The sentence begins with a modifier. So, when we get to the comma, we should stop and ask the question that the modifier raises. Here the question is,"Who or what sucks sap from young twigs?" The part immediately following the comma should answer our question in a logical manner.
A. tree growth is retarded by the woolly adelgid, causing needles to change color from deep green to grayish green and to drop
Tree growth sucks sap from young twigs?
Makes no sense. Eliminate A.
B. tree
...
A. tree growth is retarded by the woolly adelgid, causing needles to change color from deep green to grayish green and to drop
Tree growth sucks sap from young twigs?
Makes no sense. Eliminate A.
B. tree
...
