It's an amazing thing - something in the back of our ancient brain tells us to avoid bugs; to jump, scream and squish any that come near. Of course there is a good arguement for learned behaviour. A young child will most likely pick up a bug and examine it; one who sees fearful responses may respond likewise. Many of our behaviours come from observing the norms of those around us.
So, are you scared of bugs - is it a phobia? You may recall a movie called Arachnophobia? It highlighted one of the primary bug phobias - the fear of spiders (the phobia includes insects like scorpions). Most people do not like spiders and other insects. Choosing not to brush up against a spider's web or allowing some insect to scurry by at a safe distance is a normal, socially-driven discomfort with bugs. They are creepy and crawly; how are we supposed to know which ones are safe? I will discuss this to some degree when I highlight a bug.
When is it a phobia? A phobia is a irrational, intense and persistent fear. There is an excessive and unreasonable desire to avoid the object/situation. It's important to note that it is irrational - people say/think 'I should know better'. For example, a person with a wasp phobia might react by letting go of a steering wheel and losing all control of car simply to get the thing off. A key question is 'is it interfering with my life?'. If so, and you wish to change this, a psychologist can be helpful. They do systematic desensitization - the thing of fear is gradually introduced over time and eventually you are able to move past it.