The use of fenthion as a post harvest dip treatment for fresh horticultural commodities that are host to pest fruit flies has been restricted due to health concerns with residues. The restricted treatment is a single dip, flood spray or spray of fenthion (obsolete Interstate Certification Assurance Operational Procedures ICA-01, ICA-02 and ICA-03, respectively) at a rate of 412.4mg/L fenthion for 60s or more.

The focus of this report is to determine the effect of a reduced rate (100mg/L instead of 412.5mg/L) of fenthion on the mortality of Cucumber fly (Bactrocera cucumis, French) (CF) applied twice (the second dip applied 24h after the first) in Australian-grown zucchinis at four different fly developmental stages; eggs, first instar larvae, second instar larvae and third instar larvae.

Survival of insects from Control (untreated) infested fruit gave an estimate of the number of insects exposed to the test treatment in fenthion dipped fruit. Over 30,000 insects (total from three replicates in time) at each of the four immature life stages were treated. The findings were that all double dip fenthion treatments caused 100% CF mortality in zucchinis. The outcome of this experiment was that two 1 minute dips of fenthion, at a rate of 100mg/L of fenthion in water, separated by 24 hours in storage at 10°C caused 100% mortality of CF eggs and larvae in fresh zucchinis. The temperature of the dip solution was 20°C.

This information can be used by industry to support an application for the use of the double knock low dose fenthion treatment of zucchinis as a quarantine treatment against cucumber fly as a quarantine treatment for market access.

NOTE: Research on residue analyses of zucchinis treated using this new treatment were commissioned by Hannay-Douglas Pty Ltd and showed residues were well below maximum residue limits set by the APVMA in all cases.