Biology Reference
In-Depth Information
Table 18.5 Susceptibility of
T
.
ni
larvae of various ages within an instar to treatment with
ALA + Dpy
Percentage larval death over and beyond
the controls after 3 days
in the greenhouse
Stage of the
instar
Susceptibility
ranking
Before
normalization (%)
After
normalization (%)
Experiment Entry
A
1
Mid third
1
47
52
2
Early third
2
60
66
3
Late third
3
91
100
B
1
Mid third
1
47
92
2
Early third
2
39
76
3
Late third
3
51
100
C
1
Mid third
1
21
57
2
Early third
2
18
49
3
Late third
3
27
100
D
1
Mid third
1
26
54
2
Early third
2
35
73
3
Late third
3
48
100
Correlation between the stage of development within an instar
and larval death
0.739
Level of significance
1 %
Adapted from Rebeiz et al. (
1988a
)
and mortality was highly significant, with the late stage of the instar being more
susceptible to the ALA + Dpy treatment than the early and mid stages (Rebeiz
et al.
1988a
). This period of maximum susceptibility corresponded to the period
when the larvae were quiescent and the new cuticle for the next instar was being
actively synthesized beneath the old cuticle.
18.2.6 Effectiveness of the ALA + Dpy Treatment
in the Absence of a Post-spray Dark Incubation Period
To determine if a post-spray dark incubation period, was required for expression of
insecticidal activity, the mortality of second instar
T
.
ni
treated in the dark or light
was compared. In the dark treatment, treated larvae (ALA + Dpy) were subjected
to a 17-h dark incubation prior to exposure to light (dark sprays). Light treatment
consisted of spraying larvae at the beginning of the light phase of a 14-h light/10-h
dark photoperiod prior to exposure to light (light sprays). As shown in Table
18.6
,
the light sprays were as effective as the dark sprays in causing larval death. This
indicated that, although in the light Proto is destroyed as rapidly as it is formed, the
steady state formation of Proto in the light was enough to cause extensive photody-
namic damage.
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