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1.
Diazinon toxicity to a susceptible strain of house fly (Musca domestica L.) was synergized by tridiphane [2-(3,5-dichlorophenyl)-2-(2,2,2-trichloroethyl)oxirane], a herbicide synergist. Both diazinon and tridiphane were partially metabolized in the house fly by glutathione (GSH) conjugation. Synergism appeared to be due to inhibition of diazinon metabolism/detoxification. Crude glutathione S-transferase (GST) preparations from the house fly catalyzed GSH conjugation of diazinon, tridiphane, 3,4-dichloronitrobenzene (DCNB), and chloro-2,4-dinitrobenzene (CDNB). Tridiphane and the GSH conjugate of tridiphane appeared to inhibit diazinon GSH conjugation, but diazinon did not inhibit tridiphane GSH conjugation. The enzymatic rate of tridiphane GSH conjugation was 22 times the rate of diazinon GSH conjugation; therefore, attempts to assay tridiphane as an inhibitor of diazinon GSH conjugation were inconclusive because of the high concentration of tridiphane GSH conjugate produced during the assay. CDNB underwent enzymatic GSH conjugation at a rate 240 times faster than that of tridiphane and 5000 times faster than that of diazinon. GSH conjugation of CDNB was not inhibited by tridiphane, but was inhibited by the GSH conjugate of tridiphane. In vivo, the GSH conjugate of tridiphane was produced in sufficient concentration to cause the observed inhibition of diazinon metabolism and synergism of diazinon toxicity. However, the possibility that parent tridiphane caused or contributed to the inhibition of diazinon metabolism and synergism of diazinon toxicity could not be excluded. Inhibition of diazinon metabolism did not appear to be due to depletion of either GSH or GST.  相似文献   

2.
DDT-dehydrochlorinase has been isolated in a highly purified form by a procedure involving affinity chromatography, gel-permeation chromatography, and preparative isoelectrofocusing. At least two protein species appeared to possess DDT-dehydrochlorinase activity; the principal one of these was purified by a factor of 660-fold. This appeared to be dimeric with subunits of molecular weight of 23,000 and 25,000. Another protein with this activity appeared to consist of two identical subunits of Mr 25,000. The protein with greatest activity was isoelectric at pH 7.1. It was found to be homogeneous on analytical gel electrophoresis in both the presence and absence of SDS. The same protein generated a number of minor protein bands on analytical electrofocusing in polyacrylamide gels, but there is evidence that these bands may be artifactual. Both purified forms of the enzyme possessed substantial glutathione S-transferase activity with both CDNB and DCNB. An acidic protein, a dimer of subunits of Mr 23,000 had substantial GSH transferase activity with CDNB as substrate, but had no DDT-dehydrochlorinase activity.  相似文献   

3.
The relationship between glutathione S-transferase activity toward 3,4-dichloronitrobenzene and O-alkyl or O-aryl conjugation of diazinon was investigated in eight strains of house flies. No significant difference was found in the amount of O-aryl conjugation. In contrast, house flies which had higher glutathione S-transferase activity toward 3,4-dichloronitrobenzene also had higher O-alkyl conjugating activity toward diazinon. The glutathione S-transferase(s) in phenobarbital-pretreated flies degraded diazinon faster than those in the nontreated ones. The present results showed that the formation of the O-alkyl conjugate was enhanced by phenobarbital pretreatment, while the formation of the O-aryl conjugate was not affected by induction. Based on these findings, it would appear that one of the multiple forms of glutathione S-transferase is specifically induced and responsible for the increase in O-alkyl conjugation.  相似文献   

4.
Glutathione transferases have been purified to a high degree of homogeneity from three strains of house fly by a procedure involving affinity chromatography on glutathione-sulfobromophthalein conjugate immobilized on Sepharose 4B, followed by preparative isoelectrofocusing. The affinity chromatography yielded purifications of between about 10- and 100-fold, depending on the strain and the substrate with which activity was measured. Each strain was shown to possess several proteins with glutathione S-transferase activity which fell into two clearly defined groups. The first group, of relatively low isoelectric point, showed activity with CDNB but little with DCNB, p-nitrobenzylchloride, or 1,2-epoxy-3-(p-nitrophenoxy)propane, whereas the second group, of higher isoelectric points, showed substantial activity with all substrates tested. Studies on the subunit structure of these enzymes demonstrated the existence of three different sized subunits of Mr 20,000, 22,000, and 23,500. From the experimental evidence recorded here, the existence of at least three functionally different glutathione transferases is inferred.  相似文献   

5.
The effect of phenobarbital and certain pesticides on glutathione S-transferase activity was investigated. The maximum amount of enzyme induction occurred 96 hr after phenobarbital treatment. Chlorinated hydrocarbons were more effective inducers than the other pesticides evaluated. Phenobarbital treatment did not alter the apparent Km value but altered the Vmax value of glutathione S-transferase to 3,4-dichloronitrobenzene. The amount of reduced glutathione was not increased by phenobarbital treatment. Pretreatment of house flies with phenobarbital provides some protection against methyl parathion, methyl paraoxon, azinphosmethyl, and methidathion toxicity.  相似文献   

6.
A field population of the rice stem borer (Chilo suppressalis Walker) with 203.3-fold resistance to triazophos was collected. After 8-generation of continuous selection with triazophos in laboratory, resistance increased to 787.2-fold, and at the same time, the resistance to isocarbophos and methamidophos was also enhanced by 1.9- and 1.4-fold, respectively, implying some cross-resistance between triazophos and these two organophosphate insecticides. Resistance to abamectin was slightly enhanced by triazophos selection, and fipronil and methomyl decreased. Synergism experiments in vivo with TPP, PBO, and DEM were performed to gain a potential indication of roles of detoxicating enzymes in triazophos resistance. The synergism results revealed that TPP (SR, 1.92) and PBO (SR 1.63) had significant synergistic effects on triazophos in resistant rice borers. While DEM (SR 0.83) showed no effects. Assays of enzyme activity in vitro demonstrated that the resistant strain had higher activity of esterase and microsomal O-demethylase than the susceptible strain (1.20- and 1.30-fold, respectively). For glutathione S-transferase activity, no difference was found between the resistant and the susceptible strain when DCNB was used as substrate. However, 1.28-fold higher activity was observed in the resistant strain when CDNB was used. These results showed that esterase and microsomal-O-demethylase play some roles in the resistance. Some iso-enzyme of glutathione S-transferase may involve in the resistance to other insecticides, for this resistant strain was selected from a field population with multiple resistance background. Acetylcholinesterase as the triazophos target was also compared. The results revealed significant differences between the resistant and susceptible strain. The Vmax and Km of the enzyme in resistant strain was only 32 and 65% that in the susceptible strain, respectively. Inhibition tests in vitro showed that I50 of triazophos on AChE of the resistant strain was 2.52-fold higher. Therefore, insensitive AChE may also involved in triazophos resistance mechanism of rice stem borer.  相似文献   

7.
A strain of the fall armyworm, Spodoptera frugiperda (J.E. Smith), collected from corn in Citra, Florida, showed high resistance to carbaryl (562-fold) and methyl parathion (354-fold). Biochemical studies revealed that various detoxification enzyme activities were higher in the field strain than in the susceptible strain. In larval midguts, activities of microsomal oxidases (epoxidases, hydroxylase, sulfoxidase, N-demethylase, and O-demethylase) and hydrolases (general esterase, carboxylesterase, β-glucosidase) were 1.2- to 1.9-fold higher in the field strain than in the susceptible strain. In larval fat bodies, various activities of microsomal oxidases (epoxidases, hydroxylase, N-demethylase, O-demethylases, and S-demethylase), glutathione S-transferases (CDNB, DCNB, and p-nitrophenyl acetate conjugation), hydrolases (general esterase, carboxylesterase, β-glucosidase, and carboxylamidase) and reductases (juglone reductase and cytochrome c reductase) were 1.3- to 7.7-fold higher in the field strain than in the susceptible strain. Cytochrome P450 level was 2.5-fold higher in the field strain than in the susceptible strain. In adult abdomens, their detoxification enzyme activities were generally lower than those in larval midguts or fat bodies; this is especially true when microsomal oxidases are considered. However, activities of microsomal oxidases (S-demethylase), hydrolases (general esterase and permethrin esterase) and reductases (juglone reductase and cytochrome c reductase) were 1.5- to 3.0-fold higher in the field strain than in the susceptible strain. Levels of cytochrome P450 and cytochrome b5 were 2.1 and 1.9-fold higher, respectively, in the field strain than in the susceptible strain. In addition, acetylcholinesterase from the field strain was 2- to 85-fold less sensitive than that from the susceptible strain to inhibition by carbamates (carbaryl, propoxur, carbofuran, bendiocarb, thiodicarb) and organophosphates (methyl paraoxon, paraoxon, dichlorvos), insensitivity being highest toward carbaryl. Kinetics studies showed that the apparent Km value for acetylcholinesterase from the field strain was 56% of that from the susceptible strain. The results indicated that the insecticide resistance observed in the field strain was due to multiple resistance mechanisms, including increased detoxification of these insecticides by microsomal oxidases, glutathione S-transferases, hydrolases and reductases, and target site insensitivity such as insensitive acetylcholinesterase. Resistance appeared to be correlated better with detoxification enzyme activities in larval fat bodies than in larval midguts, suggesting that the larval fat body is an ideal tissue source for comparing detoxification capability between insecticide-susceptible and -resistant insects.  相似文献   

8.
Glutathione transferase (GST) was purified from the hindgut of grasshopper (Zonocerus variegatus) a polyphagous insect. The purified enzyme had a native molecular weight of 40 kDa and a subunit molecular weight of 19 kDa. The purified enzyme could conjugate glutathione (GSH) with 1-chloro-2,4-dinitrobenzene (CDNB), paranitrobenzylchloride, paranitrophenylacetate, 7-chloro-4-nitrobenzo-2-oxa-1,3-diazole (NBDCl), and 1,2-dichloro-4-nitrobenzene (DCNB) with specific activities of 3.3 ± 0.3, 0.49 ± 0.10, 0.10 ± 0.002, 1.2 ± 0.2, and 1.7 ± 0.4 μmol/min/mg protein, respectively. CDNB appears to be the best substrate with a specificity constant, kcat/Km, of 1.8 ± 0.1 × 10−4 M−1 S−1. The kinetic mechanism of Z. variegatus GST (zvGST) in the conjugation of GSH with some electrophilic substrates appears complex. Conjugation of GSH with DCNB was inhibited by high DCNB concentration, while with NBDCl, as the electrophilic substrates, different values of Km were obtained at high and low concentrations of the substrates. Cibacron blue, hematin, S-hexylglutathione, and oxidized glutathione inhibited the enzyme with I50 values of 0.057 ± 0.004, 0.80 ± 0.2, 33 ± 2 μM, and 5.2 ± 0.3 mM, respectively. The nature of inhibition by each of these inhibitors is either competitive or non-competitive at varying GSH or CDNB as substrates. NADH and NAD+ inhibited the enzyme with an I50 value of 0.4 ± 0.01 and 11 ± 1 mM, respectively. NADH at a concentration of 0.54 mM completely abolished the activity. As part of its adaptation, the flexible kinetic pathway of detoxication by zvGST may assist the organism in coping with various xenobiotics encountered in its preferred food plants.  相似文献   

9.
Methyl bromide was metabolized by susceptible and resistant strains of adult granary weevil, Sitophilus granarius (L.), mainly by conjugation with glutathione. S-Methyl glutathione and S-methyl cysteine were produced by both strains and S-methyl glutathione sulfoxide was identified as a metabolite in the resistant strain. In the untreated insects, no significant difference was observed in glutathione S-transferase activity but the resistant contained approximately twice as much glutathione per insect as the susceptible strain. When the insects were treated with methyl bromide, the glutathione content of both strains was lowered; proportionally, however, the decrease was considerably higher in the susceptible than in the resistant strain. These results indicate that conjugation of methyl bromide with glutathione is a major detoxication pathway and tolerance to this fumigant is related, in part at least, to the level of glutathione in the granary weevil.  相似文献   

10.
The effects of the herbicide antidotes CGA-92194 (α-[(1,3-dioxolan-2-yl-methoxy)-imino]benzeneacetonitrile), flurazole [phenylmethyl 2-chloro-4-(trifluoromethyl)-5-thiazolecarboxylate], dichlormid (2,2-dichloro-N,N-di-2-propenylacetamide), and naphthalic anhydride (1H,3H-naphtho(1,8-cd)-pyran-1,3-dione) on nonprotein thiol content, glutathione content, and glutathione S-transferase (GST) activity in etiolated sorghum (Sorghum bicolor L.) Moench) shoots were examined. CGA-92194 and naphthalic anhydride had no effect on nonprotein thiol or reduced glutathione (GSH) content of sorghum shoots. In contrast, dichlormid and flurazole increased nonprotein thiol content of sorghum shoots by 24 and 48%, respectively. These increases were largely attributable to an increase in GSH. The antidotes increased GST activity less than twofold when using CDNB (1-chloro-2,4-dinitrobenzene) as a substrate. In contrast, when using metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethyl)acetamide] as a substrate, the increase in GST activity in response to antidote treatment was much greater: flurazole (30-fold), CGA-92194 (20-fold), naphthalic anhydride (17-fold), dichlormid (5-fold). The degree of protection from metolachlor injury conferred by a particular antidote was strongly correlated (R2 = 0.95) with its ability to enhance GST activity, as evaluated with metolachlor as substrate. A comparison of GST activity in untreated and CGA-92194-treated seedlings, over a range of metolachlor concentrations (0.5–500 μM), indicated that the relative enhancement of enzyme activity by CGA-92194 was greater at lower metolachlor concentrations. The rate of nonenzymatic conjugation of metolachlor and GSH in vitro was much less (on a gram fresh weight equivalent basis) than the enzymatic rate. These results are consistent with the hypothesis that the above antidotes protect sorghum by enhancing GST activity which results in accelerated detoxification of metolachlor via GSH conjugation.  相似文献   

11.
Kinetic parameters were measured for glutathione S-transferase, an enzyme important in metabolic resistance to insecticides, in one susceptible and two insecticide-resistant strains of the house fly (Musca domestica L.), and in untreated and chemically induced flies. Both resistant strains differed from the susceptible strain in apparent Km values for the enzyme, while only one differed in apparent Vmax. Two of the strains were inducible with phenobarbital; the third with 3-methylcholanthrene. Kinetic analysis indicated enzyme induction was associated with changes in Km rather than Vmax, and genetic experiments showed that most variation relating to Km and Vmax was controlled by chromosome II. Based on these results, both metabolic resistance and induction of enzyme activity were associated primarily with the production of different forms of glutathione S-transferase rather than more of the enzyme present in susceptible flies.  相似文献   

12.
Glutathione S-transferases (GSTs) catalyzing the conjugation of reduced glutathione (GSH) to a vast range of xenobiotics including insecticides were investigated in the psocid Liposcelis bostrychophila Badonnel. GSTs from susceptible and two resistant strains (DDVP-R for dichlorvos-resistant strain and PH3-R for phosphine-resistant strain) of L. bostrychophila were purified by glutathione-agarose affinity chromatography and characterized by their Michaelis-Menten kinetics towards artificial substrates, i.e., 1-chloro-2,4-dinitrobenzene (CDNB), in a photometric microplate assay. The specific activities of GSTs purified from two resistant strains were significantly higher than their susceptible counterpart. For the resistant strains, GSTs both showed a significantly higher affinity to the substrate GSH while a declined affinity to CDNB than those of susceptible strain. The inhibitory potential of ethacrynic acid was very effective with highest I50 value (the concentration required to inhibit 50% of GSTs activity) of 1.21 μM recorded in DDVP-R. Carbosulfan also exhibited excellent inhibitory effects on purified GSTs. The N-terminus of the purified enzyme was sequenced by Edman degradation, and the alignment of first 13 amino acids of the N-terminal sequence with other insect GSTs suggested the purified protein was similar to those of Sigma class GSTs.  相似文献   

13.
Glutathione S-transferases (GST) from corn, giant foxtail, onion, pea, house fly, and equine liver catalyzed conjugation of tridiphane with glutathione (GSH). The conjugate was characterized by soft ionization mass spectral methods. Tridiphane and the GSH conjugate of tridiphane both inhibited GSH conjugation of atrazine in vitro (corn and giant foxtail). Tridiphane did not inhibit GSH conjugation of 1-chloro-2,4-dinitrobenzene (CDNB) in corn or giant foxtail; however, the GSH conjugate of tridiphane was a competitive inhibitor with respect to GSH and was four times more effective with extracts from giant foxtail (Ki = 2 μM) than from corn (Ki = 8 μM). The GSH conjugate of tridiphane inhibited a variety of GST enzymes with several different substrates. When compared to other inhibitors of GST, only triphenyl tin chloride was more effective than the GSH conjugate of tridiphane in inhibition of GST from giant foxtail. Both GST and GSH decreased in corn and increased in giant foxtail as tissues matured. The catabolism of the GSH conjugate of tridiphane was compared in crude enzyme systems from corn, giant foxtail, and onion. The rate of catabolism was much greater in extracts from corn leaves than from giant foxtail leaves. Inhibition of GSH conjugation of CDNB was reversed as the GSH conjugate of tridiphane was catabolized. The possibility that synergism of atrazine toxicity by tridiphane is mediated by conversion of tridiphane to a GSH conjugate is discussed in relationship to the relative rates of GSH conjugation of tridiphane and atrazine, concentrations of GSH, Ki values, tissue age, and stability of the conjugate in different tissues.  相似文献   

14.
The microsomal oxidase system in the flesh fly (Sarcophaga bullata Parker) and the black blow fly [Phormia regina (Meigen)] was examined using aldrin as substrate. In both species the oxidase requires NADPH and is inhibited by CO and by piperonyl butoxide. The enzyme activity changes significantly during larval development, reaching a maximum shortly before puparium formation then declining during the pupal stage and increasing again in adults. Dietary sodium phenobarbital at 0. 1– 0.5% increases the oxidase activity up to 60- and 56-fold in the larvae of blow flies and flesh flies, respectively.  相似文献   

15.
The effects of CDAA (N,N-diallyl-2-chloroacetamide) pretreatment on subsequent CDAA injury to corn were examined and compared with the effects of the herbicide protectant R-25788 (N,N,-diallyl-2,2-dichloroacetamide). In addition, the effects of CDAA pretreatment on subsequent CDAA metabolism were determined. It was found that 5μM CDAA protected corn from injury by 200 μM CDAA when given as a 2.5- or 1-day pretreatment. R-25788 at similar concentrations did not protect corn from subsequent R-25788 injury. Pretreatment with CDAA increased GSH levels of corn roots by 61% within 1 day, and these levels did not increase with a longer 2.5-day pretreatment with CDAA. GSH-S-transferase activity was assayed spectrophotometrically using CDNB (1-chloro-2,4-dinitrobenzene). A 1-day pretreatment with CDAA increased the root GSH-S-transferase activity by 35%, but did not affect shoot GSH-S-transferase activity. A 2.5-day pretreatment resulted in a 50% increase in root GSH-S-transferase activity but no response of the shoot enzyme was observed. Even longer pretreatments with CDAA did not result in any further increases in enzyme activity. When corn roots pretreated with CDAA for 2.5 days were excised and incubated with radiolabeled CDAA, they exhibited greater rates of uptake and metabolism than did nonpretreated roots. With in vitro studies, a fairly high rate of nonenzymatic degradation of CDAA was observed. However, the enzymatic rate was always double that of the nonenzymatic rate under the experimental conditions used. It is concluded that elevations in the GSH levels and GSH-S-transferase activities of corn roots following CDAA pretreatments may be involved in the protection of corn from subsequent CDAA injury.  相似文献   

16.
The permethrin resistant strain (TR-strain) of the beet armyworm, Spodoptera exigua (Hübner), has 92.5-fold resistance to permethrin (at LD50 level) compared to the permethrin susceptible strain (TS-strain). Bioassay involving permethrin mixed with piperonyl butoxide, an inhibitor of microsomal cytochrome P450s, significantly reduced the resistance ratio from 92.5- to 7.9-fold. However, S,S,S-tributylphosphorotrithioate and diethylmaleate which are inhibitors of esterases and glutathione S-transferase, respectively, did not affect the resistance level. These results indicate that the detoxification of permethrin in the TR-strain was primarily due to the cytochrome P450 monooxygenases. LD50 for permethrin was increased to 4.5-fold by the pre-treatment of phenobarbital in the TS-strain. The effect of induction by phenobarbital was almost completely overcome by the piperonyl butoxide treatment. However, it was observed that phenobarbital treatment did not cause any change in the toxicity of permethrin to TR strain. Since this result deviated from the expectation that the metabolism of phenobarbital in the TR-strain should be greater than that in the TS-strain, it was deemed necessary to compare the metabolism of phenobarbital between the TS- and TR-strains. Comparison was made based on the concentration of phenobarbital in the hemolymph and whole body. The results showed no significant difference in phenobarbital treatment between the two strains used in this study suggesting the possibility that the induction system in TS-strain is different from the TR-strain.  相似文献   

17.
The effect of fumigants on glutathione and glutathione S-transferase in the Khapra beetle larvae (Trogoderma granarium) was studied by fumigating for 1, 3, and 5 hr with a dose causing 100% mortality at 24 hr of exposure. Glutathione and glutathione S-transferase were assayed in the cytosol at 1, 3, and 5 hr of exposure. Time-dependent depletion of glutathione was seen for all fumigants except carbon tetrachloride and phosphine. The depletion was maximum (60–70%) in the cases of methyl bromide, methyl iodide, and acrylonitrile, and least (20–30%) in the cases of ethylene dibromide and ethylene oxide. The order of glutathione depletion by various fumigants at 5 hr exposure was methyl iodide > methyl bromide = acrylonitrile > ethylene dichloride > ethylene oxide > ethylene dibromide. Glutathione S-transferase was induced by all fumigants except ethylene dibromide, methyl bromide being more potent than methyl iodide. The enzyme induction ranged from 186% by acrylonitrile to 40% by carbon tetrachloride. Mortality above 10% correlated well with the degree of GSH depletion (r = 0.729) whereas the latter did not correlate with the transferase induction.  相似文献   

18.
The date stone beetle,Coccotrypes dactyliperda F. (Coleoptera: Scolytidae), is a primary pest of green unripe dates. The adult beetle chews a characteristically round hole in the fruit, which results in the druit dropping 1 or 2 days later. Egglaying and entire development occur within galleries produced by the female in the date stone; males are unable to penetrate the stones. In laboratory cultures and in field-collected date stones, the beetle adult females were predominant (85-93%). Overwintering probably occurs in the adult stage within date stones of fruits which had dropped and were left on the ground. At 28°C the average egg incubation period was 5.9 days, larval duration 12–15 days, and pupal development 4.0 days. Development from egg to adult for females was significantly longer than for males (24.8vs. 22.1 days). Mated females produced males and females, whereas unmated females gave rise to males only. The mean number of progeny per mated female (30.4) was significantly higher than that recorded for unmated females (6.6), but the latter lived significantly longer (73.3vs. 62.9 days).  相似文献   

19.
The correlation between the natural levels of glutathione S-transferase (GST) and the tolerance to the organophosphorus insecticides parathion-methyl and paraoxon-methyl, as well as the interaction of affinity-purified enzyme and the insecticides were investigated in order to collect further information on the role of the glutathione S-transferase system as a mechanism of defence against insecticides in insects. The studies were carried out on the larvae and pupae of the coleopteran Tenebrio molitor L, which exhibit varying natural levels of GST activity. Stage-dependent susceptibility of the insect against insecticides was observed during the first 24 h. However, 48 h after treatment, the KD50 value increased significantly due to the recovery of some individuals. Simultaneous injection of insecticide with compounds which inhibit GST activity in vitro caused an alteration in susceptibility of insects 24 or 48 h post-treatment, depending on stage and insecticide used. Inhibition studies combined with competitive fluorescence spectroscopy revealed that the insecticides probably bind to the active site of the enzyme, thus inhibiting its activity towards 1-chloro-2,4-dinitrobenzene in a competitive manner. High-performance liquid chromatography and gas chromatography revealed that T molitor GST catalyses the conjugation of the insecticides studied to a reduced form of glutathione (GSH). From the above experimental results, it is considered that GST offers a protection against the organophosphorus insecticides studied by active site binding and subsequent conjugation with GSH. © 2001 Society of Chemical Industry  相似文献   

20.
The susceptibility of tufted apple budmoth larvae, Platynota idaeusalis, to azinphosmethyl decreases with each successive instar. A comparison between fifth and third instars showed that the fifth instars have a higher level of glutathione S-transferase activity per milligram of protein, a lower content of cytochrome P-450 per milligram of protein, and absorb a lesser percentage of and LD01 dose than the third instar. Results of metabolism were consistent with these findings. In addition to these factors, the fifth instar larvae may have a threshold mechanism for eliminating penetrated azinphosmethyl from the body, unchanged. This allows the fifth instar larval population to withstand increasingly higher doses of azinphosmethyl without a proportional increase in mortality.  相似文献   

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