This can certainly be a difficult call, but I would follow the terms of the lease. Legally speaking I believe that is what would prevail in this situation.
If they were new tenants and there was a possibility that there were bedbugs there when they moved in I would treat them, but since they are long term tenants the only way that bed bugs are there now is if they brought them in, making it their responsibility.
We have terms in our lease stating that we perform general pest control twice per year, but tenant is responsible for treating pests with the exception of termites. The majority of the time pests are there because of the tenant living conditions and should not be the owner’s responsibility. If our tenants complain of pests we inspect the unit and take pictures. Nine times out of ten we can show that living conditions are the cause and put it back on the tenant to remedy. Once you make that argument and have evidence to back it up they will usually concede and take care of it. Tenant usually first try to make things the owner’s responsibility and not their own. Be kind but firm. If logic and the terms of your lease back your position, tenants will usually accept the verdict.