Relationship between red scale, Aonidiella aurantii (Maskell) (Hemiptera: Diaspididae), and its natural enemies in the upper and lower parts of citrus trees in South Africa

Abstract

AbstractIn mature Valencia orange trees at three sites in the Transvaal lowveld, there was a variable differential between levels of Aonidiella aurantii (Maskell) in the upper and lower parts of the trees. At one site there were high scale levels in the crowns of the trees as a result of past poor chemical spray procedure. In recent years, pest management has maximized on the use of natural enemies, especially Aphytis spp. However, the natural enemies have not been able to ameliorate the crown problem. At generally high levels of Aonidiella aurantii, the natural enemies were ableonly, more or less, to maintain the average overall level of the scale from one year to the next; they could not bring it down. This arose each year from too many scales escaping parasitism and reaching the relatively invincible adult stage early in the season. As the behaviour of the parasitoids was inversely density-dependent at these relatively high scale levels, this led to a greater increase in scale numbers, leading eventually to up to 90% of the population (excluding crawlers) being composed of adult females. The inverse density dependence was also partly because the majority of single scales escaped parasitoid attack. In the lowveld, it is a prerequisite that the density of A. aurantii first be lowered by an introduced natural enemy (e.g. the coccinellid Chilocorus nigritus (F.)) that attacks the adult stage and/ or by chemical means before maximal use is made of the regulatory influence of the existing natural enemies, particularly Aphytis spp.