account for other trends. In Scotland moth numbers have remained stable, but the centre of gravity of the population, which is the place where the species is most common, has moved further north.

“We can only assume that this is because of warmer temperatures,” says Shortall. “But identifying any one driver for changes in insect behaviour is next to impossible. There are too many variables: it could be climate change, urbanisation, changes in agricultural methods, the use of different pesticides, and many more.”

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