The root cause seems to be an unknown problem with bacteria that produce thiamine at the base of food chains, as well as some invasive species (which for example caused big problems in the Great Lakes in the USA). Thiamine is pretty fundamental to life so it might explain the decline in insects as well as the other effects described in the above article. Reading about thiamine antagonists it seems sulfates can destroy thiamine:
"Rumen bacteria also reduce sulfate to sulfite, therefore high dietary intakes of sulfate can have thiamine-antagonistic activities."
Sulfates in the environment are one result of burning coal. So it is possible that one side effect of burning coal for decades is the food chain has been interrupted at the bacterial level, reducing the amount of thiamine available. If true, I'm not sure if that would be easy to fix; at the very least it would probably require coal burning to stop immediately and perhaps manufacture of thiamine and distribution of it in the environment, which could take a decade and might be somewhat risky with side effects if done on a large scale. If it really is coal burning causing thiamine deficiencies over wide areas it should be easy to tell by looking for thiamine deficiencies downwind of coal burning plants.
There is a large body of existing literature linking pesticide application to decreased biodiversity. I’m not saying this speculation about thiamine is unfounded, but if there’s good supporting data for it, it’s likely to be an additive rather than alternative factor contributing to the loss of biodiversity.
Possibly they could be interrelated, if say thiamine deficiency made insects more susceptible to pesticides or pesticides also increased thiamine deficiency. The link between pesticides and decreased biodiversity would still exist, but the root cause or chain of causes might be something happening at a lower level than simple direct poisoning. This paper is interesting in that respect "A 2018 Horizon Scan of Emerging Issues for Global Conservation and Biological Diversity":
Thiamine Deficiency as a Possible Driver of Wildlife Population Declines
Evidence is increasing that a range of taxonomic groups, including bivalve molluscs, ray-finned fish, and birds across the Northern Hemisphere, are deficient in thiamine (vitamin B1) 19, 20. Thiamine is required for basic cellular metabolism and functioning of neuronal membranes. Thiamine deficiency rarely is a direct cause of mortality, but impairs health and can cause immunosuppression or leads to behavioural and reproductive problems that ultimately could cause population decline or extirpation. Days of thiamine deficiency may present long-lasting sublethal effects, which makes recognition of the extent of thiamine deficiencies more complex. The deficiencies likely are caused by insufficient dietary intake, which may be related to shifts in thiamine-producing algal populations. A recent and extensive survey along the northwest coast of the US found evidence of thiamine depletion in the water column [21]. Additionally, exposure to environmental pollutants may interfere with thiamine uptake. The extent to which thiamine deficiency may pose a substantial long-term risk to a range of species remains unclear.
End quote.
So thiamine deficiency may also be partially induced by environmental pollutants which possibly includes insecticides (though the paper gives no citations for which pollutants). The scale of the known thiamine problems, at least in water systems, are scary, as is the fact that we don't really know what the extent of it is, especially on land. My experience in research, software, and life is that there are usually multiple simultaneous problems behind observable issues.
https://www.theatlantic.com/science/archive/2021/01/vitamin-...
The root cause seems to be an unknown problem with bacteria that produce thiamine at the base of food chains, as well as some invasive species (which for example caused big problems in the Great Lakes in the USA). Thiamine is pretty fundamental to life so it might explain the decline in insects as well as the other effects described in the above article. Reading about thiamine antagonists it seems sulfates can destroy thiamine:
https://en.wikipedia.org/wiki/Thiamine#Antagonists
"Rumen bacteria also reduce sulfate to sulfite, therefore high dietary intakes of sulfate can have thiamine-antagonistic activities."
Sulfates in the environment are one result of burning coal. So it is possible that one side effect of burning coal for decades is the food chain has been interrupted at the bacterial level, reducing the amount of thiamine available. If true, I'm not sure if that would be easy to fix; at the very least it would probably require coal burning to stop immediately and perhaps manufacture of thiamine and distribution of it in the environment, which could take a decade and might be somewhat risky with side effects if done on a large scale. If it really is coal burning causing thiamine deficiencies over wide areas it should be easy to tell by looking for thiamine deficiencies downwind of coal burning plants.