I expect that the fall in ag chemicals is correlated to the increase in GM
cotton since cotton uses nearly 25% of the insecticide in the world.

Your pro-GM explanation does not explain this:

"Latin American agrochemical sales experienced the largest decline,
with sales down by 3.8% to US $3.4 billion total. The Asia/Pacific
market fell by 1.8% in 2002, to US $7.15 billion."

No Bt cotton is grown in Latin America and very little in
Asia/Pacific...

regards
Marcus