Oak Processionary Moth Threat to Human Health

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

References and Further Reading

Copyright Note

Karen does not seek or receive any commercial interest or advantage from this blog. She is not promoting any business venture. She simply loves to share fascinating facts about plants. These pages illustrate her love of plants, botany, biodiversity, gardens and creative expression. There is always so much to learn about plant diversity. This blog is designed as a showcase for photography, commentary on plants and wildlife, gardens and other places visited, horticulture and related topics. Viewpoints are her own, not those of her employer.

© Karen Andrews 2018 onwards. All rights reserved. Unauthorized use and/or duplication of this material without express and written permission from this blog’s author and/or owner is strictly prohibited. Excerpts and links may be used, provided that full and clear credit is given to Karen Andrews and BotanyKaren.net with appropriate and specific direction to the original content. 

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