and Integrated Pest Management - part - usaid

SAMPLING METIODS FOR PEST SURVEILLANCE INRICE 119
Table 2. Regression models to predict the mean brown planthopper (BPH)
density from the proportion of sample units with at least n hoppers.
Dependent variablea Regression model Predicted proportion
corresponding to ETb
Proportion of sample y = 0.603 '4 - 0.079 R + 0.012 (>1)
units with at least r = 0.96 N = 44
1 BP1
Proportion of sample y = 0.649 i-- 0.077 R - 0.265 (>1)
uniLs with at least r = 0.98 N = 44
2 BPH
Proportion of sample y = 0.548 )/I-- 0.055 R- 0.314 0.87
units with at least r = 0.98 N = 44
3 BPH
Proportion of sample y = 0.427 'Ii- 0.033 Y - 0.290 0.73
uniLs with at least r = 0.97 N = 44
4BPH
alndependent variable always: __
mean number brown planthopper/sample unit; bET
= economic threshold = 10 BPH/sample unit.
The second option consists of an alternative definition of "proportion
infested units". Instead of relating the mean number of hoppers per sample unit
to tile proportion of units with at least one insect, it might as well be related to
the proportion of units with at least 2, 3, or 4 insects. In this way, the
enumerative sampling cannot be substituted completely but the expenditure for
cotnting may a: least be ininimized.
Table 2 shows the regression models for the relationship between the mean
number of hoppers per unit and the proportion of units with at least 1,2, 3, or 4
hoppers, respectively. The last column of Table 2 gives the criterion for
evaluating [lie practicability of the models with regard to binomial sequential
sampling. Hence, sampling plans based on either model (3)or (4)will be tested.
SAMPLING FOR LEAFFOLDER
All attempts to work out a preseuce-absence sampling method for leaffolder
(Cnaphalocrocis nedinalis) have so far failed. The available information
indicates that probably only fundamental changes in the present surveillance
system could make such a method possible, namely, either by reducing the 10
tiller sample unit to one tiller or sampling larvae instead of damaged leaves.
Both alternatives would make the surveillance system more complicated, rather
than simpler. Therefore, a sequential sampling plan which requires counting of
damaged leaves was determined.