There are different types of Pesticides, here is one bit of information that I just found.
Taken from
http://feql.wsu.edu/entom490/lect4moaspray.pdf
Non-systemic (Contact Pesticides)
– Little or no ability to penetrate plant tissues
(leaves or roots) and translocate from site
of contact to distal parts of the plant
These only stay on the plant for a short period of time and then wash away.
Systemics
– Pesticide can penetrate plant tissue and be
translocated to other parts of plant
– Chemical diffuses through the cuticle of leaves
(sprays) or through the outer layer of roots (soil
applications) into the epidermal cells
– Chemical crosses into either the xylem or phloem
• Some chemicals are only xylem mobile and
thus can only move upward in the transpiration
stream from the site of first contact
Phloem mobile chemicals can move either up
or down from the site of first contact (the “true”
systemics)
These stay in the wood.