Adapting sampling effort to assess the population establishment of Torymus sinensis,the biocontrol agent of the chestnut gallwasp

Torymus sinensis Kamijo is to date the most effective controlling agent of the chestnut gallwasp Dryocosmus kuriphilus. The procedure to evaluate the parasitism rate of T. sinensis consists of dissecting D. kuriphilus galls collected in the field and checking them for its presence. Such a procedure is, however, time-consuming. Using a space-for-time substitution approach, by creating a temporal gradient within 15 sites and by means of bootstrap simulations, we analysed the effect of different sampling efforts on the precision of evaluating the success of T. sinensis release and the evolution of its parasitism rate towards biocontrol. Results show that after the first release, 200 galls are required to assess its presence with a 1% failure rate. Once T. sinensis is established, 100 galls are generally sufficient to estimate its parasitism rate by accepting a (median) percentage of error of 14% in the second year and 6% and 3% in the third and fourth year, respectively. Finally, we propose a series of design options to reduce the sampling effort according to the targeted accuracy level and as a function of the obtained parasitism rate.