Researchers say the world has lost more than one fourth of its land-based insects over the past 30 years.

The finding comes from a major study of insect populations worldwide. The researchers noted the loss of such bugs suggests a more complex problem than earlier research has shown.

Ants, bees, butterflies and other land-based bugs have been suffering population drops of about nine percent every 10 years or so. But freshwater insects, such as dragonflies and mosquitoes, have been growing in number over the years.

The researchers found that different areas have experienced different rates of insect decline. They reported their findings in Science magazine.

For the study, researchers examined 166 sets of records from a total of 1,676 sites in 41 countries. Some of the information dated as far back as 1925.

Overall, bugs are disappearing at a rate of just under one percent a year, with the strongest declines reported in the United States and Germany. That is a much smaller population decline than noted in some smaller localized studies, but it adds up to something "awfully alarming," said Roel van Klink. He is with the German Centre for Integrative Biology and was the lead author of the report.

"The decline across insect orders on land is jaw dropping," said Nick Haddad, an expert on butterflies at Michigan State University. He was not involved in the study.

Ongoing decline on land at this rate will be extremely damaging for ecological systems and for humans, Haddad told The Associated Press. He noted that insects are pollinators. They move pollen from part of the flower of a plant to another part. This causes the plant to produce fruit or seeds. Insects also help to break down dead plants and animals.

Van Klink added that insects also serve as food to many animals, making them very important to all Earth's ecosystems. “But at the same time, insects transmit terrible diseases like malaria, Zika and West Nile virus, they eat our crops and damage tree plantations,” he told the Reuters news agency.

The researchers also studied insects that spend at least part of their lives in freshwater. These bugs were found to have experienced a population increase of about 11 percent every 10 years. Freshwater covers only about 2.5 percent of the Earth’s surface, so the majority of insects live on land.

The number of insects on average has declined in the air, in the grass and soil, but not in trees or underground, the researchers found. Some estimates show that land-based insects would face a population drop of 24 percent over the next 30 years. Freshwater bugs would experience a 38 percent increase over the same period.

Insect declines are worst in North America and in parts of Europe. But the decline appears to be leveling off in the United States.

The U.S. Midwest lost 4 percent of its bugs a year. Van Klink noted that the big international losses seem to be around cities and surrounding areas as well as cropland, where bugs are losing their food and habitat.

Other scientists said the findings made sense. But they worried that the study had limited data from some large areas, such as the Middle East, South Asia and Africa.

Van Klink did not find a link to climate change in the loss of insects. But he did note the widespread effects of growing cities and loss of undeveloped areas to agriculture. The researchers said clean-water policies established in recent years were responsible for the increase in freshwater bugs.

Ann Swengel has studied butterflies for over 30 years. Swengel is a citizen scientist and a co-author of the report. She said she saw a sign of hope on a cloudy day last year in Wisconsin. She and her husband counted 3,848 monarch butterflies, providing evidence of recent efforts to improve habitats for the colorful, flying insects.

"It was absolutely magnificent," she said. "It's not too late."