While there were many species of mites, springtails, and other tiny bugs in our bin, we were missing predators. I ordered a few million nematodes (a kind of tiny roundworm) by mail. Within a few weeks (long enough for the adult flies to die off and their larva to have been eaten) we had a fly-free bin.
All multicellular organisms use cell division for growth and the maintenance and repair of cells and tissues. Single-celled organisms use cell division as their method of reproduction. Somatic cells divide regularly; all human cells (except for the cells that produce eggs and sperm) are somatic cells.
When organisms grow, it isn't because cells are getting larger. Organisms grow because cells are dividing to produce more and more cells. In human bodies, nearly two trillion cells divide every day. Watch cells divide in this time lapse video of an animal cell (top) and an E. coli bacteria cell (bottom). The video compresses 30 hours of mitotic cell division into a few seconds.