Susceptibility of Scaevola taccada (Gaertn.) Roxb. bushes to attack by the coccid Icerya seychellarum Westwood: the effects of leaf loss

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Abstract
The distribution of infestation levels of the coccid Iceryu seychellarum on bushes of Scuevola taccudu and its population fluctuations over a 10 month period on Aldabra atoll show that individual plants of S. taccada exhibit differential susceptibility to attack. Coccids on heavily and lightly infested bushes show no important differences in growth rate, fecundity or plant induced death rate. There were also no demonstrable differences in bush exposure, food quality (total and soluble nitrogen levels) or settlement behaviour of first instar larvae (crawlers). Detailed examination of Iceryu populations on individual leaf clusters showed that mortality caused by leaf fall is density dependent and can be avoided only if a large proportion of the population frequently moves from senescing to mature leaves. The proportion of the population required to move to younger leaves in order to complete development is greater in low density Zceryu populations. The effects of leaf fall are discussed with reference to the maintenance of differential susceptibility within the plant population.