Impact of network thinning on the performance of the AHDB Potatoes aphid monitoring scheme

AHDB and its predecessor organisations have funded an aphid monitoring scheme to provide information on the presence of aphid vectors as part of an integrated approach to virus management in seed potato crops. The scheme was based on voluntary participation by growers, in nine regional blocks across GB, who maintained a series of water trap sites (~100) from which weekly samples were extracted and identified. AHDB commissioned an analysis to assess the impact of changing the number of traps (from 100 per annum) on the performance of the monitoring scheme.

The effect of thinning the trapping network was assessed using four measures of performance: the detection of Myzus persicae each week that it is present; the probability that a thinning causes a delay of at least one week in the reporting of M. persicae; the effect on the identification of high risk locations within regions; the effect on the regional average estimate of cumulative Potato Virus Y (PVY) vector pressure. Three thinning regimes were examined: 

• 80% of trap locations retained (low thinning);
• 50% retained (intermediate thinning),
• 20% retained (severe thinning).

Thinning was simulated by randomly removing trap locations from historic data.

 It was estimated that:

• There would be an approximately 8%  {23%; intermediate thinning, 40%; severe thinning} probability of a delay of at least one week being introduced in the reporting of M. persicae within a given region, relative to the full historic network.
• There would be a 91% probability {72%; intermediate thinning, 43% severe thinning} that if a region previously contained a location with a high-risk value of the cumulative PVY vector pressure index it would continue to do so after thinning.
• There would be a 95% chance that the random error introduced to the regional average estimate of cumulative PVY vector pressure index will lie between -6.4-5.9, {-14.1-16.0; intermediate thinning, -19.3-34.4; severe thinning} corresponding to an approximately 3% chance {7%; intermediate thinning, 11% severe thinning} that such introduced error would be sufficient (under worse case conditions) to allow a high-risk region (in a given week) to be misclassified as zero risk.