What Causes Persistent Itching Post Insect's Bite?

My wife has had whar appears to be an insect bite of some sort. It started out like mosquito bite or stuck by a cactus spine. We tried ti find it with a needle but to no avail. This occurred in November on a hike in brushy area. Not known to have spiders apparent , with ticks. There has been no physical body reactions such as fever or illness. There was only a light itch and was decided that it wasn't bad enough to go to a doctor of which we have to go to Kingman Az 60 miles away and have PA open a clinic 1 day a week. It has grown in sizw and is not healing. Brown recluse biteswe are told starts to destry the tissue around the bit. This has not occurred. She has an appointment the 13th of Jan. withe a dermitologist.