The light-colored areas on these birch leaves were caused by birch leafminers similar to the ones depicted below. The leaf on the left is one that has been only slightly affected by leafminers while the leaf on the right has definitely been severely hurt by feeding leafminers. The leaf on the right represents a tree whose entire defense system has probably been compromised by the leafminers (but who in all likelihood was already weak before being attacked by leafminers), while the leaf on the left represents a tree that will probably not experience very much, if any, lasting negative effects of the leafminers.



Birch leafminers at work, wreaking havoc on a basically defenseless birch tree.





All of these pictures taken from the Penn State sponsored website about plant pests