INTERACTIONS BETWEEN THE ENCELIA LEAF BEETLE AND ITS HOST-PLANT, ENCELIA-FARINOSA - THE INFLUENCE OF ACIDIC FOG ON INSECT GROWTH AND PLANT CHEMISTRY

The impact of acidic deposition on interactions between the plant Ence lia farinosa and the herbivorous beetle Trirhabda geminata (Chrysomeli dae) was determined under greenhouse conditions. Acidic fogs (pH 2.75) did not significantly affect the overall foliar concentrations of wat er or soluble protein as compared with control fogs (pH 5.6). Nonethel ess, E. farinosa foliage was altered by exposure to three 3-h acinic f ogs such that growth and biomass gain by T. geminata increased by more than 30% as compared to beetles feeding on control-fogged plants. Thu s, previous indications that changes in soluble proteins or water cont ent were responsible for increased biomass gain and growth of T. gemin ata cannot be substantiated by this study. Additionally, changes in th e plant defensive chemistry were not responsible for increased herbivo re growth, as farinosin, encecalin, and euparin foliar concentrations did not vary significantly between fog treatments. Significant increas es in CO2 assimilation rates of E. farinosa exposed to acidic fogs wer e documented at 3, 7, and 21 days following treatment, suggesting that carbohydrate-based products of increased plant metabolism may have pl ayed a role (e.g. soluble carbohydrates). However, the key factors res ponsible for increasing herbivore performance on acidic-fogged E. fari nosa remain largely unknown. (C) 1997 Elsevier Science Ltd. All rights reserved.