Treatment for mites on lychee fruit prior to irradiation for improved market access (LY16002)

What was it all about?

This project ran from 2016 to 2018 to help address a potential barrier to the export of Australian lychees: the presence of common pest the lychee erinose mite (Aceria litchii).

The researchers investigated simple postharvest treatments to reliably remove the mite from fruit destined for markets including the US and New Zealand. While lychees heading to these export destinations are subject to an irradiation protocol to eradicate target quarantine pests, the non-target erinose mite is able to withstand current irradiation rates – and higher rates could damage the fruit.

In looking at additional treatments to tackle mites, the project team evaluated postharvest dipping and flood spraying with paraffinic oil, and the potential for oil in combination with physical pre- and post-treatment cleaning using high-pressure/high-volume water sprays.

The effect of the oil treatment in combination with irradiation on fruit quality was also evaluated.

The researchers found that paraffinic oil at three per cent concentration applied as a 30 second dip or flood spray reduced mites and other surface insects on fruit. Though oil alone didn’t completely eliminate the pests, the addition of pressure washing before, and two minutes after, the oil application further reduced their incidence.

The project did find that oil dips could exacerbate surface fungi (rots) from about 10 days after treatment, predominately around the stalk end of fruit, but this problem wasn’t found with the flood spraying approach. Re-skinned lychee varieties proved particularly suited to the oil treatment, with colour enhancement recorded and the fruit storing well for up to two weeks at 5°C.

Further, the oil treatment was assessed in combination with irradiation, with no adverse interaction between the two seen – and during the project, a commercial shipment of lychees treated using oil and irradiation successfully entered the US.

As part of their work, the project team established a prototype roller conveyer incorporating an in‐line water spray, oil flood spray and post-oil water spray, which significantly reduced the count of live mites. However, the two-minute time lag required for the oil to be efficacious prior to the final water spray to remove dead or dying pests did impact on packing rates.

Following this work, Hort Innovation Lychee Fund project Mite and insect disinfestation of lychee fruit using high pressure water sprays (LY18000) was established to look at optimising the efficacy of high-pressure water application, to allow higher throughput on the packing line.

Also of note, in the first year of LY16002 the project team considered the use of fumigation with ethyl formate and carbon dioxide as an alternative treatment for mites. As fruit experienced severe browning at the fumigation rates required to kill mites and other insect pests, the project moved its focus on to the oil and water method.

Full details of the project can be found in the final research report, available for download at the top of this page.