Development of a non-chemical treatment system for avocado against Queensland fruit fly for interstate trade.

SUMMARY Most avocado cultivars are known to be particularly poor hosts for fruit flies with field control treatments rarely being required if fruit are harvested at the normal commercially acceptable hard stage. However, current interstate trade and market access for avocados from areas with endemic populations of Queensland fruit fly require that chemical postharvest treatments with dimethoate or fenthion must be applied. An alternative to these chemical treatments is highly desirable given industry and consumer concerns about pesticide usage and the possibility that such treatments may not be available in the future. Studies in Western Australia demonstrated non-host status for hard Hass avocado for Mediterranean fruit fly allowing interstate trade without the need for chemical postharvest treatment. This project was designed to determine if similar non-host status for Queensland fruit fly could be demonstrated for four cultivars of avocado grown in Queensland and New South Wales i.e. Hass, Lamb Hass, Shepard and Wurtz. An additional aim was to determine if a systems approach incorporating poor host status with a short cold treatment could be used to meet interstate phytosanitary requirements for cultivars which did not meet conditional non-host status requirements. The field studies undertaken in this project provided quantitative evidence that undamaged skin and flesh of hard avocados on the tree significantly reduced both adult fruit fly oviposition and immature fruit fly development. There were differences between cultivars in suitability for fruit fly infestation with Hass and Lamb Hass at the hard stage meeting the requirements for non-hosts (provided fruit were blemish free). The thinner skinned cultivars, Shepard and Wurtz, although still very poor hosts, were slightly more susceptible to fruit fly attack and did not meet the technical requirements for non-host status. For Shepard and Wurtz avocados, cold disinfestation trials demonstrated that a combination of host resistance to fruit fly development and a short cold treatment at