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Reduction of stemborer damage by intercropping maize with cowpea
Agriculture Ecosystems & Environment ELSEVIER
Agriculture, Ecosystems and Environment 62 (1997) 13-19
Reduction of stemborer damage by intercropping maize with cowpea Henrik Skovg~d a,*, Peeter P~its b,l a Department of Population Biology, Universitetsparken 15, 2100 Copenhagen ¢), Denmark b Swedish University of Agricultural Sciences, Department of Entomology, Box 7044, 750 07 Uppsala, Sweden Accepted 17 October 1996
Abstract
The effect of intercropping maize with cowpea on the damage caused by three species of lepidopteran stemborers (Chilo partellus, Chilo orichalcociliellus, and Sesamia calamistis) was studied in Kenya. Significantly higher yields of maize (27-57%) were found in the intercrop in four out of five experiments, corresponding with significantly lower numbers (15-25%) of stemborers. A multiple regression model predicting yield per maize plant showed that 77% of the variation could be explained by site, rainfall, and the number of stemborers. Keywords: Chilo partellus; Chilo orichalcociliellus; Sesamia calamistis; Yield loss; Multiple regression model
1. Introduction Intercropping is traditionally practised by subsistence farmers in most developing countries, and is largely rooted in the experience of farmers rather than based on scientific methods (Litsinger and Moody, 1976; Okigbo and Greenland, 1976; Zethner, 1995). Crop diversity in tropical agroecosystems may provide several advantages over monocropping, by giving a higher total return in yield and acting as
* Corresponding author at: The Danish Pest Infestation Laboratory, Skovbrynet 14, 2800 Lyngby, Copenhagen-DK, Denmark. Tel.: +45 45878055; fax: + 4 5 45931155; e-mail: [email protected]. Present address: Department of Plant Science, University of British Columbia, 336-2357 Main Mall, Vancouver, B.C. V6T IZ4, Canada.
an insurance against crop failure or fluctuating market prices of single crops. Intercropping has also been suggested as a means of reducing arthropod pest infestation and subsequent damage (Dempster and Coaker, 1972; Altieri et al., 1978; Andow, 1991a). The mechanisms involved in pest suppression in intercropping remain obscure. In many cases impeded pest movements seem to be the main cause (Kareiva, 1983; Tonhasca and Byrne, 1994), but increased influence of natural enemies has also been implicated (Sheehan, 1986; Letourneau, 1987; Russell, 1989). With intercrop experiments, difficulties can arise when relating pest infestation and injury to the growth and yield of the plants. Factors such as competition between plant species and plant interactions with the soil may override the influence of the pests, leading to incomplete conclusions about intercropping. Therefore, both higher and lower yields in intercrop-
0167-8809/97/$17.00 Copyright © 1997 Elsevier Science B.V. All rights reserved. PII S 0 1 6 7 - 8 8 0 9 ( 9 6 ) 0 1 1 1 4 - 0
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H. Skovggtrd, P. Pfits /Agriculture. Ecosystems and Environment 62 (1997) 13-19
ping have been reported at reduced pest infestation levels (Altieri et al., 1978; Risch et al., 1983). World-wide, lepidopterous stemborers cause considerable yield loss of maize and sorghum (Alghali, 1986; Sharma and Sharma, 1987; Davis and Pedigo, 1990; Morallo-Rejesus et al., 1990; Van den Berg and Van Rensburg, 1991). Maize, Zea mays (L.), intercropped with cowpea, Vigna unguiculata (L.), is a traditional crop combination for smallholders in the coastal area of East Africa. The most common and serious insect pests are two pyralids, Chilo partellus (Swinhoe) and Chilo orichalcociliellus (Strand), and one noctuid, Sesamia calamistis Hampson which can be found simultaneously in the field (Mathez, 1972; Overholt et al., 1994). In a recent series of intercrop experiments, it was found that crop diversity did not influence Chilo spp. oviposition behaviour or oviposition per se (Pats et al., 1997). However, intercropping increased egg parasitism, although larval and pupal parasitism were not affected (Skovg~d and P~its, 1996). Further, when sampling data were combined, fewer larvae and pupae were found in the intercrop than in the monocrop (Skovg~d and Pats, 1996). How this occurrence of stemborers in the two cropping systems influences yield still needs to be quantified. The present study aims first, to examine to what extent intercropping maize with cowpea can constrain stemborer damage and secondly, to quantify the influence of stemborers on maize yield in the two cropping systems.
2. Materials and methods
2.1. Experimental layout Field plots were located in Mtwapa (14 km) and Kilifi (54 km) north of Mombasa on the Kenyan coast. The experiments were conducted in two consecutive seasons, that of 'short rains' 1993 (planting in Mtwapa on 14 October and Kilifi on 21 October, harvest 28-29 January) and 'long rains' 1994 (planting in Mtwapa on 11 April and in Kilifi on 9 May, harvest 6 August and 10 September). A longer duration than usual of the long rains during 1994 allowed for an additional experiment in Mtwapa (planting 3 June, harvest 10 September).
Two different types of plots were used, maize as monocrop and maize mixed intercropped with cowpea. The monocrop maize, cv. 'Pwani Hybrid' was planted at a spacing of 0.5 m within and between rows. In the intercrop every other maize plant was replaced by a cowpea plant, cv. 'Katumani 80'. Maize and cowpea were planted simultaneously. The plots were laid out in a randomised block design. Plot size in the short rains was 20 m x 20 m with three replicates per treatment, but was reduced to 10 m X 10 m plots with eight replicates per treatment in the long rains. The smaller plots were chosen to produce more uniform plants within the plot and to increase the number of replicates. In Kilifi (long rains) one intercrop plot was destroyed by grazing animals and its data were excluded from the analyses. For the additional experiment only five plots per treatment were laid out. Distance between plots was 3 m of mown grass. The soil was classified as sandy in Mtwapa and loamy sandy in Kilifi. An 18N:46P (100 kg ha -~) fertiliser mixture was added to the soil before planting. Additionally, 80 kg ha-l of N 26 was applied twice as top dressing in the monocrop and 40 kg ha-I to the maize plants in the intercrop. The first application was made 2 weeks after germination and the second at plant tasselling. Weeding was done manually. Rainfall was measured at each location. Maize cobs were harvested by hand at maturity by taking 100 plants at random in the monocrop and 50 in the intercrop. For thesmaller plots in the long rains, 50 and 25 plants were chosen. Kernel weight was determined at 13% water content.
2.2. Sampling stemborers The density of stemborers was recorded weekly by destructive sampling of plants at random in each plot. Because the monocrop contained twice the number of plants than the intercrop, and to obtain similar proportions of sampled plants in both systems, 50 maize plants in monocrop and 25 plants in intercrop were chosen during the first season. In the long rains, the number of plants sampled was reduced to 12 and six plants because of the smaller plot size. Each plant was uprooted and dissected and the number of stemborer larvae and pupae recorded. (for seasonal phenology of the stemborers, see Skovg~d and Pats, 1996).
H. Skovggtrd, P. PiJts/Agriculture, Ecosystems and Environment 62 (1997) 13-19
2.3. Statistical analyses
relationship between mean grain yield P ~ ) per plant at harvest and four explanatory variables, site (S), treatment (T), total rain (W), and mean cumulated number of stemborers (C----~,all of which are assumed to have contributed to final yield. Total rainfall from 1 week before planting to 3 months afterwards was included in the analysis to find the period giving the best prediction. Yield P ~ ) per plant was used as the dependent variable upon subjecting the data to In transformation to normalise data and to stabilise the variance about the regression line.
To analyse for the effect of cropping system (T), site (S), and experiment (E), an unbalanced nested analysis of variance was conducted for the grain yield of maize and the number of stemborers. For both tests a mixed model nested ANOVA with three levels and an interaction term was conducted. The equation is Xijkl = tx + Si -b E(i)j + Tk -b SiTk -b E(ijk)t
(1)
where Xij~I denotes either the mean grain yield at 13% water content per plant (P---Y)or the mean cumulative number of stemborers C ~ ) per plant of the ith site (random) of the jth experiment (random) of the kth treatment (systematic). /z is the parametric mean and E(ijk)l is the error term with the lth replicate. Yield was assumed to be constrained by the combined number of stemborers hosting the plant from germination and to harvest. Therefore, the mean cumulative number of stemborers C ~ ) was determined as
3. Results
The nested analyses of variance showed a significantly higher yield ( P < 0.01) and less cumulative numbers of stemborers ( P < 0.05) per plant in intercropping but no site or interaction effects were found (Table 1). In addition, a significant effect of experiment nested within site was found for yield and stemborers ( P < 0 . 0 0 1 ) , indicating that the maize yield and the incidence of stemborers was different depending on the season. The yield of the long rain experiments contributed significantly to the positive effect of intercropping with 38 and 42% higher yields than in the monocrop (Table 2). Also, higher mean yields of the intercrop were recorded in Kilifi during the short rains (27%) and in Mtwapa for the extended period (57%) (Table 2). No significant difference was found in the cumulative number of stemborers for each experiment. There was a trend in all experiments, except for Kilifi (short rains) that the cumulative number of stemborers per plant was
~B(i) C---S= i=,
15
(2) s
where i refers to the sampling occasions up to harvest (s). B{i) is the mean density of the ith occasion of alive and dead larvae and pupae per plant. Plant yield and cumulative number of stemborers per plant were In transformed to normalise data and stabilise the variance. A multiple regression (Statistical Analysis Systems Institute Inc., 1985) was applied to define the
Table 1 Nested analysis o f variance on yield a n d c u m u l a t i v e n u m b e r o f stemborers per maize plant. M o n o c r o p a n d intercrop (T), Kilifi a n d M t w a p a (S), a n d experiments ( E ) Source
Model T S
E(S) T- S Error
d.f.
6 1 t 3 1 45
Yield
Stemborers
M e a n square
F-value
P
R2
Mean square
F-value
P
R2
5.53 1.02 11.47 5.52 0.00 0.14
41.07 7.60 2.21 40.99 0.00
* * * * * NS * * * NS
84.6%
1.42 0.40 0.49 2.05 0.11 0.09
15.63 4.44 0.25 22.64 1.17
* * * • NS * * * NS
67.6%
NS, not significant; * P < 0.05; * * P < 0.01; * * * P < 0.001.
H. Skxmgdrd, P. PZtts/Agriculture, Ecosystems and Environment 62 (1997) 13-19
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Table 2 Mean ( ± SE) grain yield at 13% water content and cumulative number of stemborers per maize plant. Percentage relative to the monocrop. Rain is total precipitation for each experiment. N is number of replicates Site and treatments Kilifi (short rains) Monocrop
Yield (g)
%
Cum. stemborers
39.3 ( ± 3.6)a
%
15.8 ( ± 1.6)a
49.9 ( ± 8.5)a
Kilifi (long rains) Monocrop
93.6 ( _+5.5)a
14.6
Mtwapa (short rains) Monocrop
3
24.8 ( ± 1.8)a
129.4 ( + 9.6)b
8
Mtwapa (long rains) Monocrop
43.4 ( ± 4.6)a
8
3
19.2
-
61.5 ( + 5.1)b
Mtwapa Ext. (long rains) Monocrop
10.5 ( + 2.4)a
210 3
11.8 ( ± 1.9)a
8 - 25.4
1050
8.8 ( + 0.9)a
7
30.4 ( ± 3.7)a
5
57.1 Intercrop
25.1
13.4 ( + 1.4)a
41.7 lntercrop
873
17.9 ( + 2.4)a -
23.6 ( ± 8.0)a
- 14.5 21.2 ( ± 2.2)a
29.2 ( ± 9.3)a
Intercrop
241
18.1 ( + 1.4)a
38.3 Intercrop
- 21.4
16.5 ( ± 3.6)a
N 3
27.0 Intercrop
Rain (mm)
438
23.9 ( + 4.6)a
5
Means followed by the same letter are not significantly different at P < 0.05 (F-test).
15-25% lower in the intercrop than in the monocrop (Table 2). The highest cumulative number of stemborers was found for the extended period (long rains) which corresponded well with the lowest yield per plant (Table 2). The multiple regression model predicting grain yield per plant at harvest ~ as a function of site (S), treatment (T), rain (W), and mean cumulative number of stemborers (-C-S-) explains 91% of the variation. First and second order terms plus their first order interactions were included in the full model (F15.37 = 25.22, P < 0.001, N = 52). If only the significant terms were included the explained variation dropped to 77% (Table 3). Upon taking the ant±log, the model becomes P----Y= exp (2.9743 + 1.1325S + 0.0016W - 0.0288C--S)
(3)
where S = 1 for Kilifi and 0 for Mtwapa. The model predicts yield to decline exponentially with an increase in the cumulative number of stemborers for
both cropping systems. Omitting the stemborers from the model reduced the explained variation to 70% (/72.49 "~- 5 8 . 2 , P < 0.001, N = 52). Thus, although stemborers only contributed 7% to the explained variation this was nevertheless significant (F14,37 = 34.8, P < 0.01, N = 52). (See Mendenhall (1968) for testing linear models.) Table 3 Reduced model with Kilifi and Mtwapa as (S), cumulated rain (W), and mean cumulated density ( - ~ of the three stemborer species Chilo partellus, Chilo orichalcociliellus, and Sesamia calamistis Source
d.f. Mean F-value P square
Model 3 10.11 Intercept 1 40.17 S 1 14.40 W 1 9.13 CS 1 2.73 Error 48 0.19 **
P
***
54.60 211.40 77.72 49.28 14.73
P
R2
Parameter±SE
* * * 77.3% *** 2.9743+0.2045 *** 1.1325__.0.1285 *** 0.0016+0.0002 ** -0.0288+0.0075
H. Skovg&rd, P. Pfits / Agriculture, Ecosystems and Environment 62 (1997) 13-19
4. Discussion
Intercropping maize with cowpea significantly increased yield per plant compared with a sole crop of maize (Table 1). The number of stemborers per plant was 15-25% lower in the intercrop in four of the five experiments. Reduced density of stemborers or injury is generally argued as the reason for higher yields (this study; Lambert et al., 1987; Omolo et al., 1993). The maize plants may also have gained by intercropping with the leguminous cowpea. Intercrop results can be difficult to interpret because factors such as intra- and interspecific plant competition influence plant growth simultaneously with the pest injury (Andow, 1991b). Competition for water, nutrients or light can influence growth and yield more than the pests do (Martin et al., 1989). Taking the possible interactions of these responses with respect to pests into consideration suggests that the higher yield of maize in the present study was the result of reduced cumulative number of stemborers per plant. That maize intercropped with a non-stemborer host gives a yield advantage compared with monocropped maize is in accord with other findings (Lambert et al., 1987; Litsinger et al., 1991; Omolo et al., 1993; Ampong-Nyarko et al., 1994). However, neither Litsinger et al. (1991) nor Ampong-Nyarko et al. (1994) draw this conclusion because they present their data on an area basis. However, taking plant densities into consideration, the maize and sorghum yield per plant in the intercrop is consistently higher in their reports. The reduced model with site, rain, and stemborers as independent variables explained 77% of the total variation in yield per plant. Not surprisingly, site and total rain contributed most to the model (Table 3). Site was included as a qualitative variable because omitting this factor resulted in a significantly lower R 2 and unrealistic estimates of the model parameters. The applicability of the model may be improved if quantitative information about sites, e.g. field capacity, nutrients and organic matter in the soil could be included. Rain during the 2 first months, including the first week before planting, contributed most to the explained yield variation. From germination to tasselling, maize is drought sensitive and water shortage can result in severe yield loss (Acland, 1989). Within the range of observations, the model
17
predicts yield to increase with more rain, Especially in Kilifi, abundant precipitation and favourable soil moisture during the early growth of maize resulted in the highest yields of all experiments (Table 2). However, the full benefit of rain can be masked by damage from stemborers, which in the extended period of the long rains caused a considerable reduction in yield (Table 2). Emergence of a new, non-diapausing generation of moths, synchronised with planting, leads to a high infestation and probably explains the low yield in that experiment. Therefore, very late planting of maize cannot be recommended, even if favourable weather permits this. If the crop can tolerate a certain level of injury, then a small reduction in pest numbers at this level will have little effect on final yield (Andow, 1991b). A crop may also compensate for injury or even increase in yield when injury is slight (Bardner and Fletcher, 1974). The mean cumulative number of stemborers ~ for the two cropping systems shows a negative curvilinear relationship with yield, suggesting no compensatory response of the plants. This is in line with others who have found negative linear relationships with stemborers, based on counts of eggs, larvae, adults (Hosny and E1-Saadany, 1973; Cheshire et al., 1989; Bode and Calvin, 1990; Morallo-Rejesus et al., 1990) or indirect measures as plant injury (Davis and Pedigo, 1990) and tunnelling (Walker, 1960; MacFarlane, 1990). In this study, the negative curvilinear relationship between ~ and yield was based on the assumption that the three species of stemborers are similar with respect to the way they cause injury to the plants, and the species were therefore pooled. Similar infestation and yield curves have been shown in a comparative study of Busseola fusca (Fuller) and C. partellus in sorghum (Van den Berg and Van Rensburg, 1991). For a given set of values, e.g. Kilifi as site, 700 mm rain, and 20 stemborers per plant, Eq. (3) predicts 105 g yield per plant. Because the cumulative number of stemborers is 15-25% lower in the intercrop, about 16 borers per plant are expected instead of 20, which will increase yield to 117 g per plant or about 12% more than in the monocrop. However, without stemborer damage, Eq. (3) predicts 186 g per plant, showing the yield loss is still high at 37% in the intercrop, indicating that additional control methods are necessary.
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H. Skovg&rd, P. Piits /Agriculture, Ecosystems and Environment 62 (1997) 13-19
Intercropping maize with cowpea significantly reduced the damage by graminaceous stemborers and increased the maize yield. However, maize/cowpea intercropping as a single control method against stemborers may not be enough because although less damage is expected, the yield loss can still lie beyond what is acceptable to the farmer. Thus, intercropping as a cultural practice to control lepidopterous stemborers in maize should be regarded as one low input component in a sustainable management system.
Acknowledgements This study was funded by the Swedish Agency for Research Cooperation with Developing Countries (SAREC) and the Danish International Development Agency (DANIDA). It formed part of the project 'Pest Management on Small Scale Farms in Africa', under the auspices of the International Centre of Insect Physiology and Ecology (ICIPE)/Wageningen Agricultural University Biological Control Project. Kenya Agricultural Research Institute (KARI) in Mtwapa, and Kilifi Agricultural Institute provided field sites and weather data. Thanks are due to Peter Jolliffe for valuable comments on the manuscript.
References Acland, J.D., 1989. East African Crops. Longman Group, UK, 252 pp. Alghali, A.M., 1986. Effects of cultivar, time and amount of Chilo partellus Swinhoe (Lepidoptera: Pyralidae) infestation on sorghum yield components in Kenya. Trop. Pest Manage., 32: 126-129. Altieri, M.A., Francis, C.A., van Schoonhoven, A. and Doll, J.D., 1978. A review of insect prevalence in maize (Zea mays L.) and bean (Phaseolus vulgaris L.) polycultural systems. Field Crops Res., 1: 33-49. Ampong-Nyarko, K., Seshu Reddy, K.V., Nyang'or, R.A. and Saxena, K.N., 1994. Reduction of insect pest attack on sorghum and cowpea by intercropping. Entomol. Exp. Appl., 70: 179184. Andow, D.A., 1991a. Vegetational diversity and arthropod population response. Ann. Rev. Entomol., 36: 561-586. Andow, D.A., 1991b. Yield loss to arthropods in vegetationally diverse agroecosystems. Environ. Entomol., 20: 1228-1235.
Bardner, R. and Fletcher, K.E., 1974. Insect infestation and their effects on the growth and yield of field crops: a review. Bull. Entomol. Res,, 64: 141-160. Bode, W.M. and Calvin, D.D., 1990. Yield-loss relationships and economic injury levels for European corn borer (Lepidoptera: Pyralidae) populations infesting Pennsylvania field corn. J. Econ. Entomol., 83: 1595-1603. Cheshire, J.M., Funderburk, J.E., Zimet, D.J., Mack, T.P. and Gilreath, M.E., 1989. Economic injury levels and binomial sampling program for lesser corn stalk borer (Lepidoptera: Pyralidae) in seedling grain sorghum. J. Econ. Entomol., 82: 270-274. Davis, P.M. and Pedigo, L.P., 1990. Yield response of corn stands to stalk borer (Lepidoptera: Noctuidae) injury imposed during early development. J. Econ. Entomol., 83: 1582-1586. Dempster, J.P. and Coaker, T.H., 1972. Diversification of crop ecosystems as a means of controlling pests. In: D.P. Jones and M.E. Solomon (Editors), Biology in Pest and Disease Control. Blackwell, Oxford, pp. 106-114. Hosny, M.M. and El-Saadany, G.B., 1973. The damage-assessment and the estimation of the injury level caused by stalk borers to maize plants in Egypt. Z. Angew. Entomol., 73: 387-399. Kareiva, P., 1983. Influence of vegetation texture on herbivore populations. In: R.F. Denno and M.S. McClaire (Editors), Variable Plants and Herbivores in Natural and Managed Ecosystems. Academic Press, New York, pp. 259-289. Lambert, J.D.H., Arnason, J.T., Serratos, A., Philogene, B.J.R. and Faris, M.A., 1987. Role of intercropping red clover in inhibiting European corn borer (Lepidoptera: Pyralidae) damage to corn in Eastern Ontario. J. Econ. Entomol., 80:11921196. Letourneau, D.K., 1987. The enemies hypothesis: Tritrophic interactions and vegetational diversity in tropical agroecosystems. Ecology, 68: 1616-1622. Litsinger, J.A. and Moody, K., 1976. Integrated pest management in multiple cropping systems. In: R.I. Papendick, P.A. Sanchez and G.B. Triplett (Editors), Multiple Cropping. American Society and Agronomy, Madison, WI, pp. 293-317. Litsinger, J.A., Hasse, V., Barrion, A.T. and Schmutterer, H., 1991. Response of Ostrinia furnacalis (Guende) (Lepidoptera: Pyralidae) to intercropping. Environ. Entomol., 20: 988-1004. MacFarlane, J.H., 1990. Damage assessment and yield loses in sorghum due to the stemborer Busseola fusca (Fuller) (Lepidoptera: Noctuidae) in Northern Nigeria. Trop. Pest Manage., 36: 131-137. Martin, R.C., Arnason, J.T., Lambert, J.D.H., lsabelle, P., Voldeng, H.D. and Smith, D.L., 1989. Reduction of European corn borer by intereropping corn with soybean. J. Econ. Entomol., 82: 1455-1459. Mathez, F.C., 1972. Chilo partellus Swinh., C. orichalcociliella Strand (Lep., Crambidae) and Sesamia calamistis Hmps. (Lep., Noctuidae) on maize in the Coast Province, Kenya. Mitt. Schweiz. Ent. Ges., 45: 267-289. Mendenhall, W., 1968. Introduction to Linear Models and the Design and Analysis of Experiments. Wadsworth, CA, 465 pp. Morallo-Rejesus, B., Buctuanon, E.M. and Rejesus, R.S., 1990.
H. Skovgdrd, P. Pints/Agriculture, Ecosystems and Environment 62 (1997) 13-19 Defining the economic threshold determinants for the Asian corn borer, Ostrinia furnacalis (Guen6e) in the Philippines. Trop. Pest Manage., 36: 114-121. Okigbo, B.N. and Greenland, G.J., 1976. Integrated pest management in multiple cropping systems. In: R.I. Papendick, P.A. Sanchez and G.B. Triplett (Editors), Multiple Cropping. American Society and Agronomy, Madison, WI, pp. 63-101. Omolo, E.O., Nyambo, B., Simbi, C.O.J. and Ollimo, P., 1993. The role of host plant resistance and intercropping in integrated pest management (IPM) with specific reference to the Oyugis project. Int. J. Pest Manage., 39: 265-272. Overholt, W.A., Ogedah, K. and Lammers, P.M., 1994. Distribution and sampling of Chilo partellus (Lepidoptera: Pyralidae) in maize and sorghum on the Kenya coast. Bull. Entomol. Res., 84: 367-378. P~its, P., Ekbom, B. and Skovg~lrd, H., 1997. Influence of intercropping on the abundance, distribution and parasitism of Chilo spp. eggs (Lepidoptera: Pyralidae). Bull. Ent. Res., 87: in press. Risch, S.J., Andow, D. and Altieri, M.A., 1983. Agroecosystem diversity and pest control: Data, tentative conclusions, and new research directions. Environ. Entomol., 12: 625-629. Russell, E.P., 1989. Enemies hypothesis: A review of the effect of vegetational diversity on predatory insects and parasitoids. Environ. Entomol., 18: 590-599. Sharma, A.N. and Sharma, V.K., 1987. Studies on the economic
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injury level in maize, Zea mays L. to stemborer, Chilo partellus (Swinhoe) (Pyralidae: Lepidoptera) in India. Trop. Pest Manage., 33: 44-51. Sheehan, W., 1986. Response by specialist and generalist natural enemies to agroecosystem diversification: A selective review. Environ. Entomol., 15: 456-461. Skovg~trd, H. and Pats, P., 1996. Effect of intercropping on maize stemborers and their natural enemies. Bull. Entomol. Res., 86: 599-607. Statistical Analysis Systems Institute Inc., 1985. SAS User's Guide, 5th edn. SAS Institute Inc., Cary, NC, 956 pp. Tonhasca, Jr., A. and Byrne, D., 1994. The effects of crop diversification on herbivorous insects: A meta-analysis approach. Ecol. Entomol., 19: 239-244. Van den Berg, J. and van Rensburg, J.B.J., 1991. Comparative injuriousness of Busseola fusca (Lepidoptera: Noctuidae) and Chilo partellus (Lepidoptera: Pyralidae) on grain sorghum. Bull. Entomol. Res., 82: 137-142. Walker, P.T., 1960. The relation between infestation by the stalk borer, Busseola fusca, and yield of maize in East Africa. Ann. Appl. Biol., 48: 780-786. Zethner, O., 1995. Practice of integrated pest management in tropical and sub-tropical Africa: an overview of two decades (1970-1990). In: A.N. Mengech, K. Saxena and H.N.B. Gopalan (Editors), Integrated Pest Management in the Tropics. John Wiley, New York, pp. 1-67.
Agriculture, Ecosystems and Environment 62 (1997) 13-19
Reduction of stemborer damage by intercropping maize with cowpea Henrik Skovg~d a,*, Peeter P~its b,l a Department of Population Biology, Universitetsparken 15, 2100 Copenhagen ¢), Denmark b Swedish University of Agricultural Sciences, Department of Entomology, Box 7044, 750 07 Uppsala, Sweden Accepted 17 October 1996
Abstract
The effect of intercropping maize with cowpea on the damage caused by three species of lepidopteran stemborers (Chilo partellus, Chilo orichalcociliellus, and Sesamia calamistis) was studied in Kenya. Significantly higher yields of maize (27-57%) were found in the intercrop in four out of five experiments, corresponding with significantly lower numbers (15-25%) of stemborers. A multiple regression model predicting yield per maize plant showed that 77% of the variation could be explained by site, rainfall, and the number of stemborers. Keywords: Chilo partellus; Chilo orichalcociliellus; Sesamia calamistis; Yield loss; Multiple regression model
1. Introduction Intercropping is traditionally practised by subsistence farmers in most developing countries, and is largely rooted in the experience of farmers rather than based on scientific methods (Litsinger and Moody, 1976; Okigbo and Greenland, 1976; Zethner, 1995). Crop diversity in tropical agroecosystems may provide several advantages over monocropping, by giving a higher total return in yield and acting as
* Corresponding author at: The Danish Pest Infestation Laboratory, Skovbrynet 14, 2800 Lyngby, Copenhagen-DK, Denmark. Tel.: +45 45878055; fax: + 4 5 45931155; e-mail: [email protected]. Present address: Department of Plant Science, University of British Columbia, 336-2357 Main Mall, Vancouver, B.C. V6T IZ4, Canada.
an insurance against crop failure or fluctuating market prices of single crops. Intercropping has also been suggested as a means of reducing arthropod pest infestation and subsequent damage (Dempster and Coaker, 1972; Altieri et al., 1978; Andow, 1991a). The mechanisms involved in pest suppression in intercropping remain obscure. In many cases impeded pest movements seem to be the main cause (Kareiva, 1983; Tonhasca and Byrne, 1994), but increased influence of natural enemies has also been implicated (Sheehan, 1986; Letourneau, 1987; Russell, 1989). With intercrop experiments, difficulties can arise when relating pest infestation and injury to the growth and yield of the plants. Factors such as competition between plant species and plant interactions with the soil may override the influence of the pests, leading to incomplete conclusions about intercropping. Therefore, both higher and lower yields in intercrop-
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H. Skovggtrd, P. Pfits /Agriculture. Ecosystems and Environment 62 (1997) 13-19
ping have been reported at reduced pest infestation levels (Altieri et al., 1978; Risch et al., 1983). World-wide, lepidopterous stemborers cause considerable yield loss of maize and sorghum (Alghali, 1986; Sharma and Sharma, 1987; Davis and Pedigo, 1990; Morallo-Rejesus et al., 1990; Van den Berg and Van Rensburg, 1991). Maize, Zea mays (L.), intercropped with cowpea, Vigna unguiculata (L.), is a traditional crop combination for smallholders in the coastal area of East Africa. The most common and serious insect pests are two pyralids, Chilo partellus (Swinhoe) and Chilo orichalcociliellus (Strand), and one noctuid, Sesamia calamistis Hampson which can be found simultaneously in the field (Mathez, 1972; Overholt et al., 1994). In a recent series of intercrop experiments, it was found that crop diversity did not influence Chilo spp. oviposition behaviour or oviposition per se (Pats et al., 1997). However, intercropping increased egg parasitism, although larval and pupal parasitism were not affected (Skovg~d and P~its, 1996). Further, when sampling data were combined, fewer larvae and pupae were found in the intercrop than in the monocrop (Skovg~d and Pats, 1996). How this occurrence of stemborers in the two cropping systems influences yield still needs to be quantified. The present study aims first, to examine to what extent intercropping maize with cowpea can constrain stemborer damage and secondly, to quantify the influence of stemborers on maize yield in the two cropping systems.
2. Materials and methods
2.1. Experimental layout Field plots were located in Mtwapa (14 km) and Kilifi (54 km) north of Mombasa on the Kenyan coast. The experiments were conducted in two consecutive seasons, that of 'short rains' 1993 (planting in Mtwapa on 14 October and Kilifi on 21 October, harvest 28-29 January) and 'long rains' 1994 (planting in Mtwapa on 11 April and in Kilifi on 9 May, harvest 6 August and 10 September). A longer duration than usual of the long rains during 1994 allowed for an additional experiment in Mtwapa (planting 3 June, harvest 10 September).
Two different types of plots were used, maize as monocrop and maize mixed intercropped with cowpea. The monocrop maize, cv. 'Pwani Hybrid' was planted at a spacing of 0.5 m within and between rows. In the intercrop every other maize plant was replaced by a cowpea plant, cv. 'Katumani 80'. Maize and cowpea were planted simultaneously. The plots were laid out in a randomised block design. Plot size in the short rains was 20 m x 20 m with three replicates per treatment, but was reduced to 10 m X 10 m plots with eight replicates per treatment in the long rains. The smaller plots were chosen to produce more uniform plants within the plot and to increase the number of replicates. In Kilifi (long rains) one intercrop plot was destroyed by grazing animals and its data were excluded from the analyses. For the additional experiment only five plots per treatment were laid out. Distance between plots was 3 m of mown grass. The soil was classified as sandy in Mtwapa and loamy sandy in Kilifi. An 18N:46P (100 kg ha -~) fertiliser mixture was added to the soil before planting. Additionally, 80 kg ha-l of N 26 was applied twice as top dressing in the monocrop and 40 kg ha-I to the maize plants in the intercrop. The first application was made 2 weeks after germination and the second at plant tasselling. Weeding was done manually. Rainfall was measured at each location. Maize cobs were harvested by hand at maturity by taking 100 plants at random in the monocrop and 50 in the intercrop. For thesmaller plots in the long rains, 50 and 25 plants were chosen. Kernel weight was determined at 13% water content.
2.2. Sampling stemborers The density of stemborers was recorded weekly by destructive sampling of plants at random in each plot. Because the monocrop contained twice the number of plants than the intercrop, and to obtain similar proportions of sampled plants in both systems, 50 maize plants in monocrop and 25 plants in intercrop were chosen during the first season. In the long rains, the number of plants sampled was reduced to 12 and six plants because of the smaller plot size. Each plant was uprooted and dissected and the number of stemborer larvae and pupae recorded. (for seasonal phenology of the stemborers, see Skovg~d and Pats, 1996).
H. Skovggtrd, P. PiJts/Agriculture, Ecosystems and Environment 62 (1997) 13-19
2.3. Statistical analyses
relationship between mean grain yield P ~ ) per plant at harvest and four explanatory variables, site (S), treatment (T), total rain (W), and mean cumulated number of stemborers (C----~,all of which are assumed to have contributed to final yield. Total rainfall from 1 week before planting to 3 months afterwards was included in the analysis to find the period giving the best prediction. Yield P ~ ) per plant was used as the dependent variable upon subjecting the data to In transformation to normalise data and to stabilise the variance about the regression line.
To analyse for the effect of cropping system (T), site (S), and experiment (E), an unbalanced nested analysis of variance was conducted for the grain yield of maize and the number of stemborers. For both tests a mixed model nested ANOVA with three levels and an interaction term was conducted. The equation is Xijkl = tx + Si -b E(i)j + Tk -b SiTk -b E(ijk)t
(1)
where Xij~I denotes either the mean grain yield at 13% water content per plant (P---Y)or the mean cumulative number of stemborers C ~ ) per plant of the ith site (random) of the jth experiment (random) of the kth treatment (systematic). /z is the parametric mean and E(ijk)l is the error term with the lth replicate. Yield was assumed to be constrained by the combined number of stemborers hosting the plant from germination and to harvest. Therefore, the mean cumulative number of stemborers C ~ ) was determined as
3. Results
The nested analyses of variance showed a significantly higher yield ( P < 0.01) and less cumulative numbers of stemborers ( P < 0.05) per plant in intercropping but no site or interaction effects were found (Table 1). In addition, a significant effect of experiment nested within site was found for yield and stemborers ( P < 0 . 0 0 1 ) , indicating that the maize yield and the incidence of stemborers was different depending on the season. The yield of the long rain experiments contributed significantly to the positive effect of intercropping with 38 and 42% higher yields than in the monocrop (Table 2). Also, higher mean yields of the intercrop were recorded in Kilifi during the short rains (27%) and in Mtwapa for the extended period (57%) (Table 2). No significant difference was found in the cumulative number of stemborers for each experiment. There was a trend in all experiments, except for Kilifi (short rains) that the cumulative number of stemborers per plant was
~B(i) C---S= i=,
15
(2) s
where i refers to the sampling occasions up to harvest (s). B{i) is the mean density of the ith occasion of alive and dead larvae and pupae per plant. Plant yield and cumulative number of stemborers per plant were In transformed to normalise data and stabilise the variance. A multiple regression (Statistical Analysis Systems Institute Inc., 1985) was applied to define the
Table 1 Nested analysis o f variance on yield a n d c u m u l a t i v e n u m b e r o f stemborers per maize plant. M o n o c r o p a n d intercrop (T), Kilifi a n d M t w a p a (S), a n d experiments ( E ) Source
Model T S
E(S) T- S Error
d.f.
6 1 t 3 1 45
Yield
Stemborers
M e a n square
F-value
P
R2
Mean square
F-value
P
R2
5.53 1.02 11.47 5.52 0.00 0.14
41.07 7.60 2.21 40.99 0.00
* * * * * NS * * * NS
84.6%
1.42 0.40 0.49 2.05 0.11 0.09
15.63 4.44 0.25 22.64 1.17
* * * • NS * * * NS
67.6%
NS, not significant; * P < 0.05; * * P < 0.01; * * * P < 0.001.
H. Skxmgdrd, P. PZtts/Agriculture, Ecosystems and Environment 62 (1997) 13-19
16
Table 2 Mean ( ± SE) grain yield at 13% water content and cumulative number of stemborers per maize plant. Percentage relative to the monocrop. Rain is total precipitation for each experiment. N is number of replicates Site and treatments Kilifi (short rains) Monocrop
Yield (g)
%
Cum. stemborers
39.3 ( ± 3.6)a
%
15.8 ( ± 1.6)a
49.9 ( ± 8.5)a
Kilifi (long rains) Monocrop
93.6 ( _+5.5)a
14.6
Mtwapa (short rains) Monocrop
3
24.8 ( ± 1.8)a
129.4 ( + 9.6)b
8
Mtwapa (long rains) Monocrop
43.4 ( ± 4.6)a
8
3
19.2
-
61.5 ( + 5.1)b
Mtwapa Ext. (long rains) Monocrop
10.5 ( + 2.4)a
210 3
11.8 ( ± 1.9)a
8 - 25.4
1050
8.8 ( + 0.9)a
7
30.4 ( ± 3.7)a
5
57.1 Intercrop
25.1
13.4 ( + 1.4)a
41.7 lntercrop
873
17.9 ( + 2.4)a -
23.6 ( ± 8.0)a
- 14.5 21.2 ( ± 2.2)a
29.2 ( ± 9.3)a
Intercrop
241
18.1 ( + 1.4)a
38.3 Intercrop
- 21.4
16.5 ( ± 3.6)a
N 3
27.0 Intercrop
Rain (mm)
438
23.9 ( + 4.6)a
5
Means followed by the same letter are not significantly different at P < 0.05 (F-test).
15-25% lower in the intercrop than in the monocrop (Table 2). The highest cumulative number of stemborers was found for the extended period (long rains) which corresponded well with the lowest yield per plant (Table 2). The multiple regression model predicting grain yield per plant at harvest ~ as a function of site (S), treatment (T), rain (W), and mean cumulative number of stemborers (-C-S-) explains 91% of the variation. First and second order terms plus their first order interactions were included in the full model (F15.37 = 25.22, P < 0.001, N = 52). If only the significant terms were included the explained variation dropped to 77% (Table 3). Upon taking the ant±log, the model becomes P----Y= exp (2.9743 + 1.1325S + 0.0016W - 0.0288C--S)
(3)
where S = 1 for Kilifi and 0 for Mtwapa. The model predicts yield to decline exponentially with an increase in the cumulative number of stemborers for
both cropping systems. Omitting the stemborers from the model reduced the explained variation to 70% (/72.49 "~- 5 8 . 2 , P < 0.001, N = 52). Thus, although stemborers only contributed 7% to the explained variation this was nevertheless significant (F14,37 = 34.8, P < 0.01, N = 52). (See Mendenhall (1968) for testing linear models.) Table 3 Reduced model with Kilifi and Mtwapa as (S), cumulated rain (W), and mean cumulated density ( - ~ of the three stemborer species Chilo partellus, Chilo orichalcociliellus, and Sesamia calamistis Source
d.f. Mean F-value P square
Model 3 10.11 Intercept 1 40.17 S 1 14.40 W 1 9.13 CS 1 2.73 Error 48 0.19 **
P
***
54.60 211.40 77.72 49.28 14.73
P
R2
Parameter±SE
* * * 77.3% *** 2.9743+0.2045 *** 1.1325__.0.1285 *** 0.0016+0.0002 ** -0.0288+0.0075
H. Skovg&rd, P. Pfits / Agriculture, Ecosystems and Environment 62 (1997) 13-19
4. Discussion
Intercropping maize with cowpea significantly increased yield per plant compared with a sole crop of maize (Table 1). The number of stemborers per plant was 15-25% lower in the intercrop in four of the five experiments. Reduced density of stemborers or injury is generally argued as the reason for higher yields (this study; Lambert et al., 1987; Omolo et al., 1993). The maize plants may also have gained by intercropping with the leguminous cowpea. Intercrop results can be difficult to interpret because factors such as intra- and interspecific plant competition influence plant growth simultaneously with the pest injury (Andow, 1991b). Competition for water, nutrients or light can influence growth and yield more than the pests do (Martin et al., 1989). Taking the possible interactions of these responses with respect to pests into consideration suggests that the higher yield of maize in the present study was the result of reduced cumulative number of stemborers per plant. That maize intercropped with a non-stemborer host gives a yield advantage compared with monocropped maize is in accord with other findings (Lambert et al., 1987; Litsinger et al., 1991; Omolo et al., 1993; Ampong-Nyarko et al., 1994). However, neither Litsinger et al. (1991) nor Ampong-Nyarko et al. (1994) draw this conclusion because they present their data on an area basis. However, taking plant densities into consideration, the maize and sorghum yield per plant in the intercrop is consistently higher in their reports. The reduced model with site, rain, and stemborers as independent variables explained 77% of the total variation in yield per plant. Not surprisingly, site and total rain contributed most to the model (Table 3). Site was included as a qualitative variable because omitting this factor resulted in a significantly lower R 2 and unrealistic estimates of the model parameters. The applicability of the model may be improved if quantitative information about sites, e.g. field capacity, nutrients and organic matter in the soil could be included. Rain during the 2 first months, including the first week before planting, contributed most to the explained yield variation. From germination to tasselling, maize is drought sensitive and water shortage can result in severe yield loss (Acland, 1989). Within the range of observations, the model
17
predicts yield to increase with more rain, Especially in Kilifi, abundant precipitation and favourable soil moisture during the early growth of maize resulted in the highest yields of all experiments (Table 2). However, the full benefit of rain can be masked by damage from stemborers, which in the extended period of the long rains caused a considerable reduction in yield (Table 2). Emergence of a new, non-diapausing generation of moths, synchronised with planting, leads to a high infestation and probably explains the low yield in that experiment. Therefore, very late planting of maize cannot be recommended, even if favourable weather permits this. If the crop can tolerate a certain level of injury, then a small reduction in pest numbers at this level will have little effect on final yield (Andow, 1991b). A crop may also compensate for injury or even increase in yield when injury is slight (Bardner and Fletcher, 1974). The mean cumulative number of stemborers ~ for the two cropping systems shows a negative curvilinear relationship with yield, suggesting no compensatory response of the plants. This is in line with others who have found negative linear relationships with stemborers, based on counts of eggs, larvae, adults (Hosny and E1-Saadany, 1973; Cheshire et al., 1989; Bode and Calvin, 1990; Morallo-Rejesus et al., 1990) or indirect measures as plant injury (Davis and Pedigo, 1990) and tunnelling (Walker, 1960; MacFarlane, 1990). In this study, the negative curvilinear relationship between ~ and yield was based on the assumption that the three species of stemborers are similar with respect to the way they cause injury to the plants, and the species were therefore pooled. Similar infestation and yield curves have been shown in a comparative study of Busseola fusca (Fuller) and C. partellus in sorghum (Van den Berg and Van Rensburg, 1991). For a given set of values, e.g. Kilifi as site, 700 mm rain, and 20 stemborers per plant, Eq. (3) predicts 105 g yield per plant. Because the cumulative number of stemborers is 15-25% lower in the intercrop, about 16 borers per plant are expected instead of 20, which will increase yield to 117 g per plant or about 12% more than in the monocrop. However, without stemborer damage, Eq. (3) predicts 186 g per plant, showing the yield loss is still high at 37% in the intercrop, indicating that additional control methods are necessary.
18
H. Skovg&rd, P. Piits /Agriculture, Ecosystems and Environment 62 (1997) 13-19
Intercropping maize with cowpea significantly reduced the damage by graminaceous stemborers and increased the maize yield. However, maize/cowpea intercropping as a single control method against stemborers may not be enough because although less damage is expected, the yield loss can still lie beyond what is acceptable to the farmer. Thus, intercropping as a cultural practice to control lepidopterous stemborers in maize should be regarded as one low input component in a sustainable management system.
Acknowledgements This study was funded by the Swedish Agency for Research Cooperation with Developing Countries (SAREC) and the Danish International Development Agency (DANIDA). It formed part of the project 'Pest Management on Small Scale Farms in Africa', under the auspices of the International Centre of Insect Physiology and Ecology (ICIPE)/Wageningen Agricultural University Biological Control Project. Kenya Agricultural Research Institute (KARI) in Mtwapa, and Kilifi Agricultural Institute provided field sites and weather data. Thanks are due to Peter Jolliffe for valuable comments on the manuscript.
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