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Mussel scale (Lepidosaphes ulmi (Linnaeus)) and other scale insect pests

Mussel scale infested fruit

Nut scale on Bramley branch

Mussel scale has become an increasingly important and common pest of apple in recent years. It sometimes attacks pear.

Several other scale insects including pear scale, oyster scale and nut scale also occur locally, mainly on cider apples which are infrequently sprayed with insecticides. Mussel scale is readily distinguished from the others as it is the only one that is mussel-shaped whereas pear scale and oyster scale are roughly circular.

Wild hosts such as hawthorn may act as a source of infestation.

The main damage is caused by the presence of mussel scales on the surface of fruits at harvest. The contamination is superficial but may downgrade the fruit.

Very heavy infestations on the bark may debilitate the tree and there may be some contamination of the foliage with honeydew.

The life cycle  is limited to one generation per year. The timing of egg hatch and dispersal on the tree varies between years depending on spring temperatures.

Mussel scale crawler emergence in 2007: Numbers of crawlers captured in sticky bands in a Kent Cox orchard

Mussel scale crawler emergence in 2008: Numbers of crawlers captured in sticky bands in a Kent Cox orchard

Correct timing of insecticide sprays for control is important and should be determined by regular monitoring.

  • Weekly examination of mother scales and the surface of the bark on dry days though the critical period is required.
  • Alternatively, the branches of infested trees may be furnished with sticky bands made from double sided sticky tape and the numbers of crawlers captured counted and the bands refreshed each week.
  • By this method it is possible to ascertain when approximately 80-90% of scales have emerged.
  • This is the critical time for application of first sprays. A temperature-based forecasting model can give useful back-up.

Control  

  • Research in AHDB Horticulture Project TF 180 demonstrated that control is best achieved with  sprays of thiacloprid (Calypso) or 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 thiacloprid (Calypso) 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.
  • These products are most effective against emerged migrating crawlers and first and second stage scales that have recently settled at their feeding sites. They have little effect on scales that have not emerged from under their mother scale.
  • As the products only have effective action for a week or two, applications in the later stages of the emergence cycle catch the maximum proportion of the population at the susceptible stage.
  • Spirodiclofen (Envidor) also has some activity against mussel scale but takes one to two weeks to act so it probably has to be applied earlier in the emergence period to get maximum effect. 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 product may not be apparent till the following year.
  • Fatty acids (Savona) is currently the only insecticide recommended by the manufacturer for control of scale insects on fruit trees during the growing season. If this material is used, high volume sprays are necessary to thoroughly wet the wood, timed to control the crawlers. However, such treatment is likely to be very costly.
  • Sprays of other broad-spectrum insecticides, applied at medium to high volume at this time, are also likely to be partially effective.
  • Synthetic pyrethroids 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.
  • Note that fenoxycarb (Insegar) was ineffective in AHDB Horticulture-funded trials in 2007 and 2008.

 

Insecticides approved for use on apple which are recommended for control of scale insects or offer incidental control when applied to control other pests

Choice of insecticides – efficacy factors

Active ingredient Trade name (examples) Class Selectivity Approved for control of Safety to Typhs 
acetamiprid Gazelle neonicotinoid broad-spectrum, systemic Aphids safe
cypermethrin various pyrethroid broad spectrum Aphids, caterpillars, codling & tortrix moths, sawflies, apple sucker harmful
deltamethrin Decis etc. pyrethroid broad spectrum Aphids, apple sucker, capsids, codling & tortrix moths, sawfly harmful
fatty acids Savona soap broadspectrum Aphids, leafhoppers, mealy bugs, scale insects, spider mites, thrips harmful
pyrethrins Spruzit extract from pyrethrum broad spectrum Aphids, apple blossom weevil, caterpillars, scales insects, spider mite harmful
spirodiclofen Envidor ketoenol insecticide and acaricide partially selective Spider mites, rust mites, mussel scale harmful
thiacloprid Calypso chloro-nicotinyl broad-spectrum, systemic Rosy apple aphid. (Also likely to control capsids and sawfly, though not caterpillars or woolly aphid) safe

Choice of insecticides – Safety factors

Read and follow label before applying sprays

  Hazards Harvest interval(days)  Max. no. sprays Buffer zoneWidth (m)
Anticholin-Esterase?  Humans Fish &aquatic life Bees
acetamirpid no u h u 14 2 20
cypermethrin no h, i ed d 0 4 50
deltamethrin no h, i ed d 0 u 18
fatty acids no u h u 0 u sm
pyrethrins no h,i ed d 0 4 50
spirodiclofen no h h d 14 1 30
thiacloprid no h, i ed h 14 2 30
h=harmful, i=irritant, d=dangerous, ed=extremely dangerous, c=closed cab required for air assisted sprayers, u=uncategorised/unclassified/unspecified

Control in organic orchards

  • Priority should be given to cultural control methods and fostering natural enemies.
  • High volume sprays of fatty acids (Savona) may be used as described under ‘Chemical control’ above. The importance of correct timing of sprays is stressed. Fatty acids (Savona) is permitted for use in organic production systems, but prior approval must be given by certification bodies before application.

Further reading