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Rapid Pest Risk Analysis (PRA) for: Malacosoma parallela


This rapid PRA shows: Malacosoma parallela is a moth with nest-building larvae, which feeds on a wide range of deciduous trees and shrubs, and can cause significant defoliation. The species is native to western and central Asia and a very small part of south-east Europe, is usually found in mountainous regions, and is most common at heights over about 1600 m. 

Risk of entry

Entry is considered to be unlikely on plants for planting, as all life stages (including the egg masses) are reasonably conspicuous, and very low volumes are imported from this moth’s current range. This judgement is made with medium confidence. Entry on the other three pathways is considered very unlikely: on bark (including wood with bark), the confidence is medium, while for both cut branches and natural spread, the confidence is high. 

Risk of establishment 

Malacosoma parallela is considered moderately likely to establish outdoors in the UK, with medium confidence. It is a montane pest, most common at high altitudes in its native range, and these regions will have much hotter days (and cooler nights) compared to lower elevations, though average temperatures may be broadly comparable. Thus, at higher altitudes, insects have the potential to develop rapidly in the daytime heat: the UK is likely to have much cooler daytime temperatures allowing less development. This is considered to reduce the likelihood of establishment, as larvae may not receive sufficient warmth. Suitable hosts are found throughout the UK. Establishment in protected cultivation is considered very unlikely, with high confidence. Suitable hosts are not commonly grown under protection, and there is a possibility that winter temperatures would be too high. However, given the polyphagous nature of the species, it is possible that some woody plants gown under protection may be suitable hosts, though due to the highly conspicuous larval nest, any infestation would probably be rapidly noticed and destroyed. 

Economic, environmental and social impact

In its native range it can be a serious pest of trees, though damage appears to vary with altitude. Like many forest pests, damage appears to be cyclical and often occurs in tandem with other species, though there are very few reports of quantifiable damage attributable to this species alone.

The overall assessment is that, in susceptible areas and 13 in outbreak years, it causes very large impacts, this judgement made with medium confidence. In the UK, potential economic, environmental and social impacts are all considered to be small, these judgements all made with medium confidence. The uncertainty arises over the suitability of UK climate for damaging population levels to build up. Social impacts are due to larval nests and hairy larvae potentially causing some concern among members of the public, though the larval hairs of this species are not especially irritating.


  • Malacosoma parallela


  • Cydonia oblonga
  • Malus
  • Prunus
  • Prunus dulcis
  • Quercus
  • Quercus infectoria
  • Quercus macranthera


Type File Size
Pest Risk Analysis Download 429,76kB

PRA Area

  • United Kingdom