Oystershell scale and other scale insects - chemical control

Lime sulphur
Lime sulphur is both a fungicide and insecticide. It has long been used for the control of scale insects on dormant trees. Its use at this time also destroys lichen on the bark which may otherwise protect scale insects from control. A spray of lime sulphur in winter can assist scale control with oil sprays by removing the lichen before the oil is applied. Winter spraying with lime sulphur in the past has used rates as high as 5%. These sprays have been reported as toxic to predatory mites, such as Typhlodromus pyri, which is an important predator of European red mite and also feeds on scale crawlers. The use of lime sulphur in this way should, therefore, be carefully monitored.

Lime sulphur has been used increasingly by organic apple growers for disease control in summer. The rates used range from 1-3% and it is not known how effective these sprays are in reducing scale infestation. Research at the Clyde Research Centre of HortResearch has provided no evidence that summer use of lime sulphur as a fungicide has reduced scale infestations. Further research is needed. Lime sulphur should not be used in conjunction with oil because of phytotoxicity.

Spraying of scale-infested shelter trees can contribute significantly to improving scale control by reducing immigration of crawlers in summer. Lime sulphur application in the dormant period could be useful in reducing scale on deciduous shelter trees.