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Introduction
True fruit flies (Diptera: Tephritidae) are serious pests of horticulture worldwide, due to their broad larval host range, cosmopolitan distribution and high invasive capacity (McPheron & Steck, 1996; Malacrida et al., 2007). The major fruit fly pest in Australia is the Queensland fruit fly, Bactrocera tryoni (Froggatt). Native to Queensland, this species spread south in the 1860s (Drew, 1989) and is now found along the entire east coast of Australia (Meats, 1981) where it infests almost all horticultural crops (Drew, 1978). Genetic structuring in these east coast populations have been studied in Yu et al. (2001), Meats et al. (2003) and Sved et al. (2003).
In addition to the east coast populations, Australia has a second endemic fruit fly region, the northwestern region comprising the Northern Territory and Western Australia (fig. 1). The east and northwest regions are separated by an arid zone straddling the border between the Northern Territory and Queensland (Drew, 1989).
Fig. 1.
Map of Australia showing the sampling sites in Queensland, Western Australia and the Northern Territory. Numbers correspond to locations given in table 1. Circles indicate the regions used in the analysis. The arid region separating Queensland and the Northern Territory is shaded.
Until recently, the northwest region was considered to be relatively free from serious fruit fly pests. The main species described in the region, B. aquilonis (May), a sister species to B. tryoni, was not considered to be a pest (Drew, 1989), having been reared from just four commercial crops: peach, guava and two species of citrus (Allwood & Angeles, 1979). However, in 1985, a large increase in the range of preferred hosts was reported for B. aquilonis in the Darwin area (Smith et al., 1988). Collections of fruit before and after this date showed that 40 cultivated fruit species, including mangoes and star fruit, were now being infested. Pest flies were soon found in other major towns in the Northern Territory and, in 2000, new outbreaks of pest flies were reported in the horticultural regions of Kununurra (834 km southwest of Darwin: S. Smith, personal communication).
The cause of the increase in infestation of commercial crops since 1985 is unknown and is complicated by the fact that