African Crop Science Journal African Crop Science Society ISSN: 1021-9730 EISSN: 2072-6589 Vol. 4, Num. 3, 1996, pp. 373-382

SHORT COMMUNICATION: Influence of time of insecticide application on control of insect pests of cowpea and grain yield of cowpea at Mtwapa, Coastal province of Kenya

S. Kyamanywa^1

Department of Crop Science, Makerere University, P.O. Box 7062, Kampala, Uganda

^1 Formerly at the International Centre of Insect Physiology and Ecology (ICIPE), P.O. Box 30772 Nairobi, Kenya

(Received 12 January, 1996; accepted 29 June, 1996)

Code Number: CS96078
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Abstract

Grain yield of cowpea, the most important grain in the Coast Province of Kenya, is very low mainly because of insect pests attack. It is, however, not clear which growth development stages at which pest attack occurs lead to significant reduction in yields. Consequently, it is difficult to focus control strategies on a particular growth stage. A study was, therefore, conducted to determine the critical stage of cowpea growth at which insecticide application minimises grain yield loss due to pests. The results indicated that controlling flowering and podding pests resulted in 15 fold increase in cowpea grain yield. It is concluded that control efforts should be focused on flowering and podding pests.

Key Words: Maruca testulalis, Megalurothrips sjostedti, Ootheca sp., Pod sucking bugs, Vigna unguiculata

RESUME

Le rendement de grain du niebe, qui est le plus important dans la cote provinciale du Kenya, est surtout plus bas a cause de pestes d'insectes. Cependant, il n'y a plus de precisions sur l'etape de la croissance qui reduit sensiblement le rendement du niebe en cas d'attaque. Par consequent, il est difficile de concentrer sur les strategies des controles sur une etape de croissance particuliere. Cependant, une etude a ete menee pour determiner l'etape critique de la croissance du niebe a laquelle l'application de l'insecticide minimise pois le rendement des grains a cause de perte des pestes. Les rEsultates ont indique que le contr™le de croissance des fleurs et de la cosse resultent en 15 fois l'augmentation dans la production des grains du niebe. Il a ete conclu que les efforts des controles devrent etre concentres sur la croissance de fleurs et de la cosse.

Mots ClEs: Maruca testulalis, Megalurothrips sjostedti, Ootheca sp., sacs pour sucer de la cosse, Vigna unguiculata

Introduction

Cowpea, Vigna unguiculata (L) Walp, is the most important grain legume in the coastal province of Kenya. The coast province is the second largest producer of cowpea in Kenya after Eastern Province (Khaemba, 1980). In these provinces, cowpea is a major source of dietary protein, especially for the rural and urban poor.

The yield of cowpea in Kenya, and in the coast province in particular, is extremely low ranging from 150 - 500 kg ha^-1 compared to a yield potential of over 1500 kg ha^-1 reported from experimental stations (Khamala, 1978; Pathak and Olela, 1986; Alghali, 1992). It is now generally agreed that the major cause of low yields in cowpea is damage due to insect pests.

The cowpea crop in the coastal province is seriously attacked by several insect pests throughout its crop cycle. The important pests include Aphis craccivora, Ootheca sp., Empoasca sp. as vegetative pests; Megalurothrips sjostedti, Maruca testulalis, Clavigralla tomentosicollis; Nezara viridula and Helicorvepa armigera as flower and pod feeders.

Although it is believed that these insect pests cause serious damage to cowpea, there has been no attempt to determine which group (vegetative, flower and pod feeders) is the most important and therefore deserve highest research priority in Kenya. Previous studies have either concentrated on understanding effects of intercropping cowpea on pest populations (Amoako-Atta, 1983; Kyamanywa and Ampofo, 1988; Gethi and Khaemba, 1991) or on evaluating effects of different pesticides on a single pest species (Gethi and Khaemba, 1986). Thus, it is not clear whether it is the leaf feeders, or the flower and pod feeders that are most important in reducing overall grain yield in the coast province. Furthermore, even in situations where effects of insecticides on cowpea pests have been tested ( Koehler and Metha, 1972; Metha and Nyiira, 1973; Gethi and Khaemba, 1986), little attention has been given to determining the appropriate time of insecticide application that optimises yield of cowpea while controlling the most important group of pests. Most trials have concentrated on effects of insecticides on individual insect pest rather than dealing with groups of insects attacking different stages of cowpea. This has resulted in a lack of a comprehensive insecticide control recommendation for all the major pests attacking different growth stages of cowpea.

The objectives of this study, therefore, were; to determine the losses caused by leaf, flower and pod feeding insect pests of cowpea, and to determine the most critical stage of cowpea growth at which insecticide application minimises grain yield loss while maximising profit.

MATERIALS AND METHODS

The study was conducted at the Coast Agricultural Research Station, Mtwapa, Kenya, during the long rain season of 1992. The station is almost at sea level and experience a hot humid climate.

Three cowpea varieties, ICV-2, Katumani-80 and a land race, were used in the experiment. ICV-2 is an early maturing cultivar (60 days), well adopted to semi-arid area and yields 2000 kg ha^-1 of grain under normal management (Pathak and Olela, 1986). Katumani-80 is a variety recommended by the Kenya Agricultural Research Institute (KARI) for the semi arid and humid areas of Kenya, and yields above 1800 kg ha^-1. The land race was found during a survey to be grown by a majority of farmers in the Coast Province. It was included to determine its performance under experimental conditions and as a local control. The three varieties were planted at a spacing of 60 cm and 15 cm between and within rows, respectively.

In order to determine the stage of cowpea growth at which insect pests cause the greatest loss in grain yield, four insecticide application regimes were used:-

- Untreated (control)

- Insecticide applied once a week during the vegetative stage. It was assumed that this treatment controlled all the pests of the vegetative stage. Therefore, the yield loss in this treatment would represent the damage caused by flower and pod feeding insects.

- Insecticide applied once a week starting at flower bud initiation until pods were mature dry. This controlled the flower and pod pests, and hence, the yield loss in this treatment would be due to vegetative pests.

- Insecticide applied once a week throughout the crop cycle. This treatment was considered as the control in which all groups of insect pest attacking different stages of growth of cowpea were assumed to have been controlled. Thus, the grain yield from this treatment was assumed to be the yield of an insect-free cowpea crop.

The insecticide used was cypermethrin (Ambush CY). It was applied at the manufacturer's (Zenaca Ltd, UK) recommended rate of 100ml/20 litres of water.

The experimental design was a split-plot in which the varieties were in the main plots and the spray regimes in the subplots which measured 1.8 x 2.10 m. The treatments were replicated three times.

Population densities of M. sjostedti and M. testulalis were estimated by randomly picking 20 leaf buds or flowers of cowpea per plot, depending on stage of growth. The buds or flowers were placed in glass vials containing 50% ethanol solution and taken to a laboratory where the larvae of M. testulalis and nymphs and adults of M. sjostedti were separated from plant parts and counted. In addition, the activity of M. sjostedti in the different spray regimes was compared using white coloured water trap of size 20 x 8 x 8 cm. At 20 days after emergence of cowpea (DAE), a time when there is increased activity of M. sjostedti (Kyamanywa et al., 1993), a trap was placed in each plot and maintained at the height of the cowpea canopy. Thrips in each trap were counted at 7 days interval.

Foliar damage due to Ootheca sp. was estimated using a visual rating scale of 0-5 where: 0 = no foliage damage; 1 = 1-5% of foliage consumed; 2 = 6-25% foliage consumed; 3 = 26-50% consumed; 4 = 51-75% consumed and 5 = 76-100% consumed. The damage was assessed once a week starting 20 DAE, when the first damage symptoms were noticed. At each sampling occasion, visual damage rating was done on 20 cowpea plants per plot, from which average scores per plant were calculated for different spray regimes.

Damage to cowpea pods both by M. testulalis, and pod sucking bugs was estimated at two stages; during the podding stage when pods were still green, and at harvesting when the pods were mature dry. In the podding stage 10 plants selected at random were inspected in situ, and the total number of pods damaged by both pests recorded. Sampling was done at 39, 46 and 56 DAE. The number of peduncles per plant and their length were also recorded for 20 randomly selected plants per plot.

At maturity, plants in the two centre rows per plot, covering an area of 1.26 m2 were harvested. Pods from each plant were kept in a separate, well labelled paper bag. The pods were later sorted to determine those damaged by M. testulalis and pod sucking bugs, and those without damage. Grain weight from each of the above categories was recorded. The data were subjected to analysis of variance using Mstatc computer package.

Results

Effect of insecticide spray regimes on pest population and their damage. Insecticide spray regimes significantly (P< 0.01) affected foliar damage (Fig. 1). Foliar damage due to Ootheca sp. was consistently higher in unsprayed cowpea plots than all the other treatments, followed by the plots which were sprayed during the flowering and podding stages, and least in plots which were sprayed throughout and those sprayed during the vegetative stage. The differences were highly significant, except for those between plots sprayed in vegetative stage and those sprayed throughout (Table 1).

Figure 1

Figure 2

Table 1

Table 2

The effects of the insecticide spray regimes on damage due to M. testulalis and pod sucking bugs are shown in Table 1. The number of M. testulalis larvae was significantly higher in the plots which were sprayed during the vegetative stage and the untreated, and least in plots which were sprayed during the flowering and podding stages. A similar trend was observed at harvest (Table 2). The percentage of pods damaged by M. testulalis was highest in plots which had been sprayed during the vegetative stage and least in those sprayed throughout.

In the case of damage by pod sucking bugs, however, there were no significant differences between the different spray regimes, suggesting that, perhaps, the chemical was not effective (Tables 1 and 2). It is worth noting that the percentage of pods damaged by sucking bugs was slightly higher in plots which had been sprayed during the flowering and podding stages than in any other treatments. The percentage of pods with combined damage of M. testulalis and pod - bugs was highest in plots sprayed during the vegetative stage and in the untreated plots.

Table 3

The differences in grain yield between plots sprayed throughout and those sprayed during the flowering and podding stages were not significant. Similarly, grain yield from plots sprayed during the vegetative phase was not significantly different from the unsprayed plots. What is perhaps most significant is the fact that the grain yield per unit area from plots sprayed throughout and those sprayed during flowering and podding was over 15 times more than that from the unsprayed plots.

Relative cowpea grain yield loss under different chemical spray regimes. To calculate the yield loss caused by the insect pest complex on cowpea, yield from plots which were sprayed once every week, assumed to be insect pest free, was used for comparing with the other treatments (Table 4).

Table 4

Separation of losses caused by different pod pests indicated that the greatest loss occurred where a pod exhibited combined M. testulalis and pod sucking bug damage followed by pods which had pod bug damage alone (Table 4). It is important to note that the grain weight from undamaged pods in the different spray regimes did not differ significantly, while the weights of damaged pods varied significantly between the different spray regimes. Pods damaged by either Maruca or pod sucking bugs were heaviest in plots which were sprayed throughout the season and least in unsprayed plots.

Regression/correlation analyses (Table 5) indicated that there were significant negative relationships between thrips population density, Maruca larvae population and grain yield. However, the relationship between percentage pod damaged by pod bugs and grain yield were not significant. There was nevertheless a significant negative relationship between number of pods with combined damage of Maruca and sucking bugs, and grain yield; as the number of pods with combined damage increased the yield decreased. The multiple correlation coefficient (R=0.83) was highly significant suggesting that most of the total variation in yields of cowpea was due to linear function involving thrips, maruca population and pod sucking bug damage.

Table 5

Table 6

Figure 3

DISCUSSION

The results have shown that insect pests of cowpea caused up to 93% reduction in grain yield per unit area, if not protected. Insecticide application resulted in 15 fold increase in yield. This, as indicated by Alghali (1992), was evidence that insecticide application to control pest on cowpea was very profitable. It is, however, appropriate to establish recommendations that minimise excessive or unnecessary insecticide applications for cowpea farmers.

Applying insecticides during the vegetative stage of cowpea growth controlled damage due to Ootheca sp. and other foliage pests, but did not increase grain yield. In fact yields from these plots were not significantly different from yield of unsprayed cowpea. This suggests that the damage due to Ootheca sp. of about 25-30% defoliation observed in the unsprayed plot, did not reduce grain yield. The results, concur with those of Wein and Tayo (1978) who demonstrate that 50% defoliation of cowpea leaves in the vegetative phase did not significantly reduce grain yield.

Insecticide applied during the flowering and podding stages resulted in highest grain yield per plot, the treatment gave better yield than applying the insecticide once every week through the cowpea growing period. Similar results were reported from Nigeria by Alghali (1992). These findings suggested that insect pests of flowers and pods were most important in reducing grain weight. Correlation and regression analyses indicated that M. sjostedti, M. testulalis and the pod sucking bugs (PSB) accounted for 83% loss in grain yield.

Considering the contribution of M. testulalis and PSB to grain weight loss, it was observed that PSB reduced pod weight by about 70% and 39.2% in unsprayed and sprayed plots, as compared to M. testulalis which reduced pod weight by 27.7% and 0.82% in unsprayed and sprayed plots, respectively, suggesting the PSB were perhaps more important than M. testulalis. Furthermore, PSB did not seem to be affected by cypermethrin; there were no significant differences in number of pods damaged by PSB between sprayed and unsprayed plots. Gethi and Khaemba (1991) also did not observe significant difference in PSB population between unsprayed and plots sprayed with endosulfan. Alghali (1992), however, managed to control PSB by applying a mixture of Cypermethrin and Dimethoate. Failure of cypermethrin to control damage caused by PSB suggested that, perhaps, most of the increase in yield observed in plots sprayed at flowering was due to control of M. sjostedti and M. testulalis.

The results advocate for insecticide application during the flowering and podding stages. However, what is not clear from the present work is the optimum number of insecticide application. The four insecticide applications used in the flowering and podding stages may not have been necessary. It is, therefore, appropriate to conduct further studies to determine the frequency of insecticide application and the type of insecticide.

The significant interaction effect of varieties and insecticide regimes on yield is worth highlighting. The yields of the three varieties were not significantly different in both plots sprayed during vegetative stages and the untreated ones, but were very significant in plots sprayed throughout and those sprayed in flowering phase. This demonstrated that the improved varieties, Katumani-80 and ICV-2, were superior to the local land race under high input system only.

Based on the available literature (Alghali, 1991, 1992) and the present results, it is concluded that insect pest of flowers and pods are the most important, and control efforts should be directed at these two stages of growth.

Acknowledgements

This work was conducted at Kenya Agricultural Research Institute (KARI) - Mtwapa, under the PESTNET programme of the International Centre of insect Physiology and Ecology (ICIPE). The author is, therefore, indebted to Prof. T.R. Odhiambo, then Director of ICIPE, for the opportunity to work at ICIPE, and for the financial support. The author also acknowledges the tremendous support from KARI staff particularly Mr. K. Malinga, the Director KARI - Mtwapa, and Mr. K. Mwangi, Entomologist. The data were collected by Messrs H. Mzingirwa and J. Mutai ICIPE technicians.

REFERENCES

Alghali, A.M. 1991. Integrated pest management strategy for cowpea production under residual soil moisture in Bida area of Northern Nigeria. Tropical Pest Management 37:224-227.

Alghali, A.M. 1992. On-farm evaluation of control strategies for insect pests in cowpea with emphasis on flower thrips, Megalurothrips sjostedti Trybom (Thysanoptera). Tropical Pest Management 38:420-424.

Amoaka - Atta, 1983. Observations on the pest status of stripped bean weevil, Alcidodes leucogrammus Erichs on cowpea under intercropping systems in Kenya. Insect Science and its Application 4:351-356. Gethi, M. and Khaemba, B.M. 1986. The effect of intercropping cowpea(Vigna unguicu-lata) with maize (Zea mays) on incidence and damage by the legume pod borer (Maruca testulalis Geyer, Lepidoptera: Pyralidae) in Kenya. East African Agriculture and Forestry Journal 51:36 - 40.

Gethi, M. and Khaemba, B.M. 1991. Damage by pod sucking bugs on cowpea when intercropped with maize. Tropical Pest Management 37:236 - 239.

Koehler, C. S. and Metha, P.N. 1972. Relationships of insect control attempts by chemicals to components of yield of cowpeas in Uganda. Journal of Economic Entomology 65:1421-1427.

Khaemba, B.M. 1980. Resistance of Cowpea to the Common Pod Sucking Bugs Riptortus dentipes and Anoplecnemis curvipes (Hemiptera: Coridae) Ph.D Thesis, University of Nairobi.

Khamala, C.P.M. 1978. Pests of grain legumes and their control in Tanzania. In: Pests of Grain Legumes, Ecology and Control. Singh, S.R., Van Emden, H.F. and Taylor, T.A. (Eds.), pp. 127-134. Academic Press. London and New York.

Kyamanywa, S. and Ampofo, J.K.O. 1988. The effect of cowpea/maize mixed cropping on incident light at the cowpea canopy and flower thrips (Thysanoptera thripidae) population density. Crop Protection 7:186-189. Metha, P.N. and Nyiira, Z.M. 1973. An evaluation of five insecticides for use against pest of cowpea, Vigna unguiculata (L) Walp, with special reference to green pod yield. East Africa Agriculture and Forestry Journal 39: 99 - 104.

Pathak, P.S. and Olela, J.C. 1986. Registration of 14 cowpea cultivars Crop Science 26:647-648.

Wein, H.C. and Tayo, T.A. 1978. The effect of defoliation and removal of reproductive structure on growth and yield of tropical grain legumes. In: Pests of Grain Legumes, Ecology and Control. Singh, S.R.,Van Emden, H.F. and Taylor, T.A. (Eds.), pp. 241-252. Academic Press. London and New York.

Copyright 1996 The African Crop Science Society

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