Research wrap-up: cold and sulphur dioxide keeping table grape exports safe

With the export market for Australian table grapes booming, recently completed research is giving growers and exporters added peace of mind when it comes to the control of potential insect pests within shipments.

Hort Innovation Chief Executive John Lloyd said that Australia’s approach to biosecurity and its commitment to phytosanitary measures are among the best in the world.

“Our export partners require that there are no live insects in shipments received, and this new research confirms that existing practices in Australia’s export of table grapes do not allow for the survival of key potential pests.”

The research was funded by Hort Innovation using the table grape R&D levy and funds from the Australian Government. It looked at the use of cold storage and sulphur dioxide (SO₂) treatment on the survival of six insect pests potentially contaminating table grape exports: long-tailed mealybug (Pseudococcus longispinus), ladybird beetle (Chilocorus sp.), European earwig (Forficula auricularia), Argentine ant (Linepithema humile), Carpophilus beetle (Carpophilus hemipterus), and two-spotted spider mite (Tetranychus urticae).

Project leader Bruce Tomkins, from Victoria’s Department of Economic Development, Jobs, Transport and Resources, said that the insects’ survival was tested in conditions similar to those experienced during export shipping to China and other Asian countries.

“Current sea-freight export of table grapes involves cold, with transit temperatures generally around 1̊C, and the cartons are treated with SO₂-generating sheets, primarily for the control of fruit rot,” Bruce said. Depending on where the grapes are being exported to, sea freight can take anywhere from two to four weeks or more.

“We found that several of the insects studied are killed quite quickly with these existing practices – either cold or SO₂, with no chance of survival,” Bruce said. “And even with the ones that are highly tolerant of cold, SO₂ treatment will increase the mortality substantially.”

Specifically, the research found that after approximately two weeks of cold storage all mealybugs, ladybird beetles and Carpophilus beetles died, with or without SO₂ sheets (though the presence of SO₂ did substantially increase mortality within the first week of storage).

With two-spotted spider mites, it took eight weeks for absolutely every mite to die with or without SO₂, though 95 per cent were eliminated after four weeks.

European earwigs were the hardiest insect tested – cold alone did not kill the majority of the bugs, with 70 per cent still alive after eight weeks in cold storage. But the addition of SO₂ saw about 92 per cent of the bugs dead after four weeks.

“What this means is we can have confidence that current treatments are doing a good job of reducing the risk of insect contaminants surviving the journey to Asian export markets,” Bruce said.

The research also turned up something interesting about the distribution of SO₂ during shipment – namely that levels are substantially higher at the top of the carton, where the sheets are generally placed.

“This was something new that we found,” Bruce said. “There have been anecdotal reports of rots always seeming to occur in the bottom of boxes, which was put down to the pooling of moisture – but now we can see that this may have something to do with the SO₂ treatment not getting down into these areas.”

More research is needed to look into this, Bruce said, but solutions may include putting another generating sheet at the bottom of boxes, changing the rate of SO₂ release, or maintaining the high rate of release at the start of shipments for longer.