What was it all about?

Murcott mandarins (Citrus reticulata) are currently exported in significant amounts from Queensland to China and Thailand, but must be free from Queensland fruit fly (Qfly) to access markets.

Methyl bromide is currently the predominant fumigant for insects including Qfly but is known to cause fruit injury in Murcott mandarins.

The aim of this project was to determine the technical feasibility of low-dose methyl bromide as a quarantine disinfestation treatment for Murcott mandarins. A treatment was sought that would kill Qfly without injuring the fruit.

Four methyl bromide treatment schedules were trialled and afterwards, all fruit was stored at 5°C for 21 days to simulate shipping followed by seven days at 22°C to simulate retail sales. Fruit was then assessed for external and internal defects, skin gloss, skin colour, weight loss, acidity, total soluble solids and taste. Qfly was treated as mature larvae and effectiveness was measured by survival to pupae stage.

The results showed that none of the combinations of methyl bromide concentration, treatment duration, and temperatures tested in this research successfully killed all insects without adversely affecting the fruit.

Specifically…

• Fumigation at 16 and 18 g/m3 methyl bromide for 7 and 8 hours at 18°C did not completely disinfest the fruit of Qfly larvae
  
  
• Fumigation at 18g/m3 methyl bromide at 20°C for 8 hours was highly effective
  
  
• Fumigation at 16 and 18 g/m3 of methyl bromide for 9 and 10 hours’ duration resulted in significant injury to internal and external quality, fruit gloss and flavour.
  
  
• Results were better for fruit fumigated at 16g/m3 at 18°C for 7 and 8 hours, but there were still adverse effects on flavour.

Researchers were not certain if the damage to fruit flavour was caused by the methyl bromide, or simply by an accumulation of carbon dioxide due to the length of time the fruit remained in the treatment chamber.

They recommended further studies to determine the cause and to look at the fruit at different times in the season, from different locations, at various maturities and alternative postharvest handling situations.