A comparison of the population growth potential, in California red scale, Aonidiella aurantii (Maskell), and yellow scale, A. citrina (Croquillet) on citris

Abstract

Successful biological control of A. aurantii and A. citrina has been achieved on many citrus properties in the region of Mildura. A project was undertaken to devise methods for augmenting and extending the natural control of red scale in Victoria. A, aurantii and A. citrina were compared by compiling age-specific life and fecundity tables over a range of temperature and humidities. A mathematical analysis of these data showed that the upper thermal death point was similar for both species, A. citrina multiplied more rapidly than A. aurantii over the median range of temperatures, and the threshold of population growth occurred at about 15260C for A, aurantii and 18260C for A, citrina. An earlier study of the settling behaviour of crawlers indicated that most A. aurantii crawlers settled on the upper surfaces of leaves, due to negative geotaxis, and that crawlers of A. citrina were negatively phototropic and therefore settled on the shaded under surfaces of leaves. In A. aurantii, delayed mating caused a high initial natality. This was thought to result from continuing ovulation in the unfertilized female, a postulate reinforced by the results of an examination of the developing embryos in females mated at various ages. On the basis of this study an improved approach to red scale control is proffered, namely, that attempts be made to increase the mortality of the over- wintering insects.