Effect of sulphur dioxide and cold on survival of insects during storage of table grapes (TG15003)

What was it all about?

With Australian table grape exports increasing rapidly in recent years, this project sought to strengthen the relationship with the Chinese market. Running from 2015 to 2016, it looked at six potential insect pests of concern, producing data to demonstrate that, should they make their way into consignments, they will not survive current cold and sulphur dioxide (SO₂) treatments – giving growers, exporters and importers added peace of mind.

The insects investigated were:

• Long-tailed mealybug
• Lady bird beetle
• European earwig
• Argentine ant
• Carpophilus beetle
• Two-spotted spider mite.

The survival of these pests was tested in conditions similar to those experienced during export shipping to China and other Asian countries as described in Red blue beetle table grape disinfestation research (TG13002). 

Key findings were 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
• It took eight weeks for absolutely every two-spotted spider mite to die with or without SO₂, though 95 per cent of the mites 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 a substantial increase in mortality, with about 92 per cent of the bugs dead after four weeks.

The research also revealed that during shipment, SO₂ levels are substantially higher at the tops of table grape cartons, where the sheets are generally placed. The researchers note that this new finding could explain anecdotal reports of rots always seeming to occur in the bottom of cartons – not because moisture pooling as thought, but because of the distribution of SO₂. More research will be required to look into this, and devise possible solutions.