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Introduction
In a comprehensive study of the Queensland fruit fly, Bactrocera tryoni (Froggatt), in some of its major host plants (mulberry, nectarine and peach) in Brisbane, a significant tritrophic relationship between the fly, certain bacteria species and the host plant was defined (Drew & Lloyd, 1987). In particular, the host plant was recorded as a significant site for adult fly feeding, distribution of particular fruit fly-type bacteria, development of female flies to sexual maturity, courtship and mating, oviposition, larval and pupal development. Based on this study, the concept was proposed that the fruiting host plant is the 'centre of activity' for a fruit fly population. This concept has also been discussed by Drew (1987) and Drew & Lloyd (1989, 1991). Host plant-based courtship and mating in Tephritidae has been observed for more than 50 species (Prokopy et al. , 1971; Zwolfer, 1974; Aluja et al. , 2000), and the strong relationship between dacine species and their host plants is believed to be significant in the processes of speciation (Drew, 2004). Additionally, the understanding of these relationships is important ecological knowledge that is basic to the development of improved field pest management strategies.
In contradiction to these concepts, Raghu et al. (2002) and Raghu & Clarke (2003) reported that mating in Bactrocera cacuminata (Hering) does not occur in its primary larval host plant, Solanum mauritianum L. In a field study of the diurnal behaviour of B. cacuminata on S . mauritianum , Raghu et al. (2002) reported that they observed only feeding, resting and oviposition behaviours during the course of their experiment. Male aggregation, courtship and mating behaviour were not observed. Raghu et al. (2002) also failed to observe these behaviours on S. mauritianum in the field when fortnightly casual observations were carried out at post-sunset over an eight-month period across five different sites in southern Queensland. In another study, Raghu & Clarke (2003) examined the diurnal response of B. cacuminata in a field cage containing plastic plants on which sugar, protein, methyl eugenol and leaves and fruit of S. mauritianum had been placed. Based solely on a significantly greater proportion of flies mating at the methyl eugenol site than at the other resources in their field cage experiment, they concluded that methyl eugenol acts...