Gas-exchange responses of alfalfa and soybean treated with insecticides

Insecticides are often used to establish pest density and yield-loss relati onships to determine economic injuries. However, the impacts of insecticide s on plant gas-exchange processes are not well understood. If an insecticid e alters plant physiology, crop yield may be altered and the thresholds dev eloped with this insecticide are suspect. Therefore, it is crucial to under stand the nontarget effects of insecticides on plant physiology. We conduct ed field experiments in 1996, 1997, and 1998 to examine the photosynthetic rates, stomatal conductance, and transpiration rates of alfalfa Medicago sa tiva L., and soybean, Glycine mar (L.) Merrill, treated with insecticides c ommonly used for arthropod pest management in the midwestern United States. The insecticides were selected to represent the major classes of insectici des used on alfalfa and soybean: pyrethroids (cyfluthrin and permethrin), a n organophosphate (chlorpyrifos), carbamates (carbofuran and carbarpl), and spinosyn (spinosad). Photosynthetic rates of alfalfa were not significantl y affected in all experiments replicated over 3 yr, except 1 h after treatm ent in 1996, which showed a significant increase in photosynthetic rates fo r some insecticides. Also, stomatal conductance and transpiration rates of alfalfa did not reveal any significant differences among insecticide treatm ents. In soybean, photosynthetic rates were not significantly different for all insecticides tested either at seedling or reproductive stages. In addi tion, stomatal conductance and transpiration rates of soybean did not revea l any significant impact of insecticides at either seedling or reproductive stages. Other than a transient increase in photosynthesis observed with so me insecticides at 1 time in 1 yr (1996),we did not observe any significant effects of tested insecticides on alfalfa and soybean physiology.