Diaspidiotus perniciosus

Crop: Apple

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 exports.  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, localized pest, in all major apple growing regions. Outbreaks are often induced by indiscriminate use of pesticides. It was much more common before the widespread use of tree spray oil (TSO).

IDENTIFICATION

They are tiny, circular, grayish scales that are only just visible to the naked eye. They are protected by a thick, waxy, protective scale (a grey, conical “shell”), 1-2 mm in diameter. When crushed, they may exude a yellowish liquid.

SYMPTOMS

The red halo-like spots surrounding feeding sites on the fruit surface make them easily recognizable in the field, especially on green or yellowish parts of the fruit. In severe cases, fruits become severely misshapen. Scales can cover twigs and branches but need careful observation to be seen as they are not as conspicuous as fruit. However, they can result in dieback of the affected parts and ultimately cause the decline of the tree.

Confusion with other pests: Pest symptoms cannot be confused with those caused by other pests.

BIOLOGY

Lifecycle: San Jose scales reproduce rapidly, with multiple generations per year in warm climates. The female scale can produce hundreds of offspring, called crawlers, which are the mobile stage of the insect. After hatching, the crawlers move to a suitable feeding site on the plant and settle down, forming a protective scale covering. They then molt and continue to develop through several stages before reaching adulthood. 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 the 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 an apple pest.

MANAGEMENT PRACTICES

This pest can become a serious problem if allowed to build up and spread through an orchard. Ongoing monitoring and management are needed to prevent this from occurring.

Monitoring

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 of this pest.

Non-chemical management

• Use only clean planting material as the scale insect is a poor disperser.
• Badly infested twigs or branches should be pruned or burnt.
• The most effective control is applying Tree Spray Oil (TSO) in winter 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 liter of water or 1 liter per 35 liters) should be sprayed thoroughly to run off. Good coverage to all twigs, branches, and trunks is essential, making sure that the spray penetrates the cracks. Application of TSO may also benefit the control of mites and woolly aphids.

Chemical management

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.

Version: NPPC 2017. San Jose scale V1.0. Bhutan Pest Factsheet. www.PestsofBhutan.nppc.gov.bt. Date produced: 14 April 2017. Last date modified: 18 June 2024. Contact: nppcsemtokha@gmail.com

Image acknowledgements: NPPC.