Hello Michael, yes, known about, but Bumble bees are very sensitive to pollutants and are suffering terriably.re-introduction of Bumble bees in the area of Dungeness has gone well as new Bumble bees emerged this year, they have been extinct there for many years.
It has been found that Honey bees prefer OSR that has Nicotinids on it, trials side by side have shown that the non-nicotinid plot was not visited? There is something going on with insects and it is the subject of much research all over the world and slowly the jigsaw of results may point to what is causing it.
Just to point out that the initial Varroa infections also are a Virus vector and thats when the Israeli, and deformed wing virus and some others appeared and have carried on since.
Further thoughts have seen Queen fertility fall, causing poor mating and thence poor hive performance. At one time you could leave the hives alone and collect honey but now you are lucky to get a second year from a hive before it fails due to poor Queens. That is why I have been re-queening from known good source.
Clive