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Field use of granulosis virus to reduce initial storage infestation of the potato tuber moth, Phthorimaea operculella (Zeller), in North Africa
Agriculture. Ecosystems and Environment, 38 ( 1992 ) I ! 9 - ! 26
119
Elsevier Science Publishers B.V., Amsterdam
Field use of granulosis virus to reduce initial storage infestation of the potato tuber moth, Phthorimaea operculella ( Zeller ), in North Africa H. Ben Salaha and R. Aalbub alnstitut National de la Recherche Agronomique de Tunisie (INRAT), Route de la Soukra. 2080 Ariana, Tunisia bLaboratoo, Services: Entomology. CaliJbrnia Department of Food and Agriculture, 1220 N Sireet. Sacramento, CA 95814, USA (Accepted 24 June 1991 )
ABSTRACT Ben Salah, H. and Aalbu, R., 1992. Field use of granulosis virus to reduce initial storage infestation of the potato tuber moth, Phthorimaea operculella (Zeller), in North Africa. Agric. Ecosystems Environ., 38:119-126. Granulosis virus (a Tunisian isolate) was tested in field conditions in Tunisia, both in a spray and as an additive to a powder commonly utilized as a carrier for insecticides (magnesium silicate ), in an attempt to reduce potato tuber moth Phthorimaea operculella (Zeller) (PTM) damage at harvest. Both spray and powder treatments were applied to the surface soil, only incidentally reaching the plants. The progress of PTM infestation during storage was also examined in ,n!reated potatoes imported from fields which had had previous applications of granulog!~ "~irus (GV). Field PTM infestation of tubers at harvest was reduced significantly by use of the spray (73%) or the powder (35%). Infesting populations of potato tuber moth failed to develop in stored untreated potatoes from field areas previously treated with the spray or powder compared with potatoes from untreated areas. Field application of granulosis virus in spray form to surface soil is recommended as a method of reducing PTM infestation at harvesl in areas with Mediterranean climate conditions. Field application of the spray also has the added benefit of reducing the evolution of PTM in stored potatoes.
INTRODUCTION
One of the main factors which contributes to potato storage loss in North Africa is Potato Tuber Moth Phthorimaea operculella (Zeller) (PTM), brought into farmers' traditional storage heaps with infested tubers from the field at harvest. The rapid increase in the field populations of the potato tuber moth, most apparent in the few days prior to harvest during the main potato growing season in Tunisia (late May and June), results in the infestation of the tubers. The damage to stored potatoes caused by this initial infestation is very high, the number of infested tubers often reaching 100% in 1.5-2 months © 1992 Elsevier Science Publishers B.V. All rights reserved 0167-8809/92/$05.00
120
H. BEN SALAH AND R. AALBU
(Essamet et al., 1988). PTM damage also promotes and propagates rotting of the stock. Several practices in Tunisia have been identified as important in preventing this 'initial' infestation, e.g. deep seeding, hilling up, early harvest, irrigation until harvest, a good sorting oftubers at harvest and rapid handling (Von Arx et al., 1987 ). Even with all these precautions, however, a population of PTM usually subsists. Furthermore, farmers cannot always follow these precautionary measures because of lack of water, the need to plant the next crop, labor shortage and market price fluctuations. Much of the infestation which occurs just prior to and at harvest is caused by eggs laid either on the soil or on tubers exposed by soil fissuring. Alternatively, the highly mobile first instar larvae move on to tubers from eggs hatching nearby. These larvae are diftT:,ult to detect with the naked eye, especially when up to 5 tons of potatoes have to be hand sorted prior to storage. Previous experiments have shown that foliar insecticide sprays of deltamethrin prior to harvest are of little use in reducing tuber PTM infestation (Von Arx et ai., 1987), perhaps because the natural enemies of PTM are also killed. The potential benefits realized from reduced initial infestations would include fewer ~lan hours expended in pre-storage sorting as well as fewer treatments during storage. The following experiments utilizing a biological insecticide prio~ , harvest were carried out with this goal in mind. A granulosls virus (GV) was isolated in Tunisia and identified (see Raman and Alcazar, i 988 ). This viral isolate was tested in the laboratory at INRAT in 1989, mixed with water and a spreading agent, with promising results in terms of egg and early instar mortality (H. Ben Salah and Roux, unpublished data). The CV isolate was also utilized as an additive, mixed with different powder carriers in near traditional storage experiments in Tunisia in 1989 (R. Aalbu and H. Ben Salah, unpublished report, 1989). Granulosis virus with a carrier powder caused a significant reduction (P< 0.05 ) in PTM compared with controls. The carriers themselves, calcium carbonate (lime) and magnesium silicate (talc), partially prevented further outbreaks of PTM populations (larval spiracular blockage has been shown to result from application of fine powders such as talc (K.V. Raman, personal communication). However, no significant control of PTM was found by utilization of either talc or lime alone although in Peru a low field PTM infestation was found as a side effect of utilizing lime as a reflecting agent to reduce surface soil temperatures (Midmore, 1984). MATERIALS AND METHODS
Potato fields of the Dutch variety 'Spunta' were divided into randomized plots of 45 m "~.Plots were equally assigned as either control or treatment. Two experiments were conducted: granulosis virus applied as a liquid spray (Ex-
USE OF GRANULOSIS VIRUS TO REDUCE POTATO TUBER MOTH INFESTATION
!2 !
periment 1 ); GV applied with a carrier powder, as a dust (Experiment 2). Granulosis virus was obtained from the virus multiplication facilities at INRAT. A concentration of 0.02 larval equivalents (LE) of virus cm-3 water or carrier powder was utilized for the experiments. Treatments were applied to the fields in the mid-morning. No other insecticides were used. Fields were irrigated weekly by a channel system. Each plot contained approximately 60 plants, producing about 300 tubers. The percentages of plants or tubers damaged by PTM after each treatment were compared by univariate analyses of variance (ANOVA) followed by a Tukey multiple comparison to test whether treatments differed significantly (P< 0.05; Zar, 1974). Experiment 1: Field use of granulosis virus spray to minimize PTM damage at harvest
To examine if reduced damage by PTM at harvest can be achieved by field utilization ofgranulosis virus in liquid spray applications prior to harvest, 12 neighboring plots in Field 1 were used to test three treatments: (a) a spray of granulosis virus and water (with 2% Triton spreading agent); (b) a spray of water alone; (c) control (no liquid spray ). Four spray applications were made, 50, 75, 88 and 94 days after emergence of the plants at the rate of 500 ! ha- i using a 10 1backpack sprayer. Only the soil surface was treated, with as little spray as possible reaching the plants. During the last 4 weeks before harvest four plants and associated tubers from each plot were examined each week for the following parameters; (a) number of leaves and/or stems with PTM damage; (b) number of tubers with visible PTM infestation; (c) number of tubers exposed, by soil cracking, to the sun and showing green chlorophyll areas. Samples for these were gathered from border areas of plots. At harvest, PTM damage was assessed on 100 tubers per plot; harvest samples were taken only within the central 13.5 m 2 of the plots. Experiment 2: Field use of a barrier dust and granulosis virus as a powder application to minimize PTM damage at harvest
To examine if reduced damage by PTM at harvest can be achieved by field utilization of a powder barrier and granulosis virus treatment on the surface soil, 6 plots in each of two widely spaced lots, one in Field 1 and one in Field 2, were used to test three variables: (a) granulosis virus and talc (components mixed in water with 2% Triton spreading agent and dried in the laboratory); (b) talc alone; (c) control (no powder). Powders were applied at the rate of 1.2 tons ha -I as utilized by Midmore (1984). Talc (Mg3Si4Oio(OH)2) was utilized rather than the lime (CaCO3) utilized by Midmore (1984) after preliminary field tests suggested a more uniform ground coverage with talc. Talc was obtained locally in a 50 kg sack. The powder treatments were applied only
! 22
H. BEN SALAH AND R. AALBU
to elevated surfaces of the soil (i.e. the ridges). Three plots of GV-talc and three of control were placed in one lot and three plots of talc and 3 plots of control were placed in the other. Two treatments were made at 50 and 75 days after emergence of the plants. Treatments were applied by use of locally available hand-held flour shifting trays. At harvest, 100 tubers per plot were examined for PTM damage. Samples were taken only within the central 13.5 m 2 ofp!ots.
Experiment 3: Reduction of PTM damage during storage by field applications of granulosis virus To examine if the reduced damage at harvest level produced by field use of granulosis virus in liquid and powder applications is maintained in untreated storage, 900 tubers were obtained at harvest from the central 13.5 m 2 of plots used in the above experiments. These comprised: 300 tubers from plots sprayed with granulosis virus liquid (Experiment 1); 300 tubers from plots treated with granulosis virus and talc powder (Experiment 2); 300 tubers from control plots of the above experiments. For each treatment 100 tubers were placed, as separate storage units, in each of three untreated, large, reinforced paper bags. All tubers were examined for PTM damage at harvest (initiation of storage) and after 10, 20 and 30 days of storage. The number of PTM adults emerging from the bags and the number of rotten tubers were also recorded. RESULTS
The progress of PTM damage in the field on both plants (leaves and stems ) and tubers (Experiment 1) is presented in Table 1. Visible damage to plants and tubers was only noticed I week prior to harvest (94 days after plant surface emergence). Although a substantial number of plants in the virus-water spray plots had at least one evidence of PTM damage a week prior to harvest, few tubers were found with PTM damage at harvest. Surface tuber exposure greatly augmented in the last few weeks prior to harvest, resulting at harvest in about 25% of the tubers showing some greening. The percentage PTM damage at harvest per treatment in Experiment 1 is shown in Table 2. Spraying with granulosis virus mixed with water resulted in significantly less PTM damage at harvest than either the control or spraying with water only, Because of independent factors, such as rotation and amount of clay in the soil, PTM damage at harvest in control plots differed greatly in the two fields ( 19.77% mean damage in Field 1 and 4.33%o in Field 2 ). Nevertheless, independent one-way ANOVA analyses of treatments followed by the Tukey multiple comparison test ( (virus talc vs. control) and (talc vs. control) ) from
USEOF GRANULOSISVIRUSTO REDUCEPOTATOTUBERMOTHINFESTATION
123
TABLE i Percentage of exposed tubers and PTM damage to plants and tubers prior to harvest (Experi~nent 1 ) Time prior to harvest
Exposed tubers PTM damage on plants' GV-H20 Control H20 PTM damage on tubers 2 GV-H~O Control H20
3 weeks
2 weeks
1 week
harvest
I !.8 !
22.13
27.16
25.22
0 0 0
0 0 0
4 !.66 33.33 25.00
0 0 0
0 0 0
1.08 2.97 10.66
~ ~
Plants dead and dry 4.34 10. I I 9.33
Percentage of plants with at least one visible evidence of PTM damage. ::Percentage of tubers with damage. TABLE 2 Comparison of PTM damage at harvest for each treatment (E:~periment i ) Treatment
Count
PTM damage (%)
Tukey homogeneous groups'
GV-H20 Control H~O
3 3 3
6.00 22.16 29.33
A B B
'Significance level = 0.0006 (95% confidence level ). TABLE 3 Comparison of PTM damage at harvest for each treatment from Experiments l and 2 ( Field 1 ) Treatment
Count
PTM damage (%)
Tukey homogeneous groups'
GV-H20 GV-Talc Control H20
3 3 9 3
6.00 14.33 19.77 29.33
A A B B C
'Significance level = 0.0006 (95% confidence level).
each field resulted in no significant differences in the amount of PTM damage at harvest. However, if one considers all treatments and controls from Field 1, one-way ANOVA followed by Tukey multiple comparison test yields three significantly different groups with GV-Talc falling into two groups. This data is presented in Table 3. To investigate the evolution of PTM damage in storage for each treatment
! 24
H. BEN SALAH AND R. AALBU
(Experiment 3 ) the percentages of infested tubers were noted after 10, 20 and 30 days of storage. To examine the rate of PTM damage increase, percentage damage was linearized by transforming it to logits Logit o f x - -
(5 + (0.5 natural log ofx) ) l-x
A multiple regression analysis was then performed on the data. Table 4 summarizes the results from the regression analysis. Moth populations in potatoes from GV-H20 & GV-Talc treated fields did not increase significantly in 30 days of storage (slopes not significantly different from zero) whereas populations in potatoes from control fields did increase. Table 5 shows expected and actual numbers of rotten potatoes as well as numbers of emerging PTM adults from Experiment 3. The total observed numbers of emerging PTM adults and rotten tubers from both treatments (GV-H20 and GV-Talc) were much less than the expected numbers. TABLE 4 Comparison of the development of the PTM during potato storage (Experiment 3) by multiple regression analysis of Iogit transformed data Treatment
Mean initial PTM damage ( from fiel~ )
Mean PTM damage after 30 days
Student's t-value ( slope )
Significance level (slope)
GV-H:O GV-Talc Control
7.00 14,33 15.00
I 1.00 19.00 41.00
1.90 1.08 3.79
0.0670* 0.2850* 0.0007
*Slopes for GV-H~O & GV-Talc are not significantly different from zero at the 95% confidence level. TABLE 5 Expected number versus the actual number of rotten potatoes and emerging PTM adults from treatments in Experiments 3 Expected number* Rotten potatoes: GV-H,O GV-Talc PTM adult emergence: GV-HaO GV-Talc
Observed number
Difference
7.46 ! 5.28
I 9
6.46 6.28
10.73 2 i,97
5 14
5.73 7.97
'Expected number derived from observed number from controls at measured mean levels of initial infestation in treatments,
USE OF GRANULOSIS VI RUS TO REDUCE POTATO TUBER MOTH INFESTATION
[ 25
DISCUSSION
Granulosis virus is a member of the family Baculoviridae. This family is unusual in that it is restricted to the Insecta. For this reason it was tested and approved for field use as part of pest control programs by the World Health Organization as early as 1973 (for review see Entwistle, 1987 ). The results obtained clearly indicate that a bioinsecticide treatment prior to harvest was beneficial. Regardless of whether the treatment formula is in liquid form (GV and water) or in powder form (GV and talc), the PTM infestation at harvest was substantially reduced, even though the amount of PTM attack present on the leaves in the plots treated with GV and water was greater than in the control (Table l ). In effect, this infestation was reduced on the tubers because of the presence of the virus on the soil, which infected the PTM eggs and first instars on their way to reach the tubers. Results also indicate that the viral solution (GV and water) provided better control than viral powder (GV and talc). It is possible that due to a few heavy rains during the field treatments, the powder formulation was less effective in maintaining homogeneous ground coverage (because of panicle dispersion and coagulation). The analysis of the results in Tables 1 and 3 indicates that the higher infestation resulted from spraying plots solely with water. This may have resulted in a more humid microclimate, i.e. one more favorable to the incubation of the PTM eggs and the development of the young PTM larvae. Data obtained during storage again confirms the efficiency of the bioinsecticide treatment prior to storage. As a consequence, PTM populations failed to develop on potatoes from GV treated plots. Both treatments (GV-Water and GV-Talc) were equally effective in inhibiting the evolution of large PTM populations, as illustrated in Table 5. The difference between the expected and observed numbers of rotten tubers was almost identical in the two treatments. Similar differences were found in the number of emerging adults from the tubers (Table 5). In Tunisia, farmers generall'; like to stop watering before harvest because wet soil makes the harvesting, s~-ting and cleaning of potatoes more difficult, the cost of water is high and wat~: is not always available. Because of this, heavy soils, i.e. those with a high clay content, tend to fissure, resulting in more exposed tubers. A high number of tubers exposed to the air and sun has previously been correlated with high PTM infestation (Von Arx et al., 1987 ). These experiments do not represent the optimum algorithm in preharvest GV treatments. We believe it should be possible to reduce the number of treatments (GV-water) and still retain the same results. In effect, as illustrated in Table l, the visible symptoms of PTM infestations on tubers become apparent only near or at harvest time (one week prior to harvest in these experiments). Since PTM eggs take about a week to hatch most tuber infes-
126
H. BEN SALAH AND R. AALBU
tation occurs around 2 weeks prior to harvest. We therefore recommend use of GV-water spray (as GV-powder application is less effective and presents more financial and tactical (wind, rain etc.) problems for the farmer) applied twice, one and two weeks prior to harvest. Similar experiments should be undertaken in other countries where PTM is a serious problem. ACKNOWLEDGMENTS
We would like to thank the staff of the CPRA training center at Saida (Souibghi) where the experiments were conducted and the technical staff (Ben Temine, Hoffman and others) at the Entomology Department at INRAT for their help. K. Fuglie (CIP) is gratefully acknowledged for his review of this manuscript. This study was undertaken while R. Aalbu was part of the staff at the International Potato Center (CIP), 11 rue des Orangers, 208¢ Ariana, Tunisia.
REFERENCES
Entwistle, P.F., 1987. Viral control. In: G.A. Kerkut and L.I, Gilbert (Editors), Comprehensive Insect Physiology, Biochemistry and Pharmacology, vol. 12, Insect Control. Pergamon, Oxford, pp. 347-4 i 2, Essamet, M., yon Arx, R., Ewell, P., Goueder, J., Ben Temine, A. and Chcikh, M., 1988. Aspects techniques et et 6conomiques du problem del a teigne et du stockage des potatoes de terre de saison en Tunisie. Ann, Inst. Nat. Rech. Agron. Tunis., 61: 1-50. Midmore, D.J., 1984. Potato (Solammm spp. ) in the hot tropics I. Soil temperature effects on emergence, plant development and yield. Field Crop Res., 8:255-27 !. Raman, K.V. and Alcazar, J., 1988. Biological control of potato tuber moth Phthorimaea opercukqla (Zeller), using a granulosis virus in Peru. In: Proc. Asian Potato Association (APA), June 12-26 1988, Kuming, China, pp. 1973-1974. Von Arx, R., Goueder, J., Cheikh, M. and Ben Temine, A., 1987. Integrated control of potato tuber moth Phthorimaea operculella in Tunisia. Insect. Sci. Applic., 8: 989-994. Zar, J.H., 1984. Biostatistical Analysis. Prentice-Hall, Englewood Cliffs, NJ, p. 718.
119
Elsevier Science Publishers B.V., Amsterdam
Field use of granulosis virus to reduce initial storage infestation of the potato tuber moth, Phthorimaea operculella ( Zeller ), in North Africa H. Ben Salaha and R. Aalbub alnstitut National de la Recherche Agronomique de Tunisie (INRAT), Route de la Soukra. 2080 Ariana, Tunisia bLaboratoo, Services: Entomology. CaliJbrnia Department of Food and Agriculture, 1220 N Sireet. Sacramento, CA 95814, USA (Accepted 24 June 1991 )
ABSTRACT Ben Salah, H. and Aalbu, R., 1992. Field use of granulosis virus to reduce initial storage infestation of the potato tuber moth, Phthorimaea operculella (Zeller), in North Africa. Agric. Ecosystems Environ., 38:119-126. Granulosis virus (a Tunisian isolate) was tested in field conditions in Tunisia, both in a spray and as an additive to a powder commonly utilized as a carrier for insecticides (magnesium silicate ), in an attempt to reduce potato tuber moth Phthorimaea operculella (Zeller) (PTM) damage at harvest. Both spray and powder treatments were applied to the surface soil, only incidentally reaching the plants. The progress of PTM infestation during storage was also examined in ,n!reated potatoes imported from fields which had had previous applications of granulog!~ "~irus (GV). Field PTM infestation of tubers at harvest was reduced significantly by use of the spray (73%) or the powder (35%). Infesting populations of potato tuber moth failed to develop in stored untreated potatoes from field areas previously treated with the spray or powder compared with potatoes from untreated areas. Field application of granulosis virus in spray form to surface soil is recommended as a method of reducing PTM infestation at harvesl in areas with Mediterranean climate conditions. Field application of the spray also has the added benefit of reducing the evolution of PTM in stored potatoes.
INTRODUCTION
One of the main factors which contributes to potato storage loss in North Africa is Potato Tuber Moth Phthorimaea operculella (Zeller) (PTM), brought into farmers' traditional storage heaps with infested tubers from the field at harvest. The rapid increase in the field populations of the potato tuber moth, most apparent in the few days prior to harvest during the main potato growing season in Tunisia (late May and June), results in the infestation of the tubers. The damage to stored potatoes caused by this initial infestation is very high, the number of infested tubers often reaching 100% in 1.5-2 months © 1992 Elsevier Science Publishers B.V. All rights reserved 0167-8809/92/$05.00
120
H. BEN SALAH AND R. AALBU
(Essamet et al., 1988). PTM damage also promotes and propagates rotting of the stock. Several practices in Tunisia have been identified as important in preventing this 'initial' infestation, e.g. deep seeding, hilling up, early harvest, irrigation until harvest, a good sorting oftubers at harvest and rapid handling (Von Arx et al., 1987 ). Even with all these precautions, however, a population of PTM usually subsists. Furthermore, farmers cannot always follow these precautionary measures because of lack of water, the need to plant the next crop, labor shortage and market price fluctuations. Much of the infestation which occurs just prior to and at harvest is caused by eggs laid either on the soil or on tubers exposed by soil fissuring. Alternatively, the highly mobile first instar larvae move on to tubers from eggs hatching nearby. These larvae are diftT:,ult to detect with the naked eye, especially when up to 5 tons of potatoes have to be hand sorted prior to storage. Previous experiments have shown that foliar insecticide sprays of deltamethrin prior to harvest are of little use in reducing tuber PTM infestation (Von Arx et ai., 1987), perhaps because the natural enemies of PTM are also killed. The potential benefits realized from reduced initial infestations would include fewer ~lan hours expended in pre-storage sorting as well as fewer treatments during storage. The following experiments utilizing a biological insecticide prio~ , harvest were carried out with this goal in mind. A granulosls virus (GV) was isolated in Tunisia and identified (see Raman and Alcazar, i 988 ). This viral isolate was tested in the laboratory at INRAT in 1989, mixed with water and a spreading agent, with promising results in terms of egg and early instar mortality (H. Ben Salah and Roux, unpublished data). The CV isolate was also utilized as an additive, mixed with different powder carriers in near traditional storage experiments in Tunisia in 1989 (R. Aalbu and H. Ben Salah, unpublished report, 1989). Granulosis virus with a carrier powder caused a significant reduction (P< 0.05 ) in PTM compared with controls. The carriers themselves, calcium carbonate (lime) and magnesium silicate (talc), partially prevented further outbreaks of PTM populations (larval spiracular blockage has been shown to result from application of fine powders such as talc (K.V. Raman, personal communication). However, no significant control of PTM was found by utilization of either talc or lime alone although in Peru a low field PTM infestation was found as a side effect of utilizing lime as a reflecting agent to reduce surface soil temperatures (Midmore, 1984). MATERIALS AND METHODS
Potato fields of the Dutch variety 'Spunta' were divided into randomized plots of 45 m "~.Plots were equally assigned as either control or treatment. Two experiments were conducted: granulosis virus applied as a liquid spray (Ex-
USE OF GRANULOSIS VIRUS TO REDUCE POTATO TUBER MOTH INFESTATION
!2 !
periment 1 ); GV applied with a carrier powder, as a dust (Experiment 2). Granulosis virus was obtained from the virus multiplication facilities at INRAT. A concentration of 0.02 larval equivalents (LE) of virus cm-3 water or carrier powder was utilized for the experiments. Treatments were applied to the fields in the mid-morning. No other insecticides were used. Fields were irrigated weekly by a channel system. Each plot contained approximately 60 plants, producing about 300 tubers. The percentages of plants or tubers damaged by PTM after each treatment were compared by univariate analyses of variance (ANOVA) followed by a Tukey multiple comparison to test whether treatments differed significantly (P< 0.05; Zar, 1974). Experiment 1: Field use of granulosis virus spray to minimize PTM damage at harvest
To examine if reduced damage by PTM at harvest can be achieved by field utilization ofgranulosis virus in liquid spray applications prior to harvest, 12 neighboring plots in Field 1 were used to test three treatments: (a) a spray of granulosis virus and water (with 2% Triton spreading agent); (b) a spray of water alone; (c) control (no liquid spray ). Four spray applications were made, 50, 75, 88 and 94 days after emergence of the plants at the rate of 500 ! ha- i using a 10 1backpack sprayer. Only the soil surface was treated, with as little spray as possible reaching the plants. During the last 4 weeks before harvest four plants and associated tubers from each plot were examined each week for the following parameters; (a) number of leaves and/or stems with PTM damage; (b) number of tubers with visible PTM infestation; (c) number of tubers exposed, by soil cracking, to the sun and showing green chlorophyll areas. Samples for these were gathered from border areas of plots. At harvest, PTM damage was assessed on 100 tubers per plot; harvest samples were taken only within the central 13.5 m 2 of the plots. Experiment 2: Field use of a barrier dust and granulosis virus as a powder application to minimize PTM damage at harvest
To examine if reduced damage by PTM at harvest can be achieved by field utilization of a powder barrier and granulosis virus treatment on the surface soil, 6 plots in each of two widely spaced lots, one in Field 1 and one in Field 2, were used to test three variables: (a) granulosis virus and talc (components mixed in water with 2% Triton spreading agent and dried in the laboratory); (b) talc alone; (c) control (no powder). Powders were applied at the rate of 1.2 tons ha -I as utilized by Midmore (1984). Talc (Mg3Si4Oio(OH)2) was utilized rather than the lime (CaCO3) utilized by Midmore (1984) after preliminary field tests suggested a more uniform ground coverage with talc. Talc was obtained locally in a 50 kg sack. The powder treatments were applied only
! 22
H. BEN SALAH AND R. AALBU
to elevated surfaces of the soil (i.e. the ridges). Three plots of GV-talc and three of control were placed in one lot and three plots of talc and 3 plots of control were placed in the other. Two treatments were made at 50 and 75 days after emergence of the plants. Treatments were applied by use of locally available hand-held flour shifting trays. At harvest, 100 tubers per plot were examined for PTM damage. Samples were taken only within the central 13.5 m 2 ofp!ots.
Experiment 3: Reduction of PTM damage during storage by field applications of granulosis virus To examine if the reduced damage at harvest level produced by field use of granulosis virus in liquid and powder applications is maintained in untreated storage, 900 tubers were obtained at harvest from the central 13.5 m 2 of plots used in the above experiments. These comprised: 300 tubers from plots sprayed with granulosis virus liquid (Experiment 1); 300 tubers from plots treated with granulosis virus and talc powder (Experiment 2); 300 tubers from control plots of the above experiments. For each treatment 100 tubers were placed, as separate storage units, in each of three untreated, large, reinforced paper bags. All tubers were examined for PTM damage at harvest (initiation of storage) and after 10, 20 and 30 days of storage. The number of PTM adults emerging from the bags and the number of rotten tubers were also recorded. RESULTS
The progress of PTM damage in the field on both plants (leaves and stems ) and tubers (Experiment 1) is presented in Table 1. Visible damage to plants and tubers was only noticed I week prior to harvest (94 days after plant surface emergence). Although a substantial number of plants in the virus-water spray plots had at least one evidence of PTM damage a week prior to harvest, few tubers were found with PTM damage at harvest. Surface tuber exposure greatly augmented in the last few weeks prior to harvest, resulting at harvest in about 25% of the tubers showing some greening. The percentage PTM damage at harvest per treatment in Experiment 1 is shown in Table 2. Spraying with granulosis virus mixed with water resulted in significantly less PTM damage at harvest than either the control or spraying with water only, Because of independent factors, such as rotation and amount of clay in the soil, PTM damage at harvest in control plots differed greatly in the two fields ( 19.77% mean damage in Field 1 and 4.33%o in Field 2 ). Nevertheless, independent one-way ANOVA analyses of treatments followed by the Tukey multiple comparison test ( (virus talc vs. control) and (talc vs. control) ) from
USEOF GRANULOSISVIRUSTO REDUCEPOTATOTUBERMOTHINFESTATION
123
TABLE i Percentage of exposed tubers and PTM damage to plants and tubers prior to harvest (Experi~nent 1 ) Time prior to harvest
Exposed tubers PTM damage on plants' GV-H20 Control H20 PTM damage on tubers 2 GV-H~O Control H20
3 weeks
2 weeks
1 week
harvest
I !.8 !
22.13
27.16
25.22
0 0 0
0 0 0
4 !.66 33.33 25.00
0 0 0
0 0 0
1.08 2.97 10.66
~ ~
Plants dead and dry 4.34 10. I I 9.33
Percentage of plants with at least one visible evidence of PTM damage. ::Percentage of tubers with damage. TABLE 2 Comparison of PTM damage at harvest for each treatment (E:~periment i ) Treatment
Count
PTM damage (%)
Tukey homogeneous groups'
GV-H20 Control H~O
3 3 3
6.00 22.16 29.33
A B B
'Significance level = 0.0006 (95% confidence level ). TABLE 3 Comparison of PTM damage at harvest for each treatment from Experiments l and 2 ( Field 1 ) Treatment
Count
PTM damage (%)
Tukey homogeneous groups'
GV-H20 GV-Talc Control H20
3 3 9 3
6.00 14.33 19.77 29.33
A A B B C
'Significance level = 0.0006 (95% confidence level).
each field resulted in no significant differences in the amount of PTM damage at harvest. However, if one considers all treatments and controls from Field 1, one-way ANOVA followed by Tukey multiple comparison test yields three significantly different groups with GV-Talc falling into two groups. This data is presented in Table 3. To investigate the evolution of PTM damage in storage for each treatment
! 24
H. BEN SALAH AND R. AALBU
(Experiment 3 ) the percentages of infested tubers were noted after 10, 20 and 30 days of storage. To examine the rate of PTM damage increase, percentage damage was linearized by transforming it to logits Logit o f x - -
(5 + (0.5 natural log ofx) ) l-x
A multiple regression analysis was then performed on the data. Table 4 summarizes the results from the regression analysis. Moth populations in potatoes from GV-H20 & GV-Talc treated fields did not increase significantly in 30 days of storage (slopes not significantly different from zero) whereas populations in potatoes from control fields did increase. Table 5 shows expected and actual numbers of rotten potatoes as well as numbers of emerging PTM adults from Experiment 3. The total observed numbers of emerging PTM adults and rotten tubers from both treatments (GV-H20 and GV-Talc) were much less than the expected numbers. TABLE 4 Comparison of the development of the PTM during potato storage (Experiment 3) by multiple regression analysis of Iogit transformed data Treatment
Mean initial PTM damage ( from fiel~ )
Mean PTM damage after 30 days
Student's t-value ( slope )
Significance level (slope)
GV-H:O GV-Talc Control
7.00 14,33 15.00
I 1.00 19.00 41.00
1.90 1.08 3.79
0.0670* 0.2850* 0.0007
*Slopes for GV-H~O & GV-Talc are not significantly different from zero at the 95% confidence level. TABLE 5 Expected number versus the actual number of rotten potatoes and emerging PTM adults from treatments in Experiments 3 Expected number* Rotten potatoes: GV-H,O GV-Talc PTM adult emergence: GV-HaO GV-Talc
Observed number
Difference
7.46 ! 5.28
I 9
6.46 6.28
10.73 2 i,97
5 14
5.73 7.97
'Expected number derived from observed number from controls at measured mean levels of initial infestation in treatments,
USE OF GRANULOSIS VI RUS TO REDUCE POTATO TUBER MOTH INFESTATION
[ 25
DISCUSSION
Granulosis virus is a member of the family Baculoviridae. This family is unusual in that it is restricted to the Insecta. For this reason it was tested and approved for field use as part of pest control programs by the World Health Organization as early as 1973 (for review see Entwistle, 1987 ). The results obtained clearly indicate that a bioinsecticide treatment prior to harvest was beneficial. Regardless of whether the treatment formula is in liquid form (GV and water) or in powder form (GV and talc), the PTM infestation at harvest was substantially reduced, even though the amount of PTM attack present on the leaves in the plots treated with GV and water was greater than in the control (Table l ). In effect, this infestation was reduced on the tubers because of the presence of the virus on the soil, which infected the PTM eggs and first instars on their way to reach the tubers. Results also indicate that the viral solution (GV and water) provided better control than viral powder (GV and talc). It is possible that due to a few heavy rains during the field treatments, the powder formulation was less effective in maintaining homogeneous ground coverage (because of panicle dispersion and coagulation). The analysis of the results in Tables 1 and 3 indicates that the higher infestation resulted from spraying plots solely with water. This may have resulted in a more humid microclimate, i.e. one more favorable to the incubation of the PTM eggs and the development of the young PTM larvae. Data obtained during storage again confirms the efficiency of the bioinsecticide treatment prior to storage. As a consequence, PTM populations failed to develop on potatoes from GV treated plots. Both treatments (GV-Water and GV-Talc) were equally effective in inhibiting the evolution of large PTM populations, as illustrated in Table 5. The difference between the expected and observed numbers of rotten tubers was almost identical in the two treatments. Similar differences were found in the number of emerging adults from the tubers (Table 5). In Tunisia, farmers generall'; like to stop watering before harvest because wet soil makes the harvesting, s~-ting and cleaning of potatoes more difficult, the cost of water is high and wat~: is not always available. Because of this, heavy soils, i.e. those with a high clay content, tend to fissure, resulting in more exposed tubers. A high number of tubers exposed to the air and sun has previously been correlated with high PTM infestation (Von Arx et al., 1987 ). These experiments do not represent the optimum algorithm in preharvest GV treatments. We believe it should be possible to reduce the number of treatments (GV-water) and still retain the same results. In effect, as illustrated in Table l, the visible symptoms of PTM infestations on tubers become apparent only near or at harvest time (one week prior to harvest in these experiments). Since PTM eggs take about a week to hatch most tuber infes-
126
H. BEN SALAH AND R. AALBU
tation occurs around 2 weeks prior to harvest. We therefore recommend use of GV-water spray (as GV-powder application is less effective and presents more financial and tactical (wind, rain etc.) problems for the farmer) applied twice, one and two weeks prior to harvest. Similar experiments should be undertaken in other countries where PTM is a serious problem. ACKNOWLEDGMENTS
We would like to thank the staff of the CPRA training center at Saida (Souibghi) where the experiments were conducted and the technical staff (Ben Temine, Hoffman and others) at the Entomology Department at INRAT for their help. K. Fuglie (CIP) is gratefully acknowledged for his review of this manuscript. This study was undertaken while R. Aalbu was part of the staff at the International Potato Center (CIP), 11 rue des Orangers, 208¢ Ariana, Tunisia.
REFERENCES
Entwistle, P.F., 1987. Viral control. In: G.A. Kerkut and L.I, Gilbert (Editors), Comprehensive Insect Physiology, Biochemistry and Pharmacology, vol. 12, Insect Control. Pergamon, Oxford, pp. 347-4 i 2, Essamet, M., yon Arx, R., Ewell, P., Goueder, J., Ben Temine, A. and Chcikh, M., 1988. Aspects techniques et et 6conomiques du problem del a teigne et du stockage des potatoes de terre de saison en Tunisie. Ann, Inst. Nat. Rech. Agron. Tunis., 61: 1-50. Midmore, D.J., 1984. Potato (Solammm spp. ) in the hot tropics I. Soil temperature effects on emergence, plant development and yield. Field Crop Res., 8:255-27 !. Raman, K.V. and Alcazar, J., 1988. Biological control of potato tuber moth Phthorimaea opercukqla (Zeller), using a granulosis virus in Peru. In: Proc. Asian Potato Association (APA), June 12-26 1988, Kuming, China, pp. 1973-1974. Von Arx, R., Goueder, J., Cheikh, M. and Ben Temine, A., 1987. Integrated control of potato tuber moth Phthorimaea operculella in Tunisia. Insect. Sci. Applic., 8: 989-994. Zar, J.H., 1984. Biostatistical Analysis. Prentice-Hall, Englewood Cliffs, NJ, p. 718.