The disappearance of insects makes it difficult to grow food

In the past 20 years, many scientific articles have reported the disappearance of insects, which are less than they used to be. Both the combined weight (what scientists call biomass) and the diversity of insect species decreased.

Some studies were based on sightings by amateur entomologists, while others involved scientists counting the number of insects splattered on car windshields. Some collected flying insects in traps annually for years and weighed them.

In the last six years, these studies have become more forceful and, at the same time, more sophisticated, confirming that, although not all insect species are in decline, many are in serious trouble. A 2020 compilation of 166 studies estimated that insect populations were declining on average globally at a rate of 0.9% per year.

But the falls are irregular. Even within the same environments, populations of some insect species declined, while others remained stable and still others increased. The reasons for these differences between insects are unknown, although evidently some are more resistant than others.

New studies on insects

Until recently, much of the evidence came from protected areas in Europe and, to a lesser extent, North America. So how is the picture elsewhere? A recent study offers new data on seasonal insect migrations in East Asia. These insects, many of them pest species, fly north in the spring each year to take advantage of the new growing season and fly south in the fall to escape the cold.

A progressive drop in the huge number of these migrants indicates that insect decline is indeed a global problem.

Millions of migratory insects

Between 2003 and 2020, scientists at the Chinese Academy of Agricultural Sciences in Beijing captured nearly 3 million migratory insects in high-altitude reflector traps on Beihuang Island, off China’s northeast coast. Another 9 million insects were detected from radar records.

In total, 98 species were identified and counted, most of them pests of crops that feed on plants or insects that are their natural enemies: predators and parasites. Over the entire 18-year period, the annual count of all identified insects dropped by 7.6%, a steady downward trend of 0.4% per year.

Clearly, insect decline is happening on a massive scale in Asia, as well as Europe and North America. It seems reasonable to assume that the causes are the same. While we don’t know for sure what these causes are, it seems likely that they operate worldwide.

The study also showed that insect pests such as the black moth, whose caterpillars prey on a wide range of vegetables, are just as strongly affected by the global insect decline as non-pest species such as bees and butterflies, which were the target of most insect pests. attacks. of earlier European and American studies.

Insects are not just pests

We’re so used to thinking of insects as pests that it’s tempting to think that in a world with fewer insects, agriculture could thrive like never before.

This new study reveals why that’s not the case. The researchers used detailed entomological records from the past to construct a complex food web, showing how each of the pest insect species caught in reflector traps can be eaten by various types of predatory and parasitic insects, often referred to as “enemies”. . For example, caterpillars of black caterpillars are eaten by green lacewings, among others.

The researchers compared how quickly 124 pests subsided along with each of their natural enemies. Over the 18-year study, the abundance of natural enemy species declined at a rate of 0.65% per year, while herbivorous prey did not decline on average.

This suggests that beneficial species of natural enemies are more likely to decline than the pests they feed on. As a result, farmers must either tolerate lower yields or use even more chemical insecticides to control pests, leading to even worse declines.

Disappearance of insects, not only pesticides are responsible

While it’s tempting to point the finger at pesticides, street lights or climate change, insect declines almost certainly have multiple, overlapping causes.

The most cited suspect is agricultural intensification. This term covers a multitude of sins. Agricultural mechanization, eradication of hedges, monocultures of crops, increased use of chemical fertilizers and regular application of pesticides aim to produce fields free of weeds, pests or diseases.

Only a small variety of plants and wild animals can survive on the narrow edges of the fields and the adjacent roadsides that still remain. Another way of saying this is that farmers have made the fields unfriendly to most insects.

Intensification is designed to ensure that as much of the agricultural ecosystem’s energy flow as possible is diverted to producing crops and livestock for human consumption.

It is estimated that humans now appropriate 24% of all annual plant growth, and this increases to a staggering 69% on farmland.

These numbers practically doubled during the 20th century. No wonder insects don’t do well in landscapes like these, and farmland takes up almost 40% of the land.

Impact on biological chains due to the disappearance of insects

Insects are by far the most numerous of all animals on Earth. The estimated global total of new insect biomass growing each year is 1.5 billion tons. Most of it is immediately consumed by an ascending food chain of predators and parasites, so the imposing superstructure of all of Earth’s animal diversity is built on a foundation of insects and their arthropod relatives.

If insects decline, then other wildlife must inevitably decline as well. There is already evidence that this is happening. In North America, bird species that feed on insects have experienced an average decline in population size of nearly 10 million over the past 50 years, while those for which insects are not essential prey have not declined. In Europe, parallel declines in insectivorous swallows, house martins and swifts have been associated with insect declines.

The economic value of services provided by insects

While it’s true that some insects are a threat to humans (disease-carrying mosquitoes come to mind), the vast majority of insects are friendly: they pollinate crops, provide natural pest control, recycle nutrients, and form soil by helping to decomposition of dead insects, animals and plants.

All these processes will slow down if insects become scarce. The economic value of these services is incalculable: agriculture could not continue for long without them.

Our insect friends are being displaced. Somehow we must find ways to create more space for them.

This article was written by Stuart Reynolds, Emeritus Professor of Biology at the University of Bath.


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