Correct timing of insecticide sprays for control is important and should be determined by regular monitoring (see ‘Monitoring’). It is a common error that sprays are applied too early. Eggs in shaded places hatch well after those exposed to the sun.
- Research in AHDB Horticulture Project TF 180 demonstrated that control is best achieved with sprays of acetamiprid (Gazelle) applied from 50% crawler emergence or later, followed by a second treatment two weeks after that, particularly where heavy infestations occur.
- Where infestations are really heavy, 3 applications may be required, remembering not to exceed the maximum permitted for any one product.
- It was also found that the use of an adjuvant (Break Thru) with acetamiprid (Gazelle) achieved the same degree of control as one application on its own.
- If a single spray of one of these materials is to be used against light infestations, it is best applied at about 80-90% crawler emergence.
- This product is most effective against emerged migrating crawlers and first and second stage scales that have recently settled at their feeding sites. It has little effect on scales that have not emerged from under their mother scale.
- As the product only has effective action for a week or two, then applications in the later stages of the emergence cycle catch the maximum proportion of the population at the susceptible stage.
- It appears to give better control of scale insects on the bark than it does of those on the fruitlets, so maximum benefits of treatment with this insecticide may not be apparent till the following year.
- A full approval for spirotetramat (Batavia) on apples for the control of sucking insect pests will offer some control of mussel scale, but growers may prefer to reserve its use for more difficult to control pests such as woolly aphid and rosy apple aphid. It must be applied after flowering and works best when pests are moving from brown wood to green tissue. It will prevent population build-up but does not offer pest ‘knockdown’.
- A recent EAMU for Flipper (fatty acids) has increased the available options should growers wish to reserve other insecticides for control of pests later in the season. Although recommended for control of aphids, mites and blossom weevil, it may offer incidental control of mussel scale crawlers when applied for other pests. It is known to complement the use of Batavia as it provides quick ‘knockdown’.
- Synthetic pyrethoids are also partially effective but they should not be used because they are harmful to the orchard predatory mite Typhlodromus pyri and many other important natural enemies.
- In other European countries, white oil sprays are used at mouse ear stage, giving a fairly good control. There is no approval for such treatment in the UK.
Insecticide resistance
Insecticide resistance is not known to occur in mussel scale populations.