Combined effect of natural enemies (Hymenoptera: Aphelinidae & Coleoptera: Coccinellidae) with different niche breadths in reducing high populations of red scale, Aonidiella Aurantii (Maskell) (Hemiptera: Diaspididae)

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Abstract
Three populations of Aonidiella aurantii (Maskell) were studied on orange trees at two sites in the hot Transvaal lowveld in South Africa. The first two were on the same trees, which had severe scale in the tree crowns and a low infestation in the bottoms. The third population, at a second site, was high and relatively evenly-spread over the whole tree. In previous years, the parasitoids Aphytis africanus Quednau and A. melinus DeBach had been unable on their own to reduce the high scale populations. Once a certain scale density level on the tree framework had been breached, a proportionately large number of susceptible scales escaped parasitism and reached the gravid female stage, which is invulnerable to attack. The ineffectiveness of parasitoids in these circumstances, resulting partly from their narrow niche breadth, was compounded by peak numbers only being reached late in the season. By this time, the large number of adult scales had already rendered the fruit unmarketable. In both sites, the predator Chilocorus nigritus (F.) was introduced and the trees also received commercial applications of petroleum oil. In one site, this oil treatment was restricted to the tree crowns. Although overall parasitism of vulnerable stages was relatively unaffected by these abiotic and biotic controlling factors, there was a shift to increased percentage parasitism of the virgin female away from the other susceptible stages. In addition, it was confirmed that the broad attack niche provided by C. nigritus, which preys on all developmental stages of the scale, readily complements parasitoid activity and can be of major economic importance.

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