Dragonfly Populations Decline by 50% in Northern Mississippi and Lake Erie

DOUGLAS MAIN wrote: 'Dragonflies are declining rapidly, as seen on weather radar, with billions of dragonflies dying en masse, reflecting the plight of insects worldwide.'

In May, the Hexagenia limbata damselflies just landed on the leaves in Minnesota. The number of Hexagenia limbata dragonflies decreased by 52% between 2012 and 2019 on the upper Mississippi River. Photography: MICHAEL FRANCIS, EARTH SCENES, NAT GEO IMAGE COLLECTION

Every summer, when lakes and rivers are teeming with dragonflies, they swarm across the skies of North America. These insects are particularly abundant in the northern Mississippi River Valley and the Great Lakes, where they are aquatic larvae, transforming into sub-adults and eventually adults.

Dragonflies' massive emergence provides food for various animals, such as perch and other important economic freshwater fish, as well as birds and bats. However, new research shows that dragonfly numbers are declining. A study recently published in the Proceedings of the National Academy of Sciences indicates that since 2012, dragonfly numbers have decreased by more than 50% in the northern Mississippi and Lake Erie regions due to pollution and algal blooms.

'We were surprised to find that dragonflies are declining year after year. This is really unexpected,' said Phillip Stepanian, a bio-meteorologist at the University of Saint Mary's, the main author of the report.

A large group of dragonflies appeared black and white on the rain and snow weather radar. For a long time, meteorologists dismissed these signals as 'noise'. As an experienced meteorologist, Stepanian realized that these signals could provide valuable information, such as the numbers and movements of animals like birds and insects.

Animation shows a dragonfly swarm in late June in Lake Erie, monitored by weather radar. The number of dragonflies may exceed 20 million.

In the paper, Stepanian and his colleagues estimated dragonfly numbers using radar and validated the method by comparing it with the number of dragonfly larvae in the bottom and lake sediments.

Research shows that the number of Hexagenia damselflies in the western Lake Erie decreased by 84% between 2015 and 2019. In the northern Mississippi River Valley, the number of dragonflies decreased by 52% between 2012 and 2019.

The dramatic decline in dragonfly numbers has a significant impact on the ecosystem because insects are an important part of the food chain as prey for many predators. They also transfer tons of nutrients from the water to the land, this is a valuable ecosystem service.

Dragonflies undergo two molting periods. When they transform from larvae to sub-adults, they rest on the shore or on objects like lampposts. Several hours later, they dry out and molt again into adults, flying at dusk to mate—then die. Photography: PHILLIPS STEPANIAN

'Dragonflies play a key role in both aquatic and terrestrial ecosystems,' said Jason Hoverman, an ecologist at Purdue University.

'As a predator, its numbers are crucial, and its decline will affect the consumer's entire food web.'

Possible causes

Several possible reasons for the decline in dragonfly numbers. First, in recent years, the levels of neonicotinoid insecticides have increased in many freshwater systems in Illinois and the Midwest, these chemicals are toxic to many insects. According to a 2018 study, the levels of these insecticides in the tributaries of the five Great Lakes are 40 times higher than the environmental protection agency's water invertebrate protection standards.

Secondly, Lake Erie is plagued by algal blooms, which are caused by runoff containing fertilizers and other pollutants. Algal blooms lead to 'dead zones' with low oxygen levels, which are toxic to aquatic larvae such as dragonflies.

Thirdly, with climate change, water temperatures are also rising, which can affect animal life cycles and reduce oxygen circulation in the lake.

Kenneth Krieger, Honorary Director of the National Center for Water Quality Research, said that dragonflies can be used as a general indicator of water quality, which is why the decline in dragonfly numbers is worrying.

For humans, dragonfly swarms may be a bit annoying or very fascinating. As a predator, they provide food for many organisms, such as fish and birds. Photography: PHILLIPS STEPANIAN

'Other insect species may be experiencing similar declines for the same reasons,' added Francisco Sanchez-Bayo, an ecologist at the University of Sydney. 'Bird, frog, bat and fish populations that feed on insects in these areas will also decline, this is an unavoidable consequence.'

Insect Insights

Other insects face similar difficulties: research worldwide shows that the number of insects is declining. Last April, a study published in Biological Conservation showed that 40% of insect species are declining and may become extinct in the coming decades.

Neonicotinoid insecticides are notorious for their toxicity to aquatic insects; dragonflies seem particularly sensitive to them. Another recent study found that the use of neonicotinoids in a Japanese lake led to a reduction in aquatic invertebrates and a decrease in the number of two commercially important fish species that they feed on.

Dragonfly populations have declined in the past and recovered, but the sustained decline in recent years is worrying, Hoverman said.

'More and more research shows that insect populations are declining, and this study provides another example.'