and Integrated Pest Management - part - usaid

120 PESTICIDE MANAGEMENT AND IPM INSOUTHEAST ASIA
x 10
0 , 0.5.4
b : 0.938
:I to I.5
"t 10to:l .64 LEAFFOLDER
W W
0> 120 I Count damaged eavs
,)
-/
TREAT
1
2
3
4
5 -
13 .
21
30
15
27
39
50
m
D
"
< I--
80
40, 40
7
,, N
b,\, '11
6 39- 61
6 48 72
7 57 83
8 66 . . 94
9 75-I05
10 87.-116
93 - 127
,.J
:D
U
NO TREATMENT
2 4 6 8 10 12
NUMBER OF SAMPLE UNITS (n)
Figure 6. Sequential sampling plan for rice leaffolder in direct seeded rice: Xe =
economic threshold; a, b = intercept, slope of patchiness regression; t= value of
normal deviate for 0.90 confidence probability; nmax = 11, for allowable
confidence interval at A= x, of 10 ± 1.5.
Data collected over several seasons on leaffolder damage fitted either
Poisson, negative binomial or normal distributions, although none in a
consistent manner. Therefore, the sequential sampling approach described by
hwao (1975) was utilized. The method is based on Iwao's patchiness regression
(1968), which relates the mean-crowding index (Lloyd 1967) to mean density.
Coggin and Dively (1982) applied this method for sequential sampling plans in
wheat and barley and Bcchinski et al. (1983) in soybean. The decision lines are
calculated as the upper and lower limits of the confidence interval of a critical
density (i.e. cconomic threshold) with a predetermined confidence probability.
Figure 6 shows the obtained sequential sampling plan for leaffolder.
By setting a tolerable width of the confidence interval for the case that the
true infestation level is equal to the economic threshold, itis possible to
determine a maximum number of sample units 1o be taken (Iwao 1975). The
chosen confidence interval of 10 ± 1.5 damaged leaves resulted in a maximum of
II sample units to be examined in case the infestation level is close to the
economic threshold (see Figure 6). The sample size is,of course, lower for light
or rnorc severe infestations so that on average, considerable savings in sampling
time can be anticipated. Respective field tests are being carried out at present to