Porphyric Insecticides - Chlorophyll Biosynthesis and Technological Applications - page 401

18.2.1 Principle

Porphyric insecticides are made of compounds which force insects to accumulate
undesirable amounts of metabolic intermediates of the heme metabolic pathway,
namely protoporphyrin IX (Proto). In the light, the accumulated Proto photosensi-
tize the formation of singlet oxygen which kills treated insects by oxidation of their
cellular membranes. Photodynamic (porphyric) insecticides usually consist of a 5-
carbon amino acid, -aminolevulinic acid (ALA), the precursor of all tetrapyrroles

in plant and animal cells, and one of several chemicals referred to as modulators.
ALA and the modulators act in concert. The amino acid serves as a building block of
Proto accumulation, while the modulator alters quantitatively and qualitatively the
pattern of Proto accumulation.

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18.2.2 Demonstration of Protoporphyrin IX Accumulation
in T. ni Treated with ALA and 2,2 0 -Dipyridyl (Dpy)

Demonstration of the potential for Proto accumulation in treated insects was initially
achieved by spraying Trichoplusia ni ( T . ni ) larvae with 40 mM ALA + 30 mM
2,2,-dipyridyl (Dpy) (Rebeiz et al. 1988a ). Treated larvae were placed overnight in
darkness at 28 C in order to allow tetrapyrrole accumulation. Extraction of treated,
dark-incubated larvae with ammoniacal acetone, followed by spectrofluorometric
examination of the larval extract, revealed the accumulation of massive amounts of a
fluorescent compound which was not present in control larvae sprayed with solvent
only (Fig. 18.1 ). Following chemical derivatization coupled to spectrofluorometric
analysis, the accumulated compound was identified as a tetrapyrrole, specifically
Proto (Rebeiz et al. 1988a ).

18.2.3 Insecticidal Effects of the ALA + Dpy Treatment

Insecticidal effects of ALA + Dpy-dependent accumulation of Proto was
demonstrated by (a) inducing the accumulation of Proto in third instar T . ni .,
(b) initiating photodynamic death by exposing treated larvae to light and
(c) establishing correlations between photodynamic death and Proto accumulation,
photodynamic death and length of larval exposure to light, and Proto accumulation
and larval growth.
In initial experiments, third instar T . ni larvae were sprayed with ALA + Dpy at
a pH of 3.5 and placed in darkness overnight to allow Proto accumulation to take
place. At the end of dark incubation, some larvae were analyzed for Proto accumu-
lation while others were exposed to 14-h light/10-h dark regime to trigger photody-
namic damage. The ALA + Dpy treatment resulted in massive Proto accumulation