188blocking effect directly on the muscle. This effect is significantly inferior to the inhibitory effect observed when the muscle is stimulated via the nerve. These observations suggest that the target for these compounds is to be found in the nerve muscle junction. The effect of the analogues III and IV on the twitch tensions is of the same order of magnitude as demonstrated for I and II. It is therefore suggested that the mode of action of HI might correspond to that of I and II. The mode of action of IV seems to be more complex since this compound is the only one tested that shows an inhibitory effect on the action potential in the frog nerves. The effects of the open-chain analogues on the in vitro preparations lend no support to the hypothesis that the dithiolane moiety is the active principle and that active analogues are metabolized into dithiolanes 15. Experientia 40 (1984), Birkhguser Verlag, CH-4010 Basel/SwitzerlandThe effect level for charatoxin compares well to that of the structurally related nereistoxin on the sartorius nerve muscle preparation as it appears from figure 2. Since both compounds show insecticidal potency with similar symptoms of intoxication 6 it is indicated that further studies on the insecticidal properties of charatoxin analogues may prove fruitful.Many questions remain to be answered regarding the precise site of action of charatoxin and its analogues. The results presented here clearly point at the cholinergic transmission as the region of action. But nothing is known about the specificity of the observed inhibition. Further studies on e.g. binding affinity to acetylcholine receptors, and a more detailed electrophysiological investigation of the effect on the membrane potentials should therefore be carried out.Summary. The organophosphorus insecticides chlorpyrifos, leptophos, and phosfolan and the carbamate methomyl was found to be more toxic to larvae of a susceptible strain of Spodoptera littoralis (Boisd.) when the posttreatment temperature was increased from 20 to 35 ~ In contrast, the pyrethroids permethrin, fenvalerate, deltamethrin, cypermethrin, and flucythrinate were more toxic at 20 ~ than at 35 ~ This effect was more pronounced in the pyrethroid-resistant strains. Evidently, resistance levels were reduced at low temperature. However, the application of piperonyl butoxide or DEF in combinations with the tested pyrethroids on R-strains resulted in reducing the effect of temperature.Several factors influence insecticide toxicity, one being temperature. It has been shown that the toxicity of natural pyrethrins and DDT correlated negatively with increasing posttreatment temperature ~-3. On the other hand organophosphates have shown a positive temperature coefficienff '4'5. Carbamates were reported to have a slightly 67 negative temperature coefficient', although methomyl manifested greater toxicity at higher temperatures in some insect species 4'8'9. Recent developments in pyrethroid chemistry have resulted in synthesis of relatively stable compounds with high toxicity ...

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Incubation of Heliothis zea (Boddie) eggs on foliage of Lycopersicon hirsutum f. glabratum C.H. Mull (accession PI 134417) results in neonates with elevated levels of tolerance to the toxic effects of PI 134417 foliage attributable to 2-tridecanone found in the glandular trichomes which abound on that foliage. The neonates from such eggs are also shown to have elevated levels of tolerance to the carbamate insecticide carbaryl. Incubation of eggs in an atmosphere containing 2-tridecanone similarly produced elevated levels of tolerance to 2-tridecanone among resulting neonates, indicating that 2-tridecanone is the likely inducing agent and that exposure to 2-tridecanone vapor, which is known to emanate from PI 134417 foliage, is sufficient for induction. Analysis of the cytochrome P-450 content in gut microsomes of fifth instar larvae indicated that exposure of larvae to 2-tridecanone in artificial diet or to PI 134417 foliage resulted in significantly elevated levels of cytochrome P-450 relative to larvae fed diet without 2-tridecanone or foliage of L. esculentum which contains no 2-tridecanone. In addition, removal of the glandular trichomes from PI 134417 foliage eliminated the ability of that foliage to induce elevated levels of cytochrome P-450. These results provide circumstantial evidence that cytochrome P-450 may be involved in the induced tolerance to xenobiotics among neonates from eggs exposed to 2-tridecanone or PI 134417 foliage.

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