Estimates of natural mortality related to environmental factors in a population of citrus red scale, Aonidiella aurantii (Maskell) (Hemiptera: Dias-pididae)

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Abstract
Biological control of Aonidiella aurantii (Mask.) is notoriously difficult in the hot lowlands of Swaziland and the eastern Transvaal. In view of resistance to organophosphates in populations elsewhere in the Transvaal, there is renewed interest in the commercial application of integrated pest management on citrus. Accordingly, data from a population of A. aurantii on orange trees in the Swaziland lowlands were examined to answer questions about biological control of the pest in this climatic region. Sections of the population on twigs and fruit were treated separately, and that on the leaves was ignored as not contributing much to the economy of the whole. The infestation of fruit each year depended largely on the size of the infestation on twigs in November (early summer), when crawlers from twigs first colonised the fruitlets. Mortality on twigs was estimated, checked from predictions of the recruitment rate and related to indices of climatic and biotic factors; 73% of the variation was described by mean predator intensity, mean minimum temperature and excess rainfall above the long-term mean, all measured over the generation time. On fruit, 32% of the variation in mortality was described by mean maximum saturation deficit in the generation time, the relationship being inverse. The dryness of the lowland climate appeared to be its favourable characteristic. The population appeared to be regulated about a mean level which was unacceptably high. Predators were the most effective natural enemies, probably followed by parasitoids (Aphytis spp.) on fruit. Endoparasitoids were probably ineffective.