Effects of dispersion pattern of propargite-resistant Tetranychus urticae Koch on the efficacy of sampling plans used for resistance monitoring

Different sample sizes were used for resistance detection and documentation for random and systematic sampling plans while Tetranychus urticae adult females were either randomly or patchily distributed in a semi-field experiment. Arenas were created for random and patchy dispersion, and 10% and 20% resistance frequencies. A sample size of 50 detected resistance, i.e., detected at least one (> 1) propargite-resistant adult female mite present at a resistance frequency of approximately 10% and above. Small sample sizes, for example 50, were unpredictable either underestimated or overestimated the resistance frequency and, therefore, should not be used to document resistance frequency as high as 10%. This study confirmed that larger sample sizes are required to correctly document resistance frequency, when resistant individuals were patchily distributed and a systematic sampling plan was used.