Interactions between the coccid, Icerya seychellarum (Westw.), and its host tree species on Aldabra Atoll : I. Euphorbia pyrifolia Lam.

(i) The growth of ca. 100 individuals of a small deciduous tree Euphorbia pyrifolia, of different ages and edaphic regimes, and with different levels of infestation by the coccid Icerya seychellarum, has been studied on Aldabra Atoll over two years (1976-1978). Infestation on leaves ranged between 0 and 1.3 mg dry weight of coccids cm of total leaf area. (ii) Field observations and an experiment in which coccids were "seeded" on to transplanted trees showed a preference of coccids to feed on senescing leaves rather than on green leaves. Deterrence by green leaves is discussed. (iii) Highest infestations developed on trees which produced the most senescent leaf material early in the wet season. The rate of leaf production on the heaviest infested trees was 36% of that on lightly infested trees. (iv) The level of coccid infestation did not depend on the concentration of soluble nitrogen in senescing leaves but on the total amount of nitrogen exported. The level of soluble nitrogen in green leaves was reduced in heavily infested leaf clusters. (v) Experimental clearance of coccids by insecticide showed that 0.7 mg cm of coccids reduced the rate of leaf growth by 52%. This corresponded closely to the effect of similar levels of coccids in the field. (vi) Leaf infestation was positively correlated with the percentage of dead apices per tree and negatively correlated with the degree of branch extension. (vii) It was concluded that leaf phenology influenced the development of coccid populations and that coccids significantly reduced plant growth.