Why is it a problem? When feeding on apples this scale insect leaves red spots, and in severe cases distorts the fruit. This affects sale prices, especially for export. Complete coverage on twigs and branches can lead to dieback of affected parts and ultimately the decline of the tree. Attacks can cause defoliation of the tree within 2-3 years. However, this pest can be managed relatively easily without the use of pesticides.
Where and when is it a problem? It can be an important, localised pest, in all major apple growing regions. Outbreaks are often induced by indiscriminate use of pesticides. It was much more common prior to the widespread use of tree spray oil (TSO).
Tiny, circular, greyish scales are only just visible to the unaided eye. They are protected by a thick, waxy, protective scale (grey, conical “shell”), 1-2 mm in diameter.
The red halo-like spots surrounding feeding sites on the fruit surface makes them easily recognisable in the field, especially on green or yellowish parts of the fruit. In severe cases fruit become severely misshapen. Scales can cover twigs and branches but need careful observation to be seen as they are not as conspicuous as on fruit. However, they can result in dieback of the affected parts and will ultimately cause decline of the tree.
Confusion with other pests: Pest symptoms cannot be confused with that caused by other pests.
Lifecycle: Adult females lay eggs within their shell. During spring, young crawlers emerge and find new locations to settle and construct their own shells, initially on leaves and fruits and ultimately on twigs. Mature nymphs, pupae and adult females are all permanently fixed to the plant tissue by the stylet (beak) embedded in the host vascular system. In cooler regions there is a single generation per year, but several generations are possible in warmer climates. Young nymphs may overwinter in cracks in the bark.
Dispersal: Only the first few instars are mobile, and can be dispersed naturally from tree to tree by wind. They can also be easily moved by people. Careful orchard hygiene is therefore needed to prevent spread of the pest into and through the orchard.
When can damage be expected? Pest populations can continue to build up through the years if left unmanaged.
Hosts: The San Jose scale has a wide host range and can affect many fruit crops. However, in Bhutan it is only known as a pest of apples.
This pest can become a serious problem if allowed to build up and spread through an orchard. Ongoing monitoring and management is needed to prevent this from occurring.
Monitoring through the growing season is necessary to determine whether a winter treatment of TSO is necessary, and also to identify any hot spots within the orchard where targeted management may be necessary. Scales on twigs and branches need to be carefully observed as they are not as conspicuous as those on fruit.
Effect of variety
There are no known resistant varieties for this pest.
- Use only clean planting material as the scale is a poor disperser.
- Badly affected twigs or branches should be pruned or burnt.
- The most effective control is applying Tree Spray Oil (TSO) in winter (late February) when the pest is overwintering in the bark. TSO acts on insects by enveloping them with a thin film of oil, which disrupts their respiration and kills them by suffocation.
- Never spray when there are leaves on the tree, or when green buds are starting to emerge, as that will cause phytotoxicity. Spray every second year (or adjust following monitoring). TSO (30 ml per 1 litre of water or 1 litre per 35 litres) should be sprayed thoroughly to run-off. A good coverage to all twigs, branches and trunk is essential, making sure that the spray penetrates the cracks. Application of TSO may also benefit the control of mites and woolly aphids.
Pesticides are not necessary and should not be used. The protective shell of the scale can make control difficult. Also, pesticides will kill beneficial insects that are helping to control pest populations, which can make pest problems worse.
Image acknowledgements: NPPC.