Nutrient management may double as pest control
Plot and glasshouse trials for spider mites in maize and redlegged earth mites and aphids in canola indicate that insect pests prefer host plants affected by an imbalance of certain nutrients.
Researchers at the University of Western Australia say the findings suggest potential for a new approach to pest detection by monitoring crop nutrient status as a surrogate for identifying actual infestations.
Associate Professor Christian Nansen explains: “Soils in WA are, for example, often deficient in potassium and growers are used to applying potassium, especially to canola.
“Potassium leads to increased leaf thickness, stronger epidermal cells and decreased leaf nutrient concentrations of sugars and amino acids. This lowers the suitability of crops as hosts for pests.”
Associate Professor Nansen says the research has shown that the potassium status of crops can be characterised using reflectance values at specific wavelengths, so the researchers are now working on a reflectance-based algorithm that can predict a crop’s suitability to host insects.
He likens it to an infrared thermometer, which measures reflectance from skin to predict internal body temperature. “We hope to develop a system that can ‘take the temperature’ of crops to determine what pest-management practices are needed.
“For example, spraying is likely to be more effective (and cheaper) if we know exactly where pests are.”
Associate Professor Nansen’s PhD student Dustin Severtson, who has studied the behaviour of cabbage aphids on canola plants, says that a large area needs to be examined to make a reliable decision on whether or not to spray. He has noted that cabbage aphids initially prefer to colonise lower leaves, so are often missed: “When they appear on the canola flower heads the infestation is well established.
“Also, the bottom leaves receive less sunlight, produce less photosynthate and are more likely to be potassium deficient. This makes them more attractive to the insects. When given a choice, aphids in the trials clearly preferred canola leaves grown under a low-potassium regime.”
The researchers are now looking to reduce the attractiveness of crops to aphids and other pests by manipulating nutrient applications.
In spring, when the plant goes into reproduction mode it puts its resources into flowers and seeds. As this happens nutrient demand increases and the plant becomes potassium deficient – and more attractive to pests. A second treatment could therefore not only maximise plant health, but also discourage pests.
Last year, a GRDC-funded study by Dr David Murray found insect pests caused $360 million worth of damage in major grain crops. Pesticide treatments cost nearly $160 million. Most costly to control on canola were aphids ($4 million) and redlegged earth mites ($7.6 million).
Both Associate Professor Nansen’s position and Dustin Severtson’s PhD program are supported by the GRDC.
Associate Professor Christian Nansen
08 6488 8672
GRDC Project Code UWA00144, UWA00158
Region National, North, South, West