Eggs can be present in leaf and survive processing in small numbers. They stay dormant until conditions allow, and the threshold usually cited is warmth above roughly 22 °C (72 °F) — which is why temperature control matters more here than humidity does.
What does the damage look like?
Neat round holes about one to two millimetres across, often with a tunnel visible if the cigar is cut open, and a scatter of fine tobacco dust in the bottom of the box. The dust is frequently noticed before the holes are.
What to do about it
Isolate the affected cigars immediately — an infestation spreads through a box. The standard response is freezing, done in stages rather than abruptly: a period in a refrigerator, then the freezer, then back through the refrigerator before returning to the humidor. The graduated return is not fussiness; a wrapper taken straight from a freezer to a humidor can crack from the shock.
Prevention
Keeping storage below the hatch threshold is the whole of it. A humidor kept in a warm room in summer is the usual circumstance, and a cool cupboard is a more effective precaution than anything applied to the cigars themselves.