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Using ‘sentinel’ plants to improve early detection of invasive plant pathogens
Fig 5
Optimising the number of sentinels to include in the population.
A. The optimal number of sentinel plants to include in the population, 
, for which the maximal reduction in the EDP compared to the baseline level is achieved (if NS is also chosen optimally). Region 1: When the sampling effort was small, 
was low. 
increased for larger sample sizes (moving to the right on the figure) and smaller sample intervals (moving downwards on the figure). Region 2: 
dropped again when the sampling effort was very high. In that region, the baseline EDP was very low (Fig 2C), and the scope for reducing it insufficient to offset the increase in transmission rate caused by adding large numbers of sentinel plants into the population. Region 3: When the sample interval (Δ) was large, the optimal number of sentinel plants to include in the population was equal to the sample size (N) (contour lines are vertical). In that region, the sample interval was long enough to allow for repeated sampling of the same plants, eliminating the need for PS to exceed the sample size. Region 4: When the sample interval (Δ) was small, the optimal total number of sentinels in the population was substantially larger than the sample size. In that region, a large sentinel population was necessary to avoid frequent repeated sampling of the same plants. B. The percentage change in the EDP compared to the baseline value at the optimum, achieved when 
and 
. C. The resultant value of the EDP at the optimum, expressed as a percentage of the total crop population.
doi: https://doi.org/10.1371/journal.pcbi.1010884.g005