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Degree Name

Doctor of Philosophy




The toxicity and metabolism of isofenphos and bendiocarb were compared between southern corn rootworm (SCR) adults and larvae. In the toxicity tests, insecticide solutions were applied topically on the ventral abdomen, and the LD(,50) values were determined after mortality was evaluated 48 h posttreatment. Isofenphos and three of its metabolites (isofenphos oxon, des-N-isopropyl isofenphos, and des-N-isopropyl isofenphos oxon) were 2.5 to 3.6 times less toxic to SCR larvae than to adults. The 48-h topical LD(,50) values ranged from 1.7 to 2.7 (mu)g/g for adults, and 5.9 to 8.2 (mu)g/g for larvae. Bendiocarb was approximately seven times less toxic to SCR larvae than to adults. The 48-h topical LD(,50) values of bendiocarb to SCR adults and larvae were 1.3 and 9.1 (mu)g/g, respectively;In the metabolism studies, SCR adults and larvae were analyzed at specific time intervals after treatment with a ('14)C-labeled insecticide solution at sublethal dosage levels, at the LD(,10) dosage for isofenphos, and at the 1/10 LD(,50) dosage for bendiocarb. Most of the ('14)C was recovered from internal organic extract and container rinse fractions;The proposed major metabolic pathway of isofenphos in SCR included oxidative desulfuration of isofenphos to isofenphos oxon, hydrolysis of isofenphos oxon and/or parent isofenphos to isopropyl salicylate, and conjugation of isopropyl salicylate. Isofenphos penetrated approximately three times faster into larvae than into adults. Less accumulation of toxic compounds inside the larvae, however, resulted from a faster metabolism (1.5x) and rapid elimination of toxic parent isofenphos. Less internal toxic compound in SCR larvae may explain partially the lower isofenphos toxicity for larvae;The proposed major metabolic pathways of bendiocarb in SCR are hydrolysis of the carbamate ester bond to yield bendiocarb phenol and the subsequent conjugation of bendiocarb phenol. The major metabolites from bendiocarb were beniocarb phenol and conjugated compounds for SCR adults and larvae, respectively. For bendiocarb, the ratio of applied dose to internal toxic compound was similar to both adults and larvae. Therefore, the amount of internal toxic dose, affected by the rates of penetration, metabolism, and excretion, was not a determining factor in the selective toxicity of bendiocarb between SCR adults and larvae.



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Chu-Ying Hsin



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131 pages

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Entomology Commons