Developing sampling plans for pesticide resistance monitoring

Abstract

Resistance monitoring (detection and documentation) is considered essential to insecticide and acaricide resistance management. Detection and documentation of resistance to pesticides require effective monitoring procedures with appropriate sampling methodology and sample sizes.

The influence of different dispersion patterns of resistant individuals on the sampling efficiency of simple random and systematic sampling plans was studied using computer simulations. A patchy dispersion pattern of resistant individuals influenced the sampling efficiency of systematic sampling plan, while the efficiency of random sampling plan was independent of any patchiness. The probability of resistance detection (detecting at least one, ≥1, resistant individual) or resistance documentation (detecting ≥ 90% of resistant individuals) decreased dramatically when a systematic sampling plan was used where resistant individuals were clumped.

When resistant individuals were randomly distributed and a systematic sampling plan was used, sample sizes required to detect at least one (≥1) resistant individual with a probability of 0.95 were 300 (present at a frequency of 1 %) and 50 (10% and above). When resistant individuals were patchily distributed, sample sizes required for such detection were 6000 (1 %), 600 (10%) and 300 (20%). Sample sizes of 1200 and 600 (random dispersion, using the random sampling plan) and 3000 and 1500 (patchy dispersion, using the systematic sampling plan) were required to document resistance present at a frequency of 10% and 20%, respectively (within an acceptable error of ± 10% of the existing resistance frequency).

The sample sizes required to document resistance, obtained through computer simulations, were tested in a semi-field/glass house experiment. Due to the smaller scale of the experiment (a small finite population) the required sample sizes were decreased accordingly. Tetranychus urticae Koch was used as a model organism. Simple random and systematic sampling plans were compared for efficiency while T. urticae adult females were either randomly or patchily distributed. The glasshouse experiment confirmed that larger sample sizes were required to correctly document resistance frequency, when resistant individuals were patchily distributed and a systematic sampling plan was used.

To determine resistance frequencies in the semi-field/glasshouse experiment a discriminating concentration (DC) was required. A concentration 0.04% ai for propargite was selected as the discriminating concentration. The potential for using molecular methods to increase the precision of determining resistance frequencies was also investigated. A high salt method of DNA extraction was found suitable for extracting DNA from individual female of T. urticae. However, heterogeneity of PCR banding patterns was found. This heterogeneity could have been due to the unknown genotypes of the females used in the analyses or the presence of non-mite DNA in the samples. Poor results meant that this research was subsequently abandoned.

To support further research and to provide information for experiments conducted in this study, the life-table parameters of susceptible and propargite-resistant strains of T. urticae were studied in the presence/absence of propargite residues. The intrinsic rate of increase (rm), Rₒ and total progeny production of the resistant strain and s ♂ X R ♀ cross hybrid treated with the LC₅₀ of the susceptible strain were higher compared with the susceptible strain and R ♂ X S ♀ cross hybrid. Population projections showed that even small differences in life-table parameters have potential to significantly influence both temporal and spatial changes in resistance frequencies.

The dynamics of spatial changes in resistance frequencies were further investigated by measuring the rate and efficiency of dispersal of the susceptible and propargite-resistant strains of T. urticae. The diffusion coefficient (D) of the susceptible and propargite-resistant strains did not differ significantly (P> 0.344). However, the dispersal efficiency (percentage of adult females crossing a specific distance) of the two strains differed significantly (P< 0.005) as more susceptible mites than propargite-resistant mites crossed the specific zones after 290 and 366 degree days. The age structure (adults, immatures and eggs) data confirmed this trend. Significantly (P< 0.05) higher numbers of susceptible adults, immatures and eggs were found in the outermost zone of an arena compared to the propargite-resistant mites. In a mixed-release experiment, bioassay results of the adult females of the two strains showed a similar pattern of spread.

The results of the experiments conducted in this study indicate that the dispersion pattern of resistant individuals in a T. urticae population is likely to be contagious (patchy). The simulations showed that when the dispersion of resistant individuals is contagious a strictly random sampling plan, along with appropriate sample sizes to detect and document resistance, should be used.