The relationship between pheromone trap catch and local population density of the oak processionary moth Thaumetopoea processionea (Lepidoptera: Thaumetopoeidae)

1. 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. 2. 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. 3. 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) 4. 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. 5. 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