HONEYDEW affected cotton can create serious problems for cotton spinning mills and lead to a damaged reputation, as well as a damaged bank account, for cotton farmers.
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For the last three and a half years, the CSIRO has been working to understand what factors can reduce honeydew levels on cotton lint to provide better advice to crop managers through their project Enhancing integrated pest management in cotton systems.
Honeydew is a sticky residue that whitefly and silverleaf aphids leave behind when they feed on cotton.
Currently, farmers use sampling techniques and spray thresholds to identify when these pests need to be controlled in their cotton.
CSIRO project supervisor Lewis Wilson said although management was usually effective, there were always many situations where cotton still was contaminated due to factors such as rapid influxes of pests from nearby matured crops.
Dr Wilson and his team conducted experiments to determine whether honeydew would eventually breakdown itself given sufficient time in the field before it was harvested.
"We wanted to know how much breakdown you get of honeydew when it just sits there in the sun," he said.
"We found breakdown in the sun alone was very low but an interesting thing we did find is rainfall washes it off pretty effectively."
Dr Wilson said his team found most of the honeydew was on the outer surface of the boll, very little penetrated into the interior boll.
"We know that the lint of the cotton is hydrophobic so it repels water instead of absorbing it," he said.
"As rainwater is running down over the outside boll, the honeydew is easily washed off and that's in our favour."
A member of the project team Simone Heimoana, developed a method to contaminate cotton bolls with a known amount of artificial honeydew.
"We can then put these bolls in the field and determine how much honeydew they had on them after various periods of exposure to sunlight and some rainfall," she said.
Dr Heimoana replicated rainfall with a lateral move irrigation system that passed over the bolls, dropping water similar to raindrops as it moved.
The team was then able to measure the amount of honeydew on the cotton after they had been exposed to different amounts of rainfall.
Tests were conducted after each passing of the irrigation system, showing the amount of honeydew decrease after 15 millimetres, 30mm and so on until the cotton had received 60mm of moisture.
"We found that around 12ml to 15ml of rain will get rid of most honey dew," Dr Heimoana said.
The Enhancing integrated pest management in cotton systems project has recently been renewed for another five years and Dr Wilson said there were still questions regarding honeydew that needed to be answered.
Dr Wilson said the CSIRO wanted to find out whether the sooty moulds that grow on honeydew break down any sugar created by the insect residue and whether it occurs quick enough to be useful to crop managers.
They also want to discover whether the presence of sooty moulds on cotton causes damage to the fibres or affects the colour of the cotton as this could lead to grade reductions.
Research will continue to be conducted to discover whether there are any other options for cotton farmers to get rid of honeydew without waiting for rain.
"It's a fairly big problem for the industry and we want to know what else we can do to help," Dr Wilson said.
"We are developing potential solutions that we can test to see if they can help the industry deal with the problem, besides relying on rainfall."