Integrated control of soft scale insect on Boronia megastigma Nees in Southern Tasmania

The commercial planting of boronia (Boroniu inegastigma Nees), which is native to Western Australia, was established as an essential oil crop in Tasmania in 1985. In general, intensive cultural or monocultural practices are vulnerable to pest problems due to lack of species diversity. While psyllid pests have now been controlled, scale insects have become a serious pest in some boronia growers fields. Two soft scale insects (Hemiptera: Coccidae) attack boronia and have been identified as the black scale, Saissetiu oleae Oliver, and soft brown scale Coccus hesperidum L. At this time, black scale is considered the most serious pest in commercial plantations despite the occurrence of many natural enemies. Similarly, black scale on citrus in California USA, has remained a problem despite a complex of parasitoids and the repeated importation of new natural enemies (Daane and Caltagirone, 1990; Lampson and Morse, 1992). Saissetia oleue is an occasional pest insect and its distribution is mainly in temperate and sub-tropical regions. Daane and Caltagirone (1990) noted that it is common for orchards to have repeated outbreaks while neighbouring orchards do not. Black scale occurs throughout Australia with South Australia, New South Wales, and Victoria more affected due to their large citrus growing areas (Wilson, 1960). Soft brown scale is of European origin (McLeod and Coppel, 1966). It is now common in all tropical and subtropical regions of the world and in greenhouses in cooler regions (Ebeling, 1959; McLeod et al, 1966). Again, South Australia, Western Australia and New South Wales are more affected because of their large citrus growing areas (Wilson, 1960). Both scale species, S. oleue and C. hesperidum, are known to have a wide range of host plants. When heavily infested parts of the plant, such as foliage, shoots and fruit, are covered with sooty mould that develops on honeydew excreted by scale insects. Damage to the plant can be either direct, i.e. by sucking sap from the plant or indirect, i.e. by covering parts of the plant with sooty mould that interferes with the efficiency of photosynthesis. Heavy infestation can result in defoliation, fruit drop, twig dieback and in extreme cases, death of the plants (Ebeling, 1959). The application of insecticides to control scale insects without accurate knowledge of pest population control mechanisms could disrupt any potential regulation by natural enemies (Denach and Rosen, 1991; Paraskakis et al 1980). Hely et al. (1982) and Beattie (1991) suggested that application of insecticides against scale insects must be applied at an appropriate time and place in order to get effective results. Boronia growers in Tasmania have relied on materials such as petroleum spray oils and a mixture of these petroleum spray oils with maldison to control black scale and soft brown scale without any sound knowledge of their biology and ecology. This has led to unsuccessful results even though multiple applications have been employed annually. The aim of this study was to gain an understanding of the population dynamics and the life-cycle of both scale species, to understand their biology and to assess a range of insecticides for possible incorporation into an integrated program to control these species.