Induction and regulation of detoxification enzymes in Heliothis zea (Boddie) by plant allelochemicals and insecticides

Abstract

The relationship between the diet of phytophagous insects and susceptibility to insecticides is complex. This dissertation attempted to shed light on several aspects of this relationship with Heliothis zea. The objectives were (i) to demonstrate in a lepidopterous herbivore the relationship between adaptation to host plant toxins and subsequent effects on susceptibility to insecticides, (ii) to elucidate the mechanism of adaptation to toxic chemicals at the cellular level, and (iii) to determine the role of insect developmental hormones in this adaptive process. Data were presented that suggest the major effect of dietary toxins in H. zea is the induction of selected detoxification enzymes. Aldrin epoxidase was induced in insects reared on different cotton cultivars. Only one cultivar, RDC-102, induced glutathione S-aryl transferase activity. The insects fed on fresh flower buds metabolized malathion and permethrin at much slower rates than the insects reared on artificial diets. The plant diets also altered the route of metabolism of permethrin. The plant toxins also inhibited insect growth and developmental rates, which complicated the understanding of the relationship between diet and insecticide susceptibility. The earlier larval instars were more susceptible to methyl parathion. However, when insects were reared on artificial diet with levels of plant allelochemicals not affecting growth rate, the larvae were more tolerant to methyl parathion, but there was no change in tolerance to permethrin. A receptor system similar to the Ah system that recognized polyaromatic hydrocarbons in mammals was shown to be present in H. zea. The ligand used was TCDD. Saturable, high affinity (K[subscript d] = 5.9nM, B[subscript max] = 0.5 pmoles/mg protein) binding of TCDD was measured in the fat body cytosol. Juvenile hormone was a strong competitor for the TCDD binding site(s). DDT and gossypol also competed for the site. TCDD showed JH-like activity by blocking pupation when insects were treated with 3.1 nmoles per insect. Four distinct microsomal proteins having similar M[subscript r] and pl values were induced by both TCDD and juvenile hormone. The competition for the TCDD binding sites by juvenile hormone I suggested this material was a natural ligand for the receptor protein. Saturable, high affinity binding (K[subscript d] = 5.56nM, B[subscript max] = 1.19 pmoles/mg protein) of juvenile hormone I was demonstrated in the fat body of late fifth instar H. zea larvae. The plant allelochemical myrcene also bound to the receptor with high avidity. Affinity for the juvenile hormone binding site was as follows: juvenile hormone I > juvenile hormone III > myrcene > methoprene >TCDD. This was the first example of competition for a high affinity juvenile hormone binding protein by a plant allelochemical. The significance of this research was that the juvenile hormone receptor in H. zea appeared to play a central role in the recognition and response to plant allelochemicals and also, to man made xenobiotics.