Acidic fog-induced changes in host-plant suitability : Interactions ofTrichoplusia ni andPhaseolus lunatus

Phaseolus lunatus L. (Henderson Bush lima beans) were exposed to 2 hr acidic fogs with 2.5∶1.0 (v/v) nitrogen-sulfur ratio typical of the west coast of the United States. Fogs with pH values of 2.0 (P < 0.01,t tests), 2.5 (P < 0.05), or 3.0 (P < 0.01) increased percent total nitrogen (dry weight) of foliage as compared to plants subjected to control fogs with a pH of 6.3-6.5. Fresh weight concentrations of soluble protein and certain free amino acid concentrations were increased by plant exposure to acidic fogs with a pH of 2.5 (t tests,P < 0.05). Concentrations of free amino acids considered essential for insect growth, as well as nonessential and total free amino acids were not significantly affected by any treatment (P > 0.05,t test). Water content (%) of foliage was not changed significantly (P > 0.05,t test) by exposure to any of the fogs.Trichoplusia ni (Hübner) (Lepidoptera: Noctuidae) larvae ate significantly more foliage and gained significantly more weight on plants treated with 3.0 pH fogs (P < 0.01,t test). Several potential explanations are offered for the lack of significant weight gain by larvae on plants in which soluble protein levels, free amino acid concentrations, or percent total nitrogen contents were enhanced by acidic fogs with a pH of 2.5 and 2.0. No larval feeding preference was detected for foliage exposed to acidic versus control fogs, and no significant differences were detected in percent survival ofT. ni eggs exposed to acidic or control fogs. Some implications of acidic fogs for population dynamics ofT. ni are discussed.