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Field identification guide for Carcelia iliaca


Carcelia iliaca (C. iliaca) is a parasitoid that is associated with Oak Processionary Moth (Thaumetopoea processionea; OPM).

C. iliaca parasitises older, almost full-grown OPM larvae inside or outside OPM nests and then completes its development by overwintering in a spent OPM nest before emerging the following spring.


C. iliaca:

  • resembles a dark grey housefly or bluebottle-shaped fly
  • has a body length of approximately eight-10 millimetres
  • can be observed running over OPM nests and caterpillars
  • can be found in the general vicinity of active OPM nests.

Where C. iliaca is observed on OPM nests, caterpillars may appear agitated.

Images of C. iliaca

Carcelia iliaca predating a live OPM nest



Carcelia iliaca predating a large live OPM nest at the base of an oak tree

Carcelia ilicia predating a live OPM nest

Bottom picture: credit The Royal Parks

To request a pdf (non-accessible) version of this field guide, please contact the OPM team.


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