The influence of environmental stress on the cacao tree in predetermining the feeding sites of cacao thrips, Selenothrips rubrocinctus (Giard), on leaves and pods

Abstract

In the present study, which represents an extension of earlier work, a method is described for comparing the metabolic status of cacao leaves by measuring the time of survival of excised discs of leaf tissue, and, when used with appropriate sampling technique, is considered reliable for indicating differences in the physiological age of such tissue. Leaf-tissue samples from trees with a history of impeded growth and low yield, and wet-seasonal liability to attack by cacao thrips, Selenothrips rubrocinctus (Giard), were compared with samples from trees of the same clone growing in the same field, with a history of relatively rapid growth, superior yield and freedom from such attack. Their relative survival periods were found to be concordant with their relative physiological ages (as expressed by percentage of dry matter and of total nitrogen and response to foliar injections) and with the subsequent incidence of cacao thrips. A similar comparison, using leaf material from two clones, susceptible and relatively resistant to attack by cacao thrips, respectively, included samples from the susceptible clone collected at a locality where cacao thrips was common in the wet season, and at another where the insect was never common at any time, and samples from the resistant clone taken in the former locality from fully exposed and shaded portions of individual leaves. Leaf samples of the susceptible clone collected at the inferior site and those from the fully exposed parts of the leaf of the resistant clone were found to be similar in length of life, and were both relatively physiologically senescent and demonstrably of subnormal nutrient status, whereas the corresponding samples from leaves of the susceptible clone at the superior site, and from the shaded portions of the leaves of the resistant clone were also similar in length of life, significantly longer lived than the former samples, physiologically younger, and apparently of normal nutrient status. The distribution of cacao thrips that subsequently developed in the field on leaves of the resistant clone was confined to the fully exposed part of the leaf. Leaves of the resistant clone were found to contain more moisture and more solid matter than those from the susceptible clone. A comparison was made of leaf tissue from leaves debilitated by lime-induced chlorosis and of leaf tissue from areas of similar leaves that had recovered as a result of foliar injection with an acidified solution of ferrous sulphate, and the subsequent incidence of cacao thrips in the field on the two types of tissue was recorded. The debilitated tissue was found to be shorter-lived than the restored, and was heavily attacked, whereas the restored tissue remained uninfested. The incidence of cacao thrips on ripening pods was found to be restricted to pods with a history of relatively slow growth prior to infestation. The findings are discussed in the light of information obtained in an earlier investigation, and it is concluded that the results can be explained by assuming that establishment of a cacao-thrips colony on cacao is obligately dependent on the prior occurrence of metabolic derangement in the host leaf leading to a retardation of protein synthesis, and that in Trinidad, W.I., where the investigation was made, such derangement results from the effect on the tree of seasonally adverse factors in its physical environment.