- Susceptible species
The Siberian silk moth, also known as the Siberian coniferous silk moth, is widespread on the Asian continent. It is a destructive pest which can cause significant defoliation of coniferous trees such as pines, spruces, larches and firs in natural and planted forests. This is caused by the larvae (caterpillars) feeding on the needles. It can kill healthy trees, and has been known to kill whole forests across wide areas.
It is also a human health hazard because the hairs of its larvae contain a substance which on contact which can cause skin irritations and systemic health problems such as joint pains.
The moth is found in Russia, China, Kazakhstan, Mongolia and the Korean Peninsula, and is spreading westwards at a rate of between 12 and 50 kilometres per year. It will attack European conifer species not found in its native range, and would therefore pose a threat to our coniferous forests if it entered the UK.
Tree health - Defoliation (pictured below) is the main symptom of the larvae's presence. Trees can be defoliated two or three years in succession during outbreaks, and many trees are unable to withstand such long periods of defoliation.
Outbreaks are often followed by outbreaks of wood-boring beetles, which can kill trees which have been heavily stressed by Siberian silk moth. The resulting accumulation of dead, dying and stressed trees can then increase the risk of forest fires.
Human health - The caterpillars have stinging hairs that can cause allergic reactions in forestry workers, forest visitors and people living close to affected trees. Exposure to the hairs, or to the sticky trails which the caterpillars leave behind when moving, can cause severe dermatitis as well as systemic reactions affecting the joints and other parts of the body.
Adult moths’ colouring varies from yellow-brown to light grey, dark brown and almost black. The forewings are marked by two characteristic dark stripes and a white spot in the centre. Hind wings are the same colour as the forewings, but lack markings.
Females are approximately 40mm long with a wing span of 60-80mm. Males are approximately 30mm long with a 40-60mm wing span.
These pictures show the four main stages of the species’ lifecycle:
Siberian silk moth’s preferred host trees in its native range are Siberian pine, Siberian larch, Siberian fir, Siberian spruce, Manchurian fir, Korean pine, Dahurian larch and Yeddo spruce. However, experiments in the USA with European conifer species showed that it can develop in other species that are not found in its natural range. European conifer species grown in UK forests include Scots pine, Norway spruce and European larch.
The moth is presumed to have originated in Siberia, but has apparently been spreading westwards. The most westerly points at which outbreaks are known to have occurred are in the Perm and Udmurtia regions, north of the Caspian Sea, but males were captured in 2001 in pheromone traps more than 1000km (625 miles) further west, near Moscow.
EPPO region: Present in the eastern part of European Russia.
Asia: Present in almost all of Asian Russia except the extreme north and the Sakhalin and Kurile Islands. It is also present in Kazakhstan, China (Heilongjiang, Jilin, Liaoning and Neimenggu), the Korean Peninsula and northern Mongolia.
European Union: Officially absent from the EU, including the UK, and never recorded in the wider environment, for example, by amateur moth recorders.
The moth has been spreading westwards at an estimated rate of 12-50km (7.5–31 miles) per year, and all stages of the life cycle can be transported on plant material moving in trade, particularly plants for planting, cut branches and Christmas trees. During outbreaks especially, eggs, larvae and cocoons can be moved on bark, or wood with the bark still attached, and can be present as contaminating pests on other products. Cocoons are often moved on branches collected for firewood.
Significant control efforts are undertaken during outbreak years in Russia and other affected countries. These involve mostly aerial treatment with chemical and bacterial products. The species’ natural enemies play an important role in regulating its population density under non-outbreak conditions. The natural enemies include:
- the egg parasitoids Telenomus gracilis, Telenomus tetratomus, Trichogramma dendrolimi and Ooencyrtus pinicolus;
- the larval and pupal parasitoid Rhogas dendrolimi;
- the micro-organisms Bacillus dendrolimus, Bacillus thuringiensis (Bt), Beauveria bassiana, polyhedrosis viruses, and some other viruses. Bt in particular has been used as part of the control strategy in Russia.
Siberian silk moth was added to the European & Mediterranean Plant Protection Organisation’s (EPPO) A2 action list in 2002, and it has been recommended that EPPO member countries regulate it as a quarantine pest.
The moth is apparently spreading westwards through Europe, so carrying out surveys using pheromone traps would help to monitor its spread, and applying appropriate control measures along the border of the pest’s present range would help to prevent its introduction into new areas. (EPPO, 2005)
To prevent the introduction of the pest, it is recommended that:
- commodities, plants for planting and cut branches of host plants from infested areas should be free of soil;
- commodities should originate in pest-free areas, be produced in protected houses, fumigated, or be imported during winter;
- wood should be debarked, heat-treated, originate in a pest-free area, or be imported during winter; and
- isolated bark should be treated to destroy any insects. (EPPO, 2005).
Action to tackle a discovery of Siberian silk moth in the UK will be guided by our contingency plan.
Although Siberian silk moth is not known to be present in the UK, there is a risk of its being accidentally introduced. We therefore urge everyone, and especially importers of wood and plant materials, and tree and plant professionals, to remain vigilant for signs of it and to report it to us using our Tree Alert form.