Science into practice - mussel scale control for apples

Mussel scale is a common pest of apples (and sometimes pears) and, since the demise of winter tar-oil wash treatments, it has been increasing in importance in commercial apple orchards in the UK.

Populations on hawthorn, heather and other wild plants are believed to be the main sources of infestation. The main damage is caused by the presence of scales on the surface of fruits at harvest. The contamination is superficial but can downgrade fruit.

For heavily infested orchards, it has been estimated that it is not unusual for 10 per cent of fruit to be downgraded due to the pest. Two HDC projects - TF 178 and TF 180 - have investigated the problem and come up with some solutions.

The first field study in 2007 found that the mussel-scale crawler migration in spring lasted much longer (over four weeks) than expected and that good control was unlikely to be achieved with a single insecticide spray. The duration of the migration at a high level was much longer than had previously been understood by UK growers and advisers. Predictions using a Dutch ambient-air temperature sum model were too late.

In follow-up 2008 trials to confirm best practice, winter treatments with mineral oils with or without insecticides did not control mussel scale on the fruits or shoots. However, two sprays of Calypso or Gazelle (two weeks apart) or a single spray of Calypso with Break Thru towards the end of the mussel-scale emergence achieved a significant reduction in the number of scales on harvested fruits. The Dutch temperature sum model should be used to estimate the timing of spray applications as it was more accurate than alternative predictors.

- Horticultural Development Company

For details on all HDC activities, visit www.hdc.org.uk.


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