The Growing Threat of Oak Processionary Moths in Southern England

Origins and Spread of Oak Processionary Moths

The Oak Processionary Moth, Thaumetopoea processionea, is indigenous to central and southern Europe. It is believed to have been inadvertently introduced to the UK through eggs laid on young oak trees imported from Europe for planting. First detected in London in 2005, the moth has since expanded its presence to several locations in south and west London, as well as West Berkshire, including sites managed by the Woodland Trust. Climatic changes, such as milder winters and fewer late frosts, have potentially facilitated the moth's survival and breeding in regions beyond its native range. Current projections suggest that much of England and Wales could provide suitable conditions for the OPM to thrive.

Forestry Commission England provides detailed information on the distribution and management of OPM in the UK.

The Life Cycle and Habits of OPM Caterpillars

OPM caterpillars exhibit several distinctive behaviors and characteristics:

• Processional Movement: In late spring and early summer, they move in conspicuous nose-to-tail lines, often with a single leader and multiple caterpillars side by side.
• Diet: Their primary food source is oak leaves, but they may resort to other broad-leaved trees like sweet chestnut, hazel, beech, birch, and hornbeam when oak foliage is scarce.
• Feeding Clusters: They tend to group together while feeding and moving, which can be a striking sight.
• Seasonal Appearance: They are typically active from May to July.
• Hairs: The caterpillars possess long, white hairs that are easily distinguishable from their shorter, irritating hairs.
• Physical Description: They have a grey body, a dark head, and as they mature, a dark stripe with paler lines on each side.
• Habitat: They are rarely found on man-made structures and prefer the natural habitat of oak trees.

Impact on Tree Health

OPM caterpillars can severely impact oak trees by stripping them of their leaves. Large infestations can leave trees bare, making them more susceptible to environmental stressors like drought and flooding, as well as other pests and diseases. The loss of foliage can also have a detrimental effect on the tree's growth and overall health.

Human Health Risks

The primary concern regarding OPM is the health risk posed by the caterpillar hairs. These hairs contain thaumetopoein, an irritant that can cause skin rashes, eye and throat irritation, and in some cases, respiratory issues. The hairs can be carried by the wind, increasing the risk of contact. It is crucial for individuals who encounter OPM caterpillars to seek professional advice and avoid handling them.

For guidance on dealing with OPM caterpillars, Cedardale Tree Surgeons and Arboriculturists offer expert advice and services.

Conclusion

The Oak Processionary Moth is a growing concern in parts of the UK, with the potential to cause significant ecological damage and public health issues. Efforts to control its spread are vital to protect both the environment and the well-being of the public. Awareness and early intervention are key to managing this invasive species effectively.