Mussel scale is a small insect that attaches itself to the stems of a box plant and sucks the sap. This can cause the foliage to discolour, turning yellow and orange. If the scale are left untreated the box may start to lose leaves and will have poor growth in the Spring.
Have a close look at the stems of the box to see the adult mussel scales. The insect is tiny, only about 3mm long and is protected under a shell or scale that resembles the shape of the mussel mollusc.
Treatment with an insecticide spray can be difficult as the mussel scales are protected under their shells. The insects can be scraped off but this does risk damaging the stems of the box plants and is something I wouldn’t recommend.
The mussel scale reproduces once in a year. The eggs are protected under the scale with the female and overwinter until hatching the following year, in late May/ June.
Young mussel scale, or nymphs, crawl around the plant looking for a place to take up permanent residence and start feeding on the sap. Spraying now with an insecticide can be very effective as the nymphs haven’t formed their protective scales.
Most insecticides available from the garden centre work well, such as Scott’s Bug Clear Ultra or Provanto Ultimate Bug Killer Two or three applications maybe required to treat all the mussel scale nymphs. Best to spray in the evening to minimise the risk to beneficial insects.
Scales may remain on the stems of the box plant but these will be dead if the treatment has been successful and new growth will not be affected.