Oak Processionary Moth can cause serious defoliation of oak trees, their principal host. On the Continent they have also been associated with hornbeam, hazel, beech, sweet chestnut and birch, but usually only where there is heavy infestation of nearby oak trees.

As with many moth species, population levels tend to vary from year to year, but they also show longer term cycles in which the population builds up to high densities over a period of 3-4 years and then declines again to lower density. Although a native of central and southern Europe, the moth spread northwards during the latter half of the twentieth century. It was first recorded in The Netherlands in 1991, and subsequent reports of damaged trees indicate that numbers soared, with 1995 - 1996 and 2004 - 2005 being particularly bad years for tree damage. Populations declined between those years, but are again reaching very damaging levels in The Netherlands and neighbouring areas of Belgium and in parts of Germany.

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