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Oak processionary moth outbreak

Author:  Oliver McEvoy
  04/12/2023
Last Updated:  04/12/2023

Oliver McEvoy

The oak processionary moth caterpillars discovered on one of the trees in Dublin 15.

The oak processionary moth caterpillars discovered on one of the trees in Dublin 15.

Defoliation caused by oak processionary moth.

Defoliation caused by oak processionary moth.

The pest Thaumetopoea processionea, commonly known as the oak processionary moth (OPM), is a Protected Zone1 Quarantine Pest in Ireland.

The caterpillars (larvae) of this moth feed on the foliage of many species of oak, which can cause severe defoliation and tree mortality in certain situations. In addition, this moth also poses a potential risk for human and animal health, as when disturbed, older caterpillar larvae can release hairs that contain an irritating protein that can cause skin rashes, eye irritations and breathing difficulties in people and animals.

On 8th June 2023 a Department of Agriculture, Food and the Marine (DAFM) inspector, acting promptly on information received from a member of the public, found OPM on four adjacent oak trees in a residential area in Dublin 15. The caterpillars were removed immediately as were the four trees to eliminate any risk of the pest spreading.

DAFM set up a demarcated area around the infected site consisting of an infested zone of 500 m radius around the findings and a buffer zone of 5 km, which extended into the Phoenix Park. The demarcated area was divided into four quadrants and each quadrant was divided into a grid of 500 m × 500 m squares. A team of inspectors was assigned to each quadrant and every grid in the quadrant was surveyed. Surveying of the demarcated area began immediately, commencing from the infected site and working outwards in concentric circles. Surveying, up until mid-July, consisted of visual inspections of oak trees in the demarcated area. From mid-July onwards trapping methods were used.

A total of 5,701 visual survey points were recorded in the 5 km demarcated area which also extended to 6 km in the quadrant in the direction of the prevailing wind. In cases where there were large stands of oak trees only the perimeters of the copse of trees were recorded as survey points, thus the total number of trees surveyed in the demarcated area was greater than 6,000.

Eighty-seven traps were erected in the demarcated area (47 in the Phoenix Park and 40 in the remainder of the demarcated area). In addition, 46 traps were placed at other high-risk locations, including forests, around the country. All traps were erected for two months and sampled every two weeks and pheromone lures were replaced after 4 weeks.

To date, there have been no other findings of OPM recorded inside or outside the demarcated area. A further survey consisting of visual inspections for OPM nests will take place over the 2023 autumn/winter period.

Suspected sightings should be treated with extreme caution and notified immediately to DAFM with your contact details and geolocation of the sighting by emailing plantandpests@agriculture.gov.ie or by telephone 01-5058885.


1. A Protected Zone is an area of the EU which is free from a harmful organism even though the conditions in the country are favourable for the establishment of the pest.

Oliver McEvoy is an Agricultural Inspector in the Horticulture and Plant Health Division of the Department of Agriculture, Food and the Marine.


This article was taken from Issue 203 Winter 2023 of the ARB Magazine, which is available to view free to members by simply logging in to the website and viewing your profile area.