African Crop Science Journal, Vol. 10. No. 4, 2002, pp. 335-344
POTENTIAL IMPACT OF INTERCROPPING ON MAJOR COWPEA FIELD PESTS IN UGANDA
P. NAMPALA, M.W. OGENGA-LATIGO, S. KYAMANYWA, E. ADIPALA, N. OYOBO1 and L.E.N. JACKAI2
Department of Crop Science, Makerere University, P.O. Box 7062, Kampala,
(Received January, 2001; accepted 2 August, 2002)
Code Number: cs02032
Insect pests are perhaps the most important constraint to cowpea (Vigna unguiculata L. Walp) production. In Uganda, aphids, thrips, pod sucking bugs and pod borers are ubiquitous and very devastating, sometimes leading to total crop failure. On-farm studies were conducted at 3 sites in eastern Uganda for three consective seasons to evaluate the use of intercropping as a pest control strategy in cowpea. Two local cowpea cultivars, Ebelat (Erect) and Icirikukwai (spreading) were grown as sole crops or intercropped with a local variety of greengram (Vigna radiata) or sorghum Sorghum bicolor cv. Seredo. Aphids and thrips populations were significantly reduced in the cowpea + sorghum intercrop but were higher in the cowpea + greengram intercrop. In contrast, pod borer and pod sucking bug infestations and their associated damage were significantly higher in the cowpea + sorghum intercrop than in the other cropping systems. These results contradict previous reports and indicate that (a) not all pests are controlled by intercropping, (b) to be effective, intercropping has to be part of a pest management system that involves other control strategies, and (c) choice of a cropping system for IPM should consider the pest profile.
Key Words: Aphids, IPM, pod borers, pod sucking bugs, thrips, Vigna unguiculata
Les pestes des insectes représentent peut être la plus grande contrainte à la production du niébé (Vigna unguiculata L. Walp). En Ouganda, les alphides, thrips, les suceurs et foreurs de gousses sont omniprésent et très dévastateurs, par moments conduisent à la non production. Des essais des champs étaient conduits en trois endroits à lest de lOuganda pour trois saisons consécutives, pour évaluer lutilisation de cultures associées comme stratégie de contrôle des pestes dans le niébé. Deux variétés du niébé, Ebelat (Erect) et Icirikukwai (spreading) étaient plantées en mono et cultures associées avec des variétés locales de greengram (Vigna radiata) ou du sorgho (Sorghum bicolor cv. Seredo). Les populations des alphides et des thrips étaient significativement réduites dans le niébé associé au sorgho mais étaient élévées dans le niébé associé au greengram. Au contraire, les infestations des foreurs et des suceurs de gousses et les dommages induits étaient significativement élévés dans la combinaison niébé et sorgho que dans dautres cultures. Ces résultats contraidisent les rapports précédents et indiquent que (a) pas toutes les pestes sont contrôlées par la culture en association, (b) pour être efficace, la culture en association doit faire partie du système de gestion des pestes impliquant dautres stratégies de contrôle, et (c) le choix du système des cultures pour la gestion intégrée des pestes devra considérer le profile des pestes.
Mots Cles: Aphides, IPM, foreurs de gousses, suceurs de gousses, thrips, Vigna unguiculata
Cowpea (Vigna unguiculata L. Walp), is a major source of dietary protein in many parts of Africa (IITA, 1984). In Uganda, the crop is grown for grain and leaves in the northern and eastern parts of the country. Although yields of 2500 kg ha-1 are achievable (Rusoke and Rubaihayo, 1994) yields at farm level average only 150-400 kg ha-1 (Sabiti et al., 1994). The major constraints are pests, diseases and low yield potential of local cultivars (Adipala et al., 1997). Insect pests cause the greatest damage, and the crop is attacked by a spectrum of pest species (Rusoke and Rubaihayo, 1994; Edema and Adipala, 1996). For instance, the cowpea aphid, Aphis craccivora Koch, is a major seedling phase problem. Subsequently, the crop is attacked by flower thrips, Megalurothrips sjostedti Trybom, pod borer, Maruca vitrata Fab. and a complex of pod and seed suckers. According to Edema (1995) and Omongo et al. (1997) these insect pests are the most damaging on cowpea in Uganda. Edema and Adipala (1996) estimated that pests account for > 70% yield reduction in cowpea, while diseases were estimated to account for < 20%.
To minimise losses, farmers in eastern Uganda regularly apply insecticides (Omongo et al., 1997). However, although pesticides may maintain pest populations below economically damaging levels, insecticide use is unsustainable because of the low purchasing power of the farmer and the likely environmental harzards. Instead, a holistic approach whereby all possible strategies are used rationally and in a manner that is ecologically acceptable is required. However, prior to the development of an appropriate package, there is need to develop and test individual components. This paper reports on the use of intercropping as a component of integrated pest management (IPM) against cowpea pests.
Farmers in eastern Uganda commonly grow cowpea as an intercrop with sorghum or greengram (Adipala et al., 1997). The farmers believe that this is for purely traditional reasons, but perhaps as indicated by other studies, intercropping can also be used as a crop protection tactic against cowpea pests (Kyamanywa et al., 1993; Ogenga-Latigo et al., 1993). Indeed, Edema and Adipala (1996) have shown that aphid infestation is less in intercropped than sole cropped cowpea, but the relative infestation in the different cowpea cropping systems has not been studied in detail.
MATERIALS AND METHODS
On-farm trials were conducted during 3 consecutive cropping seasons (i.e., during the long rains (March - July) of 1997 (1997A), short rains (August - November) of 1997 (1997B), and the long rains of 1998 (1998A). The study sites were Abata (1_ 31N; 33_ 53E) in Kumi; Katukei (1_ 13N; 33_ 42E) in Pallisa; and Amuria (2_ 02N; 33_ 39E) in Katakwi, all in eastern Uganda where cowpea production is a major activity (Omongo et al., 1996).
Two local cowpea cultivars, Ebelat (erect) and Icirikukwai (spreading) were grown as sole crops or intercropped with a local variety of greengram (Vigna radiata) or sorghum cv. Seredo. Treatment combinations, therefore, were Ebelat, Icirikukwai, Ebelat + greengram, Icirikukwai + greengram, Ebelat + sorghum and Icirikukwai + sorghum. Planting was done at the onset of rains of the 1997 first (A) and second (B) rainy seasons, and the 1998A rainy season. The treatments were arranged in a RCBD with three replications. Each plot measured 5 m x 5 m, had 9 rows and all plots were separated by 1 m alleys; the blocks were 2 m apart. The intercrops were grown in additive mixtures whereby the plant population of the main crop, cowpea in this case, was maintained constant both in the mixtures and sole crops (Osiru and Wiley, 1972). The plant populations for cowpea and intercrop components were based on a spacing of 60 cm x 20 cm (Obuo et al.1998). The incidence and severity of the major insect pests were monitored throughout the study period starting 10 days after cowpea emergence DAE . Sampling for aphid infestation was done diagonally across each plot on 10 randomly tagged cowpea plants. Severity of A. craccivora attack was rated as suggested by Ogenga-Latigo et al. 1993a , with slight modification. The rating was 1 = no aphids, 2 = 1-100 aphids, 3 = 101 - 300, 4 = 301 -600, 5 = 601 - 1000 and 6 > 1000 aphids per plant. This was done weekly starting at 10 DAE. The populations of flower thrips, and Maruca pod borer were assessed at a 10 day interval starting from bud initiation approx. 30 DAE up to pod maturity. Twenty flower buds or flowers, depending on plant growth stage, were randomly picked from each plot and placed in separate vials containing 30 ethanol Kyamanywa and Tukahirwa, 1988 . These were later dissected under a binocular microscope and the number of thrips adults + nymphs and Maruca larvae recorded. Assessment of pod sucking bugs was done at pod initiation, 50 podding and at full podding. On each sampling ocassion, the middle 2 cowpea rows were examined and the number of Heteropteran adults there recorded. Damage by pod sucking bugs and Maruca pod borer was also assessed at harvest. This was done by randomly picking thirty pods from each plot sample, opening and examining them for seed damage. The percentage of shrievelled seeds was taken as a measure of pod sucking bug damage, while the percentage of seeds eaten bored was used as a measure of Maruca damage. These parameters were chosen because of their suitability in estimating seed damage Omongo, 1996 . Grain yield was also determined using a sensitive weighing scale. Land Equivalent Ratios LERs were also computed to establish the yield advantages of cowpea + sorghum and cowpea + greengram intercrops following procedures described by Willey, 1979 . Analyses of variance ANOVA were conducted using the Genstat computer package LATRES, 1993 to test the variation of pest levels and damage with location, season, cropping system and crop variety. Significant P < 0.05 treatment means were compared using FisherÕs Least Significant Difference LSD . Data for each season and location were analysed separately before pooling for cross season and location analysis.
Intercropping significantly P < 0.05 affected aphid infestation Table 1 , with the populations being consistently lower in the cowpea + sorghum intercrop than in the sole crops and cowpea + greengram intercrop. However, differences between the two cowpea cultivars were not significant. Generally, there was reduced aphid severity during 1997B, but with no consistent trend for the cowpea + sorghum mixture. For all the growing seasons, Pallisa and Kumi supported similar levels of thrips, which were significantly P < 0.05 higher than at Katakwi Table 2 . Among the cropping systems, thrips population were significantly P< 0.05 lower in the cowpea + sorghum intercrop than in the sole crop or cowpea + greengram intercrop. Additionally, thrips populations at the three locations were higher in the long rains 1997A and 1998A than in the short rains 1997B . However, there were no significant differences in the density of flower thrips on the two cowpea cultivars. Maruca pod borer infestation was lower in the short than long rains Table 3 , being significantly P < 0.05 higher in the cowpea + sorghum mixture than in the sole crop and cowpea + greengram intercrop at Kumi. For Pallisa and Katakwi, although M. vitrata infestation was higher in the cowpea + sorghum mixtures, there was no significant difference between the cropping systems.
Different species of Hemipteran pod sucking bugs were found on the cowpea plants, including Clavigralla sp., Nezara viridula, Aspavia sp., Anoplecnemis curvipes, Riptortus dentipes, Leptoglossus sp. and Dysdercus sp. The most common were Clavigralla sp., Nezara viridula, Anoplocnemis curvipes and Riptortus sp., but during data collection, no differentiation was made between the species. At all sites, infestation of pod sucking bugs was higher during the long 1997A and 1998A than short rains 1997B ; no statistical difference were detected between the sites Table 4 . However, bug infestation on cowpea was highest in the cowpea + sorghum intercrop than in the cowpea sole crops and cowpea + greengram intercrop. No significant difference was observed between the two cowpea cultivars. Seed damage by Maruca pod borer and pod sucking bugs was significantly influenced by the cropping systems in all the locations and seasons Tables 5 and 6 . Cowpea seeds from the cowpea + sorghum intercrop consistently exhibited significantly higher damage levels than those from the sole crops and the cowpea + greengrams intercrop. Also, damage by Maruca pod borer was higher in the long rains than short rains, except at Katakwi where the reverse was true Table 6 . In the case of pod bug damage, a reverse trend was observed with the short rain crop 1997B recording greater seed damage than the long rain crop 1997A and 1998 .
During the long rains 1997A and 1998 , cowpea cropping systems significantly P < 0.05 influenced cowpea grain yields being higher in the cowpea + sorghum intercrop than in the sole crop or cowpea + greengram intercrop Table 7 . In contrast, during the second rains, the differences in grain yield were not significant. However, the cowpea + sorghum intercrop still yielded higher than the cowpea sole crop and cowpea + greengram intercrops. Seasonwise, yields were significantly higher during the long than the short rains at all locations. However, there was higher yield at Katakwi than at the other 2 locations during 1997A and 1998A, but during 1997B there was generally lower yield at Katakwi than at Pallisa and Kumi. On the basis of land equivalent ratio LER , cowpea + sorghum intercrops consistently gave higher yield advantage than the cowpea + greengram mixture Table 8 . Yield advantages were highest for the Icirikuwai + sorghum mixture than the Ebelat + sorghum intercrop. Grain yield was consistently greater in 1997A than 1997B.
This study has demonstrated that cropping systems affect pest incidence and the associated damage in variously. Aphids, thrips, Maruca pod borer and pod suckers were the most abundant pests within the cowpea cropping systems studied. This is constistent with previous studies in Uganda Edema et al., 1997; Omongo et al., 1997 . During all seasons and at all locations, cropping combinations significantly influenced infestation and severity of the four major cowpea pests but the results were variable depending on the cropping system, pest and season. Mixed cropping of cowpea with sorghum reduced infestation by aphids and thrips but increased pod borer and pod sucking bug populations. On the other hand, mixed cropping of cowpea with greengram and the cowpea sole crop increased aphid and thrips infestation but reduced pod borer and pod bug population on cowpea. It is possible that the Maruca and bug populations were re-distributed between cowpea and greengram. Our results seem to support the general view that cropping systems have a reducing influence on aphid infestations. For instance, Ogenga-Latigo et al. 1992a demonstrated that intercropping beans with closely spaced maize reduced the incidence of A. fabae on beans. Similarly, Sinthanantham et al. 1990 observed lower incidence of bean aphid on beans intercropped with maize than on sole beans. More recently in Uganda, Edema et al. 1997 reported that aphid infestation was less on intercropped than sole cropped cowpea. Studies on the influence of cowpea cropping systems on thrips infestation have also been reported.
Singh and Taylor 1978 , Karel et al. 1980 and Kyamanywa and Tukahairwa 1988 have reported reduced thrips infestation in intercropped cowpea. However, Ezueh 1991 reported that infestation by cowpea flower thrips increased in + cowpea-maize intercrop. This seems to be contrary to established view. As observed in this study, and reported by Ezueh 1991 , intercropping cowpea with cereals increases infestation by Maruca borer. However, Risch et al. 1983 reported a 46 reduction of M. vitrata in a sorghum + cowpea intercrop. Similarly, Karel et al. 1980 reported less damage by pod borers on cowpea intercropped with maize, as did Ahoaka-Atta et al. 1983 . The cowpea + sorghum mixture supported the highest number of pod suckers in all seasons. Similar results have been reported elsewhere. For example, Karel et al. 1980 working in Tanzania reported increased bug infestation of cowpea intercropped with maize. More recently however, Omongo et al. 1997 working in the same areas as in our study reported that intercropping did not reduce bug numbers.
Various studies Root, 1973; Kyamanywa and Ampofo, 1988; Ogenga-Latigo et al., 1992a,b; Kyamanywa et al., 1993 have attributed variations in herbivore load in mixtures compared to monocultures to increased efficacy and/or abundance of natural enemies, difference in food or resource concentration among cropping systems, and modification of crop microclimate. However, reasons for the apparent contradictions on the effects of similar intercrops on insects over space and time are not clearly understood. Indeed some workers Risch et al., 1983 have documented that a given pest may show variable responses over space and time. The high seed damage levels in the cowpea + sorghum intercrop corresponded to the higher M. vitrata and pod bug infestations observed in this cropping system. It is possible that these differences were a result of the presence of an alternative food source greengram which attracted some individuals away from cowpea. Sorghum is not a known host of M. vitrata or pod bugs. The reduced aphid and thrips infestation in the cowpea + sorghum mixtures probably contributed to the higher yields of this cropping systems. Despite the high pest pressure during the long rains, higher yields were obtained.
Obuo et al. 1998 also reported higher yields in the long rains than in the short rains for the cowpea + sorghum intercrop. The low yields obtained during the short rains, in spite of the reduced pest problem was probably due to the low and poorly distributed rainfall. It may also have been largely due to a greater impact of pests on the crop during the short rains than during the long rainy seasons. The higher seed damage levels recorded during the second rains are supportive of this. The yield advantage from the cowpea + sorghum mixtures based on LER could have been achieved through several ways. Usually a yield advantage occurs because component crops differ in their use of growth resources in such away that when they are grown in combination, they are either able to complement each other or compete for available resources Willey, 1979 . Competition within the cowpea + greengram mixture probably explains the lack of yield advantage, whereas for the cowpea + sorghum intercrop their could have been complimentarity through greater nutrient uptake, possibly by having peak demands for nutrients especially nitrogen and water at different stages of growth leading to the high yield advantage observed. Higher yield advantages for cultivar Icirikukwai compared to Ebelat in the cowpea + sorghum intercrop confirms earlier findings that spreading cowpea cultivars are the best components in sorghum intercrops in terms of yield advantage Adipala et al., 1997; Obuo et al., 1998 . Mixed cropping of cowpea with sorghum reduced aphids and thrips infestation but increased Maruca borer and pod bug populations.
However, mixed cropping of cowpea with greengram increased aphids and thrips infestation but reduced Maruca borer and pod bug populations. Therefore, results of this study indicate that intercropping alone does not offer effective control against all the cowpea pests. However, where a given cropping system gives high yield advantage in terms of LER, and can reduce damage or the rate of damage accumulation on a crop leading to increased yields as was the case for the cowpea + sorghum intercrop, it can certainly contribute to control of a pest s in an integrated crontrol context. Therefore, a management strategy that involves other control tactics to complement intercropping is needed. It should target the most important pest in the locality, taking into account the cropping system and seasonal fluctuation of the major pests.
This study is part of the Rockefeller Foundation Forum funded project on integrated management of cowpea pests in Uganda. We thank the Foundation, farmers and extension officers for their support.
©2002, African Crop Science Society
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