Oak Processionary Moth Threat to Human Health

Oak Processionary Moth caterpillars defoliate Northern European, deciduous Oak species, Quercus spp. © Karen Netto (Andrews)

Oak Processionary Moth (OPM), Thaumetopoea processionea, is regarded not just as a threat to defoliated Oak or Quercus species, but also as a human health risk. The caterpillars are the main issue rather than the moths. They have irritating or urticating hairs that contain a toxin.

Oak Processionary Moth caterpillars with their irritating hairs The species gets its name from the way that the caterpillars migrate in procession to feed. CC Luc Hoogenstein

Human and Animal Health Risk

The allergenic protein, thaumetopoein, causes severe problems for both humans and animals. These problems can be exacerbated in dry weather, even after the irritating hairs have been shed. The main human health concerns are:

  • severe and persistent skin irritations
  • eye problems
  • sore throats
  • respiratory problems.

The adult moths produce one generation per year. They lay their eggs high in the Oak canopies to overwinter. It has been discovered that the eggs can survive harsh Northern European winters as low as 20ºC. Once the larvae emerge in time with Oak bud burst in mid or late April, they set about feeding gregariously at night. They form a communal silken nest in the tree by day. Their feeding can result in partial or complete defoliation. Their voracious appetites inhibit the Oaks’ ability to photosynthesise at the crucial growth stage early in the spring and can leave the trees susceptible to disease.

Oak Processionary Moth eggs are concealed high in the canopy where they can withstand harsh winters © Karen Netto (Andrews)

OPM UK Containment Zone

Oak Processionary Moths are well-established in Northern Europe. Considerable efforts and legislation have been put in place to contain outbreaks around London. Reported 2020 outbreaks appear down on 2019 figures (although it is possible that Coronavirus has eroded containment efforts). The adult moths emerge and fly between the end of July and mid-September. 2020’s need for vigilance is not over.

The original cause of London outbreak was traced back to imported Oak trees destined for planting. While the male Oak Processionary Moths are known to fly distances of 50-100 kms, the female of the species is regarded as a poor flier and only likely to fly 5-20 kms per year. This means that the female is unlikely to fly across the Channel to lay her eggs in British Oak trees. However, there is an established colony in Jersey.

UK’s OPM Risks

The UK risk assessment established 4 potential pathways for OPM:

  • import trade in Oak trees for planting
  • import trade in Oak cut branches
  • import trade in Oak roundwood
  • natural spread.

Britain is currently relying on imported trees to meet its climate emergency planting targets. UK nurseries are not currently growing enough trees to meet them. The alternative is to encourage more natural regeneration by allowing scrub to protect tree seedlings.

International Year of Plant Health 2020

2020 is the International Year of Plant Health. Planned events and publicity seem to have been overshadowed by human health issues due to COVID-19. OPM is both a human and plant health issue. Vigilance must continue. If you see the OPM caterpillars, do not touch them. Report them immediately to the experts on Forest Research TreeAlert link below.

© Karen Netto (Andrews)

References and Further Reading

Department for Environment, Food and Rural Affairs (DEFRA) (2020): Thaumetopoea processionea Pest Information. UK Plant Health Information Portal. Last accessed 15 May 2020.

European Food Safety Authority (EFSA) (2009): Evaluation of a pest risk analysis on Thaumetopoea processionea L., the oak processionary moth, prepared by the UK and extension of its scope to the EU territory. Scientific Opinion of the Panel on Plant Health (Question No EFSA-Q-2008-711). 26 August 2009. Last accessed 15 May 2009.

Forest Research (2020): Oak Processionary Moth (Thaumetopoea processionea). Last accessed 13 July 2020.

Forest Research (2020): TreeAlert (online reporting of dangerous tree pests and diseases). Last accessed 15 May 2020.

Legislation.gov.uk (2019): The Plant Health (Amendment) (England) Order 2019, Plant Health, England 15 July 2019 No. 1128. (re: Thaumetopoea processionea L. (the Oak Processionary Moth). Last accessed 15 May 2020.

Rahlenbeck, S., & Utikal, J. (2015). The oak processionary moth: a new health hazard?The British journal of general practice : the journal of the Royal College of General Practitioners65(637), 435–436. https://doi.org/10.3399/bjgp15X686341. Last accessed 15 May 2020

RHS (2020): Oak Processionary Moth. Last accessed 15 May 2020.

UK Moths (2020): Oak Processionary. Thaumetopoea processionea. Author: Ian Kimber. Last accessed 15 May 2020

Woodland Trust (?): Oak Processionary Moth (Thaumetopoea processionea). Last accessed 15 May 2020.

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