Porphyric Insecticides - Chlorophyll Biosynthesis and Technological Applications - page 413

Table 18.8 Tetrapyrrole accumulation accompanying the ingestion of diet supplemented with
2 mM ALA and varying Oph concentrations a

Proto content Zn-proto content

Larval b death (%)

Treatment (nmol/100 mg protein)

Control 0.6 0.0 16.0

2 mM ALA 3.6 0.0 54.0

2 mM ALA + 0.1 mM Oph 10.4 0.0 84.0

2 mM ALA + 0.4 mM Oph 46.4 2.7 100.0

2 mM ALA + 1.6 mM Oph 67.0 4.1 100.0

Adapted from Rebeiz et al. ( 1990a )
a

Fourth instar larvae were placed on control and baited diets. After 17 h in darkness the larvae were
sampled for tetrapyrrole analysis. The remaining larvae were placed in the light for observation of
photodynamic injury
b

Refers to larval death at the beginning of the fourth photoperiod, i . e . after 3 days in the growth
chamber

18.3.6 Phenomenology of Baited Food Consumption
and Photodynamic Damage in T. ni

To further the development of ALA-dependent insecticidal field strategies, various
studies were performed to determine the relationship between food intake, Proto
accumulation and photodynamic death in fourth instar T . ni larvae. The following
was observed: (a) With diet baited with 1 mM ALA and 0.5 mM Oph, at least 4 h of
feeding on the treated diet was required before exposure to light in order to achieve
photodynamic mortality rates of 90 % or better, (b) it did not matter whether food
consumption took place in the light or in darkness, (c) ingested baited food was
detoxified if the larvae were taken off the baited food and placed on untreated diet in
darkness for 4-h or longer, and (d) Proto accumulation in the body of the larvae
increased exponentially, as the concentration of ALA and Oph in the diet increased
to 1 and 0.75 mM respectively. Beyond these concentrations, the increase in Proto
accumulation slowed down considerably.

18.3.7 Inhibition by Metal Cations

of the Insecticidal Properties of Oph

It has been reported that the tetrapyrrole-inducing properties of bidentate metal
chelators, such as Dpy in plants, were not expressed in the presence of metallic cations
such as Fe ++ and Zn ++ (Duggan and Gassman 1974 ). This phenomenon was a
convenient tool to further determine whether the insecticidal properties of Oph,
another bidentate chelator, were obligatorily linked to Proto accumulation in insects.
To this effect third instar T . ni larvae were placed overnight on treated food which in
addition to ALA (1.0 mM) and Oph (0.5 mM) contained various concentrations of
FeCl 2 ,FeCl 3 and ZnCl 2 . After 17 h incubation, some larvae were monitored for Proto
accumulation while others were exposed to light to trigger photodynamic damage.