Estimating expansion of the range of oak processionary moth (Thaumetopoea processionea) in the UK from 2006 to 2019

1. The expansion of oak processionary moth (OPM) in South-East England continues despite ongoing efforts to control the pest since its introduction in 2006. 2. Using locations of OPM larval nests, supplied by the Forestry Commission and recorded as part of ongoing surveillance and control measures from 2006 onwards, we show that the expansion of the range of OPM in South-East England up to 2019 was biphasic with a higher rate of expansion from 2015 onwards. 3. The maximum rate of OPM range expansion in the United Kingdom from 2006 to 2014 was estimated as 1.66 km/year (95% CI = [1.22, 2.09]), whereas the 2015–2019 expansion rate was estimated as 6.17 km/year (95% CI = [5.49, 6.84]). This corresponds to an estimated species range distribution area of 7077 km2 in 2019. 4. To explain the faster expansion of OPM range from 2015 onwards, we discuss potential reasons that include: natural capability of species of both short- and long-distance dispersal; external factors such as environmental heterogeneity; a reduction of active control