Transient fears of this kind are common in children but your daughter seems to have worsening symptoms over a long period of time. It is likely that her problems are not going to go away on their own or with ordinary reassurance from parents.
There are, however, very good programs which can help children with these symptoms of anxiety. Speak with your pediatrician for a referral for a mental health professional with experience evaluating and treating children for fears.
In the meantime, the best you can do is try to maintain common sense by saying calmly that you don't see any bugs and to try to keep a kind, patient, supportive attitude towards your daughter. Getting outside help now is probably wise, since longstanding problems of this type can become worse and worse if untreated.