The recent decline in bugs that fly, crawl, burrow and skitter across still water is part of a gathering "mass extinction," only the sixth in the last half-billion years.

"We estimate the current proportion of insect species in decline -- 41 percent -- to be twice as high as that of vertebrates," or animals with a backbone, Francisco Sanchez-Bayo of the University of Sydney and Kris Wyckhuys of the University of Queensland in Australia reported.

"At present, a third of all insect species are threatened with extinction."

An additional one percent join their ranks every year, they estimated. Insect biomass -- sheer collective weight -- is declining annually by about 2.5 percent worldwide.

"Only decisive action can avert a catastrophic collapse of nature's ecosystems," the authors cautioned.

Restoring wilderness areas and a drastic reduction in the use of pesticides and chemical fertiliser are likely the best way to slow the insect loss, they said.

The study, to be published in the journal Biological Conservation, pulled together data from more than 70 datasets from across the globe, some dating back more than a century.

By a large margin, habitat change -- deforestation, urbanisation, conversion to farmland -- emerged as the biggest cause of insect decline and extinction threat.

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