34 Insecticide Resistance in Diamondback Moth C. N. Sun, T. K. Wu, J. S. Chen, and 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, and DDT, have been detected in the diamondback moth in Taiwan. Synergist studies have provided insufficient evidence to show significant involvement of known metabolic systems, such as microsomal oxidation, esterase hydrolysis, and glutathione conjugation in organophosphorus and carbamate resistance. Meanwhile, moderate levels of reduction of acetyl cholinesterase sensitivity to these compounds have been observed. This, however, could not account for all the resistance detected. In addition, the relationship between carbofuran and carbosulfan resistance is discussed. While pyrethroid resistance is closely associated with microsomal oxidation, indirect evidence indicates 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 compound by the insect. Diamondback moth larvae selected with fenvalerate or fenvalerate/piperonyl butoxide seem to be more susceptible to some organophos- phorus insecticides including mevinphos, profenofos, and prothiophos. Recommendations are suggested regarding the use of synergists, pyrethroids, and organophosphorus insecticides.

Introduction

In 1984, 30 insecticides including 17 organophosphorus compounds, 2 carbamates, 6 pyrethroids, 2 mixtures of organophosphorus compounds and pyrethroid, 2 organonit- rogen compounds and Bacillus thuringiensis were officially recommended for the control of diamondback moth (DBM), Plutella xylostella (L) (Lepidoptera: Yponomeutidae), in Taiwan (Table 1). Different levels of resistance to these insecticides have been detected

Table 1. Insecticides recommended for DBM control in Taiwan in 1984ª

1 . Organophosphorus compounds (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 Fenpropathrin Flucythrinate

Dimethoate + Phenthoate (1:2) Chlorpyrifos + Cypermethrin (9:1)

Cartap Thiocyclam Bacillus thuringiensis

4. Mixtures (2)

5. Others (3)

ªSource. PDAF 1984.

Fenvalerate Permethrin

3 60 Sun, Wu, Chen, and Lee

in a multiple-resistant (BC) strain from the field (Table 2). Despite the resistance, some insecticides, including dichlorvos, mevinphos, profenofos, permethrin, cypermethrin, deltamethrin, and fenvalerate still possess much higher potency than the rest against this highly resistant strain. Cartap and 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 compounds, with special reference to the pyrethroids, will be discussed.

Table 2. Toxicity of some insecticides to susceptible (FS) and multiresistant (BC) strains of DBM

RRª FS BC

LC50 (µg/ml) LC50 (mg/ml) Insecticide

Organophosphorus Cyanofenphos 1.96 100.00 > 50,000 Diazinon 29.30 12.10 413 Dichlorvos 34.00 3.20 94 Malathion 34.00 > 100.00 > 2,941 Methyl parathion 9.40 > 100.00 > 10,638 Mevinphos 16.00 2.40 150 Profenofos 0.14 0.94 6,714 Prothiophos 0.41 2.40 5,854

Carbamatesb Carbaryl 820.00 > 100.00 >122 Carbofuran 270.00 8.90 33 Methomyl 420.00 46.50 111

Pyrethroids Cypermethrin 1.29 0.67 51 9 Deltamethrin 0.40 0.74 1,850 Fenpropathrin 9.07 13.30 1,466 Fenvalerate 1.13 2.22 1,965 Flucythrinate 2.14 23.60 11,028 Fluvalinate 44.70 104.00 2,327 Permethrin 2.58 0.40 155 Phenothrin 24.00 14.30 596 Tetramethrin 61.9 17.40 281 Tralomethrin 1.64 11.10 6,768

DDT 84.60 > 100.00 >1,182 Organochlorines

Others Cartap 25.00 4.97 199 Bacillus thuringiensis 1.859.00 3.58 2

a Resistance ratio LC50 of BC strain/LC50 of FS strain b Wu T K and C N Sun Unpublished data L i u et al (1982) C h e n J S and C N Sun Unpublished data Bacillus thuringiensis Cheng Y P and C N Sun Unpublished data based on feeding and 36 h mortality

Organophosphorus and Carbamate Resistance

Metabolic mechanism

In view of the high resistance levels observed, the synergistic action of piperonyl butoxide (PB) and S,S,S-tributyl phosphorotrithioate (TBPT), inhibitors of microsomal oxidases and esterases associated with the metabolism of organophosphorus and carbamate insecticides, was rather insignificant (Table 3). This might imply that

Insecticide Resistance in DBM 361

Table 3. Synergism of several insecticides by S,S,S-tributyl phosphorotrithioate (TBPT) and piperonyl butoxide (PB) in a susceptible (FS) and a resistant (BC) strains of DBM

FS BC LC50 (µg/ml) SRª LC50 (mg/ml) SR RR

Treatment

Dichlorvos 34 4.50 132 + TBPT 30 1.1 5.69 0.8 + PB 27 1.3 4.53 1 .0

+ TBPT 12 1.3 2.19 1 .0 + PB 17 0.9 2.03 1.1

+ TBPT - - 1 .00 0.7 + PB - - 1.97 0.4

+ TBPT 470 0.9 45.30 1 .0 + PB 140 3.0 13.30 3.5

+ TBPT 250 1.1 9.62 1.3 + PB 170 1.6 4.76 2.6

Mevinphos 16 2.24 140

Profenofos 14 0.68 6,714

Methomyl 420 46.70 111

Carbofuran 270 12.30 46

a Synergism ratio: LC50 unsynergized/LC50 synergized. b Resistance ratio. LC50 of BC strain/LC50 of FS strain. La rvae 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 and 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 and glutathione-transferase in insects (Miyata et al 1981, Yeoh et al 1982), gave only a two fold increase of the toxicity of mevinphos and had practically no synergistic action on dichlorvos and 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 5.17

Mevinphos 1.75

Profenofos 0.46

a Synergism ratio: LC50 unsynergized/LC50 synergized.

+ IBP 4.1 1 1.3

+ IBP 0.85 2.0

+ IBP 0.42 1.1

Reduced sensitivity of acetylcholinesterase

We adopted the method of Main and Dauterman (1963) to determine the bimolecular rate constants for the inhibition by some insecticides of acetylcholinesterases of a susceptible (FS) and a resistant (BC) strains of DBM. Table 5 shows clearly that the acetylcholinesterase of the BC strain was indeed less sensitive to several organophos- phorus and carbamate insecticides. Our subsequent studies revealed that this reduced

362 Sun, Wu, Chen, and Lee

Table 5. Bimolecular rate constants for the inhibition by several insecticides of acetylcholinesterases of a susceptible (FS) and a resistant (BC) strains of DBM

Ki FS BC

FS/BC RRª Insecticide

Dichlorvos Mevinphos Malaoxon Methyl paraoxon Profenofos Methomyl Carbofuran Carbaryl

1 0.098 x 1

1

3.40 x 1

1 04

1 1

38 41 53 24 0.14 36 22 11

94 150

2,941

6,714 111 33

122

a Resistance ratio. b Resistance ratio for malathion and methyl parathion, respectively

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 and 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, and 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 and 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 and 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 compounds and pyrethroids.

Table 6. Toxicity of some insecticides to a susceptible (FS), a carbofuran-selected (CF), and a carbofusulfan-selected (CS) strains of DBMª

FS CF CS

RR

Carbofuran 0.1 1 18.4 167 106 964 Carbosulfan 0.12 5.55 46 20.5 171

LC50 Insecticide LC50 LC50 (mg/ml) (mg/ml) (mg/ml)

Mevinphos 0.093 1.56 17 0.95 10 Prothiophos 0.084 1.70 20 0.84 10 Permethrin 0.001 3 0.087 67 0.04 31 Cypermethrin 0.0034 0.33 97 0.13 38

ªSource: Lee and 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 organophos- phorus compounds and pyrethroids than carbosulfan selection did; and DBM selected by carbofuran was more resistant to pyrethroids than to organophosphorus compounds.

Insecticide Resistance in DBM 3 63

This could be due to overlapping of carbofuran resistance mechanisms and organophos- phorus or pyrethroid resistance mechanisms. The overlapping of carbofuran and pyrethroid resistance was more extensive than that of resistance to carbofuran and organophosphorus compounds. Microsomal oxidation could be the common mechanism for carbofuran and pyrethroid resistance. Reduced sensitivity of acetylcholinesterase, on the other hand, could be the common mechanism for carbofuran and organophosphorus resistance.

Pyrethroid Resistance

Regression of susceptibility

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 and that to cypermethrin and fenvalerate about six fold, while resistance to deltamethrin remained practically the same. Pyrethroid resistance in DBM thus appears to be quite stable and 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

Cypermethrin Deltamethrin Fenvalerate 00 677 1017 5356 639 01 41 1 1091 3060 204 02 584 1719 41 35 480 03 626 1428 2959 41 3 04 31 6 941 341 8 273 05 47 1 949 2900 553

07 536 1100 3006 404 08 740 1097 1999 21 5 09 850 783 21 53 307 10 498 - 848 1537 300 1 1 42 1 1587 1059 290 12 253 976 578 145 13 306 1056 816 67.7 14 21 4 1064 674 84.9 15 154 1338 973 49.5 16 107 922 830 70.2

LC50 (µg/ml) Generation

Permeth ri n -

06 427 1417 3797 484

ªModified from Chen and 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 and 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 and 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, and its rapid regression to the truly susceptible state for the control of DBM.

3 64 Sun, Wu, Chen, and 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

LC50 (µg/ml) Mevinphos Profenofos Prothiophos

Generation

00 254.0 2760 7910 01 22.7 21 74 - 02 12.9 2254 471 9 03 10.9 1500 2003 04 14.8 1779 1049 05 10.7 1436 1067 06 12.3 1145 1297 07 9.0 1460 923 08 5.2 1113 988 09 8.7 1047 742 10 7.4 752 1152 1 1 6.3 646 534 12 7.2 872 91 4 13 3.6 618 81 3 14 7.3 51 4 529 15 9.3 598 603 16 8.0 507 493

a Modified from Chen and Sun (1986)

Table 9. Changes of susceptibility to three carbamates of the mixed field strain of DBM upon relaxation of insecticide selection pressurea

LC50 (mg/ml) Carbofuran Carbaryl Methomyl

Generation

0 n d nd nd 1-10 nd nd nd

1 1 294 nd nd 12 116 nd nd 13 117 nd nd 14 204 nd nd 15 92 nd nd 16 1 nd nd

ªSource Chen and Sun 1986 Not determinable No mortality was recorded at 100 mg/ml C Estimated values

Non-metabolic mechanism

DDT and pyrethroid resistance High levels of resistance in DBM to the four major synthetic pyrethroids were found 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 and 1960s before DDT was banned. The absence of synergism of DDT by PB and 1,1-di- (4-chlorophenyl) ethanol, inhibitors of microsomal oxidases and DDT-dehydrochlorinase, which are involved in the degradation of DDT, raised the possibility of the existence

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 and may be similar to a previously observed non-metabolic mechanism for DDT- pyrethroid resistance in houseflies and mosquitoes (Liu et al 1982b).

Table 10. Synergism of DDT by piperonyl butoxide (PB) and 1,1-di-(Cchlorophenyl) ethanol (DMC) in a susceptible (FS) and a resistant (BC) strains of DBMª

LC50 (mg/ml) Treatment FS DDT DDT + PBC DDT + DMC

0.140 0.075 0.190

28 25 35

ªSource Liu et al 1982b 0 1 mg/ml of PB or DMC one hour before DDT treatment

b Estimated by graphic method 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, and cypermethrin was still substantial (Table 11). This may be taken as further evidence, though indirect, 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) and S,S,S-tributyl phosphorotrithioate (TBPT) in a susceptible (FS) and a resistant (BC) strains of DBM

Synergism ratio

Metabolic mechanism

Cross-resistance to permethrin and cypermethrin (67 fold and 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 and other insect pests on cruciferous vegetables.

Repeated synergist studies indicate that only permethrin could be synergized consistently and effectively in the resistant strain by the esterase inhibitors triphenyl phosphate (TPP) and TBPT (Figure 1) (Liu et al 1974, 1981). Meanwhile, PB has been found to synergize all four major pyrethroids, though to different degrees (Figure 2)

3 66 Sun , Wu, Chen, and Lee

Table 11. Synergism of fenvalerate, deltamethrin, and cypermethrin by piperonyl butoxide (PB) in a susceptible (FS) and a resistant (BC) strains of DBMª

FS BC

R

Fenvalerate 0.94 3.39 3606 + PBC 0.53 0.22 415

Deltamethrin 0.26 0.50 1923 + PB 0.06 0.04 667

Cypermethrin 1.14 0.27 239 + PB 0.79 0.10 127

Treatment LC50 LC50 (µg/ml) (mg/ml)

ªSource: Liu et al 1984. corresponding treatment. insecticide treatment

b Resistance ratio= LC50 of BC strain/LC50 of FS strain for each L a r v a e were sprayed with 0.1 mg/ml PB one hour before

Figure 2. Synergism of four pyrethroids by piperonyl butoxide (PB) in a susceptible (FS) and 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 and 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

Insecticide Resistance in DBM 367

0 I 5 0 Butacidel Fenvolerote Butacidel Deltamethrin Butacidelcypermethrin Butacidel Permethrin

Figure 3. Synergism of four pyrethroids by Butacide in a susceptible (FS) and a resistant (BC) strains of DBM

vary with the insect species and the insecticides. A possible consequence of large scale application of synergists in the field for insect control may be the emergence and subsequent intensification of certain known or even unknown resistance mechanisms. In the case of DBM, the use of PB might eventually accelerate and intensify the suspected insensitive nerve resistance mechanism and the insensitive acetylcholinesterase resistance mechanism for pyrethroids and organophosphorus/carbamate insecticides (Liu et al 1984). In practice, farmers may choose to use the synergists indiscriminately 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 and 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, rendering this insecticide practically useless (Table 12). The persistence of resistance after removal of selection pressure mentioned earlier and 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 0 1.24 2.29 1 3.69 1.98 2 8.96 2 31 3 28.20 1.37 4 100 nd 5 > 100 nd

a Modified from Chen and 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

368 Sun, Wu, Chen, and 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

Fenvalerate + 1 mg/ml PB

00 0.52 2.23 01 0.44 2.36 02 0.49 3.80 03 0.63 3.90 04 0.93 5.10 05 1.06 3.51 06 1.48 4.02 07 1.76 2.33 08 4.14 1.51 09 17.80 0.80 4.48 2.73 10 >100 n d 6.41 1.48 1 1 > 100 nd 48.90 1.13 12 '100 nd >100 nd

Fenvalerate + 5 mg/ml PB Generation

LC50 (µg/ml) Slope LC50 (mg/ml) Slope - - - - - - - - - - - -

- - - - - -

ªModified from Chen and Sun (1986). the 9th generation. Afterwards, 5 mg/ml of PB was used.

A concentration of 1 mg/ml of PB was used for selection until Not determinable.

stage of apparent adjustment, resistance to fenvalerate/PB started to increase rapidly and 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 under these conditions, the LC50 again ran over 100 mg/ml. The synergist could no longer suppress the pyrethroid resistance in DBM.

Cross resistance patterns of fenvalerate and fenvalerate/PB-selected strains Selection of the mixed field strain with fenvalerate and fenvalerate/PB resulted in these two strains developing cross resistance to cypermethrin, deltamethrin, and 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, and prothiophos. This suggests that there is probably no common mechanism between pyrethroid and 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 compounds in the fenvalerate and fenvalerate/PB-selected strains Synergists which block the esterases, such as IBP, TBPT, and 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 and permethrin in the regressed field (MD) strain and 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 and 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 kind of tolerance to this synergist per se which would render 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,

Insecticide Resistance in DBM 3 69

Table 14. Cross resistance patterns of a mixed field (PA), a regressed field (MD), a fenvalerate selected (FP) and a fenvalerate/PB selected (F/PB) strains of DBMª

PA MD FP F/PB

RR LC50 RR LC50 RR Insecticide LC50 LC50

Cypermethrin Deltamethrin Fenvalerate Permethrin Mevinphos Profenofos Prothiophos Carbaryl Carbofuran

1.02 5.36 0.64 0.25 2.76 7.91 n d nd

0.922 0.90 0.830 0.16 0.070 0.1 1 0.008 0.03 0.507 0.18 0.493 0.06

nd 100

(mg/ml) > 100 >100 >100 >100 0.12 0.95 1.80 nd nd

(mg/ml) >147 100 147 > 98 > 100 > 98 > 19 > 100 > 19 >156 > 100 > 156 0.48 0.34 1.36 0.34 0.91 0.33 0.23 2.54 0.32

nd nd

Methomyl nd nd nd nd

Not determinable No mortality was recorded at 100 mg/ml ªSource Chen and Sun (1986) Resistance ratio= LC50 of MD, FP or F/PB strain/LCso of PA strain

Table 15. Synergism of fenvalerate and permethrin by several compounds in a regressed field (MD), a fenvalerate selected (FP) and a fenvalerate/PB selected (FIPB) strains of DBMª

MD FP F/PB

SR LC50 SR LC50 S R

Treatment LC50 (µg/ml) (mg/ml) (mg/ml)

Fenvalerate 1294.0 > 100 > 100 + I B P 794.0 1.63 > 100 n > 100 nc + TBPT 914.0 1.42 > 100 nc > 100 nc + TPP 1470.0 0.88 > 100 nc > 100 nc + PB 108.0 11.98 5.43 >18.0 > 100 nc + MGK 264 164.0 7.89 12.20 >8.2 9.21 >11.0

+ IBP 111.0 1.82 77.20 1.2 75.10 1.1 + TBPT 93.8 2.15 40.40 2.3 24.10 3.5 + TPP 134.8 1.50 21.80 4.3 37.70 2.2 + PB 15.9 12.70 6.63 14.2 58.60 1.4 + MGK 264 32.6 6.20 9.65 9.8 7.30 11.5

C Larvae were sprayed with the synergist at maximal sublethal concentration one hour prior to insecticide treatment.

Permethrin 202.0 94.40 83.90

ªSource: Chen and Sun (1986).

The concentrations used are given in Table 17.

b Synergism ratio: LC50 unsynergized/LC50 synergized.

Not calculable.

Table 16. Toxicity of PB against a mixed field (PA), a regressed field (MD), a fenvalerate-selected (FP) and a fenvalerate IPB-selected (FIPB) strains of

DBMª

PA 4.52 MD 4.50 1 .0 FP 11.7 2.6 F/PB >100 > 22

ªSource Chen and Sun (1986) PA strain

b Resistance ratio= LC50 of each strain/LC50 of

370 S u n , Wu, Chen, and Lee

storage in certain tissues, and 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 indicating that DBM would gradually recover its susceptibility to the synergistic action of P B 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) and fenvalerate

selected strain (FP)

Concentration (mg/ml) MD F/PB FP

Synergist

PB 2.5 10.0 2.5 TBPT 1 .0 2.5 2.5 TPP 10.0 10.0 10.0 IBP 5.0 10.0 10.0 MGK 264 5.0 5.0 5.0

Recommendations of the Use of Synergists, Pyrethroids, and Organophosphorus Insecticides

Our discussions may be summarized as follows:

1 . Pyrethroid resistance would not decline rapidly after the application of this

2. Organophosphorus resistance seems unstable and may be reduced quite rapidly

3. Pyrethroid selected DBM does not seem to have cross resistance to some

4. Synergist PB would temporarily arrest pyrethroid resistance. 5. DBM, to which PB has lost synergistic action on pyrethroids, may still respond

to other synergists which block microsomal oxidases, such as MGK 264. 6. 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 and to some chitin synthesis inhibitors such as IKI 7899 (Sun 1983, unpublished data).

group of insecticides is terminated.

and significantly once the selection pressure is removed.

organophosphorus insecticides.

In view of these findings, we make the following recommendations:

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 compounds begin to lose effectiveness, switch

back to pyrethroids. Use piperonyl butoxide if necessary. 4. When piperonyl butoxide becomes ineffective, try to use organophosphorus

compounds again. 5. Use pyrethroids to replace the organophosphorus compounds. Use other

synergists such as MGK 264, if needed. 6. Try to use B. thuringiensis and some chitin synthesis inhibition such as IKI-7899,

CME 134, or PH70-23, between the applications of organophosphorus compounds and

Insecticide Resistance in DBM 37 1

pyrethroids. Cartap, which has a mode of action different from that of phosphorus compounds or pyrethroids, may also be used.

We realize that these recommendations are based on only limited data, and thus we have reservations regarding their general applicability. The findings discussed above may also be used as the rationale in devising mixtures of insecticides for the control of DBM.

Literature Cited

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Chen, J. S. and C. N. Sun. 1986. Resistance of diamondback moth (Lepidoptera: Yponomeutidae) to a combination of fenvalerate and piperonyl butoxide. J . Econ. Entomol. 79:22-30.

Dittrich, V. , N. Luetkemeier, and G. Voss. 1979. Monocrotophos and profenofos: two organophosphates with a different mechanism of action in resistant races of Spodoptera littoralis J . Econ. Entomol. 72:380-384.

Kono, Y., Y. Sato, and Y. Okada. 1983. Activation of an 0-ethyl-S-n-propyl phosphorothioate, TIA-230, in the central nerve of Spodoptera larvae. Pestic. Biochem. Physiol. 20:225-231.

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