Limestone can prevent canegrub control with controlled-release insecticide

Pulverised limestone applications reduced the capacity of controlled-release (CR) insecticide to prevent canegrub damage where soil pH was raised above (approximately) 6.0-6.2. Broadcast application and incorporation of >5.0 t/ha limestone shortly before planting can result in insufficient chlorpyrifos insecticide in soil to prevent greyback canegrub damage in plant cane. The effect of 2.5 t/ha of limestone was slower, and more likely to prevent canegrub control in first ratoons than in the plant crop. Soil amendment with magnesium oxide (MgO) or pulverised dolomite had the same effect. Limestone-amended soil remained above the postulated critical value pH 6.2 in the granule zone for at least 22 months. Possible strategies to apply limestone and to maintain effective canegrub control with CR insecticide are discussed.