This study focused on relationships between a tropical forest grasshopper and a major group of its host plants. Microtylopteryx hebardi (Acrididae:Ommatolampinae) at La Selva Biological Station, Costa Rica, feeds on understorey palms as well as other understorey plants. We assessed leaf damage levels in populations of three geonomoid palm species (Geonoma cuneata, Geonama congcsta, Asterogyne martiana), investigated feeding response of M. hebardi to the three palms, and examined leaf characteristics presumed to be important to herbivores. In understorey and gapedge habitats, total leaf damage did not differ significantly between palm species. In the centre of small light gaps, where M. hebardi was most abundant, total leaf damage and total herbivory on G. cuneata was equal to that on the other species, but herbivory attributable to grasshoppers was significantly greater. Female M. hebardi showed strong preference for G. cuneata over the other palm species in laboratory feeding trials. Experiments to elucidate the proximate basis for this feeding preference showed that grasshoppers preferentially bit filter paper discs containing cuticular wax extract of G. cuneata. In consumption and digestibility experiments, grasshoppers consumed more fresh weight of G. cuneata and gained more weight per unit time spent feeding on this species. Leaves of G. cuneata had the highest percentage water content, highest fresh and dry mass per unit leaf area, and the lowest percentage crude lignin among the three palms studied. These factors may partially explain the preference of M. hebardi for G. cuneata, which was reflected in higher herbivory rates incurred by G. cuneata in sites where M. hebardi was locally abundant.

Wetting of the upper leaf surface of Juglans regia L. and of model surfaces colonized by epiphytic micro-organisms was investigated by measuring contact angles of aqueous solutions buffered at different pH values. During June to October 1995, contact angles of aqueous solutions on the leaf surface of J. regia decreased by angles ranging from 12° (low pH values) to 25° at high pH values. At the end of this vegetation period, wetting was strongly dependent on pH showing significantly lower contact angles with alkaline solutions (pH 9·0) than with acidic solutions (pH 3·0). Contact angle titration measured angles on the leaf surface as a function of the pH of buffered aqueous solutions, covering a pH range from 3·0 to 11·0. Titration curves revealed inflection points around 7·5, indicating the existence of ionizable carboxylic groups at the interface of the phylloplane. Altered leaf-surface wetting properties observed on the intact leaf surface could be simulated in model experiments by measuring contact angles on artificial surfaces colonized by Pseudomonas fluorescens and by epiphytic micro-organisms isolated from the phylloplane of J. regia. Strong evidence is provided that interfacial carboxylic groups derive from epiphytic micro-organisms present on the phylloplane. Results suggest that the age-dependent increase in, and pH dependence of, wetting as leaves mature are related to the presence of epiphytic micro-organisms on the phylloplane. Ecological consequences of increased leaf-surface wetting, concerning the structure of the leaf surface as a microhabitat for epiphytic micro-organisms, are discussed.