The effectiveness of ground-based applications of Bacillus thuringiensis var. kurstaki for controlling oak processionary moth Thaumetopoea processionea (Lepidoptera: Thaumetopoeidae)

The effectiveness of ground-based applications of Bacillus thuringiensis var. kurstaki (Btk) for controlling oak processionary moth (OPM) was evaluated by comparing the numbers of larval nests on treated and untreated oak trees in Richmond Park, in west London, UK. A total of 760 oak trees (Quercus robur) were monitored over a period of 4 years from 2014 to 2017, and the effects of current treatment and previous treatment history on nest numbers were analysed using general linear mixed models. Two applications of Btk applied 7-10 days apart in the spring, just after egg hatch, reduced the numbers of larval nests by 86-94%. In 2015, the effectiveness of treatment decreased as tree height increased, whereas in 2016 and 2017 nest numbers were reduced by a similar percentage across all tree height classes. The effects of treatment were relatively short-lived, because at the scale at which the insecticide applications were carried out, trees were re-colonised rapidly by adult OPM flying in from the surrounding area. Consequently, nest numbers generally returned to pre-treatment densities within 1-2 years.