Date of Award:


Document Type:


Degree Name:

Doctor of Philosophy (PhD)


Animal, Dairy, and Veterinary Sciences

Committee Chair(s)

William A. Brindley


William A. Brindley


R. P. Sharma


S. G. Oberg


D. W. Davis


W. J. Hanson


The role of esterases and monooxygenase in the detoxication vii processes of field populations of pea aphids was studied in relation to seasonal changes, developmental stage, and treatment with DEF and piperonyl butoxide. LCSO values for 12 hour exposure periods in glass vials ra nged from 0.04-2.3 micrograms / vial and 820-4871 J.Jg/vial for paraoxon and carbaryl, respectively. Topical bioassays showed the adults to be more tolerant to paraoxon (LDSO of 1. 4-12.3 ng/aphid) toxicity than nymphs (LDSO of 0.13-4. 7 ng/aphid).

Treatment with DEF® (S,S.S-tributylphosphorotrithioate) enhanced paraoxon toxicity against the pea aphid whereas piperonyl butoxide had little effect on the enhancement of either paraoxon or carbaryl toxicity. The higher synergistic effect of DEF indicates a greater dependency upon esterases in detoxifying paraoxon. The high LC5O values of carbaryl toxicity may indicate that part of the pea aphid tolerance to carbaryl is attributable to a mechanism other than detoxication.

The percent dependency values were 29-98% and 8- 39% for paraoxon (esterases) and carbaryl (monooxygenases), respectively.

The amount of esterases in individual aphids was determined by polyacrylamide, vertical, and slab gel electrophoresis. Three esterase bands were resolved. Two were identified as carboxyl esterases which were most active in hydrolyzing both a-naphthylacetate and paraoxon. The third was classified as cholinesterase due to its small degree of hydrolysis of a-naphthyl acetate and because it had no effect upon paraoxon hydrolysis. The same electrophoretic pattern was detected in both adults and nymphs but the adults had esterases that were more effective in hydrolyzing the substrates.

Paraoxon metabolism by the pea aphids was studied in vitro and with isolated bands from electrophoresis gels. Diethylphosphoric acid (DEPA) was the only paraoxon metabolite detected. Treatment of the insects with DEF prevented DEPA formation. This effect was probably due to inhibition of the insect's esterase system. Addition of NADPH and reduced glutathione to aphid homogenates failed to increase DEPA production by the insect indicating that monooxygenases and glutathione transferases may play minor roles in paraoxon detoxication by pea aphids.



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