Beetle mania

Each year at about this time my citrus trees are invaded by these black beetles. They attack the new fruit just beginning to form, sucking it dry causing what is left to fall off the tree.
Depending upon their life cycle, they can be black as shown in this photo also orange or green.
I pick them off with a pair of tweezers but I need to be careful as they can shoot poison from their tale end.
Why does nature have these "good for nothing" insects?