Interactions between the coccid, Icerya seychellarum (Westw.) and its host tree species on Aldabra Atoll : II. Scaevola taccada (Gaertn.) Roxb.

(i) In an area of coastal scrub on Aldabra Atoll the growth and phenology of 120 bushes of Scaevola taccada was measured along three transects stretching 200 m inland from the sea, between May 1977 and November 1978. Correlations between bush parameters, the infestation by the coccid Icerya seychellarum and prevailing saline conditions were measured. Considerable death (up to 50% of labelled clusters) of S. taccada occurred in 1977-78. Leaf infestation ranged from 0 to 1.6 mg dry weight of coccids cm-2 total leaf area per cluster. (ii) Coccids fed predominantly on senescing leaves and seasonal differences in leaf infestation were closely associated with changes in leaf phenology. Under experimental conditions coccids could utilize green leaves. (iii) Infestation increased with distance away from the sea whilst the influence of salt, and its effect in increasing leaf succulence, decreased inland. Infestation was strongly negatively correlated with leaf succulence. (iv) Two experiments with S. taccada showed that coccids significantly reduced the growth of leaves and roots and increased the rate of leaf senescence. 0.5 mg cm-2 of coccids reduced leaf production by 39%. (v) In May 1977 the proportion of dead clusters was significantly positively correlated with coccid infestation and in 1978, when mean cluster death doubled, death near the coast (0-100 m) was positively correlated with leaf succulence, whilst inland (101-200 m) death significantly increased with increasing infestation. (vi) It was concluded that coccids could account for the reductions in leaf production in the field and that they contributed to much of the bush death. (vii) The effects of coccids on the leaf production of S. taccada and Euphorbia pyrifolia (Part I) closely agreed. It was estimated that 1.25 mg cm-2 of coccids can completely check leaf production.