Yes of course it's a concern. But the footprint we are talking about here is pretty small. On our 15 acres, I don't use any pesticides, or herbicides, and grow all organically. The bonsai is such a small impact, that I don't worry about it. And with the amount of investment into the trees, I can't really see doing it any other way. The bees don't pay any attention to the bonsai, as there is so much more for them around to eat, (I grow lots of comfrey for attracting them all summer) and the few bugs that die are generally not eaten by birds, as they rarely come around the trees as there again, is so much better foraging than there is around the trees.
And the results have been pretty amazing since I started using it. If the results were not so good, then I'd rethink the usage.