Flying Insects Have Decreased in Number by Over 75 Percent in Recent Decades

Entomologists have long suspected that insects have been decreasing in number over the years, but a new study shows the decline's severity.

Insects have been one of the most plentiful and successful life forms on the planet. According to the Entomological Society of America, they are the cornerstone of our ability to survive on Earth, and they outnumber humans by more than 1.6 billion to one.

It is hard to imagine what would happen if insects were not around to pollinate crops, to serve as a food source for countless animals, and to provide valuable genetic information that helps to inform life-saving research.

This rather drab, imagined world — one absent colorful butterflies and busy honeybees — may have to be considered in light of new research published in the journal PLOS ONE, which found the total flying insect biomass declined by more than 75 percent from 1989 to 2016 in Germany.

“We were highly surprised by the extent of the decline of over 75 percent, which took place in just 27 years,” lead author Caspar Hallmann of Radboud University’s Institute for Water and Wetland Research told Seeker. “And what’s more, this decline happened in nature reserves that are meant to preserve biodiversity and ecosystem functioning. This is alarming.”

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Hallmann, senior author Hans de Kroon, and their colleagues on the study long suspected that flying insect numbers were declining, based on other research and their own observations. To gain a better understanding of the situation, they measured total flying insect biomass using Malaise traps placed within 63 nature protection areas in Germany.

Malaise traps are large, tent-like structures that have been in use by scientists since the early 1930s. Insects fly into the “tent” before being funneled into a collecting vessel attached to the structure’s highest point.

Hallmann explained that the type of data provided by Malaise traps is bulky and hard to analyze at the species level.

“However,” he continued, “when you get an over 75 percent decline in total insect biomass, you know this is not due to a few or vulnerable species. This must be a decline in the entire spectrum of flying insects, from primary consumers like leaf-chewing bugs, all the way to parasitoid wasps and large bumble bees.”

The researchers chose to investigate flying insects in particular because, in Germany, out of the approximately 33,000 insect species that are present, over 90 percent are those that fly.

While no corresponding data over the same study period is available for non-flying insects, “we can just hope they are faring better, but we have no reason to believe that is the case,” Hallman said.

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Although Germany was selected as the location for the study, the findings likely apply to other countries. Many flying insect species travel long distances, going across country borders and even oceans.

The meticulous sampling of flying insects over so many sites and so many years yielded a dataset that is unique in the world, de Kroon told Seeker. In order to minimize the impact on flying insect communities, the team did not investigate each location annually.

“Therefore,” de Kroon continued, “we can only assess the overall decline over the study period, and are not able to look into the temporal variability in the rate of decline.”

He added that he and his team observed a decline in plant species richness locally in the vicinity of the traps, although that cannot yet be attributed to the decrease in numbers of insects. Prior research further shows insect-eating birds numbers have dramatically declined throughout Western Europe.

It then appears that plants, flying insects, and many birds — all of which have lives that are intertwined within their ecosystems — are decreasing in species diversity, if not in other respects.

The scientists were surprised to find that the decline in flying insects was apparent regardless of habitat, changes in weather, or land use. While each are clearly important to ecosystem health, none could fully explain the observed decrease in numbers of flying insects, which indicates that other large-scale factors are responsible for the declines.

“The protected areas in which this study was undertaken are relatively small and surrounded by agriculture,” Hallmann said. “This suggests a possible role for, for example, fertilizer and pesticide application.”

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De Kroon said, “A decline this severe calls for increased enforcement of all actions that are known to be beneficial to insects, such as establishing flower-rich patches in arable field margins, and minimizing the effects of agricultural practices such as pesticides.”

The researchers hope to determine how the decline in flying insects breaks down across the various insect families and species, each of which has their own specific ecological functions. Doing so could offer additional clues to what primary factors are responsible for the decreased numbers.

The scientists are currently investigating sites in the Netherlands to better determine how pesticides are impacting insect populations, and how those losses are affecting insectivorous bird species.