Identification
Species, host tree and behaviour together are far more reliable than appearance alone. Work through the signs below before deciding what you have found.
Six signs
Photographs on this page are illustrative and are provided to support written descriptions, not as verified reference specimens.
Grey-brown bodies covered in very long white hairs, with a darker head and a distinct dark line along the back in later stages. They are almost always found in groups rather than alone, and they are found on oak.
The behaviour the moth is named for: caterpillars move nose-to-tail in single file, sometimes in wide arrowhead formations, up and down the trunk between feeding areas and the nest.
Silken white webbing tents on the trunk or branches, tightly matted against the bark rather than loose and airy. Older nests discolour to a dirty brown and can persist long after the caterpillars have gone.
Feeding produces ragged, skeletonised leaves and, in heavier years, noticeable defoliation of parts of the canopy. Healthy mature oaks usually recover, but repeated defoliation adds stress.
Eggs are laid in neat plaques on twigs high in the canopy and are covered with scales that camouflage them against the bark. They overwinter there and hatch as the oak comes into leaf.
Several harmless native caterpillars are regularly mistaken for OPM, including tussock moths, brown-tail moth larvae and the webbing of spindle ermine. Species, tree and behaviour together tell you more than appearance alone.
Where to look
Nests are most often found on the trunk and on the underside of larger branches, frequently at a fork or in a sheltered angle of the bark. Feeding damage generally appears first higher in the canopy, which is why it can be missed from directly beneath the tree.
Step back. Looking at an oak from ten or twenty metres away, ideally with binoculars, will tell you far more than standing at the base of the trunk — and it keeps you away from any hairs being shed.
Old nests persist. A discoloured, tattered web with no visible caterpillars still contains shed hairs and should be treated as a contact hazard.
Timing
Emergence tracks oak bud burst rather than a fixed date, so what you can see depends on the season locally.
Late summer to early spring
Eggs sit in camouflaged plaques on twigs high in the canopy through autumn and winter. They are extremely difficult to see from the ground.
What this means for inspection
Winter is when climbing inspection or close canopy assessment can locate egg plaques, and when old nests are easiest to spot on bare branches.
Next steps
Send photographs for an initial opinion, and report suspected sightings through the official route.
Not sure what you've found?
Send clear photographs of the tree, the caterpillars or the suspected nest and our team will give you an initial visual opinion to help you understand your next steps.
Photo identification is preliminary and may not always be conclusive. A professional site inspection or official confirmation may be required.
Think it may be OPM?
Suspected Oak Processionary Moth sightings should be reported through TreeAlert, the official tree pest and disease reporting service. This is an external official service and opens in a new tab.
Sending us photographs does not replace official TreeAlert reporting.
