The expansion of oak processionary moth (OPM) in South‐East England continues despite ongoing efforts to control the pest since its introduction in 2006. 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. 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. 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.

1. Invasive pests pose a great threat to forest, woodland, and urban tree ecosystems. The oak processionary moth (OPM) is a destructive pest of oak trees, first reported in the UK in 2006. Despite great efforts to contain the outbreak within the original infested area of South-East England, OPM continues to spread.2. Here, we analyze data consisting of the numbers of OPM nests removed each year from two parks in London between 2013 and 2020. Using a state-of-the-art Bayesian inference scheme, we estimate the parameters for a stochastic compartmental SIR (susceptible, infested, and removed) model with a time-varying infestation rate to describe the spread of OPM.3. We find that the infestation rate and subsequent basic reproduction number have remained constant since 2013 (with R 0 between one and two). This shows further controls must be taken to reduce R 0 below one and stop the advance of OPM into other areas of England. Synthesis.Our findings demonstrate the applicability of the SIR model to describing OPM spread and show that further controls are needed to reduce the infestation rate. The proposed statistical methodology is a powerful tool to explore the nature of a time-varying infestation rate, applicable to other partially observed time series epidemic data.

Invasive pests pose a great threat to forest, woodland and urban tree ecosystems. The oak processionary moth (OPM) is a destructive pest of oak trees, first reported in the UK in 2006. Despite great efforts to contain the outbreak within the original infested area of South-East England, OPM continues to spread. Here we analyse data of the numbers of OPM nests removed each year from two parks in London between 2013 and 2020. Using a state-of-the-art Bayesian inference scheme we estimate the parameters for a stochastic compartmental SIR (susceptible, infested, removed) model with a time varying infestation rate to describe the spread of OPM. We find that the infestation rate and subsequent basic reproduction number have remained constant since 2013 (with R_0 between one and two). This shows further controls must be taken to reduce R_0 below one and stop the advance of OPM into other areas of England. Our findings demonstrate the applicability of the SIR model to describing OPM spread and show that further controls are needed to reduce the infestation rate. The proposed statistical methodology is a powerful tool to explore the nature of a time varying infestation rate, applicable to other partially observed time series epidemic data.spread and show that further controls are needed to reduce the infestation rate. The proposed statistical methodology is a powerful tool to explore the nature of a time varying infestation rate, applicable to other partially observed time series epidemic data.

Oak processionary moth (OPM) was introduced into the U.K. in 2004–2005 and pheromone traps have been used to monitor its spread and provide an early warning of the colonization of new areas. The traps capture adult male OPM, although catches are highly variable and it has proved difficult to establish a relationship between the numbers of males caught and local population densities. Surveys carried out around 260 pheromone traps as part of the U.K. OPM control programme, however, reveal a consistent linear relationship between square‐root transformed adult males per trap and cube‐root (or log10) transformed numbers of larval nests (r2 = 0.47; P < 0.001) In 91% of cases where traps captured no adult males, there were no OPM nests within 250 m and, where nests were present, the numbers of nests were low. Traps that captured more than 10–20 males were associated with counts of ≥ 50 larval nests. Defoliation and risks to human health from OPM are more closely related to nest density (i.e. the numbers of nests per tree). Consequently, identifying trap catches that might initiate a management response also requires information on the number of oak trees in the surrounding area.

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