- Email: [email protected]
Survival of the nuclear polyhedrosis virus of Heliothis armigera on crops and in soil in Botswana
JOURNAL
OF INVERTEBRATE
PATHOLOGY
Survival Heliofhis
27, 7-12
of the Nuclear armigera
Polyhedrosis
on Crops
R. E. ROOME’ Centrefor
(1976)
Virus of
and in Soil in Botswana
AND R. A. DAOUST2
Pest Research, College House, Wrights Lane, London W8 SSJ. United Kingdom
Overseas
Received
October
25, 1974
Experiments are described that record the change in activity of the nuclear polyhedrosis virus of Heliothis armigera on cotton and sorghum in the short-term and long-term survival of the virus on sorghum and in soil. Virus activity was lost rapidly on cotton but remained at a high level for up to 30 days on sorghum. Activity was still detectable on harvested sorghum more than 80 days after spraying and on the soil below, but soil activity fell to less than onethird of its original level during the winter.
INTRODUCTION Heliothis armigera is a major pest of sorghum, the main subsistence crop in Botswana. The use of a nuclear polyhedrosis virus to control this pest is being considered as part of an integrated approach to protecting sorghum. A similar virus has already been shown to control Heliothis zea successfully on sorghum in the United States (Ignoffo et al., 1965). Field trials with the virus in Botswana have shown that much greater quantities of virus are required to control Heliothis on cotton than on sorghum (Roome, 1975). The experiments recorded in this paper were carried out to measure and compare the survival of the virus on sorghum and cotton. In addition, survival of the virus in the soil surface was measured to see if this offered a possible means of carry-over of the disease ‘Seconded by the Overseas Development Administration of the United Kingdom’s Foreign and Commonwealth Office from the Centre for Overseas Pest Research to the Division of Agricultural Research, Botswana, as Entomologist in the Dryland Farming Research Scheme. Present address: Centre for Overseas Pest Research, College House, Wrights Lane, London W8 5SJ, UK Requests for reprints should be sent to R. E. Roome. ?Seconded by the United States Peace Crops to the Division of Agricultural Research, Botswana, as Assistant Entomologist. Present address: Village Park Apts., Amherst, Massachusetts 01002.
Copyright (r, 1976 by Academic Press, Inc. All rights of reproduction in any form reserved.
between seasons. Such carry-over of a nuclear polyhedrosis virus disease has been demonstrated in Canada (Jaques and Harcourt, 1971) but under soil conditions very different from those found in Botswana. MATERIALS AND METHODS Short-term persistency of the virus on sorghum and cotton was tested by allowing larvae to feed on samples of the sprayed crop at successive intervals after spraying. A range of concentrations of virus was used on two sorghum crops and one cotton crop (Table 1). Random samples of crop material, florets from the sorghum and leaves from the cotton, were taken daily after spraying. Care was taken to ensure that no cross-contamination occurred between the various concentrations. Virus activity on the samples was tested by allowing 25 3-day old Heliothis larvae to feed on each sample for 1 day. The larvae were then placed singly in tubes of uncontaminated artificial diet, and their subsequent mortality was recorded daily. Two control treatments were included in each daily test, one of unsprayed crop and a second where larvae remained continually on artificial diet. The virus suspensions used in spraying were unpurified suspensions prepared as described by Daoust and Roome (1974), with one larval equivalent (L.E.) being taken as 6 x log polyhedra of virus.
ROOME AND DAOUST TABLE 1 LTso of Virus Residues of Different Ages on Sorghum and Cotton (Expressed as Days to 50%, Mortality of Heliothis Test Larvae)’ Larvae tested per day
0
1
2
3
4
I
Sorghum Test 1 200 L.E./hectare 55 L.E./hectare 12.5 L.E./hectare Field control Lab. control
25 25 25 25 25
4.05 4.21 4.65 6.00 -
4.21 4.01 5.80 -
4.10 4.87 -
4.96 -
3.81 3.45 -
3.85 4.89 4.32 4.61
Sorghum Test 2 200 L.E./hectare 55 L.E./hectare 12.5 L.E./hectare Field control Lab. control
25 25 25 25 25
4.00 3.76 4.20 -
4.18 4.60 5.18 -
4.71 5.30 -
4.80 5.30 -
5.08 6.00 -
5.08 5.68 8.00
-
-
Cotton test 200 L.E./hectare + molasses 15% 200 L.E./hectare 55 L.E./hectare 12.5 L.E./hectare Field control Lab. control
25 25 25 25 25 25
3.18 4.71 5.00 -
5.80 6.53 -
-
8.00 -
-
Quantity of virus applied
Days after spraying
-
10
15
32
6.23 5.55
4.05 -
4.80 4.44
-
-
-
-
‘Dashes indicate tests in which 50% mortality was not achieved.
Long-term persistency of the virus was measured in soil and in samples of dust from harvested heads of both sprayed (var. segaolane)and unsprayed (var. 8D) sorghum crops. The dust samples were collected from below bags of harvested heads and were prepared for testing by passing through a 1/32in. sieve. One percent suspensionsin water were made of each of the subsamplesso obtained (two of coarse and two of fine material). Two sets of soil samples were taken, one immediately after harvest (87 days after spraying) and one after the winter (252 days after spraying). In the latter instance, the area had been cultivated to destroy tillering sorghum residues, and some soil inversion had occurred during this cultivation. Each set of soil samples consisted of six main samples, one from each of six widely separated plots in both the sprayed and the unsprayed crop areas. From each plot 20
subsamples of soil were taken, each of an area 40 cm2, to a depth of l-4 mm. The subsamples from one plot were bulked to form one main sample for testing. The soil samples were weighed and passed through a l/32-in. sieve. The coarse fraction was discarded, and the fine fraction was weighed again. A 50% suspension in water was made from each sample of fine fraction. The dust and soil suspensionswere tested for virus activity by the bioassay method described by Daoust and Roome (1974) 0.1 ml of suspensionbeing added to the surface of the diet (415 mm2) in a glass tube and one 3-day-old larva being placed in each tube. Twenty larvae were tested against each suspension, and the tests were replicated four times. Larva1 mortality was checked daily. All Heliothis larvae used in all the above tests were drawn from an insectary culture reared in continuous light at 27 * 1°C on a slightly modified Bot’s (1966) artificial diet.
SURVIVAL
OF
Heliothis
VIRUS
IN
BOTSWANA
c)cotton Key
x-x
ZOOL.E./hectare+15%molasses
x-----x o--o
2OOL.E.lhectar.z 55 L.E./hectare
O-----• AA
12.5L.E.ihectare Field control
n-----a
Lab.control
Days FIG. polyhedrosis
I,
Mortality virus;
of Heliorhis change
in mortality
armigera with
larvae increasing
fed on
sorghum
florets
age of the sprayed
RESULTS Mortality of test larvae after feeding on the sorghum from the first test fell during the first 3 days after spraying with the virus, but it rose again on the fourth day and returned to near its original level by the seventh day. Mortality also rose among the larvae on unsprayed sorghum and was high in the untreated controls on the seventh day (Fig. la).
or cotton
leaves
sprayed
with
a nuclear
virus.
In the second test on sorghum, mortality on the sprayed plots was initially high, and although a considerable fall occurred over the first 7 days with the highest treatment, treatment mortalities remained markedly higher than the control level of mortality up to day 32 (Fig. 1b). The more rapid return to high mortality levels in the first compared with the second sorghum test was probably due to the presence of a natural infestation of Hefiothis
10
ROOME AND DAOUST TABLE 2 Residues of Hefiofhis Nuclear Polyhedrosis Virus in Dust from Harvested Virus-Sprayed (Segaolane) and Unsprayed (8D) Sorghum Heads; Mortality of Test Larvae and Estimates of Virus Residues 87 Days after Spraying Sample
Segaolane dust fine coarse 8D dust fine coarse
Mean Mortality (%)
Mean trans. (angular)4
Polyhedra per gramb
91.25 85.25
71.20ab 67.68abc
9399 x 103 6851 x 103
1.6 x 1O-3 1.2 x 1o-3
52.50 33.75
46 24efg 35:l 8ghi
2419 x 103 1374 x 103
0.4 x 1o-3 0.2 x 1o-3
Larval cquivs per gramC
‘Analyzed in conjunction with data in Table 3. For method of analysis, standard error, and degrees of freedom, see Table 3 and footnote. b*cSee footnotes to Table 3.
larvae in the first test. The death of these larvae could have resulted in an increase of available, fresh virus on the sorghum tested. In the second sorghum test, no natural Heliothis infestation was present. In the cotton test, mortality due to all the test concentrations decreased rapidly in the first 4 days and subsequently did not differ from the controls (Fig. lc). Table 1 gives the LT,, (number of days taken to reach 50% mortality) of each of the test groups of larvae in the three trials. Although the changes are not clear-cut, there is an initial tendency for t,he LT,, to increase, followed by a decrease on the sorghum but not in the cotton test. High field control mortality occurred in Test 1 on sorghum on Days 1 and 7. Laboratory control mortality was also high on Day 7. The very sharp changes in mortality shown in Figure 1 suggest some variability in the susceptibility of the culture larvae used in the tests, which might have been reduced had greater replication been possible. Nevertheless, the overall tendency is for comparatively high mortality to be maintained for a long period on sorghum, while on cotton mortality due to the virus decreases very rapidly to negligible values. Table 2 shows the percentage of mortalities resulting from exposure of test larvae to samples of harvest dust taken 87 days after spraying. Mortality was higher in tests of samples of both harvest material and soil (Table 3) from the virus-sprayed
crop than from the unsprayed crop, although considerable mortality was also induced by the latter. Mortality decreased considerably in the soil samples during the winter, falling from a mean of 294 to 78 L.E./hectare in sprayed soil, and in only two of the sprayed samples taken after the winter did mortality exceed the average amount induced by the unsprayed soil (Table 3). There was little change over the winter in the low activity of the samples from unsprayed soil. As might have been expected, measurements of active polyhedra per gram were higher for the harvest dust than for the soil. DISCUSSION The results of the cotton test support those of previous workers in showing a decline in effectiveness after a few days (McKinley, 1971; Bullock, 1967), while much better persistence occurs on sorghum. The basic difference in the structure of the spray target on sorghum and cotton probably accounts adequately for the difference in apparent viability of the virus on these two crops. The compact sorghum head probably shields the virus from the sun. Growth within the head is not comparable with that of the cotton plant, and infesting larvae are concentrated in the head. Those that die of the virus disease climb first to the top of the head so that their disintegrating cadavers tend to fall back into the head. Cotton is more open and its continued. growth means
46 54efg 48:67def 56.16cde 60.52bcd 73.67a 75.70a
35.93fgh 28.82hij 22.78’j 42.84fg 18.143 19.09j
20.6Oj 4.166 46
51.25 56.25 68.75 75.00 93.75 97.50
35.00 25.00 16.25 46.25 10.00 11.25
11.25
winter
Nuclear
Mean trans. (angular)a
Before
of Heliofhis
28635 19422 11122 40587
46729 53701 76692 93209 225096 351256
0.28
0.48 0.32 0.18 0.68
0.78 0.89 1.28 1.55 3.75 5.86 2.35 X x x x 0 0 x
x x x x x x x
1O-5
1O-5 lo--’ 1O-5 1o-5
1O-5 1O-5 1O-5 1o-5 1o-5 lo-’ 1O-5
Larval equivs per gramC
spraying)
35
294
Larval equivs per hectare
15.0
21.3 25.0 30.0 25.0 18.8 25.0
83.8 63.8 26.3 17.5 20.0 12.5
Mean Mortality (%I
different (Duncan’s multiple range test). made from bioassay data (Daoust and Roome,
0
0 0
Polyhedra per gramb
(87 days after
After
1974)
where
21.84Cd 4.377 36
26.32’ 26.20’ 31.66’ 28.87’ 25.63c 25.95c
67.83a 54.46b 29.29c 21.41Cd 24.60Cd 1 1.25d
(252
Areas;
0
0
percentage
12699 16434 21165 16434 9960 16434
124749 63412 17679 8134 11371
spraying)
x x x X x X X 0
1o-5 1O-5 1o-5 1O-5 1o-5 1O-5 1O-5
1O-5
1O-5 1O-5 1o-5 1o-5 1o-5
of mortality
0.21 0.27 0.35 0.27 0.17 0.27 0.26
x X x x x 0 0.63 x
2.08 1.06 0.29 0.14 0.19
Larval equivs per gramC
days after
(8D)
Polyhedra per gramb
probit
winter
and Unsprayed
Mean trans. (angular)’
‘TABLE 3 Polyhedrosis Virus in Soil from Virus-Sprayed (Segaolane) Mortality of Test Larvae and Estimates of Virus Residue@
‘Numbers followed by the same letter are not significantly bMean mortality corrected by Abbott’s formula. Estimate and where x = log (10 X PIB/mm2). ‘One larval equivalent = 6 X log polyhedra.
Segaolane soil (sprayed) 1 2 3 4 5 6 Soil mean 8D soil (unsprayed) 1 2 3 4 5 6 Soil mean Control Standard error Degrees of freedom
Sample
Mean Mortality (%I
Residues
= 0.78
X 2.34x
33
78
Larval equivs per hectare
12
ROOME
AND
that the virus is diluted and exposed. Most of the virus falls on leaves, whereas larval feeding is concentrated on flower buds, flowers, and bolls. Larvae dying of the disease on cotton are exposed and more likely to be washed off or blown away. Although quantities of virus in the soil surface 87 days after spraying (294 L.E./ hectare) appear to be greater than those originally sprayed (200 L.E./hectare), their distribution must be very different. During spraying the virus is directed into the sorghum head so that although 200 L.E./ hectare represents the gross quantity applied, it is present in relatively concentrated pockets in each sorghum head. Virus in the soil may represent some of the original virus not deposited on the target or washed and blown off, but it mainly consists of newly formed virus from larvae dying in the head, which is washed and blown from the head before and at harvest. Its distribution must be much more diffuse than that originally on the crop. Its activity has been shown to decrease during winter, and there is a further period for attenuation between winter and the occurrence of new Heliothis infestations on the land, the effects of which were not measured. The likelihood of soil virus residues being a major factor in the initiation of economically important virus outbreaks in subsequent seasons seem small. Heliothis attacks most crops grown in Botswana; thus, crop rotation may not reduce this likelihood. Similarly, the introduction of minimum tillage practices may improve carryover of virus, but fallowing will almost certainly decrease it. These three agronomic practices are all considered important in improving dryland cropping in Botswana (Gibbon, 1973). It is possible that some carryover of virus occurs in soil and that this may infect larvae early in a following season, leading to effective virus mortality late in that season. However, it does not seem likely that this is an important contributor to control of Heliothis early in the season.
DAOUST
Heliothis larval infestations rarely last more than 30 days on sorghum, so the long survival of Heliothis nuclear polyhedrosis virus on sorghum means that one application of virus is adequate to protect this crop. ACKNOWLEDGMENTS The authors thank the Ministry of Agriculture of the Government of Botswana for permission to publish this work. They also acknowledge the work of technical assistants 0. Mpudi, B. Lekoma, J. Moruisi, S. Gunda, and S. Dihoro and International Voluntary Service Assistant Entomologist Kaye Flattery and thank Dr. R. F. Chapman and Mr. W. R. Ingram for reading and criticizing the manuscript.
REFERENCES BOG, J. 1966. Rearing Heliothis nrmigera (Hubn.) and Prodenia litura F. on an artificial diet. S. Afr. J. Agr. Sci..9,535-538.
BULLOCK, H. R. 1967. Persistence of Heliothis nuclearpolyhedrosis virus on cotton foliage. J. Invertebr. Pnthol.. 9,434~436. DAOUST, R. A., AND ROOME, R. E. 1974. Bioassay of a nuclear-polyhedrosis virus and BociNus fhuringiettsis against the American Bollworm, Heliothis armigera, in Botswana. J. Inverlebr. Pathol., 23,318-324. GIBBON, D. 1973. “Dryland Crop Production Research in Botswana: An Interim Review and Report.” Agronomist: ODA Dryland Farming Research Scheme, Agricultural Research Station, Private Bag 33, Gaborone, Botswana. IGNOFFO, C. M., CHAPMAN, A. J.. AND MARTIN, D. F. 1965. The nuclear-polyhedrosis virus of Heliorhis zea (Boddie) and Heliorhis virescens Fabricius. III. Effectiveness of the virus against field populations of Heliothis on cotton, corn and grain sorghum. J. Invertebr. Pathol.. I, 227-235. JAQUES, R. P.. AND HARCOURT, D. G. 1971. Viruses of Trichoplusia ni (Lepidoptera: Noctuidae) and Pieris rapae (Lepidoptera: Pieridae) in soil in fields of crucifers in southern Ontario. Can. Enfomol.. 103. 12851290.
D. J. 1971. Nuclear polyhedrosis virus of the cotton bollworm in Central Africa. Carton Grow. Rev., 48,297-303. ROOME, R. E. 1975. Field trial, with a nuclear polyhedrosis virus and Bacillus rhuringiensis against Helio/his armigera (Hbn.) larvae on sorghum and cotton in Botswana. Bull. Int. Res.. 65 (3) In press. Accepted and to be published September 1975. MCKINLEY,
OF INVERTEBRATE
PATHOLOGY
Survival Heliofhis
27, 7-12
of the Nuclear armigera
Polyhedrosis
on Crops
R. E. ROOME’ Centrefor
(1976)
Virus of
and in Soil in Botswana
AND R. A. DAOUST2
Pest Research, College House, Wrights Lane, London W8 SSJ. United Kingdom
Overseas
Received
October
25, 1974
Experiments are described that record the change in activity of the nuclear polyhedrosis virus of Heliothis armigera on cotton and sorghum in the short-term and long-term survival of the virus on sorghum and in soil. Virus activity was lost rapidly on cotton but remained at a high level for up to 30 days on sorghum. Activity was still detectable on harvested sorghum more than 80 days after spraying and on the soil below, but soil activity fell to less than onethird of its original level during the winter.
INTRODUCTION Heliothis armigera is a major pest of sorghum, the main subsistence crop in Botswana. The use of a nuclear polyhedrosis virus to control this pest is being considered as part of an integrated approach to protecting sorghum. A similar virus has already been shown to control Heliothis zea successfully on sorghum in the United States (Ignoffo et al., 1965). Field trials with the virus in Botswana have shown that much greater quantities of virus are required to control Heliothis on cotton than on sorghum (Roome, 1975). The experiments recorded in this paper were carried out to measure and compare the survival of the virus on sorghum and cotton. In addition, survival of the virus in the soil surface was measured to see if this offered a possible means of carry-over of the disease ‘Seconded by the Overseas Development Administration of the United Kingdom’s Foreign and Commonwealth Office from the Centre for Overseas Pest Research to the Division of Agricultural Research, Botswana, as Entomologist in the Dryland Farming Research Scheme. Present address: Centre for Overseas Pest Research, College House, Wrights Lane, London W8 5SJ, UK Requests for reprints should be sent to R. E. Roome. ?Seconded by the United States Peace Crops to the Division of Agricultural Research, Botswana, as Assistant Entomologist. Present address: Village Park Apts., Amherst, Massachusetts 01002.
Copyright (r, 1976 by Academic Press, Inc. All rights of reproduction in any form reserved.
between seasons. Such carry-over of a nuclear polyhedrosis virus disease has been demonstrated in Canada (Jaques and Harcourt, 1971) but under soil conditions very different from those found in Botswana. MATERIALS AND METHODS Short-term persistency of the virus on sorghum and cotton was tested by allowing larvae to feed on samples of the sprayed crop at successive intervals after spraying. A range of concentrations of virus was used on two sorghum crops and one cotton crop (Table 1). Random samples of crop material, florets from the sorghum and leaves from the cotton, were taken daily after spraying. Care was taken to ensure that no cross-contamination occurred between the various concentrations. Virus activity on the samples was tested by allowing 25 3-day old Heliothis larvae to feed on each sample for 1 day. The larvae were then placed singly in tubes of uncontaminated artificial diet, and their subsequent mortality was recorded daily. Two control treatments were included in each daily test, one of unsprayed crop and a second where larvae remained continually on artificial diet. The virus suspensions used in spraying were unpurified suspensions prepared as described by Daoust and Roome (1974), with one larval equivalent (L.E.) being taken as 6 x log polyhedra of virus.
ROOME AND DAOUST TABLE 1 LTso of Virus Residues of Different Ages on Sorghum and Cotton (Expressed as Days to 50%, Mortality of Heliothis Test Larvae)’ Larvae tested per day
0
1
2
3
4
I
Sorghum Test 1 200 L.E./hectare 55 L.E./hectare 12.5 L.E./hectare Field control Lab. control
25 25 25 25 25
4.05 4.21 4.65 6.00 -
4.21 4.01 5.80 -
4.10 4.87 -
4.96 -
3.81 3.45 -
3.85 4.89 4.32 4.61
Sorghum Test 2 200 L.E./hectare 55 L.E./hectare 12.5 L.E./hectare Field control Lab. control
25 25 25 25 25
4.00 3.76 4.20 -
4.18 4.60 5.18 -
4.71 5.30 -
4.80 5.30 -
5.08 6.00 -
5.08 5.68 8.00
-
-
Cotton test 200 L.E./hectare + molasses 15% 200 L.E./hectare 55 L.E./hectare 12.5 L.E./hectare Field control Lab. control
25 25 25 25 25 25
3.18 4.71 5.00 -
5.80 6.53 -
-
8.00 -
-
Quantity of virus applied
Days after spraying
-
10
15
32
6.23 5.55
4.05 -
4.80 4.44
-
-
-
-
‘Dashes indicate tests in which 50% mortality was not achieved.
Long-term persistency of the virus was measured in soil and in samples of dust from harvested heads of both sprayed (var. segaolane)and unsprayed (var. 8D) sorghum crops. The dust samples were collected from below bags of harvested heads and were prepared for testing by passing through a 1/32in. sieve. One percent suspensionsin water were made of each of the subsamplesso obtained (two of coarse and two of fine material). Two sets of soil samples were taken, one immediately after harvest (87 days after spraying) and one after the winter (252 days after spraying). In the latter instance, the area had been cultivated to destroy tillering sorghum residues, and some soil inversion had occurred during this cultivation. Each set of soil samples consisted of six main samples, one from each of six widely separated plots in both the sprayed and the unsprayed crop areas. From each plot 20
subsamples of soil were taken, each of an area 40 cm2, to a depth of l-4 mm. The subsamples from one plot were bulked to form one main sample for testing. The soil samples were weighed and passed through a l/32-in. sieve. The coarse fraction was discarded, and the fine fraction was weighed again. A 50% suspension in water was made from each sample of fine fraction. The dust and soil suspensionswere tested for virus activity by the bioassay method described by Daoust and Roome (1974) 0.1 ml of suspensionbeing added to the surface of the diet (415 mm2) in a glass tube and one 3-day-old larva being placed in each tube. Twenty larvae were tested against each suspension, and the tests were replicated four times. Larva1 mortality was checked daily. All Heliothis larvae used in all the above tests were drawn from an insectary culture reared in continuous light at 27 * 1°C on a slightly modified Bot’s (1966) artificial diet.
SURVIVAL
OF
Heliothis
VIRUS
IN
BOTSWANA
c)cotton Key
x-x
ZOOL.E./hectare+15%molasses
x-----x o--o
2OOL.E.lhectar.z 55 L.E./hectare
O-----• AA
12.5L.E.ihectare Field control
n-----a
Lab.control
Days FIG. polyhedrosis
I,
Mortality virus;
of Heliorhis change
in mortality
armigera with
larvae increasing
fed on
sorghum
florets
age of the sprayed
RESULTS Mortality of test larvae after feeding on the sorghum from the first test fell during the first 3 days after spraying with the virus, but it rose again on the fourth day and returned to near its original level by the seventh day. Mortality also rose among the larvae on unsprayed sorghum and was high in the untreated controls on the seventh day (Fig. la).
or cotton
leaves
sprayed
with
a nuclear
virus.
In the second test on sorghum, mortality on the sprayed plots was initially high, and although a considerable fall occurred over the first 7 days with the highest treatment, treatment mortalities remained markedly higher than the control level of mortality up to day 32 (Fig. 1b). The more rapid return to high mortality levels in the first compared with the second sorghum test was probably due to the presence of a natural infestation of Hefiothis
10
ROOME AND DAOUST TABLE 2 Residues of Hefiofhis Nuclear Polyhedrosis Virus in Dust from Harvested Virus-Sprayed (Segaolane) and Unsprayed (8D) Sorghum Heads; Mortality of Test Larvae and Estimates of Virus Residues 87 Days after Spraying Sample
Segaolane dust fine coarse 8D dust fine coarse
Mean Mortality (%)
Mean trans. (angular)4
Polyhedra per gramb
91.25 85.25
71.20ab 67.68abc
9399 x 103 6851 x 103
1.6 x 1O-3 1.2 x 1o-3
52.50 33.75
46 24efg 35:l 8ghi
2419 x 103 1374 x 103
0.4 x 1o-3 0.2 x 1o-3
Larval cquivs per gramC
‘Analyzed in conjunction with data in Table 3. For method of analysis, standard error, and degrees of freedom, see Table 3 and footnote. b*cSee footnotes to Table 3.
larvae in the first test. The death of these larvae could have resulted in an increase of available, fresh virus on the sorghum tested. In the second sorghum test, no natural Heliothis infestation was present. In the cotton test, mortality due to all the test concentrations decreased rapidly in the first 4 days and subsequently did not differ from the controls (Fig. lc). Table 1 gives the LT,, (number of days taken to reach 50% mortality) of each of the test groups of larvae in the three trials. Although the changes are not clear-cut, there is an initial tendency for t,he LT,, to increase, followed by a decrease on the sorghum but not in the cotton test. High field control mortality occurred in Test 1 on sorghum on Days 1 and 7. Laboratory control mortality was also high on Day 7. The very sharp changes in mortality shown in Figure 1 suggest some variability in the susceptibility of the culture larvae used in the tests, which might have been reduced had greater replication been possible. Nevertheless, the overall tendency is for comparatively high mortality to be maintained for a long period on sorghum, while on cotton mortality due to the virus decreases very rapidly to negligible values. Table 2 shows the percentage of mortalities resulting from exposure of test larvae to samples of harvest dust taken 87 days after spraying. Mortality was higher in tests of samples of both harvest material and soil (Table 3) from the virus-sprayed
crop than from the unsprayed crop, although considerable mortality was also induced by the latter. Mortality decreased considerably in the soil samples during the winter, falling from a mean of 294 to 78 L.E./hectare in sprayed soil, and in only two of the sprayed samples taken after the winter did mortality exceed the average amount induced by the unsprayed soil (Table 3). There was little change over the winter in the low activity of the samples from unsprayed soil. As might have been expected, measurements of active polyhedra per gram were higher for the harvest dust than for the soil. DISCUSSION The results of the cotton test support those of previous workers in showing a decline in effectiveness after a few days (McKinley, 1971; Bullock, 1967), while much better persistence occurs on sorghum. The basic difference in the structure of the spray target on sorghum and cotton probably accounts adequately for the difference in apparent viability of the virus on these two crops. The compact sorghum head probably shields the virus from the sun. Growth within the head is not comparable with that of the cotton plant, and infesting larvae are concentrated in the head. Those that die of the virus disease climb first to the top of the head so that their disintegrating cadavers tend to fall back into the head. Cotton is more open and its continued. growth means
46 54efg 48:67def 56.16cde 60.52bcd 73.67a 75.70a
35.93fgh 28.82hij 22.78’j 42.84fg 18.143 19.09j
20.6Oj 4.166 46
51.25 56.25 68.75 75.00 93.75 97.50
35.00 25.00 16.25 46.25 10.00 11.25
11.25
winter
Nuclear
Mean trans. (angular)a
Before
of Heliofhis
28635 19422 11122 40587
46729 53701 76692 93209 225096 351256
0.28
0.48 0.32 0.18 0.68
0.78 0.89 1.28 1.55 3.75 5.86 2.35 X x x x 0 0 x
x x x x x x x
1O-5
1O-5 lo--’ 1O-5 1o-5
1O-5 1O-5 1O-5 1o-5 1o-5 lo-’ 1O-5
Larval equivs per gramC
spraying)
35
294
Larval equivs per hectare
15.0
21.3 25.0 30.0 25.0 18.8 25.0
83.8 63.8 26.3 17.5 20.0 12.5
Mean Mortality (%I
different (Duncan’s multiple range test). made from bioassay data (Daoust and Roome,
0
0 0
Polyhedra per gramb
(87 days after
After
1974)
where
21.84Cd 4.377 36
26.32’ 26.20’ 31.66’ 28.87’ 25.63c 25.95c
67.83a 54.46b 29.29c 21.41Cd 24.60Cd 1 1.25d
(252
Areas;
0
0
percentage
12699 16434 21165 16434 9960 16434
124749 63412 17679 8134 11371
spraying)
x x x X x X X 0
1o-5 1O-5 1o-5 1O-5 1o-5 1O-5 1O-5
1O-5
1O-5 1O-5 1o-5 1o-5 1o-5
of mortality
0.21 0.27 0.35 0.27 0.17 0.27 0.26
x X x x x 0 0.63 x
2.08 1.06 0.29 0.14 0.19
Larval equivs per gramC
days after
(8D)
Polyhedra per gramb
probit
winter
and Unsprayed
Mean trans. (angular)’
‘TABLE 3 Polyhedrosis Virus in Soil from Virus-Sprayed (Segaolane) Mortality of Test Larvae and Estimates of Virus Residue@
‘Numbers followed by the same letter are not significantly bMean mortality corrected by Abbott’s formula. Estimate and where x = log (10 X PIB/mm2). ‘One larval equivalent = 6 X log polyhedra.
Segaolane soil (sprayed) 1 2 3 4 5 6 Soil mean 8D soil (unsprayed) 1 2 3 4 5 6 Soil mean Control Standard error Degrees of freedom
Sample
Mean Mortality (%I
Residues
= 0.78
X 2.34x
33
78
Larval equivs per hectare
12
ROOME
AND
that the virus is diluted and exposed. Most of the virus falls on leaves, whereas larval feeding is concentrated on flower buds, flowers, and bolls. Larvae dying of the disease on cotton are exposed and more likely to be washed off or blown away. Although quantities of virus in the soil surface 87 days after spraying (294 L.E./ hectare) appear to be greater than those originally sprayed (200 L.E./hectare), their distribution must be very different. During spraying the virus is directed into the sorghum head so that although 200 L.E./ hectare represents the gross quantity applied, it is present in relatively concentrated pockets in each sorghum head. Virus in the soil may represent some of the original virus not deposited on the target or washed and blown off, but it mainly consists of newly formed virus from larvae dying in the head, which is washed and blown from the head before and at harvest. Its distribution must be much more diffuse than that originally on the crop. Its activity has been shown to decrease during winter, and there is a further period for attenuation between winter and the occurrence of new Heliothis infestations on the land, the effects of which were not measured. The likelihood of soil virus residues being a major factor in the initiation of economically important virus outbreaks in subsequent seasons seem small. Heliothis attacks most crops grown in Botswana; thus, crop rotation may not reduce this likelihood. Similarly, the introduction of minimum tillage practices may improve carryover of virus, but fallowing will almost certainly decrease it. These three agronomic practices are all considered important in improving dryland cropping in Botswana (Gibbon, 1973). It is possible that some carryover of virus occurs in soil and that this may infect larvae early in a following season, leading to effective virus mortality late in that season. However, it does not seem likely that this is an important contributor to control of Heliothis early in the season.
DAOUST
Heliothis larval infestations rarely last more than 30 days on sorghum, so the long survival of Heliothis nuclear polyhedrosis virus on sorghum means that one application of virus is adequate to protect this crop. ACKNOWLEDGMENTS The authors thank the Ministry of Agriculture of the Government of Botswana for permission to publish this work. They also acknowledge the work of technical assistants 0. Mpudi, B. Lekoma, J. Moruisi, S. Gunda, and S. Dihoro and International Voluntary Service Assistant Entomologist Kaye Flattery and thank Dr. R. F. Chapman and Mr. W. R. Ingram for reading and criticizing the manuscript.
REFERENCES BOG, J. 1966. Rearing Heliothis nrmigera (Hubn.) and Prodenia litura F. on an artificial diet. S. Afr. J. Agr. Sci..9,535-538.
BULLOCK, H. R. 1967. Persistence of Heliothis nuclearpolyhedrosis virus on cotton foliage. J. Invertebr. Pnthol.. 9,434~436. DAOUST, R. A., AND ROOME, R. E. 1974. Bioassay of a nuclear-polyhedrosis virus and BociNus fhuringiettsis against the American Bollworm, Heliothis armigera, in Botswana. J. Inverlebr. Pathol., 23,318-324. GIBBON, D. 1973. “Dryland Crop Production Research in Botswana: An Interim Review and Report.” Agronomist: ODA Dryland Farming Research Scheme, Agricultural Research Station, Private Bag 33, Gaborone, Botswana. IGNOFFO, C. M., CHAPMAN, A. J.. AND MARTIN, D. F. 1965. The nuclear-polyhedrosis virus of Heliorhis zea (Boddie) and Heliorhis virescens Fabricius. III. Effectiveness of the virus against field populations of Heliothis on cotton, corn and grain sorghum. J. Invertebr. Pathol.. I, 227-235. JAQUES, R. P.. AND HARCOURT, D. G. 1971. Viruses of Trichoplusia ni (Lepidoptera: Noctuidae) and Pieris rapae (Lepidoptera: Pieridae) in soil in fields of crucifers in southern Ontario. Can. Enfomol.. 103. 12851290.
D. J. 1971. Nuclear polyhedrosis virus of the cotton bollworm in Central Africa. Carton Grow. Rev., 48,297-303. ROOME, R. E. 1975. Field trial, with a nuclear polyhedrosis virus and Bacillus rhuringiensis against Helio/his armigera (Hbn.) larvae on sorghum and cotton in Botswana. Bull. Int. Res.. 65 (3) In press. Accepted and to be published September 1975. MCKINLEY,