6 fropical Pest Management 26 (1): 3 - 7 :...
1
6
fropical Pest Management 26 (1): 3 - 7 :
3
0 Crown copyright, 1980
000
00
Trials with Decamethrin* for the Control of
Hehthis armigera? on Tomatoes in Senegal
E. F. Collingwood and L. Bourdouxhe
Centre pour le Développement de
SHorticulture,
Projet TF/SEN 13 (BELj,
6. P 154, Dakar, Senegal.
Summary. Trials carried out during 1976 and 1977 have demonstrated the high
insecticidal activity of decamethrin against Heliothis armigera, one of the major pests
of the tomato trop in the Cap Vert region of Senegal. Acephate, carbaryl and
tetrachlorvinphos also gave acceptable levels of control, whereas Baci/us thuringiensis,
dichlorvos and dimethoate were unsatisfactory. The effectiveness of all the products
was diminished when tomatoes were irrigated by means of overhead rotary sprinklers.
Introduction
The tomato is grown on a fairly extensive scale in the Cap Vert region of Senegal which comprises the peninsula
extending for a distance of 40 km or SO to the east of Dakar: cropping estimates show that in the 1974 season for
exampie, between 400 and 500 ha of tomatoes were grown, both for local consumption and for export to Europe
between December and June.
One of the most serious constraints on production is the damage caused by the tomato fruit worm, Heliothis
armigera (Hübn.), which attacks the Young tomato fruits and renders them unsaleable: at the highest point of the
attack it is not uncommon to find up to 85% of the fruits damaged. By calculating the percentage of damaged fruits
at regular intervals it is possible to construct a curve reflecting the rise and fall of the population at different times of
the year.
Figure 1 shows two such curves based on weekly counts made on several hundred fruits harvested from
unsprayed plots of tomatoes in the Cap Vert region during 1976 and 1977. From this it cari be seen that the attack
started about midJanuary, and rose to a peak during May/June and March/ApriI in 1976 and 1977 respectively.
Between mid-August and mid-December in both years, a negligible amount of damage was recorded.
Experience of tomato growing in this region since 1972 suggests that the pattern of population development
is similar in other years, though precise information is not available for 1972-75. However, the months during
which the population of H. armigera is highest coincide with the most important production period. Hence, the use
of insecticides has to be considered to protect the main tomato trop.
The majority of the farms in the Cap Vert region are small, many being of the order of 2000-4000 m2, and
the farmers themselves have little or no experience in handling pesticides. One of the primary considerations,
therefore, in selecting insecticides for tria1 was that they should have a low or a relatively low mammalian toxicity.
Another important factor which has to be considered is that of irrigation. Some of the tomato crops are grown
on land with a fairly high water-table and, apart from watering-in at transplanting, receive no irrigation during the
whole of their growth cycle; whereas other crops, grown on fairly light, well-drained soil are watered several times
per week by means of watering-caris fitted with a coarse sprinkler-rose. Thus, one of the 1977 trials was carried out
on a tomato trop which was irrigated three times per week by means of overhead rotary sprinklers.
*it was learnt while this article was in press that decamethrin, although widely used, has not been accepted as a common name for
NRDC 161 by ISO.
tlepidoptera: Noctuidae, Melicleptriinae.
TropicaF,Pest Management Vol. 26 No. 1, March 1980
. s
100.
90.
,. . .
,.-. -*
,a’
‘.
.’
\\
l ’
;
80.
e
70.
;
60.
.
ul
*Z
J
50.
.
II
z
B
lt
40.
E
4
i’
P
:.I
i
,,
<.
..1976
08
30.
;1
.-- - --.1977
: ,
20.
.
10.
;,.,...
%.\\
O -
j
‘f
Irn
T-y--T-m ’ j ’ j ‘a’mo ‘n ‘d’
Fig. 1. Percentage of tomato fruits damaged !y the tomato fruitworm Heliothis armigera during 1976 and 1977.
In 1973, Elliott et a/. succeeded in synthesising a photostable pyrethroid, thus opening up the possibility of
using this category of insecticide for the control of certain insect pests of agricultural Icrops.
Towards the end of 1975 another photostable pyrethroid, decamethrim (supplied by PROCIDA) became
available in limited quantities and a series of tria’s was planned to compare this material with other selected
insecticides in 1976 and 1977. The 1977 selection included two insecticides of short persistence for use if necessary
during the harvesting period.
z
Materials .and methods
The trials described below were carriecl out at the Centre pour le Développement de I’Horticulture which is
in the Cap Vert region approximately 10 km east of Dakar.
1976 triai
The site used for this tria1 consisted of a fairly sandy topsoil with a heavier underlying layer rich in organic
matter. Irrigation was not necessary during the growing period. Four insecticides were included in this tria1 namely,
acephate, decamethrin, dimethoate and tetrachlorvinphos.
The tria1 comprised a randomised block design with four replications: treatment plots were 5 m x 2 m and
were each planted with two lines of ten tomato plants, cultivar UHN II (VFN determinate), which has a determin-
ate growth form. The transplanting date was 1 April which, due to the rather wet state of the soil, was several weeks
later than originally planned.
Insecticides were applied once every week during the late afternoon by means of a knapsack sprayer, in a
volume of water equivalent to 2000 //hIa. The first of seven weekly applications was made on 10 May: the timing of
the first spray for this and subsequent trials ‘was edetermined by the date of h,atching of Heliofhis eggs and the
Decamethrin for Heliothis control - Collingwood and Bourdouxhe
5
T A B L E 1 . E F F E C T I V E N E S S O F VARIOUS I N S E C T I C I D E S A G A I N S T
H E L I O T H I S A R M I G E R A , 1 9 7 6 T R I A L ( N O N - I R R I G A T E D )
Mean n u m b e r
Dosage of active
Average
of healthy
Insecticide
ingredient per
P ercentage
fruits per
hectare (g)
attack
plot
Decamethrin
2 5
0.1
3 5 6 a
Tetrachlorvinphos
1 5 0 0
5 . 7
3 0 8 ab
Acephate
7 5 0
8 . 0
2 9 5 b
Dimethoate
4 0 0
1 7 . 9
2 1 9 c
Control (untreated)
-
6 1 . 6
9 8 d
-
Means not followed by the same letter are significantly different (P=O.O5) (based on Duncan’s multiple
range test 1.
appearance of the first young caterpillars on the plants. Harvesting began on 8 June and continued until 22 July by
which time the trop was completely mature. At the peak of the production period harvesting was carried out twice
per week.
Results of the 1976 tria1 are shown in Table 1, which also indicates the dosages of insecticide used in this trial.
1977 trials
During the 1977 season two trials were carried out, one with no irrigation during the growing season, the other
irrigated three times per week by means of overhead rotary sprinklers, giving a total of approximately 30 mm of
water per week.
For the two 1977 triais the tomato cultivar ‘ROSS~I’ (VFN determinatel was used: this cultivar produces fairly
large numbers of medium-sized fruits compared with the fewer and larger fruits of UHN 11. This is reflected in the
considerably larger total number of fruits harvested in the 1977 trials, compared with 1976.
All treatments in the 1977 trials were applied in a volume of water equivalent to 1000 llha.
Non-irriga ted tria1 1977
In this trial, the following insecticides were compared:
(a) acephate, Baciifus thuringiensis,” carbaryl, decamethrin, dichlorvos, dimethoate and tetrachlorvinphos, all applied
once per week
(b) decamethrin 2, applied once every two weeks.
The layout of the tria1 was essentially the same as that used in 1976. Plants of tomato cv.
Rossol were trans-
planted on 30 November 1976. The first insecticide application was made on 3 January 1977, there being a total of
eight treatments with the weekly-applied insecticides, and four with the decamethrin applied at fortnightly intervals.
The first of five weekly harvests was carried out on 25 February.
Dosages and results are shown in Table 2.
lrrigated tria1 1977
Insecticides evaluated were the same as described in the tria1 above, except that the 25 g a.i./ha rate of
decamethrin applied every two weeks was replaced by a weekly application of 15 g a.i./ha.
Transplanting date was 8 January: the first of six weekly treatments was applied on 2 March and the first of
five weekly harvests was carried out on 23 March. As in the other two trials, harvesting was continued until the trop
reached maturity. There were only three replications of each treatment in the irrigated tria1 as space was limited.
Dosages and results are shown in Table 3.
“Bacih’us thuringiensis Berliner, var. Kurstaki, serotype 3a, 3b; - 16,000 international units of potency per mg of product.
i
6
Tropical Pest Management Vol. 26 No. 1, March 1980
TABLE 2. EFFECTIVENESS OF VARIOUS INSECTICIDES AGAINST
HELIOTHISARMIGERA, 1977
TRIAL (NON-IRRIGATED)
Mean number
Dosage of active
Average
of healthy
Insecticide
ingredient per
pe rcentage
fruits per
hectare (g)
attack
plot
Decamethrin 1
;!5
0 . 2
3207 a
Decamethrin 2 *
2 5
4.1
1160b
Tetrachlorvinphos
15CI0,
5 . 2
1147 b
Acephate
750
7 . 4
1120 bc
Carbaryl
1500
8.9
1100 bc
Dimethoate
760
30.6
848 d
Dich lorvos
750
33.3
814 d
Bacillus thuringiensis
800 (productl
44.4
6 6 8 e
Control (untreated)
_-
70.0
360 f
Means not followed by the same letter are significantly different (P=O.O5) (based on Duncan’s multiple range
tastl.
* Decamethrin 2, treatment applied once every two weeks.
TABLE 3. EFFECTIVENESS OF VARIOUS INSECTICIDES AGAINST
HELIOTHISARMIGERA, 1977 TRIAL (IRRIGATED)
Mean number
Dosage of active
Average
of healthy
Insecticide
ingredient per
percentage
fruits per
hectare (g)
attack
plot
Decamethrin
2 5
10.4
1079 a
Decamethrin
1 5
12.4
1064 a
Acephate
750
1 9 . 0
969 b
Tetrachlorvinphos
1 5 0 0
22.3
937 b
Carbaryl
1 5 0 0
20.5
926 b
Dichlorvos
750
43.5
687 c
Dimethoate
750
44.2
683 c
Bacillus thuringiensis
800 (product)
54.9
537 d
Control (untreated)
-
67.6
389 e
Means not followed by the same letter are significantly different fP=O.OO) (based on Duncan’s multiple
range test).
Statistical analyses were carried out to compare the numbers of healthy fruits harvested from the various
treatments. Following Bartlett’s tests of homogeneity of variantes, the non-transformed data were subjected to an
analysis of variante and Duncan’s multiple range test to separate the significant means (P=O.Q5).
Results and discussion
Considering first of all the non-irrigated 1976 trial. All four insecticides tested gave an appreciable degree of
control of H. armigera. Outstanding amongst them was decamethrin which, applied weekly at a rate of 25 g a.i./ha
diminished the attack from 61.6% (untreated) to 0.1% of damaged fruits. The two products acephate and
tetrachlorvinphos, with 8% and 5% of damage respectively would also be readily accepted locally as efficient
insecticides.
Decamethrin for Heliothis control - Collingwood and Bourdouxhe
7
In the 1977 non-irrigated trial, the decamethrin again gave the best results, but with a significant difference
between weekly and two-weekly application.
Of the other insecticides tested, tetrachlorvinphos and acephate were not significantly different from
decamethrin applied every two weeks, carbaryl came next, with a level of control which was still satisfactory.
Dichlorvos, with a higher mammalian toxicity than the other products was included because of its shorter
persistence. At a dose of 750 g a.i./ha it gave unsatisfactory results but is perhaps worth testing again at a higher
dosage rate.
Dimethoate was included in the tria1 because it is readily available locally and fairly cheap, rather than for its
qualitises in controlling Lepidoptera. It performed well in 1976 (17.9% damage). However, this performance is
difficult to understand in relation to the 1977 results which, despite an increased dosage rate* showed a considerably
higher percentage of damage (3O.ti%, non-irrigated trial). The results of the 1977 tria1 are, in fact more in line with
those observed when dimethoate has been used on a field scale.
The seventh product tested, the biological insecticide, B. thuringiensis, is very safe frorn the point of view of
mammalian toxicity and residues, and would be an ideai product for use locally. Results obt.ained under the con-
ditions of the 1977 trials were somewhat disappointing, although the dosage used (800 g product /ha) was perha,ps
rather low. lrî addition it is known that several factors such as the variety and serotype of B. thuringiensis used, the
Formulation of the product, the addition of various adjuvants, etc. cari influence the effectiveness of this type of
insecticide (Falcon 1971), thus the results obtained with this sole variety and a single dosage rate should not ba
interpreted too rigidly.
lrrigated tria1 1977
Comparison of the results af the two 1977 trials shows broadly the same order of insecticidal activity in both
trials. It ic apparent, however, that the overhead irrigation has led to a reduction in the effectiveness of all the
insecticides, The same effect would almost certainly be found on the farms where irrigated crops are grown, as these
are watered at least once per day by means of watering-caris. A modified system of watering is being studied at the
Centre, but it may be difficult to supplant the long-established traditional method.
Considerrng the trials as a whole, it is evident that the four best products were acephate, carbaryl, decamethrin
and tetrachlorvinphos. Of these decamethrin showed the highest insecticidal activity. A weekly application of
15 g a.i./ha could probably be recommended where overhead irrigation is used, although this has only been tested
in a single trial: where tomatoes are irrigated other than by overhead irrigation, an application rate of 25 g a.i./ha
every two weeks would appear to be adequate.
The quantity of active ingredient involved in the decamethrin treatment is very small, and when distributed
over the foliage of a tomato trop cari probably be counted in parts per billion in terms of residue. This, in conjun,-
tion with a moderate mammalian toxicity probably makes decamethrin a candidate for consideration as an insecticide
for use even during the period of harvest, and it has been authorised for use on vegetable crops in France with zero
delay between treatment and harvest.
Acknowledgments
These trials were carried out as part of the programme of the horticultural project TF/SEN 13 (BEL) which is
managed jointly by the Ministry of Rural Development of the Government of Senegal and the Food and Agriculture
Organisation, and which is funded in part by the Belgian Government. We wish to thank the Government of Senegal
for permission to publish this paper.
ELLIOTT, M.. FARNHAM, A. W., JANES, N. F., NEEDHAM, P. H., PULMAN, D. A. and STEVENSON, J. H. (1973). A photo-
stable pyrethroid. Nature 246: 169-170.
FALCON, L. A. (1971). Use of bacteria for microbial control of insects. Pp. 67-90. In Microbial conrrd of insects and mites. Ed.
H. D. BURGESS and N. W. HUSSEY. Academic Press, London, New York, San Francisco.
---
-
l Chemical analysis of the dimethoate used in the 1977 triais showed no reduction in the percentage of active ingredient.
6
fropical Pest Management 26 (1): 3 - 7 :
3
0 Crown copyright, 1980
000
00
Trials with Decamethrin* for the Control of
Hehthis armigera? on Tomatoes in Senegal
E. F. Collingwood and L. Bourdouxhe
Centre pour le Développement de
SHorticulture,
Projet TF/SEN 13 (BELj,
6. P 154, Dakar, Senegal.
Summary. Trials carried out during 1976 and 1977 have demonstrated the high
insecticidal activity of decamethrin against Heliothis armigera, one of the major pests
of the tomato trop in the Cap Vert region of Senegal. Acephate, carbaryl and
tetrachlorvinphos also gave acceptable levels of control, whereas Baci/us thuringiensis,
dichlorvos and dimethoate were unsatisfactory. The effectiveness of all the products
was diminished when tomatoes were irrigated by means of overhead rotary sprinklers.
Introduction
The tomato is grown on a fairly extensive scale in the Cap Vert region of Senegal which comprises the peninsula
extending for a distance of 40 km or SO to the east of Dakar: cropping estimates show that in the 1974 season for
exampie, between 400 and 500 ha of tomatoes were grown, both for local consumption and for export to Europe
between December and June.
One of the most serious constraints on production is the damage caused by the tomato fruit worm, Heliothis
armigera (Hübn.), which attacks the Young tomato fruits and renders them unsaleable: at the highest point of the
attack it is not uncommon to find up to 85% of the fruits damaged. By calculating the percentage of damaged fruits
at regular intervals it is possible to construct a curve reflecting the rise and fall of the population at different times of
the year.
Figure 1 shows two such curves based on weekly counts made on several hundred fruits harvested from
unsprayed plots of tomatoes in the Cap Vert region during 1976 and 1977. From this it cari be seen that the attack
started about midJanuary, and rose to a peak during May/June and March/ApriI in 1976 and 1977 respectively.
Between mid-August and mid-December in both years, a negligible amount of damage was recorded.
Experience of tomato growing in this region since 1972 suggests that the pattern of population development
is similar in other years, though precise information is not available for 1972-75. However, the months during
which the population of H. armigera is highest coincide with the most important production period. Hence, the use
of insecticides has to be considered to protect the main tomato trop.
The majority of the farms in the Cap Vert region are small, many being of the order of 2000-4000 m2, and
the farmers themselves have little or no experience in handling pesticides. One of the primary considerations,
therefore, in selecting insecticides for tria1 was that they should have a low or a relatively low mammalian toxicity.
Another important factor which has to be considered is that of irrigation. Some of the tomato crops are grown
on land with a fairly high water-table and, apart from watering-in at transplanting, receive no irrigation during the
whole of their growth cycle; whereas other crops, grown on fairly light, well-drained soil are watered several times
per week by means of watering-caris fitted with a coarse sprinkler-rose. Thus, one of the 1977 trials was carried out
on a tomato trop which was irrigated three times per week by means of overhead rotary sprinklers.
*it was learnt while this article was in press that decamethrin, although widely used, has not been accepted as a common name for
NRDC 161 by ISO.
tlepidoptera: Noctuidae, Melicleptriinae.
TropicaF,Pest Management Vol. 26 No. 1, March 1980
. s
100.
90.
,. . .
,.-. -*
,a’
‘.
.’
\\
l ’
;
80.
e
70.
;
60.
.
ul
*Z
J
50.
.
II
z
B
lt
40.
E
4
i’
P
:.I
i
,,
<.
..1976
08
30.
;1
.-- - --.1977
: ,
20.
.
10.
;,.,...
%.\\
O -
j
‘f
Irn
T-y--T-m ’ j ’ j ‘a’mo ‘n ‘d’
Fig. 1. Percentage of tomato fruits damaged !y the tomato fruitworm Heliothis armigera during 1976 and 1977.
In 1973, Elliott et a/. succeeded in synthesising a photostable pyrethroid, thus opening up the possibility of
using this category of insecticide for the control of certain insect pests of agricultural Icrops.
Towards the end of 1975 another photostable pyrethroid, decamethrim (supplied by PROCIDA) became
available in limited quantities and a series of tria’s was planned to compare this material with other selected
insecticides in 1976 and 1977. The 1977 selection included two insecticides of short persistence for use if necessary
during the harvesting period.
z
Materials .and methods
The trials described below were carriecl out at the Centre pour le Développement de I’Horticulture which is
in the Cap Vert region approximately 10 km east of Dakar.
1976 triai
The site used for this tria1 consisted of a fairly sandy topsoil with a heavier underlying layer rich in organic
matter. Irrigation was not necessary during the growing period. Four insecticides were included in this tria1 namely,
acephate, decamethrin, dimethoate and tetrachlorvinphos.
The tria1 comprised a randomised block design with four replications: treatment plots were 5 m x 2 m and
were each planted with two lines of ten tomato plants, cultivar UHN II (VFN determinate), which has a determin-
ate growth form. The transplanting date was 1 April which, due to the rather wet state of the soil, was several weeks
later than originally planned.
Insecticides were applied once every week during the late afternoon by means of a knapsack sprayer, in a
volume of water equivalent to 2000 //hIa. The first of seven weekly applications was made on 10 May: the timing of
the first spray for this and subsequent trials ‘was edetermined by the date of h,atching of Heliofhis eggs and the
Decamethrin for Heliothis control - Collingwood and Bourdouxhe
5
T A B L E 1 . E F F E C T I V E N E S S O F VARIOUS I N S E C T I C I D E S A G A I N S T
H E L I O T H I S A R M I G E R A , 1 9 7 6 T R I A L ( N O N - I R R I G A T E D )
Mean n u m b e r
Dosage of active
Average
of healthy
Insecticide
ingredient per
P ercentage
fruits per
hectare (g)
attack
plot
Decamethrin
2 5
0.1
3 5 6 a
Tetrachlorvinphos
1 5 0 0
5 . 7
3 0 8 ab
Acephate
7 5 0
8 . 0
2 9 5 b
Dimethoate
4 0 0
1 7 . 9
2 1 9 c
Control (untreated)
-
6 1 . 6
9 8 d
-
Means not followed by the same letter are significantly different (P=O.O5) (based on Duncan’s multiple
range test 1.
appearance of the first young caterpillars on the plants. Harvesting began on 8 June and continued until 22 July by
which time the trop was completely mature. At the peak of the production period harvesting was carried out twice
per week.
Results of the 1976 tria1 are shown in Table 1, which also indicates the dosages of insecticide used in this trial.
1977 trials
During the 1977 season two trials were carried out, one with no irrigation during the growing season, the other
irrigated three times per week by means of overhead rotary sprinklers, giving a total of approximately 30 mm of
water per week.
For the two 1977 triais the tomato cultivar ‘ROSS~I’ (VFN determinatel was used: this cultivar produces fairly
large numbers of medium-sized fruits compared with the fewer and larger fruits of UHN 11. This is reflected in the
considerably larger total number of fruits harvested in the 1977 trials, compared with 1976.
All treatments in the 1977 trials were applied in a volume of water equivalent to 1000 llha.
Non-irriga ted tria1 1977
In this trial, the following insecticides were compared:
(a) acephate, Baciifus thuringiensis,” carbaryl, decamethrin, dichlorvos, dimethoate and tetrachlorvinphos, all applied
once per week
(b) decamethrin 2, applied once every two weeks.
The layout of the tria1 was essentially the same as that used in 1976. Plants of tomato cv.
Rossol were trans-
planted on 30 November 1976. The first insecticide application was made on 3 January 1977, there being a total of
eight treatments with the weekly-applied insecticides, and four with the decamethrin applied at fortnightly intervals.
The first of five weekly harvests was carried out on 25 February.
Dosages and results are shown in Table 2.
lrrigated tria1 1977
Insecticides evaluated were the same as described in the tria1 above, except that the 25 g a.i./ha rate of
decamethrin applied every two weeks was replaced by a weekly application of 15 g a.i./ha.
Transplanting date was 8 January: the first of six weekly treatments was applied on 2 March and the first of
five weekly harvests was carried out on 23 March. As in the other two trials, harvesting was continued until the trop
reached maturity. There were only three replications of each treatment in the irrigated tria1 as space was limited.
Dosages and results are shown in Table 3.
“Bacih’us thuringiensis Berliner, var. Kurstaki, serotype 3a, 3b; - 16,000 international units of potency per mg of product.
i
6
Tropical Pest Management Vol. 26 No. 1, March 1980
TABLE 2. EFFECTIVENESS OF VARIOUS INSECTICIDES AGAINST
HELIOTHISARMIGERA, 1977
TRIAL (NON-IRRIGATED)
Mean number
Dosage of active
Average
of healthy
Insecticide
ingredient per
pe rcentage
fruits per
hectare (g)
attack
plot
Decamethrin 1
;!5
0 . 2
3207 a
Decamethrin 2 *
2 5
4.1
1160b
Tetrachlorvinphos
15CI0,
5 . 2
1147 b
Acephate
750
7 . 4
1120 bc
Carbaryl
1500
8.9
1100 bc
Dimethoate
760
30.6
848 d
Dich lorvos
750
33.3
814 d
Bacillus thuringiensis
800 (productl
44.4
6 6 8 e
Control (untreated)
_-
70.0
360 f
Means not followed by the same letter are significantly different (P=O.O5) (based on Duncan’s multiple range
tastl.
* Decamethrin 2, treatment applied once every two weeks.
TABLE 3. EFFECTIVENESS OF VARIOUS INSECTICIDES AGAINST
HELIOTHISARMIGERA, 1977 TRIAL (IRRIGATED)
Mean number
Dosage of active
Average
of healthy
Insecticide
ingredient per
percentage
fruits per
hectare (g)
attack
plot
Decamethrin
2 5
10.4
1079 a
Decamethrin
1 5
12.4
1064 a
Acephate
750
1 9 . 0
969 b
Tetrachlorvinphos
1 5 0 0
22.3
937 b
Carbaryl
1 5 0 0
20.5
926 b
Dichlorvos
750
43.5
687 c
Dimethoate
750
44.2
683 c
Bacillus thuringiensis
800 (product)
54.9
537 d
Control (untreated)
-
67.6
389 e
Means not followed by the same letter are significantly different fP=O.OO) (based on Duncan’s multiple
range test).
Statistical analyses were carried out to compare the numbers of healthy fruits harvested from the various
treatments. Following Bartlett’s tests of homogeneity of variantes, the non-transformed data were subjected to an
analysis of variante and Duncan’s multiple range test to separate the significant means (P=O.Q5).
Results and discussion
Considering first of all the non-irrigated 1976 trial. All four insecticides tested gave an appreciable degree of
control of H. armigera. Outstanding amongst them was decamethrin which, applied weekly at a rate of 25 g a.i./ha
diminished the attack from 61.6% (untreated) to 0.1% of damaged fruits. The two products acephate and
tetrachlorvinphos, with 8% and 5% of damage respectively would also be readily accepted locally as efficient
insecticides.
Decamethrin for Heliothis control - Collingwood and Bourdouxhe
7
In the 1977 non-irrigated trial, the decamethrin again gave the best results, but with a significant difference
between weekly and two-weekly application.
Of the other insecticides tested, tetrachlorvinphos and acephate were not significantly different from
decamethrin applied every two weeks, carbaryl came next, with a level of control which was still satisfactory.
Dichlorvos, with a higher mammalian toxicity than the other products was included because of its shorter
persistence. At a dose of 750 g a.i./ha it gave unsatisfactory results but is perhaps worth testing again at a higher
dosage rate.
Dimethoate was included in the tria1 because it is readily available locally and fairly cheap, rather than for its
qualitises in controlling Lepidoptera. It performed well in 1976 (17.9% damage). However, this performance is
difficult to understand in relation to the 1977 results which, despite an increased dosage rate* showed a considerably
higher percentage of damage (3O.ti%, non-irrigated trial). The results of the 1977 tria1 are, in fact more in line with
those observed when dimethoate has been used on a field scale.
The seventh product tested, the biological insecticide, B. thuringiensis, is very safe frorn the point of view of
mammalian toxicity and residues, and would be an ideai product for use locally. Results obt.ained under the con-
ditions of the 1977 trials were somewhat disappointing, although the dosage used (800 g product /ha) was perha,ps
rather low. lrî addition it is known that several factors such as the variety and serotype of B. thuringiensis used, the
Formulation of the product, the addition of various adjuvants, etc. cari influence the effectiveness of this type of
insecticide (Falcon 1971), thus the results obtained with this sole variety and a single dosage rate should not ba
interpreted too rigidly.
lrrigated tria1 1977
Comparison of the results af the two 1977 trials shows broadly the same order of insecticidal activity in both
trials. It ic apparent, however, that the overhead irrigation has led to a reduction in the effectiveness of all the
insecticides, The same effect would almost certainly be found on the farms where irrigated crops are grown, as these
are watered at least once per day by means of watering-caris. A modified system of watering is being studied at the
Centre, but it may be difficult to supplant the long-established traditional method.
Considerrng the trials as a whole, it is evident that the four best products were acephate, carbaryl, decamethrin
and tetrachlorvinphos. Of these decamethrin showed the highest insecticidal activity. A weekly application of
15 g a.i./ha could probably be recommended where overhead irrigation is used, although this has only been tested
in a single trial: where tomatoes are irrigated other than by overhead irrigation, an application rate of 25 g a.i./ha
every two weeks would appear to be adequate.
The quantity of active ingredient involved in the decamethrin treatment is very small, and when distributed
over the foliage of a tomato trop cari probably be counted in parts per billion in terms of residue. This, in conjun,-
tion with a moderate mammalian toxicity probably makes decamethrin a candidate for consideration as an insecticide
for use even during the period of harvest, and it has been authorised for use on vegetable crops in France with zero
delay between treatment and harvest.
Acknowledgments
These trials were carried out as part of the programme of the horticultural project TF/SEN 13 (BEL) which is
managed jointly by the Ministry of Rural Development of the Government of Senegal and the Food and Agriculture
Organisation, and which is funded in part by the Belgian Government. We wish to thank the Government of Senegal
for permission to publish this paper.
ELLIOTT, M.. FARNHAM, A. W., JANES, N. F., NEEDHAM, P. H., PULMAN, D. A. and STEVENSON, J. H. (1973). A photo-
stable pyrethroid. Nature 246: 169-170.
FALCON, L. A. (1971). Use of bacteria for microbial control of insects. Pp. 67-90. In Microbial conrrd of insects and mites. Ed.
H. D. BURGESS and N. W. HUSSEY. Academic Press, London, New York, San Francisco.
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l Chemical analysis of the dimethoate used in the 1977 triais showed no reduction in the percentage of active ingredient.