According to this study:
http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0185809More than 75 percent decline over 27 years in total flying insect biomass in protected areas - Germany.
Notable highlights from the study:
"Our results demonstrate that recently reported declines in several taxa such as butterflies [7, 25–27, 58], wild bees [8–14] and moths [15–18], are in parallel with a severe loss of total aerial insect biomass, suggesting that it is not only the vulnerable species, but the flying insect community as a whole, that has been decimated over the last few decades. The estimated decline is considerably more severe than the only comparable long term study on flying insect biomass elsewhere [28]."
Anecdotally, anyone who lives in the northern hemisphere can tell you that when driving there are FAR FEWER insects splatting on the windshields compared to just 10 or 20 years ago. I've personally noticed a dramatic decline in insect population in the Northeast US in just 10 years. It seems likely (although I don't have any data so this is an anecdotal claim) that across the entire northern hemisphere insect populations have declined by this same 75% level.
The important take home consequences of this study (i.e., my thoughts):
1. There are no pesticides being used in the reserves in which this study was carried out, therefore it's almost certain that the cause is somehow related to other factors, i.e., exponential temperature rise. I personally live in an area with zero agriculture, and have noticed a dramatic decrease in insect densities as well.
2. Insects have existed on this planet far, far longer than primates. If insects cannot survive a 1.5C rise in global average temperature (above the 1750/1850 pre industrial baseline), it's difficult to imagine primates surviving much longer. Especially considering that primates require insects to survive.
3. Insects represent the foundation of the food-chain for all northern hemisphere biomes, with a 75% decline, it's likely that over the next several years die offs in many of the major larger terrestrial vertebrates will take place at a similar clip.
We're only at 1.5C above pre industrial baseline as of 2018. 75% decline in the biomass of any population of animals on the planet is extreme, especially considering that it's across all families, genus and species of insects, not just a single group. It appears as if the insect organisms cannot keep up with the exponential rise in global average temperature in the northern hemisphere. Insects are ectothermic invertebrates, which means that they are particularly well adapted to (but also sensitive to) variations in temperature. If an ectothermic invertebrate can't keep up with climate change, its difficult to imagine that endothermic vertebrates like mammals will survive.
This appears to be the prelude to an extinction level event that is unprecedented since the Permian mass extinction.
The insects are the canary in the terrestrial coal mine (similarly, Coral reefs are the canary in the oceanic coal mine), so to speak - mammals (i.e., humans), reptiles, amphibians, and birds are next.
