Review of the outbreak threshold for Queensland fruit fly (Bactrocera tryoni Froggatt)

Fruit flies cause losses in horticultural produce across the world and are a major quarantine concern for most countries. Queensland fruit fly (Qfly) is a native to Australia and is also present in a small number of Pacific Island countries. The detection of Qfly in recognized pest free areas triggers quarantine restrictions from domestic and international markets. In Australia, the detection of five male flies has been taken to indicate an outbreak (i.e. unacceptable risk). Matching the domestic standard, many countries have accepted the 5-fly limit as a quarantine threshold. But some other countries have set the detection of two male flies, or even a single fly, as the threshold for an outbreak. This different standard creates an administrative complexity for exporters and trade regulators. In this paper, we review the published science covering the impediments to pest establishment. Outbreak data from Victoria and New South Wales during 2007 and 2009 are reviewed in relation to the 2-fly and 5-fly thresholds. Large volumes of fruit have been traded within Australia and internationally based on the 5-fly threshold without incident and there is no evidence that the 2-fly threshold is more appropriate. While Qfly is recognized as being capable of longer distance dispersal than some other fruit fly species, it is also recognized as a poor colonizer. The 5-fly threshold is proposed as the most appropriate threshold for imposition of quarantine restrictions and is recommended as a universal standard for harmonization of quarantine regulations.