There are many theories about the loss of bees, everything from wireless signals, climate, disease, parasites, insecticides and loss of habitat. I think its probably a combination of most of these stressors on the bees.
In America bees are dying at some of the highest rates ever recorded—as high as 42% of U.S. bee colonies collapsed in 2015, well above the average 31 percent that have been dying each winter for nearly a decade.
Varroa mite, seems to be a major catalyst for problems with bees around the world. Not only are they themselves destructive to bees but they also carry viruses which are passed onto the bees. It seems that the combination of natural disease and pests when combined with pesticides is having a devastating affect on the bees.
Over time bees will adapt and have success in dealing with Veroa but this takes time and generations of bee learning. However we need to protect our bees now and that will have to take many forms as well as limiting insecticides shown to kill bees.
Veroa now surrounds us as it is in Papua New Guinea and New Zealand. It is unfortunately only a matter of time before this pest makes it onto our shores. It will make a difference to honey produced and of course the number of hives available once the incursion occurs.
In Australia 50% of pollination is done by feral Bee hives, all of these will die and it will take a number of years for beekeepers to increase bee numbers to help mitigate their loss.
It will most likely have an effect on crop returns as less pollination will eventually reduce the size of harvested crops and will make crops weaker. So there will be a period where food costs will increase until beekeepers can catch up. The cost of bee pollination will also increase as demand for farmed bees will increase (no longer any free pollination).