Porphyric Insecticides - Chlorophyll Biosynthesis and Technological Applications - page 406

Table 18.4 Susceptibility of various instars of T. ni to treatment with ALA + Dpy

Larval death over and beyond
the controls after 3 days
in the greenhouse (%) Experiment Entry Instar

A 1 1st 76

2 2nd 42

3 3rd 43

B 1 1st 64

2 2nd 63

3 3rd 27

C 1 1st 75

2 2nd 28

3 3rd 11

D 1 1st 70

2 2nd 23

3 3rd 11

0.897

Correlation coefficient between the
rank of the instar and larval death

Level of significance 0.1 %

Adapted from Rebeiz et al. ( 1988a )

accumulation and larval death in ALA + Dpy treatments, as a result of the greatly
improved conversion of exogenous ALA to Proto (Table 18.3 ). There also appeared to
be a positive relationship between Dpy concentration and enhanced Proto formation
(Table 18.3 ,D).

18.2.5 Effect of Age on T. ni Herbicidal Susceptibility

The possible effect of insect age on susceptibility toward porphyric insecticides
was investigated by comparing the mortality of first, second, third and fourth T . ni
instars, sprayed with ALA (40 mM) + Dpy (30 mM) at pH 3.5. After 17-h dark
incubation, the treated larvae were exposed to light. As shown in Table 18.4 , the
susceptibility of T . ni to the ALA + Dpy treatment was inversely related to the rank
of the instar, with young, first instars being most susceptible and older fourth instars
being least susceptible (Rebeiz et al. 1988a ).
The relative susceptibility of the stage of development within a particular instar
was also investigated. The mid, early, and late stages of every instar were assigned a
rank of 1, 2, or 3, one denoting the least susceptible and three denoting the most
susceptible stages. To test the relationship between larval death and the early, mid
and late stages of a particular instar, irrespective of stage-dependent susceptibility,
percent death values within an experiment (one for each instar) were normalized to
a value of 100 %. The latter represented the percent death for the late stage within
each instar. As shown in Table 18.5 , the relationship between age within an instar