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Susceptibility of some bruchid beetles of stored pulses to powders containing pyrethrins and piperonyl butoxide
J.
1970, Vol. 6, pp. 71-77.
stored Prod. Res.,
Pergamon
Press.
Printed
in Great
Britain.
Susceptibility of Some Bruchid Beetles of Stored Pulses to Powders Containing Pyrethrins and Piperonyl Butoxide A. J. S. WEAVING* Research
Laboratories,
(First received 30 April,
Pyrethrum
Marketing
1969, and in&al
form
Board
of Kenya
30 November,
1969)
Abstract-The susceptibility of some bruchid beetles to pyrethrins and pyrethrins synergised with piperonyl butoxide was determined under laboratory conditions by exposing Acanthoscelides obtectus, zabrotes subfasciatus and Callosobruchus chinensis to talcbased powders admixed with wheat. Wheat gave more consistent results than pea-beans A 48-hr exposure period was used for obtaining owing to better adhesion properties. results of practical value, but the results were less consistent than those obtained from 6-day exposures. The 48-hr exposures showed that the three species have similar pyrethrins susceptibility with LD~,,Sbetween 6.6 and 8-O ppm pyrethrins, while C. chinensis showed the greatest response to the synergist. The optimum synergist ratio for A. obtectus and <. subfasciatus was 1: 10. The 6-day exposures showed similar susceptibilities, but indicated a higher factor of synergism for A. obtectus, and an optimum synergist ratio of at least 1: 15. It is suggested that a 0.2% pyrethrins powder of 1: 10 to 1: 15 synergist ratio, applied to give 2 ppm of pyrethrins would be suitable for practical control of A. obtectus and <. subfasciatus. A 0 *2% pyrethrins powder with a 1: 5 synergist ratio, already marketed, could also be expected to give good control at the 2.0 ppm pyrethrins level assuming that ‘knock-down’ is sufficient to prevent infestation. The latter assumption was supported by small-scale infestation tests. INTRODUCTION
LLOYD and HEWLETT (1958) h ave shown that two common bruchid pests of stored pulses, Acanthoscelides obtectus (Say) and Callosobruchus chinensis (L.), are very susceptible to solutions of pyrethrins or pyrethrins-piperonyl butoxide mixtures in oil. In tests using insecticide dusts SALAS and RUPPEL (1959) found that a 1: 17 mixture of pyrethrins and piperonyl butoxide applied at 0.5 to 2 * 0 parts of pyrethrins per million gave satisfactory control of A. obtectus and zabrotes subfasciatus (Boh.). These latter species are considered to be the principal pests of stored beans, and although both occur in East Africa, A. obtectus is much more common in Kenya (McFarlane, J. A., personal communication). Callosobruchus spp. are less important as stored bean pests but should be considered as potential pests (HOWE and CURRIE, 1964). * Present
address:
P.O.
Box 97, Chiredzi,
Rhodesia. 71
72
A. J. S. WEAVING
The object of this work was to determine the susceptibility to pyrethrins of the more important species of bruchids and to establish factors of synergism for these insects when piperonyl butoxide is used as synergist. MATERIAL AND METHODS Inset ts <. subfasciatus, A. obtectus and C. chinensis were reared at 85°F and 75 per cent r.h. Approximately 300 unsexed adults were placed in each culture jar containing 500 g of Mexico 142 pea-beans, or cowpeas in the case of C. chinensis. All insects were sieved off when the F, adults began to emerge, and the cultures then held for 4 to 5 days. Insects emerging during this period were used for bio-assay and were therefore all adults up to 5 days old. Insecticides
Talc was used as filler and was impregnated with partially dewaxed pyrethrum extract and, in synergised powders, piperonyl butoxide. Chloroform was used as diluent. All powders were impregnated to give O-2 per cent (w/w) pyrethrins. Methods of assay
Wheat was sterilized by heating in an oven at 100°C for 3 hr and was held for at least 48 hr at ambient temperature and relative humidity to minimize any drying effect that may have occurred. 50 g lots of sterilized wheat were placed in widemouthed screw-top glass jars and the powders added on a weight/weight basis to yield the required dosage level of pyrethrins. This method of varying the dosage level has been regularly used at these laboratories for assaying pyrethrum against weevils and was chosen in preference to that of using fixed quantities of powders containing different concentrations of active constituents. There are drawbacks to both methods. The former shows a fall off in adhesion to grain above a certain level of application, while the latter yields dusts differing in adhesion properties according to the concentration of insecticide, combined with a tendency to ‘balling’ at higher dosage levels. The jars were rolled mechanically for 15 min to ensure thorough admixture. Fifty insects were placed in each jar which was then covered with nylon gauze. After a given exposure period on the treated grain at the same temperature and humidity as stated above, the contents of each jar were sieved through a 10 mesh screen. Insect mortality was assessed by direct inspection, the criterion for death being no visible movement after prodding. All insects otherwise affected were classed as ‘knocked-down’. Each treatment was replicated three times. Unimpregnated talc controls for all dosage levels were not included in order to keep the assays to a manageable size. A separate experiment was conducted to assess the effect of talc on mortality within the range of doses used. In all assays mortality was corrected for that in the untreated controls using Abbott’s formula. Wheat, already shown to have no effect on natural mortality in the experiments described, was used instead of pea-beans because previous bio-assay tests showed that treated pea-beans gave inconsistent results and curved regression lines with potency falling off at the higher dosage rates. Adhesion was determined by weighing the wheat and pea-beans before and after admixture with known weights of
Susceptibility of Some Bruchid Beetles of Stored Pulses
73
powder. After admixture, grain was carefully decanted from the jars for weighing, Weight changes in controls during the leaving any residual powder behind. experiment were used to correct the weights of treated grain. Three adhesion tests were carried out, the first with two replications and the remainder with three. All regression lines were fitted to data points according to the method outlined by BUSVINE (1957). Exposure periods of 6 days and 48 hr were chosen: the 6-day exposure is standard for weevil assays in these laboratories but the 48-hr exposure is considered to be more applicable to practical conditions. Separate assays were conducted for each exposure period. Oviposition tests were carried out with the above methods of admixture using beans instead of wheat. Impregnated powders were compared with untreated controls and unimpregnated filler made up of 50 per cent talc and 50 per cent pyrethrum mart as recommended for field trials. Residual levels of pyrethrins in the mart, which is the finely ground remains of pyrethrum flowers after extraction of the insecticide, were allowed for when impregnation was carried out. Jars were held for 6 weeks before the sieving and examination of the beans. The numbers of adults introduced were subtracted from the numbers sieved off giving the numbers of F, adults. The extent of the primary infestation produced by the initially introduced insects was determined from the proportion of beans with emergence holes while the proportion of beans with adhering eggs indicated the total-level of infestation. RESULTS
The _. . which when within
effect of exposing <. subfasciatus to unimpregnated talc is shown in Table 1 _ _. _. indicates that a natural mortality of approximately 10.8 per cent occurs adults are confined on wheat for six days and that the addition of pure talc, the stated range of doses, does not increase this figure significantly,
TABLE
1. AVERAGE
MORTALITY
OF zabrotes
subfmciatus
EXPOSED
FOR 6 days
ONWHEATTREATEDWITHUNIMPREGNATEDTALC
Results powder to explaining properties
Weight of talc/50 g wheat
Mortality
(g)
(%I
o-0 o-03 0.04 0.06 0.09 0.12 0.17
IO-8 11.5 13.4 7.9 5.5 16.3 10.6
from the adhesion tests, given in Table 2, show that adhesion of the pea-beans after admixture decreased as the rate of application increased, the shape of the regression lines. Wheat showed more constant adhesion throughout the range tested and resulted in a better fit.
A. J. S. WEAVING
74
TABLE 2. Per cent ADHESIONOF POWDERS CONTAINING 0.2% PYRETHRINSAND 1 *O% PIPERONYL BUTOXIDETO WHEAT AND PEA-BEANS Weight of powder added to 50 g lots of grain (g)
Pea-beans S.E. Wheat S.E.
0.0125
0.025
0.05
0.1
o-2
0.3
80.8 5.64 73.2 2.81
75.1 5.59 69.8 3.29
72.2 2.74 77.2 1.20
64.1 0.86 74.9 2.45
44.3 1.47 77.6 1.19
30.7 1.36 79-l 1.93
A significant difference in the susceptibility of male and female <. subfasciatus to pyrethrins is shown in Table 3. Similar tests with the other species were not conducted. TABLE 3. SUSCEPTIBILITYOF MALE AND FEMALE zabrotes subfasciatus TO POWDERS CONTAINING 0’2% PYRETHRINSAND 1.0% PIPERONYL BUTOXIDE LDbO (ppm pyrethrins)
Male Female Factor
0.62 0.99 x1-60
LD~
(ppm pyrethrins) l-23 l-75 x1.42
Tables 4 and 5 summarize the results from the tests carried out to determine the 50 and 90 per cent lethal doses of synergised and unsynergised pyrethrins and the accompanying factors of synergism. These results indicate that all species were susceptible to pyrethrins and showed a good response to the synergist. Columns A, B and C represent individual tests as it was not possible to compare all synergist ratios in one experiment. The additional effect of ‘knock-down’ by pyrethrins is shown in Table 6 which contains figures extracted from assays designed to yield mortality regression lines. Affected bruchids include both ‘knocked-down’ and dead insects. The results demonstrate the rapidity with which insects were affected by small doses of pyrethrins before mortality had occurred. The effects of treatment with some pyrethrum powders on oviposition and infestation by C. chinensis and <. subfasciatus are shown in Table 7. DISCUSSION
Results from the 48-hr exposure tests (Table 4) show that <. subfmciatus and obtectus have similar LD~~S but A. obtectus shows greater susceptibility to pyrethrins at the LD~,, level. LD,,,, values are, however, less reliable statistically, and difficulty was experienced with <. subfmciatus because of inconsistent results as indicated by the wider range of the fiducial limits and by the bracketed figures in Table 4. It was not found possible to obtain accurate data for the 1: 10 and 1: 15 synergist ratios. Until further work is done it is wiser to assume similar susceptibility to A.
Susceptibility of Some Bruchid Beetles of Stored Pulses TABLE
4.
DOSAGE-MORTALITY
BRUCHIDS AFTER
48 hr
75
DATA AND FACTORS OF SYNERGISM FOR THREE SPECIES OF
EXPOSURE TO WHEAT TREATED WITH PYRETHRINS SYNERGISED WITH PIPERONYL BUTOXIDE
Treatment
Test
(ppm
LDso pyrethrins)
95% Confidence limits for LD6,,
Factor
of
synergism
(ppm
LDm pyrethrins)
A. obtectus: Pyrethrins alone
-
20.00 17.62 12.63
1.74 4.47 5.83 5.63
11.80 3.75 1.82 2.16
8.32-8.44 7.57-7.62
-
34.13 31.26
6~12-6.20 1.72-2.23 0.12-0.60
1,36 3.87 (28.1 I)*
21.66 (32.68)* (14.35)*
A B C
8.93 8.40 6.59
8-92-8.94 8.39-8.41 6.58-6.60
A B C C
5.13 1.88 1.13 1.17
5.11-5.16 1.88- 1.89 1.05- 1.21 1*05-1.19
A B
8.38 7.59
Pyrethrins +pipbut.
1:
1
1:5 I:10 1:15 <. subfasciatus:
Pyrethrins alone Pyrethrins + pipbut. 1: 1 1:5 I:10 C. chine&
6.16 1.96 (O-27)*
:
Pyrethrins alone Pyrethrins + pipbut.
A B B
I :1 1:5
A
6.65
6.64-6.66
A A
2.67 1.16
2.63-2~72 1.07-1.27
22.13 z-49
6.49
5.73
2.41
* Bracketed figures indicate inconsistencies in the results with <. subfasciatus. Regression lines for the 1:5 and 1: 10 ratios were parallel to each other but not with that for pyrethrins alone, resulting in an unusually high factor of synergism for the 1: 10 synergist ratio and LD@~ values that are apparently not consistent with other tests. Heterogeneity is indicated by the wide range of the fiducial limits for these ratios.
pyrethrins for these two species as indicated by the LD~,, values. The optimum synergist ratio appears to be one part of pyrethrins to ten parts of piperonyl butoxide. This was not determined for <. ~~b&~ckztzlsfor the reasons given above though the results indicated that factors of synergism were similar for these two species. C. chinenris showed similar pyrethrins susceptibility and in limited tests factors of synergism were higher than those for A. obtectus and <. subfaciatus. Results from the 6-day exposure tests (Table 5) give a better comparison between A. obtectus and 2. subfasciatus. Again there was no substantial difference in pyrethrins susceptibility though in this case <. subfasciatus showed a steeper regression line. A. obtectus gave higher factors of synergism and for both species the optimum synergism was one part of pyrethrins to fifteen parts of piperonyl butoxide.
A.J.
76
S. WEAVING
TABLE 5. DOSAGE-MORTALITY DATA AND FACTORSOF SYNERGISM FOR 2 SPECIESOF BRUCHIDSAFTER6 days EXPOSURETO WHEAT TREATEDWITH PYRETHRINS SYNERGXSED WITH PIPERONYLBUTOXIDE
95%
Confidence limits for LDbO
L%.o Treatment
Test (ppm pyrethrins) -____---
Factor of synergism ----__--
LDeo (ppm pyrethrins)
-
10.09 11.11
A. obtectus: Pyrethrins alone Pyrethrins +pipbut. 1: 1 1:5 1:lO 1:15
A B
3.28 3.47
3.27-3.28 3.41-3.53
A A B B
2.36 0.99 0.46 0.35
2.35-2-37 o-99-0.99 0.45-0.46 o-35-0.35
A B
3.97 3.68
3-93-4.02 3-68-3.69
A A B B
2.38 1.09 0.77 0.59
2.38-2.38 l-081.09 0.77-0.77 0.59-O-62
-
4.48 1.72 0.83 0.68
l-39 3.31 7.54 9.91
<. subfasciatus: Pyrethrins alone
-
7.76 7.85
1.67 3.64 4.78 6.24
3.44 1.68 1.33 O-85
Pyrethrins
+ pipbut. 1: 1 1:5 1:lO 1:15
TABLE 6. PERCENTAGEOF BRUCHIDSDEAD AND KNOCKED-DOWN AFTEREXPOSURE FOR 48 hr TO WHEAT TREATEDWITH PYRETHRINS Dead or knocked-down (%) -____-_____ Dose ____ (ppm pyrethrins) C. chinensis <. subfasciatus
Treatment Pyrethrins alone
Pyrethrins +pipbut.
2.00 3.00 5.00 1: 1 1:5
1:lO
Untreated
83.6
77.9 89.2 100.0
1.50 0.50 1.00 2.00 0.25 o-50 l-00
100~0
-
73.0 99.2 -
39.9 94.2 100.0 2.7 67.3 100.0
0.00
9.7
0.0
Factors of synergism and LD 9o figures for the 48-hr exposure periods should form the basis of field trials and control recommendations; rapid ‘knock-down’ and kill are essential to prevent oviposition as the effects of pyrethrins on fertility and emergence are not understood. On the basis of using the optimum synergist ratio (1: 10 to 1: 15)) a dosage rate of 2 ppm of pyrethrins would be expected to give excellent control of A. obtectus and
Susceptibility TABLE
of Some Bruchid
Beetles of Stored
Pulses
7. INFESTATION BYTWO BRWCHIDS OF COWPEAS OR MEXICAN TREATED WITH PYRETHRUM POWDERS
Treatment
Mean No. F, adults
2. subfasciatus on Mexican Control Filler Treated* C. chinensis on cowpeas: Control Filler Treated*
142 pea-beans 309 147 0
77
142 PEA-BEANS
Mean y’ primary infestation of beans
Mean y0 beans bearing eggs
50.2 15.1 0
99.1 82.3 0
72.6 0 0
100.0 0 0
:
311 0 0
* Includes the following treatments: 0.1% pyrethrins-to-5% piperonyl butoxide at l-25 pyrethrins. 0.2% pyrethrins + 1 *O% piperonyl butoxide at 2 a50 ppm of pyrethrins.
ppm of
C. chinensis. This dose may be suitable against <. subfmciatus but this requires confirmation. The powders on the market containing 0 * 2 y0 pyrethrins and l-0 y0 piperonyl butoxide may be appropriate for Bruchid control. Although some control could be expected at the 2 ppm of pyrethrins level, the present results indicate that a powder with a 1: 5 synergist ratio would have to be used at a higher rate, i.e. 3 * 75 ppm, to give 90 per cent kill within 48 hr, compared with l-82 ppm if a 1: 10 synergist ratio was used. The above conclusions are based on mortality only. Before death occurs, insects exposed to pyrethrins pass through a ‘knocked-down’ phase when they are unable to carry out normal activities. Insects are initially only slightly affected but prior to death are nearly motionless. Biological studies are required to determine whether females in this condition can oviposit and whether any resulting eggs are viable. Results from the small scale tests given in Table 7 indicate that a powder with 0 * 1 y0 pyrethrins and 0 - 5 y0 piperonyl butoxide will prevent infestation by <. subfmciatus and C. chinemis when applied at the rate of l-25 ppm of pyrethrins. As indicated by the mortality results of the above assays, this is likely to be caused by rapid ‘knock-down’ rather than kill. The assumption that ‘knocked-down’ females cannot produce an infestation therefore seems plausible since most of these were moribund and unlikely to recover. Further work is required on this aspect. Results in Tables 6 and 7 show that a powder with a 1:5 synergist ratio, applied to give 2 ppm of pyrethrins, would be expected to give excellent control and that a rate of application of 1 ppm of pyrethrins might prove satisfactory. REFERENCES BUSVINE, J. R. (1957) A Critical Review of the Techniques for Testing Insecticides. Commonwealth Institute of Entomology, London. HOWE, R. W. and CURRIE, J. E. (1964) Some laboratory observations on the rates of development, mortality and oviposition of several species of Bruchidae breeding in stored pulses. Bull. ent. Res. 55, 437-477. LLOYD, C. J. and HEWLETT, P. S. (1958) The relative susceptibility to pyrethrum in oil of Coleoptera and Lepidoptera infesting stored products. Bull. ent. Res. 49, 177-185. SALAS, L. and RUPPEL, R. F. (1959) Efectividad de insecticides aplicados en polvo para controlar las principales plagas de1 frijol y de1 maiz almacenados, en Colombia. Agricultura trap. 15, 93-108.
1970, Vol. 6, pp. 71-77.
stored Prod. Res.,
Pergamon
Press.
Printed
in Great
Britain.
Susceptibility of Some Bruchid Beetles of Stored Pulses to Powders Containing Pyrethrins and Piperonyl Butoxide A. J. S. WEAVING* Research
Laboratories,
(First received 30 April,
Pyrethrum
Marketing
1969, and in&al
form
Board
of Kenya
30 November,
1969)
Abstract-The susceptibility of some bruchid beetles to pyrethrins and pyrethrins synergised with piperonyl butoxide was determined under laboratory conditions by exposing Acanthoscelides obtectus, zabrotes subfasciatus and Callosobruchus chinensis to talcbased powders admixed with wheat. Wheat gave more consistent results than pea-beans A 48-hr exposure period was used for obtaining owing to better adhesion properties. results of practical value, but the results were less consistent than those obtained from 6-day exposures. The 48-hr exposures showed that the three species have similar pyrethrins susceptibility with LD~,,Sbetween 6.6 and 8-O ppm pyrethrins, while C. chinensis showed the greatest response to the synergist. The optimum synergist ratio for A. obtectus and <. subfasciatus was 1: 10. The 6-day exposures showed similar susceptibilities, but indicated a higher factor of synergism for A. obtectus, and an optimum synergist ratio of at least 1: 15. It is suggested that a 0.2% pyrethrins powder of 1: 10 to 1: 15 synergist ratio, applied to give 2 ppm of pyrethrins would be suitable for practical control of A. obtectus and <. subfasciatus. A 0 *2% pyrethrins powder with a 1: 5 synergist ratio, already marketed, could also be expected to give good control at the 2.0 ppm pyrethrins level assuming that ‘knock-down’ is sufficient to prevent infestation. The latter assumption was supported by small-scale infestation tests. INTRODUCTION
LLOYD and HEWLETT (1958) h ave shown that two common bruchid pests of stored pulses, Acanthoscelides obtectus (Say) and Callosobruchus chinensis (L.), are very susceptible to solutions of pyrethrins or pyrethrins-piperonyl butoxide mixtures in oil. In tests using insecticide dusts SALAS and RUPPEL (1959) found that a 1: 17 mixture of pyrethrins and piperonyl butoxide applied at 0.5 to 2 * 0 parts of pyrethrins per million gave satisfactory control of A. obtectus and zabrotes subfasciatus (Boh.). These latter species are considered to be the principal pests of stored beans, and although both occur in East Africa, A. obtectus is much more common in Kenya (McFarlane, J. A., personal communication). Callosobruchus spp. are less important as stored bean pests but should be considered as potential pests (HOWE and CURRIE, 1964). * Present
address:
P.O.
Box 97, Chiredzi,
Rhodesia. 71
72
A. J. S. WEAVING
The object of this work was to determine the susceptibility to pyrethrins of the more important species of bruchids and to establish factors of synergism for these insects when piperonyl butoxide is used as synergist. MATERIAL AND METHODS Inset ts <. subfasciatus, A. obtectus and C. chinensis were reared at 85°F and 75 per cent r.h. Approximately 300 unsexed adults were placed in each culture jar containing 500 g of Mexico 142 pea-beans, or cowpeas in the case of C. chinensis. All insects were sieved off when the F, adults began to emerge, and the cultures then held for 4 to 5 days. Insects emerging during this period were used for bio-assay and were therefore all adults up to 5 days old. Insecticides
Talc was used as filler and was impregnated with partially dewaxed pyrethrum extract and, in synergised powders, piperonyl butoxide. Chloroform was used as diluent. All powders were impregnated to give O-2 per cent (w/w) pyrethrins. Methods of assay
Wheat was sterilized by heating in an oven at 100°C for 3 hr and was held for at least 48 hr at ambient temperature and relative humidity to minimize any drying effect that may have occurred. 50 g lots of sterilized wheat were placed in widemouthed screw-top glass jars and the powders added on a weight/weight basis to yield the required dosage level of pyrethrins. This method of varying the dosage level has been regularly used at these laboratories for assaying pyrethrum against weevils and was chosen in preference to that of using fixed quantities of powders containing different concentrations of active constituents. There are drawbacks to both methods. The former shows a fall off in adhesion to grain above a certain level of application, while the latter yields dusts differing in adhesion properties according to the concentration of insecticide, combined with a tendency to ‘balling’ at higher dosage levels. The jars were rolled mechanically for 15 min to ensure thorough admixture. Fifty insects were placed in each jar which was then covered with nylon gauze. After a given exposure period on the treated grain at the same temperature and humidity as stated above, the contents of each jar were sieved through a 10 mesh screen. Insect mortality was assessed by direct inspection, the criterion for death being no visible movement after prodding. All insects otherwise affected were classed as ‘knocked-down’. Each treatment was replicated three times. Unimpregnated talc controls for all dosage levels were not included in order to keep the assays to a manageable size. A separate experiment was conducted to assess the effect of talc on mortality within the range of doses used. In all assays mortality was corrected for that in the untreated controls using Abbott’s formula. Wheat, already shown to have no effect on natural mortality in the experiments described, was used instead of pea-beans because previous bio-assay tests showed that treated pea-beans gave inconsistent results and curved regression lines with potency falling off at the higher dosage rates. Adhesion was determined by weighing the wheat and pea-beans before and after admixture with known weights of
Susceptibility of Some Bruchid Beetles of Stored Pulses
73
powder. After admixture, grain was carefully decanted from the jars for weighing, Weight changes in controls during the leaving any residual powder behind. experiment were used to correct the weights of treated grain. Three adhesion tests were carried out, the first with two replications and the remainder with three. All regression lines were fitted to data points according to the method outlined by BUSVINE (1957). Exposure periods of 6 days and 48 hr were chosen: the 6-day exposure is standard for weevil assays in these laboratories but the 48-hr exposure is considered to be more applicable to practical conditions. Separate assays were conducted for each exposure period. Oviposition tests were carried out with the above methods of admixture using beans instead of wheat. Impregnated powders were compared with untreated controls and unimpregnated filler made up of 50 per cent talc and 50 per cent pyrethrum mart as recommended for field trials. Residual levels of pyrethrins in the mart, which is the finely ground remains of pyrethrum flowers after extraction of the insecticide, were allowed for when impregnation was carried out. Jars were held for 6 weeks before the sieving and examination of the beans. The numbers of adults introduced were subtracted from the numbers sieved off giving the numbers of F, adults. The extent of the primary infestation produced by the initially introduced insects was determined from the proportion of beans with emergence holes while the proportion of beans with adhering eggs indicated the total-level of infestation. RESULTS
The _. . which when within
effect of exposing <. subfasciatus to unimpregnated talc is shown in Table 1 _ _. _. indicates that a natural mortality of approximately 10.8 per cent occurs adults are confined on wheat for six days and that the addition of pure talc, the stated range of doses, does not increase this figure significantly,
TABLE
1. AVERAGE
MORTALITY
OF zabrotes
subfmciatus
EXPOSED
FOR 6 days
ONWHEATTREATEDWITHUNIMPREGNATEDTALC
Results powder to explaining properties
Weight of talc/50 g wheat
Mortality
(g)
(%I
o-0 o-03 0.04 0.06 0.09 0.12 0.17
IO-8 11.5 13.4 7.9 5.5 16.3 10.6
from the adhesion tests, given in Table 2, show that adhesion of the pea-beans after admixture decreased as the rate of application increased, the shape of the regression lines. Wheat showed more constant adhesion throughout the range tested and resulted in a better fit.
A. J. S. WEAVING
74
TABLE 2. Per cent ADHESIONOF POWDERS CONTAINING 0.2% PYRETHRINSAND 1 *O% PIPERONYL BUTOXIDETO WHEAT AND PEA-BEANS Weight of powder added to 50 g lots of grain (g)
Pea-beans S.E. Wheat S.E.
0.0125
0.025
0.05
0.1
o-2
0.3
80.8 5.64 73.2 2.81
75.1 5.59 69.8 3.29
72.2 2.74 77.2 1.20
64.1 0.86 74.9 2.45
44.3 1.47 77.6 1.19
30.7 1.36 79-l 1.93
A significant difference in the susceptibility of male and female <. subfasciatus to pyrethrins is shown in Table 3. Similar tests with the other species were not conducted. TABLE 3. SUSCEPTIBILITYOF MALE AND FEMALE zabrotes subfasciatus TO POWDERS CONTAINING 0’2% PYRETHRINSAND 1.0% PIPERONYL BUTOXIDE LDbO (ppm pyrethrins)
Male Female Factor
0.62 0.99 x1-60
LD~
(ppm pyrethrins) l-23 l-75 x1.42
Tables 4 and 5 summarize the results from the tests carried out to determine the 50 and 90 per cent lethal doses of synergised and unsynergised pyrethrins and the accompanying factors of synergism. These results indicate that all species were susceptible to pyrethrins and showed a good response to the synergist. Columns A, B and C represent individual tests as it was not possible to compare all synergist ratios in one experiment. The additional effect of ‘knock-down’ by pyrethrins is shown in Table 6 which contains figures extracted from assays designed to yield mortality regression lines. Affected bruchids include both ‘knocked-down’ and dead insects. The results demonstrate the rapidity with which insects were affected by small doses of pyrethrins before mortality had occurred. The effects of treatment with some pyrethrum powders on oviposition and infestation by C. chinensis and <. subfasciatus are shown in Table 7. DISCUSSION
Results from the 48-hr exposure tests (Table 4) show that <. subfmciatus and obtectus have similar LD~~S but A. obtectus shows greater susceptibility to pyrethrins at the LD~,, level. LD,,,, values are, however, less reliable statistically, and difficulty was experienced with <. subfmciatus because of inconsistent results as indicated by the wider range of the fiducial limits and by the bracketed figures in Table 4. It was not found possible to obtain accurate data for the 1: 10 and 1: 15 synergist ratios. Until further work is done it is wiser to assume similar susceptibility to A.
Susceptibility of Some Bruchid Beetles of Stored Pulses TABLE
4.
DOSAGE-MORTALITY
BRUCHIDS AFTER
48 hr
75
DATA AND FACTORS OF SYNERGISM FOR THREE SPECIES OF
EXPOSURE TO WHEAT TREATED WITH PYRETHRINS SYNERGISED WITH PIPERONYL BUTOXIDE
Treatment
Test
(ppm
LDso pyrethrins)
95% Confidence limits for LD6,,
Factor
of
synergism
(ppm
LDm pyrethrins)
A. obtectus: Pyrethrins alone
-
20.00 17.62 12.63
1.74 4.47 5.83 5.63
11.80 3.75 1.82 2.16
8.32-8.44 7.57-7.62
-
34.13 31.26
6~12-6.20 1.72-2.23 0.12-0.60
1,36 3.87 (28.1 I)*
21.66 (32.68)* (14.35)*
A B C
8.93 8.40 6.59
8-92-8.94 8.39-8.41 6.58-6.60
A B C C
5.13 1.88 1.13 1.17
5.11-5.16 1.88- 1.89 1.05- 1.21 1*05-1.19
A B
8.38 7.59
Pyrethrins +pipbut.
1:
1
1:5 I:10 1:15 <. subfasciatus:
Pyrethrins alone Pyrethrins + pipbut. 1: 1 1:5 I:10 C. chine&
6.16 1.96 (O-27)*
:
Pyrethrins alone Pyrethrins + pipbut.
A B B
I :1 1:5
A
6.65
6.64-6.66
A A
2.67 1.16
2.63-2~72 1.07-1.27
22.13 z-49
6.49
5.73
2.41
* Bracketed figures indicate inconsistencies in the results with <. subfasciatus. Regression lines for the 1:5 and 1: 10 ratios were parallel to each other but not with that for pyrethrins alone, resulting in an unusually high factor of synergism for the 1: 10 synergist ratio and LD@~ values that are apparently not consistent with other tests. Heterogeneity is indicated by the wide range of the fiducial limits for these ratios.
pyrethrins for these two species as indicated by the LD~,, values. The optimum synergist ratio appears to be one part of pyrethrins to ten parts of piperonyl butoxide. This was not determined for <. ~~b&~ckztzlsfor the reasons given above though the results indicated that factors of synergism were similar for these two species. C. chinenris showed similar pyrethrins susceptibility and in limited tests factors of synergism were higher than those for A. obtectus and <. subfaciatus. Results from the 6-day exposure tests (Table 5) give a better comparison between A. obtectus and 2. subfasciatus. Again there was no substantial difference in pyrethrins susceptibility though in this case <. subfasciatus showed a steeper regression line. A. obtectus gave higher factors of synergism and for both species the optimum synergism was one part of pyrethrins to fifteen parts of piperonyl butoxide.
A.J.
76
S. WEAVING
TABLE 5. DOSAGE-MORTALITY DATA AND FACTORSOF SYNERGISM FOR 2 SPECIESOF BRUCHIDSAFTER6 days EXPOSURETO WHEAT TREATEDWITH PYRETHRINS SYNERGXSED WITH PIPERONYLBUTOXIDE
95%
Confidence limits for LDbO
L%.o Treatment
Test (ppm pyrethrins) -____---
Factor of synergism ----__--
LDeo (ppm pyrethrins)
-
10.09 11.11
A. obtectus: Pyrethrins alone Pyrethrins +pipbut. 1: 1 1:5 1:lO 1:15
A B
3.28 3.47
3.27-3.28 3.41-3.53
A A B B
2.36 0.99 0.46 0.35
2.35-2-37 o-99-0.99 0.45-0.46 o-35-0.35
A B
3.97 3.68
3-93-4.02 3-68-3.69
A A B B
2.38 1.09 0.77 0.59
2.38-2.38 l-081.09 0.77-0.77 0.59-O-62
-
4.48 1.72 0.83 0.68
l-39 3.31 7.54 9.91
<. subfasciatus: Pyrethrins alone
-
7.76 7.85
1.67 3.64 4.78 6.24
3.44 1.68 1.33 O-85
Pyrethrins
+ pipbut. 1: 1 1:5 1:lO 1:15
TABLE 6. PERCENTAGEOF BRUCHIDSDEAD AND KNOCKED-DOWN AFTEREXPOSURE FOR 48 hr TO WHEAT TREATEDWITH PYRETHRINS Dead or knocked-down (%) -____-_____ Dose ____ (ppm pyrethrins) C. chinensis <. subfasciatus
Treatment Pyrethrins alone
Pyrethrins +pipbut.
2.00 3.00 5.00 1: 1 1:5
1:lO
Untreated
83.6
77.9 89.2 100.0
1.50 0.50 1.00 2.00 0.25 o-50 l-00
100~0
-
73.0 99.2 -
39.9 94.2 100.0 2.7 67.3 100.0
0.00
9.7
0.0
Factors of synergism and LD 9o figures for the 48-hr exposure periods should form the basis of field trials and control recommendations; rapid ‘knock-down’ and kill are essential to prevent oviposition as the effects of pyrethrins on fertility and emergence are not understood. On the basis of using the optimum synergist ratio (1: 10 to 1: 15)) a dosage rate of 2 ppm of pyrethrins would be expected to give excellent control of A. obtectus and
Susceptibility TABLE
of Some Bruchid
Beetles of Stored
Pulses
7. INFESTATION BYTWO BRWCHIDS OF COWPEAS OR MEXICAN TREATED WITH PYRETHRUM POWDERS
Treatment
Mean No. F, adults
2. subfasciatus on Mexican Control Filler Treated* C. chinensis on cowpeas: Control Filler Treated*
142 pea-beans 309 147 0
77
142 PEA-BEANS
Mean y’ primary infestation of beans
Mean y0 beans bearing eggs
50.2 15.1 0
99.1 82.3 0
72.6 0 0
100.0 0 0
:
311 0 0
* Includes the following treatments: 0.1% pyrethrins-to-5% piperonyl butoxide at l-25 pyrethrins. 0.2% pyrethrins + 1 *O% piperonyl butoxide at 2 a50 ppm of pyrethrins.
ppm of
C. chinensis. This dose may be suitable against <. subfmciatus but this requires confirmation. The powders on the market containing 0 * 2 y0 pyrethrins and l-0 y0 piperonyl butoxide may be appropriate for Bruchid control. Although some control could be expected at the 2 ppm of pyrethrins level, the present results indicate that a powder with a 1: 5 synergist ratio would have to be used at a higher rate, i.e. 3 * 75 ppm, to give 90 per cent kill within 48 hr, compared with l-82 ppm if a 1: 10 synergist ratio was used. The above conclusions are based on mortality only. Before death occurs, insects exposed to pyrethrins pass through a ‘knocked-down’ phase when they are unable to carry out normal activities. Insects are initially only slightly affected but prior to death are nearly motionless. Biological studies are required to determine whether females in this condition can oviposit and whether any resulting eggs are viable. Results from the small scale tests given in Table 7 indicate that a powder with 0 * 1 y0 pyrethrins and 0 - 5 y0 piperonyl butoxide will prevent infestation by <. subfmciatus and C. chinemis when applied at the rate of l-25 ppm of pyrethrins. As indicated by the mortality results of the above assays, this is likely to be caused by rapid ‘knock-down’ rather than kill. The assumption that ‘knocked-down’ females cannot produce an infestation therefore seems plausible since most of these were moribund and unlikely to recover. Further work is required on this aspect. Results in Tables 6 and 7 show that a powder with a 1:5 synergist ratio, applied to give 2 ppm of pyrethrins, would be expected to give excellent control and that a rate of application of 1 ppm of pyrethrins might prove satisfactory. REFERENCES BUSVINE, J. R. (1957) A Critical Review of the Techniques for Testing Insecticides. Commonwealth Institute of Entomology, London. HOWE, R. W. and CURRIE, J. E. (1964) Some laboratory observations on the rates of development, mortality and oviposition of several species of Bruchidae breeding in stored pulses. Bull. ent. Res. 55, 437-477. LLOYD, C. J. and HEWLETT, P. S. (1958) The relative susceptibility to pyrethrum in oil of Coleoptera and Lepidoptera infesting stored products. Bull. ent. Res. 49, 177-185. SALAS, L. and RUPPEL, R. F. (1959) Efectividad de insecticides aplicados en polvo para controlar las principales plagas de1 frijol y de1 maiz almacenados, en Colombia. Agricultura trap. 15, 93-108.