Agri-Science Queensland research horticulturist Russ McCrystal has just finalised an Australian Sweet Potato Growers (ASPG) supported Horticulture Australia Limited project with a charter to assess the efficacy of available insecticides for the control of wireworm, root knot nematode and sweet potato weevil.
Mr McCrystal said the challenge for the largely Queensland-based industry now valued in excess of $50 million annually was to identify new "soft options" to manage sweet potato soil insect pests now regarded as the greatest risk factor for market failure in the rapidly expanding sweet potato supply chain.
Mr McCrystal said a combination of all year around sweet potato production; the stable sub-tropical environment in the major production regions; and the difficulties in managing volunteer post-harvest sweet potato growth were contributing to the ideal conditions for continuous and rapid soil pest cycles.
"As a consequence, the industry is constantly putting the few permitted insecticides available under maximum working pest pressure throughout the year," Mr McCrystal said.
"To manage the inherent risk of developing resistance to these insecticides, there is a need to pursue a range of alternative chemistry and introduce appropriate guidelines for managing potential insecticide and nematicide resistance in sweet potato pests.
"The grower survey also recommends future research to investigate better management of post-harvest crop residue as the current break crops such as forage sorghum are not able to successfully out-compete sweet potato volunteer regrowth.
"There is no doubt that crop regrowth is a major contributor to the viability of the pest cycles in the Bundaberg, Rockhampton, Lockyer Valley and Cudgen (Tweed) production areas."
Mr McCrystal said that when the four year project was launched in 2006, growers were continually reporting increased incidences of major soil insect damage.
To control wireworms, root knot nematodes and sweet potato weevil, growers were applying multiple broad spectrum insecticides that were incorporated into the soil prior to planting followed by foliar sprays during crop development.
"Our survey found growers were often applying insecticides as an insurance cover when it was possibly the wrong choice of insecticide or it was not needed."
The Agri-Science Queensland research, a part of the Department of Employment, Economic Development and Innovation (DEEDI), reviewed the efficacy of insecticides on wireworm, nematicides on root knot nematode and insecticides on sweet potato weevil.
The project has recommended that the industry should pursue changes to current application methods for insecticide use to control wireworm.
"We found that some insecticides are more effective at controlling wireworm when applied at reduced rates through the drip irrigation at strategic times during crop development rather than incorporating the insecticide into the soil prior to planting," Mr McCrystal said.
"An alternative nematicide was trialled and found to be effective against root knot nematode. The minor use permission for its use was successfully pursued on behalf of the sweet potato industry.
"This outcome was necessary to manage the risk of degradation or resistance issues developing for the only registered nematicide in the sweet potato crop.
"A new insecticide applied to the soil prior to planting has shown to be effective against sweet potato weevil. Because this product works systemically in the plant, it appears to have low impact on beneficial insects which will be a very good IPM fit for the crop," he said.
Pheromone technology to attract and destroy sweet potato weevil was successfully demonstrated at a Rossmoya district on-farm trial. As a result, the grower adoption of weevil pheromone technology has increased from 6 per cent in 2006 to 80pc in 2010.
Prior to the project, no growers were reported to be taking nematode counts before planting but in 2010, 30pc of growers were now assessing nematode populations.
Mr McCrystal said the survey identified a significant decrease in the use of organophosphate-based insecticides incorporated into the soil prior to planting in the past four years.
"While this is regarded a good outcome, it has led to an over-dependence on bifenthrin which controls both wireworm and sweet potato weevil as a soil incorporated or foliar application," Mr McCrystal said.
Mr McCrystal said the industry adoption of the grower survey research and development recommendations would make a valuable contribution towards developing a strong integrated approach to pest management in the crop.
This would further secure the reputation of the sweet potato industry as "clean and green" in the heavily regulated Australian horticultural production environment and the demanding domestic market place.
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