African Crop Science Journal African Crop Science Society ISSN: 1021-9730 EISSN: 2072-6589 Vol. 7, Num. 4, 1999, pp. 479-486

African Crop Science Journal, Vol. 7. No. 4, pp. 479-486, 1999

Integrated management of major field pests of cowpea in Eastern Uganda

P. Nampala, M.W. Ogenga-Latigo, S. Kyamanywa, E. Adipala, J. Karungi, N. Oyobo¹, J.E. Obuo² and L.E.N. Jackai³
Department of Crop Science, Makerere University, P.O. Box 7062, Kampala, Uganda
¹District Agriculture Office, P.O Box 26, Pallisa, Uganda
²Serere Agricultural and Animal Production Research Institute, P. O. Soroti, Uganda
³International Institute of Tropical Agriculture, Oyo Road, PMB 5320, Ibadan, Nigeria

Code Number: CS99038

ABSTRACT

This study, which was done on-farm and in phases, aimed at developing a farmer and environmentally friendly management package against major cowpea pests. The pests considered were Aphis craccivora, Megalurothrips sjostedti, Maruca vitrata and pod sucking bugs. One trial examined the potential of intercropping in the management of the major field pests of cowpea. Another trial studied the effect of seed dressing and host resistance on cowpea field pest infestation. The best treatments from these two trials were combined (integrated pest management) and tested together with a minimum foliar insecticide application. Intercropping cowpea and sorghum combined with carbofuran "seed dressing" and minimal insecticide spray application (spraying once at budding, flowering and at podding) increased grain yields (ca 1350 kg ha^-1) markedly compared to the unsprayed plots (ca 200 kg ha^-1), those receiving foliar insecticide spray (ca 1000 kg ha^-1) or carbofuran seed dressing only (ca 400 kg ha^-1).

Key Words: Grain yield, insecticide sprays, intercropping, seed dressing, Vigna unguiculata

RÉSUMÉ

Cette étude, qui a été conduite en milièu réel et en phases, visait au développement des packets de gestion contre les principaux pestes du niébe qui sont acceptables par les écologies et les agriculteurs. Les pestes considérés étaient Aphis craccivora, Megalurothrips sjostedti, Maruca vitrata et les punaises suceuses de gousses. Un essai examinait le potential d’association dans le contr™le de principaux pestes de champs du niébé. Un autre essai a étudié l’effet du traitement de semence et de la résistance de l’h™te sur l’infestation de peste de champ du niébé. Les meilleurs traitements de deux essais ont été combinés (gestion intégrée de peste) et testés ensemble avec l’application minimale d’insecticide foliaire. L’association du niébé avec le sorgho combinée avec le traitement de semences au carbofuran et l’application de la pulvérisation minimale d’insecticide (pulvérisation une fois au bourgeonnement, à la floraison et à la formation des gousses) a augmenté le rendements en grains (ca 1350 kg ha^-1) remarquablement comparé aux parcelles non pulvérisées (ca 200 kg ha^-1), à celles recevant la pulverisation d’insecticide foliaire (ca 1000 kg ha^-1) ou le traitement des semences au carbofuran seul (ca 400 kg ha^-1).

Mots Clés: Rendement en grains, association, traitement de semences, Vigna unguiculata

Introduction

In Uganda, and probably elsewhere in the world, cowpea (Vigna unguiculata L. Walp) pest incidence and diversity dictate that a single control strategy is unlikely to produce satisfactory results. Even though pesticides offer "immediate" control, pests respond differently to different insecticides. Besides, chemicals are expensive and are not environmentally friendly. As a result the "best mix" approach (IPM) is currently advocated for.

Integrated pest management (IPM) involves the most logical combination of different compatible tactics for the control of pests (Brader, 1979). However, because the efficacy of individual components have to be tested before they can be consolidated into a management package, researchers often hesitate to devote much effort to integrate packages. In an earlier study we evaluated the effectiveness of intercropping in reducing pest infestation on cowpea. Two local cowpea cultivars, Ebelat (erect) and Icirikukwai (semi-erect) were grown as sole crops or intercropped with a local variety of greengram (Vigna radiata) or sorghum cv. Seredo. Additionally, we also tested the efficacies of carbofuran and endosulfan seed dressings on cowpea field pests. Carbofuran (applied as a soil-drench) significantly reduced aphid infestation but not late season pest infestation. Subsequently, the most promising treatments (cowpea/sorghum intercrop and carbofuran seed dressing) were combined and tested together with a minimum insecticide spray application recommended in our program (Karungi et al., 2000b).

Our objective was to compare the effect of combining intercropping, seed dressing and minimum insecticide spray (IPM) against the sole treatments and the unsprayed control on field pest infestation of cowpea. The pests considered were aphids (Aphis craccivora Koch), thrips (Megalurothrips sjostedti Trybom), pod borer (Maruca vitrata Fabricius) and pod sucking bugs, basing on earlier studies (Omongo et al., 1998) which indicated that they were the major field pests of cowpea in Uganda.

Materials and Methods

Establishment of experiments. On-farm trials were established during the second (short) rains of 1997 and first (long) rains of 1998 in Katukei (Pallisa) and Abata (Kumi) in eastern Uganda. Cowpea is grown intensively in these areas, often with insecticide sprays (Omongo et al., 1998). At each site, plots were arranged in a randomised complete block design of three replications, each replicate plot measuring 5 x 5 m.

Two cowpea cultivars: Ebelat (erect) and Icirikukwai (spreading) were used in the study. These cowpea cultivars were planted as sole crops or intercropped with sorghum and received the following treatments: cowpea/sorghum, cowpea/sorghum + seed dressing with carbofuran, cowpea/sorghum + foliar spray (minimum spray schedule), cowpea + foliar spray and cowpea/sorghum + seed dressing with carbofuran + foliar spray. Minimum foliar spray schedule involved a single tank mix of dimethoate and ambush applied at budding, flowering and podding (Karungi et al. 2000b). Dimethoate (40% EC), applied at 400g ai ha^-1was directed at the pyrethroid-resistant pod sucking heteroptera while ambush (Cypermethrin) at 200g ai ha^-1was directed at aphids, thrips and Maruca. Carbofuran was applied at a rate of 1.5g per 1m row which was considered as a seed dressing treatment (Oonyu, 1992).

Land was prepared to a fine tilth, and planting was done at the on-set of rains. At Katukei, plantings were done on 27 July 1997 and 23 April 1998 for the short and long rainy seasons, respectively. The corresponding dates for Abata were 28 July 1997 and 24 April 1998. Three to four seeds were planted per hill at a spacing of 60 x 20 cm but were later thinned to two seedlings per hill when plants were about 15 cm tall. The plant populations were based on those established as most suitable for cowpea/sorghum intercrop in Uganda (Adipala et al., 1997; Obuo et al., 1998). The plots were kept weed-free by regular hand hoeing.

Data collection and analyses. Pest infestations were monitored throughout the study period. At each site and season, there were 5 readings for aphid infestation, 6 for thrips, 7 for Maruca and 4 for pod sucking bugs. Population densities of aphids were estimated using a visual rating of 1-6, where:1= no aphids, 2= 1-100, 3= 101-300, 4= 301-600, 5= 601-1000 and 6 => 1000 aphids per plant. Thrips and pod borers populations were recorded from samples of 20 flower buds or flowers per plot depending on cowpea growth stage. Sampling for pod bugs was carried out using the direct count method (Olatunde et al., 1991), by carefully walking along the two middle rows of each plot and counting the number of bugs seen. At plant maturity, plot yields (grains) and seed damage (number of bored and shrivelled seeds) were determined.

Pest and yield data were subjected to analysis of variance (ANOVA) using the PROC. ANOVA procedure of Genstat (Lawes Agricultural Trust Rothamsted Experimental Station, 1993), and differences among the treatment means compared using Fishers’ Protected Least Significant Difference (LSD) test at 5% probability level. Separate analyses were done for each sampling time, site and season, but subsequently, data were pooled for locational and seasonal comparisons.

The costs of insecticide application for the different treatments are presented in Table 1. The cost-benefit analysis was used to calculate the profitability (marginal returns) of each treatment following procedures outline by Alghali (1992) as:

Marginal returns =	Value of yield gain per insecticide treatment
	Cost of insecticide treatment

Table 1. Cost of insecticide application used in calculating marginal returns

Results

Lowest aphid infestation was recorded in treatments that had carbofuran seed dressing followed by treatments where insecticide sprays were imposed. In contrast, the highest was in the cowpea/sorghum intercrop that received no insecticide application (Table 2). There were no significant cultivar differences with respect to aphid infestation. At both locations, aphid infestation was higher during the long rains, but with no marked difference between the two locations.

Table 2. Effect of combining cowpea/sorghum intercrop, carbofuran seed dressing and minimum insecticide application on aphid infestation¹ of two cowpea cultivars at two sites in Uganda

¹ Visually rated as 1-6, where 1= no aphids, 2= 1-100, 3= 101-301, 4= 301-600, 5= 601-1000 and 6 > 1000 aphids per plant

Application of foliar insecticide sprays reduced thrips infestations as compared to sole use of carbofuran seed dressing or cowpea/sorghum intercrop (Table 3). Again, there was no significant (P > 0.05) difference in cultivar susceptibility to thrips, nor between locations. However, thrips infestation was slightly higher during the long than short rains.

Table 3. Effect of combining cowpea/sorghum intercrop, carbofuran seed dressing and minimum insecticide application on thrips infestation of two cowpea cultivars at two sites in Uganda¹

Maruca infestation and seed damage were significantly (P < 0.05) influenced by the treatment combinations. Treatments where only carbofuran was applied and the cowpea/sorghum intercrop recorded significantly higher populations than those receiving foliar sprays (Table 4). Similarly, seed damage was higher in treatments receiving seed dressing alone and in the cowpea/sorghum intercrop than where foliar insecticide sprays were applied (Table 5). As with thrips, Maruca infestation was generally higher during the long than short rains, with Pallisa having slightly higher populations than Kumi. Cultivar Ebelat recorded higher seed damage than Icirikukwai but the difference between the two cultivars was generally non-significant.

Table 4. Effect of combining cowpea/sorghum intercrop, carbofuran seed dressing and minimum insecticide application on Maruca vitrata infestation of two cowpea cultivars at two sites in Uganda¹

¹ Number of Maruca per 20 flowers

Table 5. Effect of combining cowpea/sorghum intercrop, carbofuran seed dressing and minimum insecticide application on mean percentage seeds eaten (damage due to Maruca vitrata activity) at sites in Uganda

Pod bugs infestation varied significantly (P < 0.05) between treatments (Table 6). Plots treated with carbofuran only and cowpea/sorghum intercrop alone had significantly (P < 0.05) higher populations of pod bugs than those which received foliar insecticide application. Likewise, percentage of shrivelled seeds was higher in treatments with sole treatment of carbofuran or cowpea/sorghum intercrop, than in those which received foliar insecticide spray (Table 7). There were, however, no significant seasonal, locational or cultivar differences.

Table 6. Effect of combining cowpea/sorghum intercrop, carbofuran seed dressing and minimum insecticide application on pod sucking bug infestation of two cowpea cultivars at two sites in Uganda¹

¹ Pod sucking bugs visually counted from 2 middle rows per plot

Table 7. Effect of combining cowpea/sorghum intercrop, carbofuran seed dressing and minimum insecticide application on mean percentage shrivelled seeds (damage due to pod sucking bugs) at two sites in Uganda

With respect to yield, plots which received carbofuran plus foliar sprays or foliar spray alone gave significantly higher yields than plots where carbofuran or cowpea/sorghum mixture was the sole treatment (Table 8). There was no significant difference in grain yields of the two cultivars. However, basing on pooled yield data there were positive marginal returns from plots that received foliar insecticide sprays, whereas the untreated plots and those treated with carbofuran alone gave no positive returns (Table 9).

Table 8. Effect of combining cowpea/sorghum intercrop, carbofuran seed dressing and minimum insecticide application on cowpea grain yields (kg ha^-1) at two site in Uganda

Table 9. Mean grain yields and marginal returns from two cowpea cultivars grown under different pest management methods¹

¹Pooled data for two seasons and two locations
²E = Ebelat., I = Icirikukwai
³Value of cowpea at the prevailing time was 1500 Uganda shillings (UShs.); 1$ = 1300 UShs.

Discussion

Results obtained in this study demonstrate the importance of integrating a number of control measures in order to achieve effective management of cowpea pests. The cowpea/sorghum intercrop alone did not provide sufficient control against the pests , except for aphids. Similarly, inclusion of carbofuran seed dressing significantly reduced aphid infestation but did not control thrips, Maruca and pod bugs infestation. Probably, the systemic effect of carbofuran did not persist long enough to affect the late season pests. The spray treatments controlled the late season pests but not aphids. However, where intercropping was combined with seed dressing and foliar insecticide sprays, grain yields were greatly increased compared to the unsprayed plots or the sole insecticide treatment. Economic analysis also revealed that the IPM treatment was most profitable.

These results suggest that the strategic use of insecticides in cowpea pest management is probably inevitable. However, the efficacy of the insecticide treatment is greatly enhanced by use of other management practices such as close spacing (Karungi et al., 2000a) intercropping and seed dressing. In a related study, Karungi et al. (2000b) demonstrated that combining minimal insecticide application (spraying once at budding, flowering and podding) and early planting and close spacing gave better pest control and increased grain yield markedly, than the sole insecticide treatment. Alternatively, a combination of seed dressing with carbofuran or other similar insecticide (e.g., carbosulfan) would avoid the need for foliar sprays during the vegetative stage, thus allowing farmers to eat the tender leaves and green pod.

Other studies on cowpea pest management suggest that cowpea production can not be sustained economically without the use of insecticides (Jackai et al., 1985; Kyamanywa, 1996; Bottenberg et al., 1997; Jackai and Adalla, 1997). While this may be so, we have demonstrated that a more economic approach is to combine the chemical application with other management strategies.

Acknowledgement

The Rockefeller Foundation under the Forum on Agricultural Resource Husbandry funded the study.

References

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