Mussel scale, which had been absent for decades, has become quite a serious problem in some areas over the past five years or so, causing the downgrading of fruit and weakening fruit bud.
The pest is most prevalent in orchards along the south-facing Greensand escarpment that extends across most of Kent, which is a favoured fruit site because it is virtually frost-free and well drained.
Control of the pest is difficult because its larvae or crawlers - the stage most susceptible to insecticides - emerge from beneath the scales over such a long period - up to eight weeks in May and June. Most appear in a four-week period.
A recent HDC-funded project, run by East Malling Research entomologist Dr Jerry Cross, has identified the best insecticides and timings to achieve optimum control of the crawlers. He found that the maximum kill was obtained with Calypso plus Break-thru (a silicone wetter) applied at 90 per cent crawler emergence, or two Calypso or Gazelle (no wetter) sprays at 50 per cent and 90 per cent crawler emergence two weeks apart. A winter spray of Certis winter oil proved less effective. A temperature-based Dutch model was used to decide when to spray.
Unless killed, the crawlers mature and send tendrils into the fruit, bark and buds to suck the sap. They produce a hard scale, protecting them from insecticide. Females lay eggs under their scales that hatch into the dispersal stage crawlers.