and Integrated Pest Management - part - usaid

114 PSTICIDE MANAGENTIr AND IPM IN SOUThEAST ASIA
as a group without differentiating between species. The number of deadhcarts per
sample unit (= I hill) is commonly recorded (Anon 1985).
As the basis for a binomial seCuential sampling plan, a model was derived
using data from field trials and surveys; data which would allow a prediction of
the average nunmer of (eadcearts per sample unit from the proportion of infested
sample units. Similar models were described by Sterling (1975) and Ingram and
Green (1972) for cotton pests. The stemborer model is shown in Figure 1.
As shown by the coefficient of determination and confirmed by an analysis
of the residuals, the model provides a sufficiently reliable prediction. It is valid
for up to 3.5 deadhearts per hill and thus includes the currently recommended
economic threshold of 1.5 deadlhearLs per hill in transplanted rice (Anon 1985).
According to the model, the economic threshold corresponded to a
proporlion of 0.68 infested hills. For calculating the sequential decision lines,
the lower limit, i.e., highest infestation level that can be tolerated without
control measures, was sci at 0.50 or 50% infested hills. This seemed to be a
practical compromise betveen accuracy (i.e. probability of wrong decisions) and
feasibility (i.e. average number of samples to be taken). Based on the same
considerations, the acceptable risk levels were set at a = 0.20 for spraying
when actual pest density is still below the economic threshold, and 13= 0.10 for
not-spraying when the economic threshold has already been surpassed. The
resulting decision lines are shown graphically and in sampling table form in
Fiure 2.
-J
D'.
ID a o °o
00
00.
0 0 a0 OD 0
CO 0 0 00 W D
0 0 D
roo 0
I 2 3
MEAN NUMBER DEAUHEARTS / HILL ( )
Figure 1. The relationship between the proportion of hills infested by stemborer
and the mean number of deadhearts per hill in transplanted rice.