Insecticide Resistance in Diamondback Moth

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1 Insecticide Resistance in Diamoback Moth 34 C. N. Sun, T. K. Wu, J. S. Chen, a W. T. Lee Department of Entomology, National Chung Hsing University, Taichung, Taiwan 40227, ROC Abstract High levels of resistance to the major categories of insecticides, ie, organophosphorus, carbamates, pyrethroids, a DDT, have been detected in the diamoback moth in Taiwan. Synergist studies have provided insufficient evidence to show significant involvement of known metabolic systems, such as microsomal oxidation, esterase hydrolysis, a glutathione conjugation in organophosphorus a carbamate resistance. Meanwhile, moderate levels of reduction of acetyl cholinesterase sensitivity to these compous have been observed. This, however, could not account for all the resistance detected. In addition, the relationship between carbofuran a carbosulfan resistance is discussed. While pyrethroid resistance is closely associated with microsomal oxidation, iirect evidence iicates nerve insensitivity may also be a contributing factor. Synergist piperonyl butoxide may temporarily obliterate pyrethroid resistance. But this effect disappears quite quickly, probably because of the development of resistance to this specific compou by the insect. Diamoback moth larvae selected with fenvalerate or fenvalerate/piperonyl butoxide seem to be more susceptible to some organophosphorus insecticides including mevinphos, profenofos, a prothiophos. Recommeations are suggested regarding the use of synergists, pyrethroids, a organophosphorus insecticides. Introduction In 1984, 30 insecticides including 17 organophosphorus compous, 2 carbamates, 6 pyrethroids, 2 mixtures of organophosphorus compous a pyrethroid, 2 organonitrogen compous a Bacillus thuringiensis were officially recommeed for the control of diamoback moth (DBM), Plutella xylostella (L) (Lepidoptera: Yponomeutidae), in Taiwan (Table 1). Different levels of resistance to these insecticides have been detected Table 1. Insecticides recommeed for DBM control in Taiwan in 1984ª 1. Organophosphorus compous (17) Acephate Cyanofenphos Cyanophos Diazinon Dichlorvos Mephosfolan Methamidophos Methidathion Mevinphos Naled Phenthoate Pirimiphos-methyl Profenofos Prothiophos Pyridaphenthion Quinalphos Salithion 2. Carbamates (2) Carbofuran Methomyl 3. Pyrethroids (6) Cypermethrin Deltamethrin Fenvalerate Fenpropathrin Flucythrinate Permethrin 4. Mixtures (2) Dimethoate + Phenthoate (1:2) Chlorpyrifos + Cypermethrin (9:1) 5. Others (3) Cartap Thiocyclam Bacillus thuringiensis ªSource. PDAF 1984.

2 3 60 Sun, Wu, Chen, a Lee in a multiple-resistant (BC) strain from the field (Table 2). Despite the resistance, some insecticides, including dichlorvos, mevinphos, profenofos, permethrin, cypermethrin, deltamethrin, a fenvalerate still possess much higher potency than the rest against this highly resistant strain. Cartap a B. thuringiensis are also effective. No resistance to the latter is detected in the BC strain. In the following, the resistance to each group of compous, with special reference to the pyrethroids, will be discussed. Table 2. Toxicity of some insecticides to susceptible (FS) a multiresistant (BC) strains of DBM Insecticide FS LC50 (µg/ml) BC LC50 (mg/ml) Organophosphorus Cyanofenphos > 50,000 Diazinon Dichlorvos Malathion > > 2,941 Methyl parathion 9.40 > > 10,638 Mevinphos Profenofos ,714 Prothiophos ,854 Carbamatesb Carbaryl > >122 Carbofuran Methomyl Pyrethroids Cypermethrin Deltamethrin ,850 Fenpropathrin ,466 Fenvalerate ,965 Flucythrinate ,028 Fluvalinate ,327 Permethrin Phenothrin Tetramethrin Tralomethrin ,768 Organochlorines DDT > >1,182 Others Cartap Bacillus thuringiensis a Resistance ratio LC50 of BC strain/lc50 of FS strain b Wu T K a C N Sun Unpublished data Liu et al (1982) Chen J S a C N Sun Unpublished data Bacillus thuringiensis Cheng Y P a C N Sun Unpublished data based on feeding a 36 h mortality RRª Organophosphorus a Carbamate Resistance Metabolic mechanism In view of the high resistance levels observed, the synergistic action of piperonyl butoxide (PB) a S,S,S-tributyl phosphorotrithioate (TBPT), inhibitors of microsomal oxidases a esterases associated with the metabolism of organophosphorus a carbamate insecticides, was rather insignificant (Table 3). This might imply that

3 Insecticide Resistance in DBM 361 Table 3. Synergism of several insecticides by S,S,S-tributyl phosphorotrithioate (TBPT) a piperonyl butoxide (PB) in a susceptible (FS) a a resistant (BC) strains of DBM Treatment FS BC LC50 (µg/ml) SRª LC50 (mg/ml) SR RR Dichlorvos TBPT PB Mevinphos TBPT PB Profenofos ,714 + TBPT PB Methomyl TBPT PB Carbofuran TBPT PB a Synergism ratio: LC50 unsynergized/lc50 synergized. b Resistance ratio. LC50 of BC strain/lc50 of FS strain. Larvae were sprayed with TBPT or PB at maximal sublethal concentrations one hour prior to insecticide treatment. For FS strain: TBPT µg/ml, PB 0.1 mg/ml; for BC strain: TBFT 0.25 mg/ml, PB 1.0 mg/ml. microsomal oxidation was only slightly involved in the resistance to methomyl a carbofuran. The slight antagonistic action of PB on profenofos might be due to the blockage by this synergist of its activation pathway. Another synergist, 0, O-disopropyl- S-benzylthiophosphate (IBP), which was reported to inhibit both carboxylesterases a glutathione-transferase in insects (Miyata et al 1981, Yeoh et al 1982), gave only a two fold increase of the toxicity of mevinphos a had practically no synergistic action on dichlorvos a profenofos (Table 4). These results prompted us to investigate whether reduced sensitivity of acetylcholinesterase to these insecticides might be a resistance factor. Table 4. Synergism of several insecticides by 0,O-diisopropyl-S-benzylthiophosphate (IBP) in a resistant (BC) strain of DBM Treatment LC50 (mg/ml) S Rª Dichlorvos IBP Mevinphos IBP Profenofos IBP a Synergism ratio: LC50 unsynergized/lc50 synergized. Reduced sensitivity of acetylcholinesterase We adopted the method of Main a Dauterman (1963) to determine the bimolecular rate constants for the inhibition by some insecticides of acetylcholinesterases of a susceptible (FS) a a resistant (BC) strains of DBM. Table 5 shows clearly that the acetylcholinesterase of the BC strain was ieed less sensitive to several organophosphorus a carbamate insecticides. Our subsequent studies revealed that this reduced

4 362 Sun, Wu, Chen, a Lee Table 5. Bimolecular rate constants for the inhibition by several insecticides of acetylcholinesterases of a susceptible (FS) a a resistant (BC) strains of DBM Insecticide Dichlorvos Mevinphos Malaoxon Methyl paraoxon Profenofos Methomyl Carbofuran Carbaryl a Resistance ratio. FS Ki BC FS/BC x x b Resistance ratio for malathion a methyl parathion, respectively RRª ,941 6, sensitivity was mainly due to a lower affinity of this enzyme for these insecticides. Profenofos, an 0-ethyl S-n-propyl phosphorothiolate, displays high levels of activity against both susceptible a resistant DBM (Table 2). Yet it is not a potent inhibitor of acetylcholineesterase in vitro (Table 5). Similar results were obtained in Spodoptera littoralis (Dittrich et al 1979). Recently, Kono et al (1983) suggested that profenophos, a other 0-ethyl S-n-propyl phosphorothiolate insecticides were activated oxidatively in the central nervous system of the insects to inhibit acetylcholinesterase. This is also in accordance with the slight antagonism of profenofos by PB as shown in Table 3. Nevertheless, this mechanism still could not account for the extremely high levels of resistance to methyl parathion a malathion. Carbofuran vs carbosulfan resistance Selection of the susceptible FS strain with carbofuran for seven generations resulted in about 170 fold resistance to the selection agent as well as approximately 50 fold resistance to the pro-insecticide carbosulfan (Table 6). Similar selection with carbosulfan resulted in 170 fold resistance to this pro-insecticide a about 1000 fold resistance to carbofuran. The reasons for this unique cross resistance between these two carbamates are being investigated. Meanwhile, attention is drawn to the cross resistance to organophosphorus compous a pyrethroids. Table 6. Toxicity of some insecticides to a susceptible (FS), a carbofuran-selected (CF), a a carbofusulfan-selected (CS) strains of DBMª FS CF CS Insecticide LC50 LC50 LC50 (mg/ml) (mg/ml) (mg/ml) RR Carbofuran Carbosulfan Mevinphos Prothiophos Permethrin Cypermethrin ªSource: Lee a Sun, unpublished data. Resistance ratio: LC50 of CF or CS strain/lc50 of FS strain. Carbofuran selection apparently made the DBM more resistant to both organophosphorus compous a pyrethroids than carbosulfan selection did; a DBM selected by carbofuran was more resistant to pyrethroids than to organophosphorus compous.

5 Insecticide Resistance in DBM 3 63 This could be due to overlapping of carbofuran resistance mechanisms a organophosphorus or pyrethroid resistance mechanisms. The overlapping of carbofuran a pyrethroid resistance was more extensive than that of resistance to carbofuran a organophosphorus compous. Microsomal oxidation could be the common mechanism for carbofuran a pyrethroid resistance. Reduced sensitivity of acetylcholinesterase, on the other ha, could be the common mechanism for carbofuran a organophosphorus resistance. Regression of susceptibility Pyrethroid Resistance Upon relaxation of the insecticide selection pressure, the mixed field (MD) strain still retained its resistance to the four major pyrethroids for about 10 generations (Table 7). By the 16th generation, its resistance to permethrin was reduced about nine fold a that to cypermethrin a fenvalerate about six fold, while resistance to deltamethrin remained practically the same. Pyrethroid resistance in DBM thus appears to be quite stable a lingers on for a period of time after the removal of selection pressure. Table 7. Changes of susceptibility to four pyrethroids of the mixed field strain of DBM upon relaxation of insecticide selection pressure LC50 (µg/ml) Generation Cypermethrin Deltamethrin Fenvalerate Permeth ri n ªModified from Chen a Sun (1986). Table 8 shows the regression of susceptibility of the same strain to three organophosphorus insecticides over the same period of time. Organophosphorus resistance in DBM seems to be less persistent than pyrethroid resistance. A greater reduction of resistance, 32 fold for mevinphos, 5 fold for profenofos a 16 fold for prothiophos, was observed. The susceptibility to mevinphos of this regressed field strain was comparable to that of the susceptible FS strain (Table 2). In view of the great differences in susceptibility to many insecticides between the FS strain a the local BC strain (Table 2) (Liu et al 1982a), this is a truly unique phenomenon. Advantage should be taken of the instability of mevinphos resistance, a its rapid regression to the truly susceptible state for the control of DBM.

6 3 64 Sun, Wu, Chen, a Lee Although the regression of susceptibility to carbamates might also have occurred upon the relaxation of selection pressure from the mixed field strain of DBM, data were not available due to the limitations of the bioassay method (Table 9). Table 8. Changes of susceptibility to three organophosphorus insecticides of the mixed field strain of DBM upon relaxation of insecticide selection pressurea Generation LC50 (µg/ml) Mevinphos Profenofos Prothiophos a Modified from Chen a Sun (1986) Table 9. Changes of susceptibility to three carbamates of the mixed field strain of DBM upon relaxation of insecticide selection pressurea Generation LC50 (mg/ml) Carbofuran Carbaryl Methomyl ªSource Chen a Sun 1986 Not determinable No mortality was recorded at 100 mg/ml C Estimated values Non-metabolic mechanism DDT a pyrethroid resistance High levels of resistance in DBM to the four major synthetic pyrethroids were fou only three to four years after the introduction of these pyrethroids to Taiwan (Liu et al 1981). A similarly high level of DDT resistance also existed in the field (Liu et al 1982a). We suspected that this rapid onset of pyrethroid resistance was due to widespread application of DDT on vegetables during the 1950s a 1960s before DDT was banned. The absence of synergism of DDT by PB a 1,1-di- (4-chlorophenyl) ethanol, inhibitors of microsomal oxidases a DDT-dehydrochlorinase, which are involved in the degradation of DDT, raised the possibility of the existence

7 Insecticide Resistance in DBM 365 of a non-metabolic mechanism of DDT resistance in this insect (Table 10). This mechanism might also play an important role in DBM resistance to synthetic pyrethroids a may be similar to a previously observed non-metabolic mechanism for DDTpyrethroid resistance in houseflies a mosquitoes (Liu et al 1982b). Table 10. Synergism of DDT by piperonyl butoxide (PB) a 1,1-di-(Cchlorophenyl) ethanol (DMC) in a susceptible (FS) a a resistant (BC) strains of DBMª Treatment DDT DDT + PBC DDT + DMC LC50 (mg/ml) FS ªSource Liu et al 1982b b Estimated by graphic method 0 1 mg/ml of PB or DMC one hour before DDT treatment C Larvae were sprayed with Pyrethroid resistance insuppressible by metabolic inhibitors Recent synergist studies revealed that esterase hydrolysis contributed only to a moderate extent to permethrin resistance in DBM (Figure 1). With PB to suppress the oxidative degradation, the residual resistance to fenvalerate, deltamethrin, a cypermethrin was still substantial (Table 11). This may be taken as further evidence, though iirect, for the possible existence of a non-metabolic mechanism for pyrethroid resistance in this insect pest. Cypermethrin Figure 1. Synergism of four pyrethroids by triphenyl phosphate (TPP) a S,S,S-tributyl phosphorotrithioate (TBPT) in a susceptible (FS) a a resistant (BC) strains of DBM Synergism ratio Metabolic mechanism Cross-resistance to permethrin a cypermethrin (67 fold a 97 fold respectively) of carbofuran-selected DBM (Table 6) suggests that the high levels of pyrethroid resistance detected in the field shortly after their introduction to Taiwan could have arisen, in part, from previous uses of carbamate insecticides for the control of DBM a other insect pests on cruciferous vegetables. Repeated synergist studies iicate that only permethrin could be synergized consistently a effectively in the resistant strain by the esterase inhibitors triphenyl phosphate (TPP) a TBPT (Figure 1) (Liu et al 1974, 1981). Meanwhile, PB has been fou to synergize all four major pyrethroids, though to different degrees (Figure 2)

8 3 66 Sun, Wu, Chen, a Lee Table 11. Synergism of fenvalerate, deltamethrin, a cypermethrin by piperonyl butoxide (PB) in a susceptible (FS) a a resistant (BC) strains of DBMª FS Treatment LC50 LC50 (µg/ml) (mg/ml) Fenvalerate PBC Deltamethrin PB Cypermethrin PB ªSource: Liu et al correspoing treatment. insecticide treatment b Resistance ratio= LC50 of BC strain/lc50 of FS strain for each Larvae were sprayed with 0.1 mg/ml PB one hour before BC R Figure 2. Synergism of four pyrethroids by piperonyl butoxide (PB) in a susceptible (FS) a a resistant (BC) strains of DBM (Liu et al 1981, 1984). Fenvalerate, of the four pyrethroids tested, was most drastically synergized by Butacide, a tank-mix formulation of PB, mixed a applied simultaneously with these pyrethroids at varying ratios (Figure 3). These studies all imply that oxidative degradation is the most important metabolic mechanism in the pyrethroid resistance in this insect. Use of PB to overcome pyrethroid resistance The use of synergists which interfere with the detoxication of insecticides to cope with a resistance problem is expected to be more effective where one dominant metabolic mechanism exists for resistance to a number of insecticides. The optimal synergist may

9 Insecticide Resistance in DBM I 5 0 Butacidel Fenvolerote Butacidel Deltamethrin Butacidelcypermethrin Butacidel Permethrin Figure 3. Synergism of four pyrethroids by Butacide in a susceptible (FS) a a resistant (BC) strains of DBM vary with the insect species a the insecticides. A possible consequence of large scale application of synergists in the field for insect control may be the emergence a subsequent intensification of certain known or even unknown resistance mechanisms. In the case of DBM, the use of PB might eventually accelerate a intensify the suspected insensitive nerve resistance mechanism a the insensitive acetylcholinesterase resistance mechanism for pyrethroids a organophosphorus/carbamate insecticides (Liu et al 1984). In practice, farmers may choose to use the synergists iiscriminately with all their insecticides. The long term consequences of applying synergists at rates up to 10 times the dose of the insecticides should be carefully assessed. Selection with fenvalerate a fenvalerate/pb Selection of the regressed field strain (MD strain at the 10th generation) with fenvalerate increased the LC50 from 1.24 mg/ml to more than 100 mg/ml in four generations, reering this insecticide practically useless (Table 12). The persistence of resistance after removal of selection pressure mentioned earlier a the subsequent rapid recurrence of resistance constitute the major obstacles in the use of pyrethroids for DBM control. Table 12. Changes of susceptibility to fenvalerate of regressed field strain (at 10th generation) upon selection with fenvalerateª Generation LC50 (mg/ml) Slope > 100 a Modified from Chen a Sun (1986) b Not determinable The first eight generations of selection of the mixed field strain with fenvalerate/pb (PB at 1 mg/ml) caused only about three fold increase of LC50 (Table 13). After this

10 368 Sun, Wu, Chen, a Lee Table 13. Changes of susceptibility to fenvalerate + piperonyl butoxide (PB) of the mixed field mixed field strain of DBM upon selection with fenvalerate/pb Generation Fenvalerate + 1 mg/ml PB Fenvalerate + 5 mg/ml PB LC50 (µg/ml) Slope LC50 (mg/ml) Slope > > '100 >100 ªModified from Chen a Sun (1986). A concentration of 1 mg/ml of PB was used for selection until the 9th generation. Afterwards, 5 mg/ml of PB was used. Not determinable. stage of apparent adjustment, resistance to fenvalerate/pb started to increase rapidly a the LC50 jumped to more than 100 mg/ml in the 10th generation. Starting with the 10th generation, the concentration of PB used in selection was increased to 5 mg/ml. Within two generations of selection uer these coitions, the LC50 again ran over 100 mg/ml. The synergist could no longer suppress the pyrethroid resistance in DBM. Cross resistance patterns of fenvalerate a fenvalerate/pb-selected strains Selection of the mixed field strain with fenvalerate a fenvalerate/pb resulted in these two strains developing cross resistance to cypermethrin, deltamethrin, a permethrin (Table 14). At a concentration of 100 mg/ml, no mortality was observed for any one of these pyrethroids. However, compared to the original mixed field strain, the selected strains were generally more susceptible to the three organophosphorus insecticides tested, mevinphos, profenofos, a prothiophos. This suggests that there is probably no common mechanism between pyrethroid a organophosphorus resistance in DBM. It also offers the possibility of alternating organophosphorus insecticides with pyrethroids in the field for the control of this insect pest. Again, due to the limitation of the bioassay technique, it is not clear if carbamate susceptibility in these two strains was affected. Synergism by several compous in the fenvalerate a fenvalerate/pb-selected strains Synergists which block the esterases, such as IBP, TBPT, a TPP, enhanced the toxicity only of permethrin to any noticeable extent (Table 15). However PB, which inhibits microsomal oxidase, produced significant synergism of both fenvalerate a permethrin in the regressed field (MD) strain a the fenvalerate selected (FP) strain but not in the fenvalerate/pb selected strain. However another microsomal oxidase inhibitor, MGK 264, was less synergistic than PB in the regressed MD a fenvalerate selected FP strains. lt definitely exhibited more synergism in the fenvalerate/pb selected FP/PB strain. A subsequent experiment designed to test the toxicity of PB to the four strains of DBM revealed that the strain selected with fenvalerate/pb was much less susceptible to this synergist (Table 16). Therefore, it seems that DBM selected with pyrethroid/pb has evolved a certain ki of tolerance to this synergist per se which would reer it ineffective as a synergist. Like other methylenedioxyphenyl synergists, PB is an inhibitor as well as a substrate of microsomal oxidases (Casida 1970). Rapid excretion,

11 Insecticide Resistance in DBM 3 69 Table 14. Cross resistance patterns of a mixed field (PA), a regressed field (MD), a fenvalerate selected (FP) a a fenvalerate/pb selected (F/PB) strains of DBMª PA MD FP F/PB Insecticide LC50 LC50 RR LC50 RR LC50 RR (mg/ml) (mg/ml) Cypermethrin > 100 > Deltamethrin >100 > 98 > 100 > 98 Fenvalerate >100 > 19 > 100 > 19 Permethrin >100 >156 > 100 > 156 Mevinphos Profenofos Prothiophos Carbaryl Carbofuran 100 Methomyl ªSource Chen a Sun (1986) Resistance ratio= LC50 of MD, FP or F/PB strain/lcso of PA strain Not determinable No mortality was recorded at 100 mg/ml Table 15. Synergism of fenvalerate a permethrin by several compous in a regressed field (MD), a fenvalerate selected (FP) a a fenvalerate/pb selected (FIPB) strains of DBMª Treatment LC50 MD FP F/PB LC50 LC50 SR SR (µg/ml) (mg/ml) (mg/ml) Fenvalerate > 100 > IBP > 100 n > 100 nc + TBPT > 100 nc > 100 nc + TPP > 100 nc > 100 nc + PB >18.0 > 100 nc + MGK > >11.0 Permethrin IBP TBPT TPP PB MGK ªSource: Chen a Sun (1986). b Synergism ratio: LC50 unsynergized/lc50 synergized. C Larvae were sprayed with the synergist at maximal sublethal concentration one hour prior to insecticide treatment. The concentrations used are given in Table 17. Not calculable. SR Table 16. Toxicity of PB against a mixed field (PA), a regressed field (MD), a fenvalerate-selected (FP) a a fenvalerate IPB-selected (FIPB) strains of DBMª PA 4.52 MD FP F/PB >100 > 22 ªSource Chen a Sun (1986) b Resistance ratio= LC50 of each strain/lc50 of PA strain

12 370 Sun, Wu, Chen, a Lee storage in certain tissues, a conjugation preceded by oxidation are possible causes for the tolerance to PB in this insect (Casida 1970, Yang 1976). In addition, there is preliminary evidence iicating that DBM would gradually recover its susceptibility to the synergistic action of PB not long after the termination of its application. Table 17. Concentrations of synergists used in bioassays for regressed field strain (MD), fenvalerate/pb selected strain(f/pb) a fenvalerate selected strain (FP) Synergist Concentration (mg/ml) MD F/PB FP PB TBPT TPP IBP MGK Recommeations of the Use of Synergists, Pyrethroids, a Organophosphorus Insecticides Our discussions may be summarized as follows: 1. Pyrethroid resistance would not decline rapidly after the application of this group of insecticides is terminated. 2. Organophosphorus resistance seems unstable a may be reduced quite rapidly a significantly once the selection pressure is removed. 3. Pyrethroid selected DBM does not seem to have cross resistance to some organophosphorus insecticides. 4. Synergist PB would temporarily arrest pyrethroid resistance. 5. DBM, to which PB has lost synergistic action on pyrethroids, may still respo to other synergists which block microsomal oxidases, such as MGK DBM seems gradually to regain its susceptibility to PB after the use of this synergist is terminated. 7. DBM resistance to conventional insecticides has little or no cross resistance to B. thuringiensis a to some chitin synthesis inhibitors such as IKI 7899 (Sun 1983, unpublished data). In view of these fiings, we make the following recommeations: 1. Use pyrethroids only when organophosphorus insecticides are no longer effective. 2. When pyrethroid resistance starts to show, shift to organophosphorus insecticides such as mevinphos, profenofos or prothiophos. 3. When these organophosphorus compous begin to lose effectiveness, switch back to pyrethroids. Use piperonyl butoxide if necessary. 4. When piperonyl butoxide becomes ineffective, try to use organophosphorus compous again. 5. Use pyrethroids to replace the organophosphorus compous. Use other synergists such as MGK 264, if needed. 6. Try to use B. thuringiensis a some chitin synthesis inhibition such as IKI-7899, CME 134, or PH70-23, between the applications of organophosphorus compous a

13 Insecticide Resistance in DBM 37 1 pyrethroids. Cartap, which has a mode of action different from that of phosphorus compous or pyrethroids, may also be used. We realize that these recommeations are based on only limited data, a thus we have reservations regarding their general applicability. The fiings discussed above may also be used as the rationale in devising mixtures of insecticides for the control of DBM. Literature Cited Casida, J. E Mixed-function oxidase involvement in the biochemistry of insecticide synergists. J. Agric. Food. Chem. 18: Chen, J. S. a C. N. Sun Resistance of diamoback moth (Lepidoptera: Yponomeutidae) to a combination of fenvalerate a piperonyl butoxide. J. Econ. Entomol. 79: Dittrich, V., N. Luetkemeier, a G. Voss Monocrotophos a profenofos: two organophosphates with a different mechanism of action in resistant races of Spodoptera littoralis J. Econ. Entomol. 72: Kono, Y., Y. Sato, a Y. Okada Activation of an 0-ethyl-S-n-propyl phosphorothioate, TIA-230, in the central nerve of Spodoptera larvae. Pestic. Biochem. Physiol. 20: Liu, M. Y., Y. J. Tzeng, a C. N. Sun Diamoback moth resistance to several synthetic pyrethroids. J. Econ. Entomol. 74: Liu, M. Y., Y. J. Tzeng, a C. N. Sun. 1982a. Insecticide resistance in the diamoback moth. J. Econ. Entomol. 75: Liu, M. Y., C. N. Sun, a S. W. Huang. 1982b. Absence of synergism of DDT by piperonyl butoxide a DMC of the diamoback moth (Lepidoptera: Yponomeutidae). J. Econ. Entomol. 75: Liu, M. Y., J. S. Chen, a C. N. Sun Synergism of pyrethroids by several compous in larvae of the diamoback moth (Lepidoptera: Plutellidae). J. Econ. Entomol. 77: Main, A. R. a W. C. Dauterman Determination of the bimolecular rate constant for the reaction between organophosphorus inhibition a esterases in the presence of substrate. Nature 198: Miyata, T., H. Sakai, T. Saito, K. Yoshioka, K. Ozaki, Y. Sasaki, a A. Tsuboi Mechanism of joint toxic action of Kitazin P with malathion in the malathion resistant green rice leafhopper, Nephotettix cincticeps Uhler (Hemiptera: Deltocephalidae). Appl. Entomol. Zool. 16: PDAF Plant Protection Manual. Provincial Department of Agriculture a Forestry, Chung Hsing Village, Nantou, Taiwan, ROC. 354 pp. Yang, R. S. H Enzymatic conjugation a insecticide metabolism. pp In C. F. Wilkinson (ed) Insecticide Biochemistry a Physiology. Plenum Press, New York. Yeoh, C. L., E. Kuwano, a M. Eto Effects of the fungicide IBP as a synergist on the metabolism of malathion in insects. J. Pestic. Sci. 7:31-40.