Abstract Eight of 9 insecticides used for pest control in pecan [Carya illionoensis (Wang.) K. Koch] suppressed net photosynthesis (Pn) of mature leaves in the orchard after a single spray treatment. Reductions were on the order of about 20% of pretreatment levels within 1 day after treatment. Sprays continued to suppress Pn for several days after application. Leaves exhibited no visible damage as a result of spray treatments. Emulsifiable concentrate formulations tended to be more detrimental to Pn than were wettable powder formulations. Four applications of fen valerate and carbaryl at 14-day intervals suppressed Pn by about 20% for the duration of the 55- day treatment period. Thus, repeated applications did not influence leaves in an additive or synergistic manner, and leaves did not adapt and become insensitive to the insecticide sprays.