Gas-Exchange Responses of Alfalfa and Soybean Treated with Insecticides

Insecticides are often used to establish pest density and yield-loss relationships to determine economic injuries. However, the impacts of insecticides on plant gas-exchange processes are not well understood. If an insecticide alters plant physiology, crop yield may be altered and the thresholds developed with this insecticide are suspect. Therefore, it is crucial to understand the nontarget effects of insecticides on plant physiology. We conducted field experiments in 1996, 1997, and 1998 to examine the photosynthetic rates, stomatal conductance, and transpiration rates of alfalfa, Medicago sativa L., and soybean, Glycine max (L.) Merrill, treated with insecticides commonly used for arthropod pest management in the midwestern United States. The insecticides were selected to represent the major classes of insecticides used on alfalfa and soybean: pyrethroids (cyfluthrin and permethrin), an organophosphate (chlorpyrifos), carbamates (carbofuran and carbaryl), and spinosyn (spinosad). Photosynthetic rates of alfalfa were not significantly affected in all experiments replicated over 3 yr, except 1 h after treatment in 1996, which showed a significant increase in photosynthetic rates for some insecticides. Also, stomatal conductance and transpiration rates of alfalfa did not reveal any significant differences among insecticide treatments. In soybean, photosynthetic rates were not significantly different for all insecticides tested either at seedling or reproductive stages. In addition, stomatal conductance and transpiration rates of soybean did not reveal any significant impact of insecticides at either seedling or reproductive stages. Other than a transient increase in photosynthesis observed with some insecticides at 1 time in 1 yr (1996), we did not observe any significant effects of tested insecticides on alfalfa and soybean physiology.