Biochemical characteristics of insecticide resistance in the fall armyworm, Spodoptera frugiperda (J.E. Smith)

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 K_m 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.     

Interactions of allelochemicals with detoxication enzymes of insecticide-susceptible and resistant fall armyworms

The induction of glutathione S-transferases and microsomal oxidases by host plants and allelochemicals was examined in sixth-instar larvae of insecticide-susceptible and resistant strains of the fall armyworm, Spodoptera frugiperda (J. E. Smith). Among 11 host plants studied, parsnip and parsley were the best inducers of glutathione S-transferase, resulting in increases of 39- and 19-fold, respectively, compared with the artificial diet. The inducer in parsnip leaves was identified by mass spectrometry, high-pressure liquid chromatography, gas chromatography, and thin-layer chromatography as xanthotoxin, a furanocoumarin. Xanthotoxin also showed a bimodal effect on the microsomal oxidase systems, increasing cytochrome P-450 content and heptachlor epoxidase activity but inhibiting aldrin epoxidase, biphenyl 4-hydroxylase, and p-chloro-N-methylaniline N-demethylase. Using indole 3-acetonitrile, indole 3-carbinol, and flavone as inducers, the inducing pattern of glutathione S-transferases was the same toward 3,4-dichloronitrobenzene, 1-chloro-2,4-dinitrobenzene, and methyl iodide. Microsomal oxidase and glutathione S-transferase were also inducible by host plants and allelochemicals in larvae of a carbaryl-resistant strain.     

Induction of detoxifying enzymes by allelochemicals and host plants in the fall armyworm

The effect of various plant substances and host plants on the microsomal oxidases and glutathione S-transferase was investigated in the fall armyworm (Spodoptera frugiperda (J. E. Smith)) maintained on a meridic diet. The glucosinolate, sinigrin, and the hydrolytic products of glucosinolates, β-phenylethylisothiocyanate, indole 3-acetonitrile, and indole 3-carbinol, and flavone were found to be potent inducers of the glutathione S-transferase in the armyworm. An 18-fold increase in the transferase activity was observed when larvae were fed a diet containing 0.2% indole 3-acetonitrile for 2 days. These compounds, with the exception of β-phenylethylisothiocyanate which appeared to be inhibitory, also stimulated the microsomal aldrin epoxidase significantly. In all instances, no induction of the microsomal oxidase or glutathione S-transferase was observed by the plant hormones, indole 3-acetic acid and gibberellic acid; the terpenoids, stigmasterol, sitosterol, and β-carotene; the polyphenolic gossypol; and the flavonol, quercetin; some of them were found to be inhibitory. Using corn, potato, and sweet potato as inducers of various microsomal oxidases, it was found that the inducing pattern of the N-demethylase was different from the two epoxidases and O-demethylase. Corn leaves were the most active compared with other aerial parts of corn (silks, developing corn, and husk) in inducing the microsomal oxidase. The microsomal oxidase in the younger larvae appeared to be less inducible by host plants than in the older larvae.     

Detoxication capacity in the honey bee,Apis mellifera L

Various detoxifying enzymes, including microsomal oxidases, glutathione S-transferases, esterases, epoxide hydrolase, and DDT-dehydrochlorinase, were assayed in adult worker bees (Apis mellifera L.) using midguts as the enzyme source. A cell-free system was used for all enzyme assays, except that microsomal oxidases required intact midgut because of the inhibitor encountered. Midgut microsomal preparations contained mainly cytochrome P-420, the inactive form of cytochrome P-450, which may explain the low microsomal oxidase activity in microsomes. All enzymes studied were active, suggesting that the high susceptibility of honey bees to insecticides is not due to low detoxication capacity. Sublethal exposure of honey bees to various insecticides had no effect on these enzyme activities, with the exception of permethrin which significantly stimulated the glutathione S-transferase, and malathion, which significantly inhibited the α-naphthylacetate esterase and carboxylesterase.     

Esterase-mediated malathion resistance in the human head louse, Pediculus capitis (Anoplura: Pediculidae)

Resistance in a dual malathion- and permethrin-resistant head louse strain (BR-HL) was studied. BR-HL was 3.6- and 3.7-fold more resistant to malathion and permethrin, respectively, compared to insecticide-susceptible EC-HL. S,S,S-Tributylphosphorotrithioate synergized malathion toxicity by 2.1-fold but not permethrin toxicity in BR-HL. Piperonyl butoxide did not synergize malathion or permethrin toxicity. Malathion carboxylesterase (MCE) activity was 13.3-fold and general esterase activity was 3.9-fold higher in BR-HL versus EC-HL. There were no significant differences in phosphotriesterase, glutathione S-transferase, and acetylcholinesterase activities between strains. There was no differential sensitivity in acetylcholinesterase inhibition by malaoxon. Esterases from BR-HL had higher affinities and hydrolysis efficiencies versus EC-HL using various naphthyl-substituted esters. Protein content of BR-HL females and males was 1.6- and 1.3-fold higher, respectively, versus EC-HL adults. Electrophoresis revealed two esterases with increased intensity and a unique esterase associated with BR-HL. Thus, increased MCE activity and over-expressed esterases appear to be involved in malathion resistance in the head louse.     

Phenobarbital induction of permethrin detoxification and phenobarbital metabolism in susceptible and resistant strains of the beet armyworm Spodoptera exigua (Hübner)

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.     

Status of Insecticide Resistance in Spodoptera litura in Andhra Pradesh,India

Twenty-two strains of the tobacco caterpillar, Spodoptera litura (F.) (Lepidoptera: Noctuidae), collected from groundnut crops of eight locations in Andhra Pradesh, India, between 1991 and 1996 were assayed in the F1 generation for resistance to commonly used insecticides. Resistance levels ranged as follows: cypermethrin, 0·2- to 197-fold; fenvalerate, 8- to 121-fold; endosulfan, 1-to 13-fold; quinalphos, 1- to 29-fold; monocrotophos, 2- to 362-fold and methomyl, 0·7- to 19-fold. In nearly all strains pre-treatment with the metabolic inhibitor, piperonyl butoxide, resulted in complete suppression of cypermethrin resistance (2- to 121-fold synergism), indicating that enhanced detoxification by microsomal P450-dependent monooxygenases was probably the major mechanism of pyrethroid resistance. Pre-treatment with the synergist DEF, an inhibitor of esterases and the glutathione S-transferase system, resulted in a 2- to 3-fold synergism with monocrotophos indicating that esterases and possibly glutathione S-transferases were at least to some extent contributing to organophosphate resistance. © 1997 SCI.     

The effects of dicofol on induction of hepatic microsomal metabolism in rats

In a comparative study, the induction effects of dicofol, technical Kelthane, and DDT on hepatic microsomal and cytosolic enzyme activities in rats were compared with those effects produced by phenobarbital (PhB) and β-naphthoflavone (BNF). Male rats (ca. 250 g) were injected (ip) for 4 consecutive days with 1.0 ml of vehicle containing either dicofol (1.5, 15.0, 29.5, or 59.0 mM, Kelthane (dicofol content equal to 29.5 or 59.0 mM), DDT (59.0 mM), or BNF (36.7 mM). Liver weights, microsomal protein, and cytochrome P-450 concentrations and microsomal and cytosolic enzyme specific activities were measured. Dicofol produced dose-related increases in all of the parameters measured except liver weight and cytosolic epoxide hydrolase activity. At a concentration of 59.0 mM, dicofol increased the concentrations of microsomal protein (1.7-fold) and cytochrome P-450 (2.9-fold), and the specific activities of cytochrome c reductase (1.6-fold), ethoxycoumarin O-deethylase (2.3-fold), aminopyrine N-demethylase (3.0-fold), microsomal epoxide hydrolase (2.6-fold), and glutathione S-transferase (2.9-fold). The induction potency of dicofol was equivalent to Kelthane, DDT, and PhB at equimolar (59.0 mM) concentrations of chemical.     

Resistance selection and biochemical mechanism of resistance to two Acaricides in Tetranychus cinnabarinus (Boiduval)

A Tetranychus cinnabarinus strain was collected from Chongqing, China. After 42 generations of selection with abamectin and 20 generations of selection with fenpropathrin in the laboratory, this T. cinnabarinus strain developed 8.7- and 28.7-fold resistance, respectively. Resistance to abamectin in AbR (abamectin resistant strain) and to fenpropathrin in FeR (fenpropathrin resistant strain) was partially suppressed by piperonyl butoxide (PBO), diethyl maleate (DEM) and triphenyl phosphate (TPP), inhibitors of mixed function oxidase (MFO), glutathione S-transferases (GST), and hydrolases, respectively, suggesting that these three enzyme families are important in conferring abamectin and fenpropathrin resistance in T. cinnabarinus. The major resistant mechanism to abamectin was the increasing activities of carboxylesterases (CarE), glutathione-S-transferase (GST) and mixed function oxidase (MFO), and the activity in resistant strain developed 2.7-, 3.4- and 1.4-fold contrasted to that in susceptible strain, respectively. The activity of glutathione-S-transferase (GST) in the FeR strain developed 2.8-fold when compared with the susceptible strain, which meant the resistance to fenpropathrin was related with the activity increase of glutathione-S-transferase (GST) in T. cinnabarinus. The result of the kinetic mensuration of carboxylesterases (CarE) showed that the structure of CarE in the AbR has been changed.     

Effect of celangulin V on detoxification enzymes in Mythimna separata and Agrotis ypsilon

Celangulin V (CA-V), a β-dihydroagrofuran sesquiterpene polyol ester, is extracted from the root bark of Chinese bittersweet, Celastrus augulatus Maxim. It exhibited selective toxicity against different insects. By CO-difference spectral and biochemical method, the effects of CA-V on two kinds of detoxification enzymes, cytochrome P450 (P450 and NADPH-cytochrome P450 reductase) and glutathione S-transferase, were investigated in oriental armyworm, Mythimna separata and black cutworm, Agrotis ypsilon. CA-V showed higher induction against P450 of M. separata than that of A. ypsilon. Treated by CA-V, the maximum absorption of M. separata increased 1.2 and 0.8 nm than the control, respectively. Meanwhile, compared with the control, the P450 content and NADPH-P450 reductase activity in treated M. separata larvae increased 1.46-, 2.26- and 1.26-, 2.56-fold, respectively. But in treated A. ypsilon larvae, they all increased a little more than those of control. So far as M. separata and A. ypsilon, whether there is exposure of CA-V or not, the P450 content and GST activity in A. ypsilon were obviously higher than those in M. separata. It suggested that the content or activity difference of these two kinds of detoxification enzymes may have important roles in the selective toxicity of CA-V in M. separata and A. ypsilon.     

Induction of glutathione S-transterase by fumigants in larvae of the Khapra beetle,Trogoderma granarium (E.)

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.     

Beta-cypermethrin resistance associated with high carboxylesterase activities in a strain of house fly, Musca domestica (Diptera: Muscidae)

A housefly strain, originally collected in 1998 from a dump in Beijing, was selected with beta-cypermethrin to generate a resistant strain (CRR) in order to characterize the resistance and identify the possible mechanisms involved in the pyrethroid resistance. The resistance was increased from 2.56- to 4419.07-fold in the CRR strain after 25 consecutive generations of selection compared to a laboratory susceptible strain (CSS). The CRR strain also developed different levels of cross-resistance to various insecticides within and outside the pyrethroid group such as abamectin. Synergists, piperonyl butoxide (PBO) and S,S,S-tributyl phosphorotrithioate (DEF), increased beta-cypermethrin toxicity 21.88- and 364.29-fold in the CRR strain as compared to 15.33- and 2.35-fold in the CSS strain, respectively. Results of biochemical assays revealed that carboxylesterase activities and maximal velocities to five naphthyl-substituted substrates in the CRR strain were significantly higher than that in the CSS strain, however, there was no significant difference in glutathione S-transferase activity and the level of total cytochrome P450 between the CRR and CSS strains. Therefore, our studies suggested that carboxylesterase play an important role in beta-cypermethrin resistance in the CRR strain.     

Metabolic resistance mechanisms to phosalone in the common pistachio psyllid, Agonoscena pistaciae (Hem.: Psyllidae)

The common pistachio psyllid, Agonoscena pistaciae, is the most damaging pest of pistachio in Iran, and is generally controlled by insecticides belonging to various classes especially, phosalone. The toxicity of phosalone in nine populations of the pest was assayed using the residual contact vial and insect-dip methods. The bioassay results showed significant discrepancy in susceptibility to phosalone among the populations. Resistance ratio of the populations to the susceptible population ranged from 3.3 to 11.3. The synergistic effects of TPP, PBO and DEM were evaluated on the susceptible and the most resistant population to determine the involvement of esterases, mixed function oxidases and glutathione S-transferases in resistance mechanisms, respectively. The level of resistance to phosalone in the resistant population was suppressed by TPP, PBO and DEM, suggesting that the resistance to phosalone is mainly caused by esterase detoxification. Biochemical enzyme assays revealed that esterase, glutathione S-transferase and cytochrome P450 monooxygenase activities in the resistant population was higher than that in the susceptible. Glutathione-S-transferases play a minor role in the resistance of the pest to phosalone.     

Glutathione S-transferase in the Australian sheep blowfly,Lucilia cuprina (Wiedemann)

Glutathione S-transferase in the Australian sheep blowfly, Lucilia cuprina, was studied using 3,4-dichloronitrobenzene (DCNB) and 1-chloro-2,4-dinitrobenzene (CDNB) as substrates. The optimum pHs for enzyme activity were 7.5–8.0 and 6.7–7.4 for DCNB and CDNB conjugations, respectively. Inclusion of glutathione and bovine serum albumin in the homogenizing buffer protected the glutathione S-transferase from inhibition by endogenous compounds present in extracts of final instar larvae and of adults less than 7–8 days old. Conjugation activities for DCNB and CDNB increased throughout larval development to reach a peak early in the pupal stage. Activity then decreased through the remainder of the pupal stage and for the first 6–7 days after emergence of the adult. Almost all of the decrease in activity during the first 6 days of the adult occurred in the abdomen, which accounted for 85% of total activity in the adult female at emergence but only 47% at 6 days. Larval DCNB conjugation activity was localized almost entirely in the fat body (94%), whereas only 50% of the CDNB conjugation activity was in the fat body with the remainder in the cuticle (25%), gut (15%), and blood (10%). Adult and larval enzyme was induced ca. three- to four-fold by sodium phenobarbital. The induction was associated with changes in apparent V_max rather than apparent K_m, suggesting that phenobarbital caused increased production of forms of enzymes already present rather than inducing synthesis of altered or new forms.     

Comparative metabolism of atrazine and EPTC in proso millet (Panicum miliaceum L.) and corn

The purpose of this study was to examine the differential activities of proso millet (Panicum miliaceum L.) and corn (Zea mays L.) with respect to atrazine [2-chloro-4-(ethylamino)-6-(isopropylamino)-S-triazine] and EPTC (S-ethyldipropyl thiocarbamate) metabolism. GSH-S-transferase was isolated from proso millet shoots and roots. When assayed spectrophotometrically using CDNB (1-chloro 2,4-dinitrobenzene) as a substrate, the shoot enzyme had only 10% of the activity of corn shoot enzyme while the root enzyme had 33% the activity of corn root enzyme. However, when proso millet shoot GSH-S-transferase was assayed in vitro using ¹⁴C-ring-labeled atrazine, it degraded the atrazine to water-soluble products at the same rate as the corn shoot enzyme. Incubation of excised proso millet and corn roots with [¹⁴C]EPTC indicated that uptake of EPTC was similar in both plants. However, proso millet metabolized the EPTC to water-soluble products at only half the rate of corn. Glutathione levels of proso millet roots were 35.9 μg GSH/g fresh wt, compared with 65.4 μg GSH/g fresh wt for corn. However, a 2.5-day pretreatment with R-25788 (N,N-diallyl-2-2-dichloroacetamide) elevated proso millet GSH levels to 62.7 μg GSH/g fresh wt. R-25788 did not elevate the activity of proso millet GSH-S-transferase, in contrast to its effects on corn. We conclude that differences in response to atrazine and EPTC in proso millet and corn are a result of their differential metabolism.     

Induction of glutathione S-transferase by phenobarbital and pesticides in various house fly strains and its effect on toxicity

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 K_m value but altered the $\text{V}{}_{\mathrm{<mtext>max</mtext>}}$ 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.     

Triazophos resistance mechanisms in the rice stem borer (Chilo suppressalis Walker)

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 V_max and K_m 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.     

Variability in resistance-related enzyme activities in field populations of the tarnished plant bug, Lygus lineolaris

Widespread use of Bt crops for control of lepidopterous pests has reduced insecticide use and provided the tarnished plant bug the opportunity to become a serious pest on mid-South cotton. Organophosphate insecticides have predominantly been used against plant bugs in recent years due to the reduced efficacy of other insecticides. In this study, a biochemical approach was developed to survey enzymatic levels associated with organophosphate resistance levels in field populations of the tarnished plant bug. Forty-three populations were collected from the delta areas of Arkansas, Louisiana, and Mississippi. Three esterase substrates and one substrate each of glutathione S-transferase (GST) and acetylcholinesterase (AChE) were used to determine corresponding detoxification enzyme activities in different populations. Compared to a laboratory susceptible colony, increases up to 5.29-fold for esterase, 1.96-fold for GST, and 1.97-fold for AChE activities were detected in the field populations. In addition to the survey of enzyme activities among the populations, we also examined the susceptibility of major detoxification enzymes to several inhibitors which could be used in formulations to synergize insecticide toxicity against the target pests. As much as 52-76% of esterase, 72-98% of GST, and 93% of AChE activities were inhibited in vitro. Revealing variable esterase and GST activities among field populations may lead to a better understanding of resistance mechanisms in the tarnished plant bug. This study also reports effective suppression of detoxification enzymes which may be useful in future insecticide resistance management program for the tarnished plant bug and other Heteropteran pests on Bt crops.     

Lack of cross-resistance to indoxacarb in insecticide-resistant Spodoptera frugiperda (Lepidoptera: Noctuidae) and Plutella xylostella (Lepidoptera: Yponomeutidae)

Two field strains of the fall armyworm, Spodoptera frugiperda (JE Smith), collected from corn in north Florida showed high resistance to carbaryl (626- and 1159-fold) and moderate resistance to parathion-methyl (30- and 39-fold) as compared with a laboratory susceptible strain. A field strain of the diamondback moth, Plutella xylostella (L.) collected from cabbage in north Florida and selected for 20 generations with permethrin showed high resistance to permethrin (987-fold) as compared with a susceptible strain. However, in all instances, no cross-resistance to indoxacarb, a novel oxidiazine insecticide, was observed in these two species. Biochemical studies revealed that, in S. frugiperda, activities of detoxification enzymes (microsomal oxidase, glutathione S-transferase and general esterase) were significantly higher in the field strains than in the susceptible strain, indicating that these detoxification enzymes were not actively involved in the resistance to indoxacarb. The lack of cross-resistance between indoxacarb and permethrin in P. xylostella further supports the notion that the mode of action of these insecticides on the insect sodium channel is different.     

Characterization of multiple glutathione transferases from the house fly, Musca domestica (L)

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 M_r 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.