Cybermites were cybernetic insects, controlled and created by the Cybermen. According to the Doctor, they were a heavily upgraded form of Cybermat. Even one could partially convert people, but to have a full upgrade would take a very large swarm. Because of this, they usually travelled in small swarms, and went directly for the victim's head. They were not used in the upgrade process for a full cyber-conversion.
A small Cybermite horde could restore and enhance some functionality of a broken, obsolete Cyberman shell, but not completely reconstitute one. They would use it as a nest, waiting until a susceptible target for conversion came near. Then, the Cybermites would puppeteer the Cyberman husk to restrain them, and scurry out of the casing for the act of converting the captive. (TV: Nightmare in Silver)
After Hedgewick's World of Wonders was destroyed with the three million Cybermen on it, it was assumed the revival of the Cybermen was thwarted. However, a single Cybermite survived, drifting in space. (TV: Nightmare in Silver)