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Oystershell scale and other scale insects - chemical control |
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Oils
A wide variety of oils are available for the control of
scale insects. Details of their characteristics and specific uses
are available in the New Zealand Agrichemical Manual. Oils can be
phytotoxic and care should be taken to select the right oil for
each application, including a check on the compatibility with
other materials being applied. Summer use of oil may be
phytotoxic if applied close to some fungicides, and only
narrow-range light oils, such as Sunspray® or Excel®
should be used at this time. Oil should not be used at green-tip
on oil sensitive cultivars such as Cox's Orange. An application
of insecticide alone should be applied instead. Oils control
scale insects mainly by smothering. This is assisted by the
penetrative and spreading ability of oil on the surface of
plants. Coverage and penetration are particularly important for
scale insects which live in cracks and crevices on the bark of
apple trees and other hosts. High volume application is also very
important in ensuring effective spray coverage.
In IFP, the use of mineral oils at green tip is based on monitoring and may be combined with insecticide (either insect growth regulator or organophosphate). The combination of oil penetration and insecticide toxicity is particularly effective at this time. Mineral oils applied in summer can also assist scale control if used at 2%. However, care is needed to avoid phytotoxicity, particularly in relation to fungicide usage. A single application is usually all that is possible, and this may not protect the crop throughout the crawler settlement period.
Mineral oils are also permitted in organic production but the preferred approach is to use vegetable oils as commercial formulations become available which are effective and non-phytotoxic. Canola and soybean oils are showing promise, particularly when used in the dormant and green tip period. Lime sulphur is another scale control option for organic growers but it should not be used in conjunction with oil. Oil should also not be used within 14 days of a sulphur spray.
Spraying of scale-infested shelter can contribute significantly to improving scale control. Because shelter species often provide a refuge for beneficial insects, the preferred IFP insecticide treatments are 2% oil or 2% oil + Applaud® in spring (oil alone for organic production). 2% Oil is also effective in summer if applied at the start of crawler release.