Leaves detaching

 I have plants dropping their leaves in large amounts. Thought it was the 
water quality at first, until I looked at several leaves under a microscope 
and noticed at the damaged portion there were little ?(bugs) there. Some 
types of plants are not affected. 

   a.) What are they ?
   b.) How do I kill them ? (with no risk to plants)

Note: Eight month old 100 gallon Jungle Tank (doing real good minis the