African Crop Science Journal, Vol. 7. No. 4, pp. 487-495, 1999
Population dynamics of selected cowpea insect pests as influenced by different management practices in eastern Uganda
J. Karungi, M.P. Nampala, E. Adipala, S. Kyamanywa and M.W. Ogenga-Latigo
Code Number: CS99039
Population dynamics of the cowpea aphid (Aphis craccivora), bud thrips (Megalurothrips sjostedti), legume pod borer (Maruca vitrata), and pod sucking bugs were studied at three sites in eastern Uganda. The management practice which combined early planting, close spacing and minimum insecticide application (spraying once at budding, flowering and podding stages) was the most effective in reducing all pest infestations followed by sole foliar sprays. Seed dressing reduced aphid infestation but increased infestations of thrips, Maruca and pod sucking bugs to levels even higher than in the control. In comparison to the control, intercropping reduced pest infestations of all the pests but was not as effective as the other control measures.
Key Words: Aphids, combined control measures, foliar sprays, intercropping, legume pod borer, pod sucking bugs, seed dressing, thrips
La dynamique de la population des pucerons (Aphis craccivora), borers des gousses (Maruca vitrata), de thrips (Megalurothrips sjostedti), et de pucerons suceurs de gousses a été étudiée dans 3 sites de la zone Est de lUganda. La gestion des méthodes combinant la plantation précoce, lespacement serré et une application minimum dinsecticide (application une fois au stade des bourgeons, floraison et de formation de gousses) a été la plus efficace dans la réduction de linfestation de tous les pestes suivi de lapplication foliaire seule. Le traitement de semences a réduit linfestation des pucerons mais a favorisé laugmentation de linfestation des thrips, maruca et des pucerons suceurs de gousses à un niveau plus élevé que le traitement temoin. En comparaison avec le temoin, lassociation de cultures a réduit linfestation de tous les pestes mais na pas été aussi efficace comme dautres mesures de contrôle.
Mots Clés: Pucerons, méthodes de contrôle combinées, pulvéristion foliaire, association de cultures, borers de gousses, pucerons suceurs de gousses, traitement des semences
Cowpea, Vigna unguiculata (L) Walp, is a very important legume crop especially in the hot areas with poor soils (Rachie, 1985). However, because of a complex of insect pests, yields are low, often <240 kg ha-1 (Rachie, 1985). Insects attack cowpea from seedlings to harvest and can cause economic damage at all stages of plant growth and in storage (Singh and Allen, 1980). The high losses have contributed immensely to cowpea occupying a subsidiary position in the farming systems of several countries in the tropics. To increase hectarage of this food crop, researchers and farmers must minimise the damage caused by insect pests. As a basis for designing appropriate management packages against major pests of cowpea in Uganda, field experiments were conducted to monitor population dynamics of the pests under different control measures. The pests considered were Aphis craccivora Koch, Megalurothrips sjostedti Trybom, Maruca vitrata Fabricius and pod sucking bugs. These are the major pests of cowpea in eastern Uganda (Omongo et al., 1997).
Materials and Methods
Four on-farm trials were conducted in eastern Uganda at three sites, Abata (1º31N; 33º53E) in Kumi; Katukei (1º13N; 31º42E) in Pallisa, and Amuria (2º02N; 33º39E) in Katakwi district. The trials were done for three consecutive seasons, during the long rains of 1997 (April-July), short rains of 1997 (August-November) and the long rains of 1998 (April-July). Two local cowpea varieties, Ebelat and/or Icirikukwai, were used in the trials. Ebelat (erect) has white relatively big seeds and takes about 70 days to maturity. In the absence of insect pests, it can yield more than 2000 kg ha-1, but without pest control, yields rarely exceed 500 kg ha-1 (Rusoke and Rubaihayo, 1994). Icirikukwai (spreading) has small whitish-brown seeds, also with 70 days maturity period. The local varieties together with two introduced varieties IT82D-716 and SVU-116 were used in one trial. The cowpeas were planted at a spacing of 60 cm between rows and 20 cm within the rows; three seeds were planted per hole but later, at two weeks after planting, thinned to two plants per hill. At each site, there were two weedings, when the plants were 2 and 5 weeks old.
Intercropping trial. The two local cowpea varieties Ebelat and Icirikukwai were grown as sole crops and as intercrops with a local variety of greengram (Vigna radiata) and sorghum cv. Seredo. The treatment combinations were Ebelat, Icirikukwai, Ebelat/greengram, Ebelat/sorghum, Icirikukwai/greengram, and Icirikukwai/sorghum. The intercrops were grown as 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 six treatment combinations were grown in randomised complete blocks replicated three times. Each plot measured 5 x 5 m, with 2 m alleys between plots.
Seed dressing trial. Prior to planting, cowpea seeds were dressed with carbofuran dust (Furadan 5G) at a rate of 1.5 g per 1 m row. Two erect cultivars: IT82D-716 (introduced) and Ebelat (local), and two spreading cultivars: SVU-116 (introduced) and Icirikukwai (local) were grown in a factorial arrangement, with cultivars in the main plots and plots receiving carbofuran and the control (untreated), in the subplots. Each subplot measured 5 x 5 m, with 2.5 m alleys between plots. The plots were replicated three times.
Foliar sprays trial. A randomised complete block experiment of three replicates
was established. Each block consisted of five treatment plots, each 3 x 3 m,
and were separated by 1.5 m alleys. Blocks were 2.0 m apart and each plot was
planted with Ebelat. A cocktail mixture of dimethoate (200 g a.i. ha-1)
and cypermethrin (200 g a.i. ha-1) were used to spray the cowpeas.
The cocktail mixture was applied at four regimes as follows:
Integrated pest management. This trial was arranged in a randomised complete
block design with treatments replicated three times. Each plot measured 3 x
3 m, with 1.5 m alleys between plots and 2.0 m between blocks. The two local
cultivars, Ebelat and Icirikukwai were used in the study. A cocktail
mixture of dimethoate (200 g a.i. ha-1) and cypermethrin (200 g a.i.
ha-1) was applied to control insect infestation on the two cowpea
varieties. The cocktail is moderately effective against major pests of cowpea
(Alghali, 1992); dimethoate was included to control the synthetic pyrethroid
resistant pod sucking bugs. The treatments studied were as detailed below;
At each site the cowpeas were planted at the on-set of the rains.
Data collection and analysis. For each experiment, aphid infestation was assessed once every 10 days starting 10 days after emergence (DAE). On each sampling day, visual infestation rating was done on 10 randomly chosen cowpea plants per plot, and average scores per plant calculated. Aphis craccivora population densities were estimated using a visual rating scale of 1-6, where: 1 = no aphids; 2 = 1-100 aphids; 3 = 100-300 aphids; 4 = 300-600 aphids; 5 = 600-1000 aphids, and 6 = >1000 aphids per plant.
Population densities of Megalurothrips sjostedti were estimated by randomly picking 20 racemes/flowers of cowpea per plot, depending on the stage of growth. The racemes/flowers were placed in glass vials containing 50% ethanol solution. Subsequently, nymphs and adults of M. sjostedti were separated from the plant parts and counted. The infestation was assessed once every 10 days from 20 DAE. Population densities of Maruca vitrata were similarly estimated by randomly picking 20 racemes/flowers per plot, depending on the stage of growth. The infestation was assessed once every 10 days starting at 20 DAE. Population densities of pod sucking bugs were recorded on a plot basis by moving around each plot and counting the number of pod sucking bugs seen. The infestation was assessed once every 10 days starting at 50 DAE.
For each insect species the insect counts at each sampling date were pooled across seasons and sites and plotted against days after cowpea emergence.
For all management options, peak aphid activity was at 30 DAE. In comparison to the other management practices, the intercropping trial had the highest aphid populations (Fig.1). Seed dressing reduced aphid infestations but was not as effective as foliar sprays. The trial combining different management options had the least aphid populations.
In the case of thrips infestation, peak activity was at 50 DAE. Highest thrip infestations were found in the seed dressing trial, higher than the control. Thrips populations were also high in the intercropping trial (Fig.2). The trial combining different tactics and that of foliar sprays markedly reduced the thrips infestations.
Peak Maruca infestation varied depending on the pest management option. In the case of the trial combining different management options, and of foliar sprays and the control, peak Maruca activity was at 50 DAE, but was earlier (40 DAE) for the seed dressing trial, and later (60 DAE) for the intercropping trial. Overall, the seed dressing trial had the highest Maruca population, higher than the control. The trial combining different management options recorded the least Maruca infestation (Fig.3).
Peak pod sucking bugs activity was at 70 DAE for all control measures, except for the intercropping and control trials which was at 60 DAE. The trial combining different management measures, and the foliar sprays trial had the least pod sucking bugs infestation (Fig. 4).
The highest grain yields were in the plots receiving combined control measures followed by those receiving foliar sprays alone (Table 1). Though grain yields from plots receiving seed dressing and intercropping were higher than the control, yields from these plots were very low (Table 1).
TABLE 1. Grain yields of cowpea as influenced by different management
1Pooled data for three locations and three seasons
For intercropping to be effective in reducing aphid infestations, the non host crop in the mixture must interfere with the aphids visual stimulation so as to hinder colonisation of the host crop (Kennedy et al., 1961; Ogenga-Latigo et al., 1993; Naidu et al., 1998). In our study, except for the unsprayed control, the intercropping trial had the sharpest rise in aphid populations and the highest peak infestation, 300-600 aphids per plant. We speculate that since the crops in the mixture were planted simultaneously, by the time the sorghum reached optimum size to interfere with aphids visual stimulation, the aphids had already established themselves. Foliar sprays were better than seed dressing in controlling aphids because they provided direct contact with the aphids. In our study, we used a contact insecticide (cypermethrin) which is efficacious, especially since the aphids soft bodies provide no resistance to the chemical activity.
Thrips populations were highest in the seed dressing trial, higher than the unsprayed control. Nampala et al. (1999) attributed this to the fact that seed dressing promotes vigorous cowpea plant growth, because of reduced aphid and nematode infestation. Also, carbofuran itself is known to have a stimulating effect on plant growth (Jackai and Adalla, 1997). Thus, the vigorous plants attracted heavy thrips infestation.
Peak Maruca infestation was earliest and highest in the seed dressing trial probably due to the fact that, because of the vigorous growth due to carbofuran effect, plants attained denser canopies. Denser canopies are known to favour Maruca multiplication because they provide hiding places for the larvae from predators; dense canopies also protect the larvae from dessication (Oghiake et al., 1992). In the intercropping trial, Maruca populations continued to increase with time because as the season progressed, the sorghum in the mixture was increasing in size thus creating denser plant canopy, an environment which, as stated earlier, favours Maruca infestation.
Next to the unsprayed control, peak pod sucking bugs infestation was earliest (60 DAE) and highest in the intercropping trial. This was probably because of the favourable micro-climate in mixtures which encouraged fast multiplication of the pod sucking bugs. According to Gethi and Khaemba (1991), shading by the taller crop (sorghum) in the mixture provides cooler conditions which favour the bugs. Unlike the other control measures where the pod bugs populations continued to rise, after peaking at 60 DAE in the intercrop the pod bugs populations started to fall. This fall in pod bugs population is probably due to reduced resource concentration (Ogenga-Latigo et al., 1993).
Our results clearly show that population build-up of the different insect pests depended on management practice. Overall, however, treatment containing foliar insecticide sprays recorded the least pest populations. Therefore, to keep populations of cowpea insect pests to a minimum, foliar sprays are invariably an essential component of any pest management strategy. However, foliar sprays are only recommended for the more important post-budding pests (Karungi et al., 2000). For vegetative pests (aphids), a chemical-free component such as intercropping and other cultural practices can be used. This would allow the farmers to harvest cowpea leaves for spinach without fear of pesticide poisoning (Nampala et al., 1999).
The results of this study clearly demonstrate that IPM tactics do not only reduce pest infestation, but also result in yield benefit. There are also environmental benefits since the IPM package actually involved fewer sprays. These are key elements in IPM protocals (Alghali, 1991). In our case the IPM package enables the farmers to harvest cowpea leaves for "spinach", without being exposed to chemical poisoning, which is a major concern in Uganda.
This study is part of a broader Rockefeller Foundations Forum funded project on integrated management of cowpea pests in Uganda (Grant RF 9500#77). We thank the Foundation, farmers and extension officers for their support. Additional support to the first author was provided by the Kampala office of the United States Agency for International Development (USAID).
©1999, African Crop Science Society
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