Nature in my Yard

I wrote last month about the insects in my neighbor's cherry tree, and since then I've tried to notice details of nature in my own yard.  We've lived in this house for 35 years, and I've noticed that each year the insect population changes.  Some years there are lots of ordinary hornets, while other years bald-faced hornets predominate.  Some years there are lots of grasshoppers, other years very few, and so on.  This year we've had a huge number of large orange-red flies that look like bees at first glance.  I don't remember having seen these at all before, and now we have dozens of them.  They fly like bees and feed on flower blossoms like bees, but a close up look shows that they are flies.  Just why this population explosion is happening this year is a complete mystery, at least to me!