Date of Award:

5-1-1992

Document Type:

Dissertation

Degree Name:

Doctor of Philosophy (PhD)

Department:

Biology

Department name when degree awarded

Biology

Committee Chair(s)

William A. Brindley

Committee

William A. Brindley

Committee

Diane G. Alston

Committee

Edward W. Evans

Committee

Thomas A. Grover

Committee

Dennis L. Welker

Abstract

Populations of Lygus hesperus in areas where applications of the organophosphate insecticide, trichlorfon, had been used for control of this pest have elevated LC50 values when assessed by a contact bag bioassay. Biochemical investigation using ⍺-naphthyl acetate as an esterase substrate revealed that resistant populations contain a larger proportion of individuals with higher esterase activity. The same populations also had reduced acetylcholinesterase (AChE) sensitivity to inhibition by paraoxon. The population structure regarding resistance level could be easily visualized from the distribution patterns of esterase activity and AChE insensitivity, or its heterogeneity estimated by the slope of the bioassay line. The susceptible population was very homogeneous while the resistant populations were more heterogeneous. A positive correlation was found between the resistance levels of populations and their esterase activities. Thus, esterases may be detoxifying and/or sequestering agents in L. hesperus. Altered AChE was another resistance mechanism. When the insensitivity of AChE was considered with esterase activity, a better correlation coefficient was obtained. Thus, altered AChE was consi-dered as another resistance mechanism. Homogenates of adult Lygus bugs were analyzed by nondenaturing polyacrylamide gel electrophoresis. Staining with ⍺-naphthyl acetate and ⍺-naphthyl propionate revealed six zones of esterase activity. These isozymes were characterized by inhibition properties and substrate specificities. The esterases are carboxylesterases and have their best activities toward aromatic naphthyl esters with short side chains. The molecular weights of the major esterase isozymes were estimated using non-denaturing polyacrylamide gel electrophoresis with reference to standard protein markers. The molecular weights were between 98000 and 140000. The frequency of one Est-3 allozyme, Est-3105, was found to be positively correlated with trichlorfon resistance in this insect. In the most resistant population, it had a frequency of 0.297. In a susceptible population, the frequency was only 0.036. Esterase polymorphisms were noticed for these populations and their importance was discussed. Developmental variability and distribution of the esterase isozymes were investigated. Homogenates of older females and of eggs contained a heavily stained zone (Est-1) at the cathodal end of the gel.

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