There are two basic concepts that need to be defined:
- Economic Injury Level (EIL) = the lowest density of pest which causes economical loss.
- Control Threshold (CT) = density of pest from which control measures have to be applied to prevent the population from reaching the EIL
- CT1 (as regards the first chemical treatment, the one that would be applied on the larvae of first generation) = it is the pest level by the end of the development of hibernating larvae which, predictably.
- CT2 (as regards the second chemical treatment, the one that would be applied on the larvae of second generation) = it is the pest level by the end of the development of larvae of the generation which, predictably, will cause a pest density equal to EIL at harvest time.
- CT3 (as regards the third chemical treatment, the one that would be applied on the larvae of third generation = it is the pest level by the end of the second generation which, predictably, will cause a pest density equal to EIL at harvest time.
Establishing the CT1 is not possible, since the appropriate methodology is not available to assess, with reliability, the density of hibernating larvae in different areas between the months of April and May. Therefore, the application is restricted to CT2 and CT3.
Calculations are based on the regressions, obtained experimentally in the Delta, between the pest level by the end of the season (which we will substitute by the EIL value) and the pest level by the end of the 1st and 2nd generation. The CT can be given as canes attacked per m2 (CATm2) and as a percentage of 1 m2 plots with some attacked canes (%PAT).
SEQUENTIAL SAMPLINGS FOR LOCALIZATION OF AREAS WITH PEST LEVELS HIGHER THAN THE CONTROL THRESHOLD
The objective is to optimize a methodology which helps detect, at the right time and in a simple way, the areas where the CT is surpassed and therefore it is necessary to apply special control measures.
Sequential sampling has three properties which render it interesting to use in the situations described:
- It makes it possible to set the accuracy of the estimation in advance.
- The pest level is not established in an absolute way. It is given according to a reference level previously established (the CT in our case).
- The number of samples to be examined varies depending whether the pest level to be estimated is very near the reference level (CT). Therefore, in areas with attack levels very much under or over the CT, the final result will be reached with few samples.
In a sequential sampling, the sample units are progressively examined until a point is reached when there is enough information to give the result with the error probabilities established beforehand.
When the data come from a normal sampling (count of number of individuals in every sample unit), the sequential sampling has to take into account the spatial distribution of the organism to be studied.
However, sequential sampling can also be applied in cases when in every sample unit, only the presence/absence of individuals which are the object of the sampling is determined (positive binomial sampling).
The results obtained from research work on C.suppressalis at the Delta, make it possible for either type of sequential sampling to be used nowadays.