The cross-resistance spectrum of Sitophilus granarius (L.) (Coleoptera: Curculionidae) heterozygous for pyrethrin resistance

THE CROSS-RESISTANCE SPECTRUM SITOPHILUS GRANARIUS (L.) (COLEOPTERA: CURCULIONIDAE) HETEROZYGOUS FOR PYRETHRIN RESISTANCE

OF

A. J. PRICKETT Ministry of Agriculture. Fisheries and Food, Slough Laboratory. London Road, Slough. Berks. England Abstract-Dose-response data were obtained from the F, progeny of crosses between virgin Siropkilus yrarlarius (L.) from pyrethrin-resistant and susceptible strains. These data were compared with data from the parental strains. The insects were tested with pyrethrins, bioresmethrin, malathion, pirimiphos-methyl, lindane. DDT and propoxur. The exposure method was confinement of the insects on filter papers impregnated with insecticide in oil. The pyrethrin-resistant strain showed resistance to all seven insecticides. The probit lines of the heterozygotes fell approximately half-way between those of the resistant and susceptible strains for all the compounds tested. R-strain males responded to lindane more slowly than did the females, whereas there was no significant difference in the rate of response of the sexes of the S-strain.

INTRODUCTION A DISCRIMINATINGdose technique is an efficient method for routine testing for insecticide resistance in stored-product insects (CHAMP, 1968; CHAMP and CAMPBELL-BROWN, 1970). In this type of test, insects are exposed to a dose of insecticide approximately equal to the ED99.9 of a known susceptible strain. Any insects not responding to the test indicate the possible presence of resistance, and further confirmatory tests are conducted. If all the insects in the sample respond the population is classed as susceptible. However, under certain conditions this test can fail to detect the presence of resistance genes in the sample. When the frequency of the resistance gene in the population being sampled is low, the sample may contain some insects that are heterozygous for resistance, but the frequency of homozygous resistant insects may be so low that none occur in the sample. If this occurs with a recessive resistance gene, the heterozygous insects will respond to the discriminating dose and the test will therefore indicate susceptibility. This may result in the original population being treated with the insecticide against which the gene confers resistance, and thus selection for that resistance will occur. However, if the resistance were detected at the low frequency, the use of an appropriate alternative insecticide may either successfully eradicate the population, or reduce it so drastically that the resistance may be lost from the population by chance. Dose-response data for insects heterozygous for resistance are therefore required to ensure that the results of discriminating dose tests are not misinterpreted. LLOYD and SHAW (1968) crossed a pyrethrin-resistant strain of Sitophilus grunurius (L.) with an insecticide-susceptible strain. The F, progeny were treated with pyrethrins and their response was intermediate when compared with the two parent strains. The results suggested that the resistance was inherited in an autosomal semi-dominant manner. The resistant strain was shown to be cross-resistant to a wide range of insecticides (LLOYD, 1969a, b. 1973). Historically, this strain was taken into laboratory culture in 1954 and was subjected to 30 selections with pyrethrins during 44 generations. by which time its resistance level was x 138 when measured by topical application tests (LLOYD, 1969a). Subsequently the strain has not been selected and its level of resistance appears to have remained stable. It may therefore be assumed that this strain is homozygous for the pyrethrin-resistance gene(s), and that LLOYD‘S cross-resistance data are descriptive of homozygous insects. 19

20

A. J. PRICKETT

The present work investigates the responses of S. grarwius adults heterozygous pyrethrin-resistance to insecticides chosen from the four major groups-pyrethroids, organophosphorus compounds, organochlorines and carbamates.

for

MATERIALS AND METHODS The pyrethrin-resistant strain (R-strain) of S. granarius was crossed with a laboratory strain (S-strain) with no known resistance to insecticides. Single wheat grains were taken from cultures and placed in individual glass tubes (50 mm by 12 mm dia) closed with muslin. Virgin adults were collected within 2 days of emergence from these grains. Their sex was determined and they were held separately in similar clean tubes containing food (two wheat grains) for 7 days to mature. Single-pair crosses of each strain and reciprocal crosses between the two strains were set up in glass tubes (75 mm by 25 mm dia) closed with muslin and containing wheat grain to a depth of approximately 40mm. These adults were removed from the grain after 4 weeks. The F, progeny were collected after a further 4 weeks and again 2 weeks later. The progeny were held in lots of 500 on fresh grain (approx 300g) for two weeks until tested with insecticides. All insects were bred and tested at 25’C and 709, r.h. in constant light. The method of treatment was similar to that described by CHAMP (1968), in which adult insects were confined on filter papers impregnated with insecticide in oil. The insecticide concentrates were dissolved in a 3 : 1: 1 mixture by volume of petroleum ether (6&8O”C b.r.), acetone and Shell Risella 17 oil. Whatman No. 1 filter papers (70 mm dia) were each treated with 0.5 ml of solution. applied by pipette, and allowed to dry overnight. The dose levels quoted in this paper refer to the concentration of insecticide in the oil alone. One hundred insects, in 4 replicates of 25, were exposed to each dose level and to control filter papers treated with the solvent mixture alone. The insecticide concentrates used were natural pyrethrins (99%). bioresmethrin (939,). malathion (96%) pirimiphos-methyl (860/b), lindane (pure), DDT (pure) and propoxur (pure). The pure compounds were considered as loo”,; for the purpose of formulating solutions. The exposure period was 24 hr except for DDT (48 hr) and propoxur (6 hr). The number of insects knocked down was assessed at the end of the exposure period. Probit analysis was used to estimate the dose-response regression lines and to test these lines for parallelism. Where two or more replicates out of the four at a dose level gave nil or complete responses, all replicates at that dose level were combined. If all four replicates showed a nil or complete response, they were excluded from the analysis. Expected responses of less than 45d or more than 96% were included in the estimation of the regression line but excluded from the chi-squared tests for goodness of fit. The relative levels of resistance (resistance factors) were calculated from probit lines constrained to a common slope and their 957: fiducial limits were estimated according to the method described by FINNEY (1971). The parental and F, genotypes are referred to as SS (fully susceptible), RR (fully resistant), SR (F, of SS; x RR,“) and RS (F, of RR; x SST). Insects were weighed in batches of 25, to the nearest 0.1 mg. RESULTS The responses of the parental S- and R-strains and of the progeny of the reciprocal crosses between them are shown in Table 1. Heterogeneity of response was detected in 2 of the 28 tests (RS x bioresmethrin, p = 0.03; SS x pirimiphos-methyl, p = 0.05) and the fiducial limits of the KD~~ values were widened accordingly. The regression lines of the F, progenies, the parents, and both the F, and parents, were tested for parallelism (Table 2). No significant differences were detected between the slopes of the Fi for each insecticide. The slope of the R-strain was not significantly different from that of the S-strain with lindane. DDT, malathion, pirimiphos-methyl, or propoxur. However. the slope of the R-strain was less than that of the S-strain with pyrethrins (p < 0.001) and bioresmethrin (p < 0.001). Although none of these four lines showed significant heterogeneity (Table I). visual examination of them suggested that the responses of the R-strain were beginning to plateau at the higher dose levels. The dose-response data for the R-strain with pyreth-

The Cross-Resistance

T~BI.I:

I Rt:s~o~st

OF S. yrumuks

Spectrum

of Sitophrlus

yrurwirr.s

(L.)

R-STRAIN, S-STRAIT AND F, TO SEVEN wstu~~mt-s:

21

PROBIT REGK~SSION DAIA

Cross lnsecticlde P\rerhrins

Blorrsmethrin

Llndanc

DDT

Malathion

Pirimiphos-methyl

Propoxur

x ; sxs SxR RxS RxR sxs SxR RxS RxR sxs SxR RXS RxR SXS SxR RxS RxR sxs SxR RxS RxR sxs SxR RXS RxR srs Sr R R*S RxR

7.04 9.50

IO.4 34.2 0.630 1.67 I .68 6.60 0.0488 0.103 0.149 0.221 1.57 4.00 5.15 6.15 0.0935 0.152 0.166 0.347 0.0705 0.109 0.109 0.190 0.0660 0.117 0.121 0.15 1

rins was re-analysed, using the resulting slope was 4.29 4 0.49 the slope of 4.20 h 0.29 for the correct. There were insufficient

95”” Fiducial

limits

Slope & SE

I .9O 8.75 9.50 31.3 0.595 1.57 1.55 5.95 0.0437 0.09 I5 0.139 0200 1.35 3.27 4.x 5.15 0.0X80 0.147 0.161 0.335 0.0680 0. I06 0.106 0.184 0.0605 0.111 0.114 0.215

2.19 10.3 11.3 31.4 0.670 I .78 1.81 7.60 0.0526 0.112 0. I60 0.23x I.84 5.37 9.35 7.45 0.0975 0.17 0. I72 0.358 0.0775 0. I I 2

320 3.10 2.73 1.77 5.53 4.66 5.70 3.35 6.34 4.W 4.66 5.04 I .99 2.1x 1.7x 2.3s 9.74 9.9x 9.6X I I .02 IO.84 IO 12 Y.69 IO’)2 3.38 ‘-I _5.-j.31 5.62

0.113 0.196 0.710 0. I?4 0.17Y 027x

A2

0.29 0 IX n 17 0.X 0.43 0.29 0.43 0.36 0.X7 0.5J 0.3X 0.57 0. I7 0.29 (1.X II.24 I.20 0.70 0.71 I .Oh 0.X? 0.67 0.77 0.74 0.39 ox 0.35 0.8 I

ri./.

I’

I2.50 14.25 23.81 31.1x 26.52 77.01 38.57 ‘3.07 17.36 9.84 14.12

IO.80 I X-M 1 I.84 I ?.-I4 II.17 7.33 20.93 15.99 l5.N 27.51 16.17 12.2-I IO.25 12.07 23.57

19.90 7.17

lower four dose levels and discarding the top three. The (x2 = 10.72. I1$ = 14, p = 0.71) which was very similar to S-strain, indicating that the above interpretation may be data to repeat this procedure with bioresmethrin.

'I'MII-2. RISPONSF OF S. yrum~riusR-STRAW, S-STRAIN AND F, TOSI:V~N INSECTICIIWS:ASAL~SIS OI.PARALI I,I.ISM OF PROBIT REGRESSION DATA

Insecticide Pyrethrins

Bloresmethrln

l.lndane

DDT

M&thlon

Pvimiphos-methyl

Propox-ur

Comparison* F, P F, F, P F, F* P F, F, P F, F, P F, F, P F, F, P F,

Common

slope + SE

2.9 I 3.27 3.20 4.9 I 4.41 4.12 4.75 5.49 5.00 1.97 2.14 2.0X 9.x4 IO.60 IO.03 9.92 10.8X 10.36 5.76 4.64 5.07

0.12 0.16 0.1 I 0.76 0.N 0.27 0.31 0.44 0.75 0. I Y 0. IJ 0 II 0.51 0.x0

+ P

+ P

+ P

+ P

+ P

+ P

+ P

* F, = Comparison of SR and RS. P = Comparison of SS. SR, RS. RR. k, - P = Comparison

of SS and

0.43 ii.49 0.53

0.36 0.23 0.35 0.20 RR

2

Parallelism [l./.

2.69 19.01

24.65 0.34 ‘1.57 2x.13 0.0 I I .71) 2.75 1.55 1.84 -1.50
1 3 I

3 1

I .oi

P 0.10 <0.01)1 <0001 0.5::



0 57 11.‘1 0.17 0 2I > o.?r; 0-i I

I .hO < 0.0I I .h3

3 I

0 :x 0.7h >(I,45 O.h6 > O.~J!

4.44

3

0.20 I).22

I.OY 0.0’) < 0.0 I

22

A. J. PRICKETT

Pirimiphos methyl Malathion

Propoxur DDT Lindane

Bioresmethrin

Pyrethrins

1

I

I

I

.

2

4

8

16

Resistance

factor

FIG. 1. Comparisons of S. granarius homozygous (RR) and heterozygous (RS, SR) for pyrethrinresistance, with a susceptible strain (SS). Resistance factors and their 950j0 fiducial limits calculated at KD5,,. RS = progeny of R; x SJ. SR = progeny of Sy x R,J.

In all tests the KD~~ of the F1 fell between those of the parents. The resistance factors, or relative tolerances, and their 95% fiducial limits were calculated from the KD=,~ values estimated from the lines constrained to parallelism (FINNEY, 1971). The resultant values are illustrated in Fig. 1, in which the bars of the histogram are scaled logarithmically to better illustrate the relative positions of the probit lines and to avoid the apparent distortion created by quoting anti-log values. The resistance factors of the Fr progenies were similar and approximately half that of the R-strain for pyrethrins, bioresmethrin, malathion, pirimiphos-methyl, and propoxur. With lindane and DDT the resistance factor of the SR genotype was also approximately half that of the R-strain, but the RS genotype fell between these two. Due to the wide fiducial limits with DDT, RS was not significantly different from either SR (t = 1.15, d.jY = 26, p = 0.26) or RR (r = 1.83, d.f = 24, p = 0.08) but SR was significantly less than RR (t = 2.99, d.jY = 24, p = 0.0006). With lindane, the resistance of RS was greater than the SR (t = 5.02, d.jI = 28, p < 0.001) and less than RR (t = 5.05, d$ = 28. p < 0.001). To test the relative tolerances of the sexes of the parent strains to lindane, 300 each of So, S&, R?, Rd were exposed to three dose levels which straddled the KD~~ values, and their responses analysed (Table 3). No formal statistical comparison of the ~~~~ values was necessary to show that whereas there was no difference between the sexes of the S-strain, the ~~~~ of the R$ was significantly greater than that of the RQ Neither strain showed a significant departure from parallelism between the sexes (S, x2 = 0.10, d.$ = 1, TABLE 3A. RESPONSEOF S. granarius R-STRAIN AND S-STRAIN MALES AND FEMALES TO LINDANE; PROBIT REGRESSIONDATA Strain

KDso

S; SS

0.0401 0.0404

R; R,J

0.0990 0.208

957: Fiducial limits

0.0369, 0.0372,

Slope k SE

0.0435 5.38 0.0438 5.27 Test of parallelism 0.0375. 0. I39 3.21 0.177. 0.234 3.90 Test of parallelism

+ 0.51 & 0.49 t 0.83 & 0.50

x2

d,f

P

6.59 5.97 0.10 4.69 6.84 1.73

7 7

0.53 0.54 0.75 0.70 0.55 0.19

I 7 7 1

The Cross-Resistance

TAME

Spectrum

= = = =

granarius

23

(L.I

3B. RELATIVE TOLERANCES OF THE SEXES ot R-STRAII*; AND S-STRAIN S. grunurirrs TO LINDANF Relative tolerance at KDSO

Comparison s, R, Sj S,

of Sirophilus

SJ Rj R; R,.

95”” Fiducial limits

1.01 1.83 5.1 2.x

0.90. 1.50, 4.1. 2.0.

1.13 7.33 6.1 3.6

p = 0.75; R, x2 = 1.73. d.& = 1, p 7 0.19). The lindane KD~~ values for the parental strains (Table 1) were higher than those for the sexes tested separately (Table 3A). This may have been due to differences in methodology. The former values were obtained from the progeny of single pairs whereas the latter were from mass cultures tested at a different time. Comparisons of the weights of the R- and S-strains, their sexes, and the F1 progenies are shown in Table 4. All four genotypes were shown to have significantly different weights in the order from lightest to heaviest of SS, SR, RS, RR. Whereas in both strains the male on average weighed more than the female, the difference was not significant in the S-strain (p = 0.08) but was in the R-strain (p = 0.009). TARLE 4A. WEIGHTS OF S.grunarius WEEKS,

Subject

WEIGHED

R-STRAIN. S-STRAIN AND F, AT AGE 24 IN RATCHESOF 25 INSEcTS

Batch weight (mg) Mean + SE

ss

65.42 77.41 87.49 101.40 63.7 67.4 65.5 94.5 101.9 98.2

SR RS RR

S, SS Mean R1 R; Mean

+ k + * f f + f f f

Average individual weight (mg)

Number of batches 32 32 3’ 32 8 8 16 4 4 8

0.80 0.67 0.60 0.89 1.2 1.4 1.0 1.1 1.2 1.6

2.62 3.10 3.50 4.06 2.55 2.70 2.62 3.78 4.07 3.93

TABLE 4B. COMPARISONS OF THE MEAN BATCH WEIGHTS OF S. grannrius Comparison

SS:SR SR:RS RS:RR Si :S.j R;:RJ

r value

n,r.

P

II.3 11.0

62 62


12.8 1.90 3.86

62 14 6


TABLE 5. THEINFLUENCEOFEXPOSUREMETHOD UPON THE RESISTANCEFACTORSOF R-STRAIN S. grunarius

Method Topical application (LD)* Film tesr (KD) Hard filter papers* Soft filter papers

Pyrethrins

DDT

Resistance factors Lindane Malathion

149

29.5

6.8

6.9

17.7 (24)t 16.8 (24)

23.4 (72) 3.9 (48)

7.3 (7) 4.5 (24)

5.3 (5) 3.7 (24)

* Data from LLOYD and WILLIAMS (1973). t Duration of exposure (hr).

24

A. J.

PRICKETT

DISCUSSION

The metabolism of pyrethrin I and DDT has been examined in this resistant strain and a different susceptible strain by ROWLANDSand LLOYD(1969, 1976). Both strains metabolised pyrethrin I oxidatively; the R-strain produced approximately three times the amount of the two major metabolites found in the S-strain. DDT was also oxidatively metabolised in the R-strain, with trace amounts of DDE produced by dehydrochlorination. The S-strain showed only a small amount of dehydrochlorination of DDT to DDE. An oxidative mechanism has been shown in Trihofium custunetrm to confer resistance to a number of organophosphorus compounds, including malathion (DYTE et al., 1970). Lindane metabolism would probably require dehydrochlorination before oxidation. In 1973 LLOYD showed that with pyrethrins and bioresmethrin, topical application of the synergist piperonyl butoxide (PB) was capable of reducing the LD,, of the R-strain to that of the S-strain without synergist, thus overcoming the resistance. It should however be noted that synergism also occurred in the S-strain. Metabolic studies demonstrated that PB caused a pronounced inhibition of oxidative attack in the R-strain. LLOYD (1969b) also demonstrated that PB synergised DDT in the R-strain. KUMARet ul. (1967) tested propoxur-resistant S. granariLfs (which were cross-tolerant to DDT) with propoxur and propoxur plus PB and showed that the synergist reduced the LD~~ of the R-strain to that of a comparative S-strain without synergist. This may indicate that oxidative metabolism was involved. It may therefore be postulated that one or several oxidation mechanisms confer true cross-resistance to all the compounds tested. Comparison of resistance factors obtained in the present work with those of LLOYDand WILLIAMS(1973), reproduced in Table 5, demonstrates the high degree to which resistance factors can be influenced by the method of treatment and time of exposure. With topical application, followed by an end-point assessment of response, comparisons of resistance factors may be made between different strains treated with the same insecticide. With treated substrate tests, allowances should be made for possible changes of these factors associated with change in the duration of exposure. The resistance factors of the F, (Fig. 1) are approximately half those of the R-strain. with the exception of RS with lindane and possibly DDT. Thus the cross-resistance spectrum of the F1 heterozygotes was similar to the R-strain but only half its magnitude. Examination of the regression data of the responses to lindane of the sexes (Table 3) showed that R; were more resistant than the R i but that there was no significant difference between SJ and S,. At the end of the test the sexes were redetermined to ensure there had been no errors. Further investigation of the data, which was recorded up to 6 days after removal of the insects from the treated filter papers, showed that by the 6th day the response of Rj was not significantly different from that of the R$. Thus the relative resistance of the R; compared to the R$ was due to a difference in the rate of response. This may agree with HEWLETT’S(1974) interpretation of LLOYD’S(1969a) data concerning the response of the R-strain to DDT, which was that ‘his data showed evidence of a bimodal distribution of time to death, owing, possibly, to a difference between the sexes’. LLOYDand PARKIN (1963) suggested that both the males and females of the R-strain contributed to the increased weight of the F, heterozygotes, and that therefore this heavier weight depended upon an autosomal factor or factors. The replicated data obtained in the present work confirms this view, but also shows that RS individuals were significantly heavier than SR. Comparing the average individual weights (Table 4) of SS, SR. RS, RR insects with their KD~,, values for lindane (Table 1) produced a correlation coefficient of 0.999. Not only is this correlation too good to occur by chance, but also the regression line indicates that insects with an average weight of 2.23 mg have a zero KD~~! Consequently, although the differences in size of the four genotypes appear to be sufficient to account for the differences in susceptibility, the correlation must at present be considered spurious. The anomaly of the RS response to lindane, and possibly DDT, falling between SR and RS suggested sex-linkage. Although this is not primarily a genetical paper this

The Cross-Resistance

Spectrum

of Sitophrlus

grunurius

(L.)

75

anomaly will now be considered. In S. yranarius, females are XX and males XY with the Y chromosome being very small (SMITH, 1952). A dominant factor on the X chromosome which confers additional resistance to lindane would result in both sexes of RS, but only the females of SR. having this resistance. Thus the mean response of RS would fall between that of the SR and RR. In S. oryzae DDT resistance has been shown to be controlled by one or more sex-linked semi-dominant factors in a strain which was also resistant to lindane (CHAMP, 1967). Further investigation is required to elucidate the present responses to lindane and DDT and possible similarities between the two species. The results support the view that the major oxidative resistance mechanism(s) is autosomal and semi-dominant. and confers resistance to all the insecticides tested. Additional resistance to lindane, and possibly DDT, may be conferred by a sex-linked gene. The absence of described mutants in S. yrunuvius suggests that genetic markers revealed by a electrophoretic method are required for a full genetic analysis of the resistance in this strain. Since the resistance in the R-strain is not recessive it should be possible to detect the presence of heterozygotes in a discriminating dose test using any of the seven insecticides. REFERENCES CHAMP, B. R. (1967) The inheritance of DDT resistance in Sitophilus oryxe (L.) (Coleoptera. Curculionidae) in Queensland. J. stored Prod. Rex 3. 321-334. CHAMP. B. R. (1968) A test method for detecting insecticide resistance in Sirophilus oryxe (L.) (Coleoptera. Curculionidae). .I. srored Prod. Res. 4, 175-178. C‘HAMP. B. R. and CAMPBELL-BROWN. M. J. (1970) Insecticide resistance m Australian Triholium cusroneurn (Hrrbstt 1. A test method for detectmg insecticide resistance. J. SOW{ Pmd. Rrs. 6. 53%70.

Dvrc. C. E.. ROWLANDS. D. G., DALY, J. A. and BLACKMAN, D. G. (1970) A new lype of resistance in rust-red flour beetles. Pest Infar. Rrs. 1969. p. 4142. FINNEY. D. J. (1971) Probit analysis (3rd Edn). Cambridge University Press. Cambridge. England. HEWLETT, P. S. (1974) Time from dosage to death in beetles, Triholium c~astuneum, treated with pyrethrins or DDT. and its bearing on dose-mortality data. J. stored Prod. Rex 10, ‘7-41. ii< MAR. V. and MORRISON. F. 0. (1967) Carbamate and phosphate resistance in adult granary weevlls. J. (YO)I. Ellr. 60, 143&1434. I.I.OYII. C. J. (1969a) Studies on the cross-tolerance to DDT-related compounds of a pyrethrin-reslstant strain of SlrqMus grcmtrrius (L.) (Coleoptera. Curculionidae). J. stored Prod. Rrs 5. 337 -356. LLOYD, C. J. (1969b) The synergism of DDT, deutero-DDT, and methoxychlor in a pyrethrin-resistant strain of Sirophilus granarius (L.) (Coleoptera, Curculionidae). J. stored Prod, Res. 5, 357-363. L.I OYI). C. J. (1973) The toxicity of pyrethrins and five synthetic pyrethrolds, to Triholium cusrunwm (Herbst). and susceptible and pyrethrin-resistant Sirophihrs yrunurius (L.). J. srured Prod Res. 9, 77--92. I.I(IYII. C. J. and PARKIN. E. A. (1963) Further studies on a pyrethrum-resistant strain of the granary weecll. Sfr,~phii~s ;lrctn(tr’ilts (L.). J. Sci. Fd Ayric. 14, 655-663. 1.1 OYI). C. J. and SHAW. D. D. (1968) Genetics. Pyrethrin resistance m grain weevils. Pesr Z?lfksr. Rrs. 1967. 36. II OYI). C. J. and WIL.LIAMS. V. (1973) Detection of resistance m grain weevils. Pc.$r Ir$w. Conrrol 1968-71). 112 123. ROWLANDS. D. G. and LLOYD, C. J. (1969) DDT metabolism in susceptible and pyrethrin-resistant Sirophilus grtmurius (L.) (Coleoptera; Curculionidae). J. srored Prod. Res. 5, 413-415. ROWLANIX. D. G. and LLOYD, C. J. (1976) Incidence of resistance in the United Kingdom. The gram weevli. Metabolism and synergism of pyrethroids in susceptible and resistant strains. Pest I+sr. Cotlrrol 1971~ 1973. 75 77 SUII-II. S. G. (1952) The cytology of Siropkilus (Culundru) oryzau (L.). S. qrunurius (L.). and some other Rhychophora (Coleoptera). Cyroloyia 17. 50-70.