What was it all about?

Kelly’s citrus thrip is a key pest of citrus in the Riverland-Sunraysia region. It is known to have reduced the packout of export quality fruit and to render some fruit unsaleable in Navel and Valencia oranges, lemons and grapefruit.

Control programs rely heavily on organophosphate insecticides, but populations are developing resistance resulting in increased spraying and disruption of integrated pest management.

Earlier work demonstrated that applying compost could reduce the emergence of the thrips from soil by more than half, by boosting populations of soil-dwelling predatory mites. This project set out to investigate how long these benefits last.

Researchers applied composts of recycled green organics, grape mark and animal manure then assessed yields and the size and quality of harvested fruit.

The key outcomes were…

• Compost maintained higher soil moisture levels in the top 25 cm of soil compared to untreated controls, for at least six years and probably significantly longer than that
  
  
• Composted soils had significantly higher nitrogen and other nutrient levels
  
  
• Plants from the earlier trials that revealed the protective effects of compost were found to still be protecting against Kelly’s citrus thrips up to four years after application, particularly where composted green waste was used
  
  
• Fruit from trees treated with compost were larger diameter, and in most cases fruit density was also higher
  
  
• Benefits achieved from the composted green waste appear to be more robust and long term than from other composts used, particularly in the case of animal manure which had little effect beyond three years.

Use of compost in production could be a way for growers to avoid increased spraying of insecticides, which is not only costly but disrupts integrated pest management.