You can imagine how upset I was when I opened up the closed suit bag holding my Oleg Cassini suit and found that moths had eaten a couple of holes in the pants. Figuring it still would be cheaper to reweave the holes, rather than buying a new suit, I shelled out something like $15 to have each hole filled. But a couple of years later, the moths struck back and there were new holes. Since my original reweaver had gone out of business, I found a new one. I lamented to my new launderer/tailor about battling moths and wondered why they chose to eat up my best suit which was in a suit bag, but not touch my other suits which were not.
His response was very interesting, one that I hadn't heard before, but which I have since confirmed to be accurate. For clothing to be susceptible to being eaten by moths, it has to be 100 percent wool. If the material is a wool blend, the moths don't care about it. That's why the moths concentrated on my good suits. Interestingly, subsequently I purchased a very heavy winter overcoat in the Bronx, paying only $75 for this item which had most of its tags removed. I was puzzled when the moths attacked this item, though it was so thick that it was still usable despite a little damage. Turns out that unbeknownst to me, until I took it to the cleaners, this was a pure wool overcoat.
Even after learning the distinction between wool garments and wool blends, I wondered why encasing the suit in a nice suit bag didn't protect it from the moths, while my cheaper everyday suits that I never bagged survived. He explained that the danger was not from moths flying around your closet. Rather the danger was from moth eggs that are deposited on the wool garment. So if your wool suit already has moth eggs on them, sealing it in a bag provides no protection.
Furthermore, he added two important points. Moth eggs are everywhere, so leaving my wool suit outside insures that it would attract some eggs. But if you actually go out and wear the suit, as opposed to leaving it hanging stationary in the closet, somehow the broader airing prevents the moth eggs from hatching. So it was the act of bagging it that ensured my suit would provide a meal for hungry moths.
The last piece of useful information is that dry cleaning a suit will kill the moth eggs, so if you bag the suit as soon as it is cleaned, you can safely store it away. Another launderer I spoke with indicated that this will protect the suit for at least a year. So for seasonally worn wool clothing, annual cleaning and bagging will let you outsmart those pesky moths.