Humidity isn't that big of a factor, but temperature is. Eggs (if they are present) will hatch if the temperature gets above the recommended 70 degrees ...
If one cigar has them and the beetle has already left (via the exit hole), all the cigars in the humidor are in jeopardy.
A good source of info is 'Cigar 101' ... and it's on the CigarsInternational web site.
Here's a link on what thay have to say about beetles:
http://www.cigarsinterna...om/html/cig101_7mob.asp Cigar beetles hatch from eggs that are laid on the tobacco leaves. The curing and the manufacturing process kills many of the eggs, but sometimes a few survive.
The eggs hatch into worms that eat their way out of your cigar. They emerge as small brown beetles. The eggs usually will not hatch if the temperature in the humidor stays below 75 degrees.
How to prevent further attacks ---
The way to get rid of tobacco beetles is to freeze them. But don't just go popping your prize cigars in the freezer. There are steps you should follow to protect your cigars during the freezing process.
Place all the cigars from the affected humidor in ziplok freezer bags (double bag them if possible). Be sure to squeeze out as much air as possible before closing the bag(s).
Place them in the freezer for three to four days, then place them in the refrigerator for one day.
Take them out of the refrigerator and leave them at room temperature for one day (still in the bag(s).
Thoroughly clean out the inside of your humidor with a vacuum cleaner (a small crevice tool will allow you to get into the corners of your humidor).
Take them out of the bag and place them back in your humidor.