You have to disclose them. I would imagine how much it affects the resale value of your home depends on:
1) The amount of damage
2) The buyer
It might also depend on the buyer's home inspector. I looked at a home that had been treated for termites. The inspector thought it was no big deal and told me how his home once had them.
I recently had my 15 yr old home treated for termites after succumbing to the Orkin sales pitch. They do my quarterly pest control service and the guy that did the estimate for the termite treatment has serviced my house for years. Although there was no evidence of termites in my house, I believed the guy when he said it is only a matter of when not if it would happen. Anyway, seeing as how I will probably be selling the house and trading up in another 2-3 years I decided to go ahead and get the treatment done as a preventive measure and the deal includes a warranty that's renewable each year for 10 yrs. So, I was thinking this would be a good selling point when it comes time to sell the house. Was my logic reasonable, or would a termite treatment flag my house as one that has had termites and maybe turn it into a negative?