It was inevitable that OPM would eventually be introduced into the United Kingdom on infested oak tree planting material and in retrospect equally inevitable that it would establish and spread as it subsequently has. At that time (2005/6) the UK was a member of the European Union and as such, bound by the doctrine of free trade in goods and services among member states: someone had apparently forgotten to mention that living plant material can carry pests and diseases. By 2010 oak processionary moth had a firm foothold in London, was spreading rapidly and clearly not to be shifted. Pesticide spraying had been ongoing for almost five years and having some impact, but not enough to significantly slow the spread of OPM, let alone rid the UK of this alien invasive insect pest.
T. Mabbett (Wed,) studied this question.