Hey Luc, mature trees and low yields are a bummer. That's a long dry season you got, I figured your green season was a little longer. How are you irrigating? You may have to let them drought stress for the more normal period of time instead of the long 8 months they are experiencing now. Maybe after 4 months or so start irrigating. Also during the dry season you could apply potassium chlorate or KCLO3. The potassium chlorate is supposed to be applied when the trees are not flushing, just flushed, or getting ready to flush, from what I've been told. This application is supposed to help increase the crop load and crop consistency. I'll send you a pdf. file of the research, if your interested.
The Africanized bee problem sucks too, seems like the long dry season would curb a large pollinator population as well.