Effects of simulated acidic precipitation on plant-insect interactions in agricultural systems: corn and black cutworm larvae

Effects of simulated acidic precipitation on food utilization, growth, development, and herbivory of black cutworms (Agrotis ipsilon) on corn (Zea mays L.) in the most economically important life-history combinations of plant and insect were investigated in a series of laboratory and greenhouse experiments. A nutritional-index technique was used in petri-dish experiments with fourth, fifth, and sixth instar black cutworms to determine if acid precipitation could affect the amount of food consumed via affects on insect physiological parameters. Larvae were fed corn plants that were watered two times weekly with simulated precipitation (pH 2.8, 4.2, and 5.6) until the larvae pupated (total deposition of 8-10 cm of precipitation). The inverse relationship between the index of approximate digestibility (AD), which was higher in the pH 2.8 and 5.6 treatments, and the efficiency of conversion of digested food indices (ECD), which was lowest in the pH 2.8 treatment, suggested a stress effect of high acidity. A series of six greenhouse pot experiments was conducted to assess effects of acid precipitation on black cutworms and their interactions with growing corn plants. Fourth, fifth, and sixth instar larvae were placed in pots containing soil and corn plants, which were initially in the first or second leaf stage, and simulated acid precipitation (pH 2.8, 4.2, and 5.6) was applied two times per week until the larvae pupated (8-16 cm total deposition). Significant trea tment effects were observed in the sixth instar larvae on two-leaf corn experiment, where the larvae grew larger and developed faster in the pH 2.8 and 4.2 treatments than in the 5.6 treatment. Acid precipitation treatments did not significantly affect black cutworm damage to corn plants in any of these experiments.