Life cycle

Mussel scale has one generation per year.  Eggs are laid in the autumn and are deposited by the female under the scale shell before she dies.

• Eggs hatch in late May or early June and the first stage nymphs, known as crawlers, disperse over the tree during a period of approximately 4-6 weeks, though small numbers continue to emerge for several more weeks.
• The timing of migration varies between years depending on spring temperatures.
• When they have found a suitable feeding site, they settle mainly on the bark but also on fruitlets where they remain for the rest of their life.
• Each then moults to a second instar and then a third instar nymph, both stages being sedentary remaining in the same place and protected by the mussel-shaped scale formed from wax and the cast nymphal skin.
• In late August and September, each female deposits up to 80 eggs beneath the scale, then dies.
• The scale remains attached to the bark and protects the eggs through the winter.
• Although males appear in some races of mussel scale, only females occur on fruit crops and reproduction is entirely parthenogenetic.

Pest status

Mussel scale

• A minor but common pest of apple and sometimes pear.

Other scale insects

• Oyster scales, pear scale and nut scale are also minor pests of fruit trees in the UK but occur less frequently than mussel scale.
• The San José scale is a destructive and widespread pest of fruit trees but has not yet been recorded in Britain.

Other hosts

Mussel scale

• Occurs on many other woody host plants. Fruit crop hosts include apple, pear, cherry, plum, bilberry and less frequently currant and gooseberry.
• Other hosts include blackthorn, cotoneaster, hawthorn, heath, heather and many others.
• Populations on hawthorn, heather and other wild plants are believed to be the main sources of infestation of orchards.

Other scale insects

• In the UK, oyster scale occurs on apple, apricot, birch, cherry, currant, nectarine, peach, pear and plum. Birch is considered to be the original host.
• Pear scale occurs on apple, peach, pear and poplar.
• Nut scale occurs on apple, pear and plum.

Varietal susceptibility

All the commonly grown apple varieties are susceptible to mussel scale.

Distribution

Widespread and common. Low levels of infestation occur in many orchards.

Recognition

Adults
2.0-3.5 mm long, flat and mussel–shaped, grey to yellowish brown. Found on the bark and fruits of apple trees.

Nymphs (crawlers)
Oval, pale yellowish brown.

Eggs
Minute, oval and white. Deposited beneath the scale.

Other pests with which mussel scale may be confused

• Only mussel scale is mussel-shaped.
• Pear scale and oyster scale individuals sometimes settle on fruits mainly round the calyx, but are roughly circular.

Damage

• 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.
• At first, the upper surface of the leaves assumes a glistening, sticky appearance but it later becomes unsightly with the growth of sooty mould fungi on the honeydew.

Monitoring

Visual inspection

• Record the percentage of fruits at harvest contaminated by mussel scale.
• If the level is economically significant (e.g. >1%), then insecticidal treatment may be justified in the dormant period or after blossom the following year.
• The bark of apple trees should be examined in the dormant period (early on if a tar oil winter wash treatment is to be applied) for signs of infestation.

Monitoring crawler emergence

• 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 clear sticky tape (see image) and the numbers of crawlers captured counted.
• The bands should be refreshed each week.
• By this method it is possible to ascertain when approximately 80-90% of scales have emerged, the critical time for application of first sprays.

Forecasting

A mass hatch of the eggs of mussel scale often occurs in a short time period of a few days in late May or June.

• Insecticide sprays need to be targeted against the young crawlers that emerge.
• Mature larvae are protected by their outer scale and are much less susceptible to insecticides.
• For this reason, pinpointing the timing of the mass hatch is helpful to time sprays correctly.
• The timing of the hatch and the optimum timing for application of sprays can be predicted from temperature sums.
• First emergence occurs at 151 day-degrees above a threshold of 8^oC after 1 January.
• Mass egg hatch occurs at about 190 day-degrees. 90% hatch occurs at 230 day-degrees and is considered to be the optimum timing for application of commonly used pesticides.
• Early hatched nymphs may reach the second instar stage by this time but these are still susceptible to the commonly used insecticides.

Chemical control

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.
• 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 have 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 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.

Cultural control

• There are few obvious cultural control measures for this pest.
• Isolation from hawthorn and other trees that are wild hosts will reduce the probability of infestation developing.
• Mussel scale populations tend to be greatest in old orchards where the pest has been allowed to increase without check over a number of seasons.
• Physical destruction of colonies or their removal may be possible but is unlikely to be economic.

Natural enemies

Scale insect populations are host to a complex of natural enemies.

Parasitic wasps
The minute chalcid wasp Aphytis mytilaspidis is a common external parasite of mussel, oyster and pear scales.

• The egg of the parasite, usually one per scale, is laid under the waxy scale, close to the body of the insect.
• The wasp has two generations per year and can feed on the second nymphal stage as well as on the adult female.
• The greatest extent of parasitism of mussel scale recorded is 26%, but in most cases parasitism is much lower.

Several other species of parasitic wasp also attack mussel and other scale insects.

• Levels of parasitism can be assessed by looking for small circular holes in the old scales from which the adult wasps have emerged.
• However, natural populations of the parasitic wasps do not constitute a significant or reliable regulatory mechanism.

Predatory insects

• Ladybird adults and larvae, mirid and anthocorid bugs, earwigs and predatory mites often destroy large numbers of scales, particularly the vulnerable young stages.

Biological control

Biological control of mussel scale has not been developed.

• However, the parasitic wasp Encarsia perniciosi has been introduced to control the San José scale with some success in other European countries.

 

Further reading

Ben-Dov, Y. & Hodgson, C. J. 1997. World Crop Pests, Volume 7B. Soft Scale Insects – Their Biology, Natural Enemies and Control. Amsterdam, Elsevier.

Helsen, H. H. M., Blommers, L. H. M. & Trapman, M. C. 1996. Timing observation and control of mussel scale Lepidosaphes ulmi. IOBC wprs Bulletin 19(4), 145-149.

Kosztarab, M. & Kozar, F. 1988. Scale insects of central Europe. Akademiai Kiado, Budapest, 456 ppRosen, D. 1990. World Crop Pests, Volume 4B. Armored Scale Insects – Their Biology, Natural Enemies and Control. Amsterdam, Elsevier.