80% drop recorded in insect biomass

7th June 2017
Malaise trap as used by the Krefeld Society

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There has been lots of talk and concern about decline and loss of species in the UK and beyond, with the usual focus on economic  insects like honeybees, and the iconic rare species on which conservationists tend to concentrate their attention.  We are certainly aware of population declines in these species, but it has been more difficult to put figures on the overall decline (or increase) in total numbers of insects or their biomass. After all – as rare insects decline, perhaps the more common ones simply become more abundant?

In an article in the online Science magazine of the prestigious American Association for the Advancement of Science, Gretchen Vogel recently pulled together data and opinions from several sources, especially some long term German studies by the Krefeld Entomological Society.  To say with any confidence what has been happening to insect populations, accurate, regular repeated sampling is needed.  This isn’t often possible for professionals, because the time frame of research grants is so limited, and many of the best studies, like this, are conducted by skilled amateurs.

Using Malaise traps, which very efficiently sample flying insects, the Krefeld group carefully replicated sampling in semi-natural habitats and have datasets going back to 1989 with preserved bulk samples which can be compared across the years.  They compared results from 1989 and 2013.  For two traps set between May and October, the total catch dropped from 2.54 kg to only 550 gm, which is a 78% decline.  Thinking this might be an anomaly, the subsequent years’ catch in 2014 was found to be equally as bad.
This is a shocking and unexpected figure.  Even if you aren’t too keen on flying bugs, they are the food for pretty well all the other creatures in gardens and nature reserves.  As Professor Dave Goulson of Sussex University put it: "If you're an insect-eating bird living in that area, four-fifths of your food is gone in the last quarter-century, which is staggering," Dave is working with the Krefeld group to get the most out of their exceptional data, but assigning one or more causes to the observed decline will be very difficult.

Across much of the world, we are seeing a steep decline in numbers of flying insect feeding birds like swifts and swallows.  If the data from Germany are widely applicable, insect biomass decline may be a major factor.  From my own experience, comparing driving at night in the late 1950’s with the present, the decline in night flying moths is spectacular – it’s rare for me to see any moths now, and insect-splattered windscreens are a thing of the past.

It’s well worth reading Gretchen Vogels’ article to pick up on other related observations, and some speculation on causes.