D McC Newbery1, M G Hill2, P G Waterman3. 1. Department of Biological Science, Stirling University, FK9 4LA, Stirling, Scotland. 2. Entomology Division, D.S.I.R., Private Bag, Auckland, New Zealand. 3. Phytochemistry Research Laboratory, Department of Pharmaceutical Chemistry, University of Strathclyde, Gl 1XW, Glasgow, Scotland.
Abstract
The relative susceptibility of thirty-tree species to feeding by the coccid, Icerya seychellarum, (from a previous atoll survey) were correlated with four morphological characters (leaf lamina and midrib thicknesses, density of hairs on leaf underside and presence of waxy upper leaf surface) and the concentrations of six mineral elements (N, P, K, Ca, Mg and Na) and three measures of chemical defense (total phenols, condensed tannins and alkaloids). For twenty-two species four phenological variables, from a separate two year study, were also correlated with susceptibility.Lamina thickness, presence of dense hairs and the concentrations of alkaloids or condensed tannins were significantly correlated with host susceptibility. The ideal host has thick, evergreen and hairless leaves. Some host species with high concentrations of secondary chemicals were also highly susceptible and a new hypothesis suggests that stylet manoeuvrability enables a coccid to avoid these substances in thick, but not thin, leaves.Experimental 'seeding' of crawlers on to eleven host species showed marked differences in susceptibility in different stages of the life-cycle: presence of dense hairs appeared to deter settlement whilst thick lamina enhanced coccid survival.
The relative susceptibility of thirty-tree species to feeding by the coccid, Icerya seychellarum, (from a previous atoll survey) were correlated with four morphological characters (leaf lamina anpan>d midrib thicknesses, density of hairs on leaf underside anpan>d presence of waxy upper leaf surface) anpan>d the concentrations of six mineral elements (N, P, K, Ca, pan> class="Chemical">Mg and Na) and three measures of chemical defense (total phenols, condensed tannins and alkaloids). For twenty-two species four phenological variables, from a separate two year study, were also correlated with susceptibility.Lamina thickness, presence of dense hairs and the concentrations of alkaloids or condensed tannins were significantly correlated with host susceptibility. The ideal host has thick, evergreen and hairless leaves. Some host species with high concentrations of secondary chemicals were also highly susceptible and a new hypothesis suggests that stylet manoeuvrability enables a coccid to avoid these substances in thick, but not thin, leaves.Experimental 'seeding' of crawlers on to eleven host species showed marked differences in susceptibility in different stages of the life-cycle: presence of dense hairs appeared to deter settlement whilst thick lamina enhanced coccid survival.