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Integrating planting date with insecticide spraying regimes to manageinsect pests of cowpea in north-eastern NigeriaAlpha Y. Kamaraa; Friday Ekelemea; Lucky O. Omoiguia; Tahirou Abdoulayea; Paul Amazaa; DavidChikoyea; Ibrahim Y. Dugjeb

a International Institute of Tropical Agriculture, Croydon, UK b Department of Crop Production,University of Maiduguri, PMB, Maiduguri, Nigeria

Online publication date: 13 July 2010

To cite this Article Kamara, Alpha Y. , Ekeleme, Friday , Omoigui, Lucky O. , Abdoulaye, Tahirou , Amaza, Paul ,Chikoye, David and Dugje, Ibrahim Y.(2010) 'Integrating planting date with insecticide spraying regimes to manageinsect pests of cowpea in north-eastern Nigeria', International Journal of Pest Management, 56: 3, 243 — 253To link to this Article: DOI: 10.1080/09670870903556351URL: http://dx.doi.org/10.1080/09670870903556351

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Integrating planting date with insecticide spraying regimes to manage insect pests of cowpea in

north-eastern Nigeria

Alpha Y. Kamaraa*, Friday Ekelemea, Lucky O. Omoiguia, Tahirou Abdoulayea, Paul Amazaa, David Chikoyea

and Ibrahim Y. Dugjeb

aInternational Institute of Tropical Agriculture, Ibadan, Nigeria, C/o IITA Ltd, Carolyn House, 26 Dingwall Road, CroydonCR93EE, UK; bDepartment of Crop Production, University of Maiduguri, PMB 1069, Maiduguri, Nigeria

(Received 19 June 2008; final version received 14 December 2009)

We sought to establish the most effective combination of planting dates with insecticide spraying regimes for themanagement of insect pests of cowpea in the savannas of northeast Nigeria. The results after 3 sprays, made onceeach at the bud initiation, flowering, and podding stages, did not differ significantly from those after 2 sprays, madeonce each at flowering and podding, in terms of reducing insect pest population and increasing grain yield. Despitethe reduction in insect infestation, delaying planting beyond mid-August reduced cowpea grain yield by 12.3%, onaverage. The yield of the medium-maturing variety IT89KD-391 was significantly higher when planted inmid-August and sprayed twice than when planted on the earlier or later dates. The yield of the indeterminatelate-maturing variety ITKD89-288 was higher when planted in early August and sprayed thrice. Early- and medium-maturing cowpea varieties should therefore be planted in mid-August and sprayed twice. Late-maturingindeterminate varieties should be planted in early August and sprayed thrice.

Keywords: cowpea; insect pests; insecticide spray regime; planting date; grain yield

1. Introduction

Cowpea (Vigna unguiculata (L.) Walp.) is one of themost important food and forage legumes in the semi-arid tropics. It is a valuable and dependable commod-ity crop for farmers and grain-traders, with approxi-mately 8.4 million ha grown worldwide and an annualproduction of over 3.05 million Mt (Singh 2005; Timkoand Roberts 2007). It is grown mostly by poor farmersin developing countries, with 80% of productionoccurring in the dry savannas of tropical West Africa(Singh 2005). The main cowpea-growing countries areNigeria, Niger, and Brazil (Quin 1997). Cowpea iswidely grown in the Guinea and Sudan savannas ofNigeria where Borno State is the major cowpeaproducer.

Despite cowpea’s importance and high yieldpotential in the Nigerian savannas, insect pest attackis a major constraint upon production (Singh et al.1990). Severity can vary, and sometimes lead to totalyield loss (Singh and Allen 1980). Yield losses of up to70%, from insect pests alone, have been reported(Rusoke and Rubaihayo 1994). In some areas, thelosses caused by insect pests account for a reduction ingrain yield as much as 80% (ICIPE 1980). Cowpeagrowers in Borno State are at risk of losing the entirecrop to insect pests in most growing seasons. The mostdamaging of all insect pests are those that attack thecrop during the flowering and podding stages (Jackai

et al. 1985). Worldwide, insect pests, especially Aphiscraccivora Koch, Megalurothrips sjostedti Trybom,Maruca virata Fab., and a complex of pod-feedingbugs cause the greatest yield reductions (Alghali 1992a;Omongo et al. 1997). In a recent study, Kamara et al.(2007) reported that flower thrips, the legume pod-borer (Maruca) and a range of pod-feeding bugs werethe major insect pests of cowpea in the dry savannas ofnortheast Nigeria. Maruca larvae damage flower buds,flowers, green pods, and seeds (Singh and Jackai 1985).Thrips start to attack at flower initiation, causingflower bud abortion (Akingbohungbe 1982). Adultsand nymphs of pod bugs remove sap from green pods,causing abnormal pod and seed formation (Singh andJackai 1985). High levels of insect resistance are notavailable in current cultivars (Oghiakhe et al. 1995;H.A. Ajeigbe, personal communication). The develop-ment of integrated insect pest management strategies isthe key for successful cowpea production.

Insecticide application is the most widely knownmeans of a pest control method in cowpea (Matteson1982); it is not otherwise feasible to grow cowpeacommercially (Jackai et al. 1985). Farmers can improveyield 10-fold if insecticides are used (Singh and Jackai1985). In Tanzania, Price et al. (1983) obtained seedyield increases using deltamethrin, monocrotophos,dimethoate and endosulfan at both the pre-and post-flowering phases. In Nigeria, Alghali (1992b) obtained

*Corresponding author. Email: a.kamara@cgiar.org

International Journal of Pest ManagementVol. 56, No. 3, July–September 2010, 243–253

ISSN 0967-0874 print/ISSN 1366-5863 online

� 2010 Taylor & Francis

DOI: 10.1080/09670870903556351

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yield increases of 50–200% following the application ofinsecticide, once each at flowering and podding stages.Spraying once at flowering stage increased grain yieldby 75%; 2 sprays, once each at flowering and poddingstages, significantly, reduced insect pest populationlevels and increased grain yield by 126% (Kamara et al.2007). In Kenya, Kyamanywa (1996) obtained a15-fold increase in grain yield after two sprays, onceeach at flowering and podding stages. Some farmers inNigeria have resorted to the indiscriminate use ofinsecticides to reduce pest damage (Kamara et al.2007), sometimes applying as many as 8–10 sprays perseason. However, most Nigerian farmers are resource-poor, and require pest management strategies that arecost-effective and sustainable. The use of insecticidesmust be minimized because of high costs and harmfuleffects on human health and the environment(Giliomee 1997). To increase effectiveness and reduceoveruse, their application should be integrated withother cultural practices for insect pest management incowpea.

In recent years, planting date has been identified asan important component of integrated pest manage-ment practices (IPM). It has been suggested thatadjusting of planting dates could cause asynchronybetween crops and insect pests (Pedigo 1989). Forexample, in the Delmarva Region, USA, plantingcowpea early in combination with an application ofinsecticide resulted in a much higher grain yield thanafter planting late (Javaid et al. 2005). Similarly, inUganda, Karungi et al. (2000a) reported that earlyplanting reduced levels of infestation by aphids, thripsand pod-feeding bugs but increased levels of infestationby Maruca. There is usually a build-up of pests as theseason progresses, that causes most damage to late-planted cowpea. Early sowing has also been reportedto enable the crop to escape high temperatures duringthe flowering stages when the crop is sensitive to heat(Hall 1992; Ismaila and Hall 1998). Taylor (1978) andAkingbohungbe (1982) suggested that differences inplanting dates could be explored as they might offersome scope in avoiding various insect pests. Culturalpractices, when combined with insecticides, are prob-ably effective against some pests, and could be used ascomponents of IPM (Javaid et al. 2005). Conflictingresults regarding the effects of date of planting oncowpea pest infestation have, however, been reportedin the Nigerian savannas. In an earlier study in Nigeria,the grain yield of cowpea planted early in the seasonwas higher than from the late planted crops (IITA1982). The increased yield was attributed to low pestpopulation levels, and the crop was allowed to maturebefore the rains ceased. Some authors have argued thatearly planting in West Africa (before August) willallow the plant to flower in September when rainfall isheavy and cowpea flowering would usually requirefrequent spraying. Also, most of the cowpea varietiesgrown by the farmers are photosensitive and should be

planted late in the season to enhance early flowering(H. A. Ajeigbe, personal communication). Farmers aretherefore usually advised to plant their cowpea aroundlate July to early August.

Cowpea is a popular crop in northeast Nigeria, butthere is little information on the interaction betweenplanting date and insecticide spray regime on insectpest incidence and grain yield of cowpea in the region.To increase and sustain cowpea production, reducelosses due to pests, and to obtain economical yields,there is a need to develop pest management strategieswith a judicious use of insecticide, integrated with hostplant resistance and planting date. The aim of thisstudy was to identify the appropriate combination ofplanting date and spray regime for the control of insectpests of cowpea in the savannas of northeasternNigeria. The specific aims were as follows: (1) todetermine the appropriate planting date for contrastingvarieties that would reduce insect pest attacks andincrease the grain yield of cowpea and (2) to determinethe minimum level of a cost effective insecticide sprayregime for contrasting cowpea varieties over a range ofplanting dates.

2. Materials and methods

2.1. Study area

The field experiments were conducted under rain-fedconditions in 2006 and 2007 at Miringa (108730N,128140E) in the northern Guinea savanna (NGS), andat Damboa (118150N, 128760E) in the Sudan savanna(SS) of northeast Nigeria. The soil at Miringa is sandyclay with organic matter of 15.8 g kg71, P 1.7 mgkg71, K 0.31 Cmol kg71 and pH of 5.67. At Damboa,the soil is sandy loam with organic matter 8.0 g kg71,P 2.1mg kg71, K 0.36 Cmol kg71 and pH of 5.93.Both locations have a unimodal rainfall distribution.In Damboa, total annual rainfall was 894.0 mm in2006 and 843.3 mm in 2007. In Miringa, total annualrainfall was 1275.9 mm in 2006 and 1269.9 mm in 2007(Figure 1). Minimum and maximum temperatures were21.9 and 37.98C in 2006 and 22.9 and 36.58C in 2007, atDamboa. In Miringa, these were 23.7 and 36.98C in2006 and 25.8 and 35.88C in 2007.

2.2. Cowpea cultivars, planting dates and insecticidetreatments and experimental design

Two improved cowpea cultivars that differ in maturity(selected on the basis of differences in growth habit anddays to maturity), 4 insecticide spraying regimes, and 3planting dates were compared in a factorial experimentarranged in a split-plot design. The varieties wereIT89KD-288 (indeterminate and late maturing) andIT89KD-391 (determinate and early/medium matur-ing) obtained from the Cowpea Breeding Programmeat IITA. The insecticide spraying regimes employedwere: no spraying, spraying once at the onset of

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flowering (1 spray), spraying once at 50% floweringand once at 50% podding (2 sprays), and spraying onceeach at flower bud initiation, 50% flowering, and 50%podding (3 sprays). Planting dates were August 9th,15th, and 21st. Each treatment plot consisted of 4 rows5 m long and 0.75 m apart. The plots were separatedby 2-m alleys and were covered with polythene sheetsduring spraying to protect the plots that were notsprayed. For all treatments, before planting, each fieldwas disc-harrowed and ridged. Three seeds of thecowpea varieties were sown at an intra-row spacing of0.25 m. The plants were later thinned to two plantsper stand at 2 weeks after planting (WAP). Immedi-ately after planting, paraquat (1:1-dimethyl-4, 40-bipyridinium dichloride) was applied at the rate of276 g a.i. L71 and followed by hoe-weeding 3 weekslater to control weeds. Fertilizer at 15 kg each of N, P,and K was applied at 1 WAP, in the form of NPK15:15:15. A standard formulation, cypermethrinþdimethoate (Best Action) at the rate of 30 þ 250 ga.i. per L was used and delivered with a knapsacksprayer during each period of spraying to control insectpests.

2.3. Insect infestation and grain yield assessment

Two inner rows were selected for sampling insects inthe plots. Ten days after each spray, 20 flowers werepicked from each plot during the morning and wereplaced in vials containing 30% ethanol to determine

the number of flower thrips and Maruca. Flowersamples were taken twice at 10-day intervals. At theUniversity of Maiduguri the flowers were dissected andthe Maruca larvae and thrips nymphs and adults wereidentified to species level, separated from plant partsand counted. Pod-feeding bugs (Clavigralla tomentosi-collis Stal, C. shadabi Dolling, and Anoplocnemiscurvipes (Fab.)) were counted in the two middle rowswhen the insects were observed on the field (Amatobi1994). The two central rows of each plot wereharvested when pods of the first flush were matureand dry. Grain yield was reported on a 100% drymatter basis.

Data were subjected to ANOVA using the PROCMIXED (SAS 2001) procedure. Insect counts weresquare root transformed (Steel and Torrie 1980) beforeanalysis. Replication was treated as a random effectand planting date, spraying regime and cultivar weretreated as fixed effects in determining expected meansquare and appropriate F-tests in the ANOVA.Differences between two treatment means were com-pared with Students’ t-test based on the standard errorof the difference (SED) at 5% level of probability.

2.4. Economic analysis of cowpea production underdifferent spraying regimes

Partial budgeting using gross margin analysis com-pared treatments of different cowpea varieties based ondifferent planting dates, locations, and spraying

Figure 1. Weekly rainfall in 2006 and 2007 in Miringa and Damboa.

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regimes. The gross margin is usually referred to asreturns over variable costs: and it serves as a proxymeasure of profitability (Amaza 2000). It is equal tothe gross value of a cowpea variety output minus allthe variable costs incurred on it during the productionyear. In the context of smallholder agriculture, thismargin is also the return to farmers’ capital andmanagement ability.

GM ¼X

piqi �X

rjxj ð1Þ

where GM is the Cowpea gross margin; pi is the Unitprice of cowpea output i; qi is the quantity of cowpeaoutput i; rj is the unit cost of the variable input j; xj isthe quantity of the variable input j.

This analysis accounts for only part of the costsbecause the aim is to compare the profitability ofdifferent treatments. Therefore, the analysis comparestreatments based on returns to family labour, capital,and management. Thus, the variable costs deductedhere include only the cost of seeds, chemicals, andlabour for spraying as they alone varied acrosstreatments. The other variable inputs were constantacross treatments, since the planting density and cropmanagement (other than planting date and number ofsprays) were similar for all treatments, including thezero spray for both varieties. Constant costs acrosstreatments are not taken into account because they willnot affect the final ranking of the different options.

Data from 36 cowpea plots with two cowpeavarieties (IT89KD-288 and IT89KD-391) (18 plotsfrom each of the two agroecological zones, (SS andNGS) were used for the economic analysis. Using anexchange rate of US $1 ¼ N126, the cowpea price was$ 0.44 kg71 for brown seeds and $0.36 kg71 for whiteseeds. Cowpea grain was evaluated at $0.89 kg71

(brown) and $ 0.79 kg71 (white). For each spraying,

costs were US $6.8 ha71 for chemical US$ 12 ha71 forlabour.

3. Results

3.1. Insect pests

3.1.1. Thrips

The three-way interaction of year, planting date, andinsecticide spray was significant at both localities. AtDamboa, the interactions year 6 planting date 6variety and planting date 6 insecticide spray 6 vari-ety interactions were significant. At Miringa, theinteraction between year, insecticide spray, and varietywas significant (Table 1). The population levels ofthrips at all planting dates were generally higher inDamboa than in Miringa (Table 2) and higher in 2006than in 2007 when averaged across planting dates andinsecticide spray applications at both locations. Ingeneral, the population level decreased significantlywith the increase in spraying frequency at bothlocations. At Damboa, the population level wassignificantly higher on cowpea planted on August15th than when planted on August 9th and 21st in allinsecticide spray treatments in 2006: however, in 2007,the population level was higher on cowpea planted onAugust 9th. At this locatity, the population leveldecreased as planting date was delayed, irrespective ofthe number of sprayings. Thrips were most abundantin the unsprayed treatment at Damboa at all plantingdates in 2007. In relation to the unsprayed treatment,one spray of insecticide on cowpea planted on August9th in 2006 had an effect on population levels of thripssimilar to that of 2 sprays on cowpea planted onAugust 21st. In 2007 in Damboa, 3 sprays on cowpeaplanted on August 9th had an effect on populationlevels similar to 1–3 sprays on cowpea planted onAugust 21st. The population levels were 1.5 times

Table 1. Probability of F-values of response of thrips, Maruca, pod-sucking bugs, and grain yield of cowpea to planting dates,spraying regime and variety at Damboa and Miringa.

Effect

Thrips/20 flowers Maruca/20 flowers PSB/m2 Grain yield (kg/ha)

Damboa Miringa Damboa Miringa Damboa Miringa Damboa Miringa

Year (Y) 50.0001 50.0001 0.1090 50.0001 50.0001 50.0001 0.1448 0.0043Planting date (P) 0.0639 0.0004 0.0349 0.3207 0.0007 0.1198 0.0424 0.0043Y 6 P 50.0001 0.0001 0.0118 0.0004 0.0018 0.1514 0.0317 0.0006Spray regime (S) 50.0001 0.0008 0.0499 50.0001 50.0001 50.0001 50.0001 50.0001Y 6 S 0.6271 50.0001 0.0092 50.0001 50.0001 50.0001 50.0001 50.0001P 6 S 0.1797 0.0332 0.4242 0.0087 0.2575 0.0582 0.4426 0.0040Y 6 P 6 S 0.0005 0.0437 0.1020 0.0654 50.0001 0.0068 0.0004 0.3970Variety (V) 0.0039 0.8268 0.5139 0.6852 50.0001 0.0012 0.0001 0.0016Y 6 V 0.0001 0.4723 0.7569 0.5409 50.0001 0.3149 0.5055 0.0104P 6 V 0.0024 0.6452 0.2970 0.4217 50.0001 0.8996 0.7693 0.4117S 6 V 0.4437 0.1303 0.5144 0.8806 50.0001 0.1519 0.4070 0.7031P 6 S 6 V 0.0136 0.5792 0.7088 0.5428 0.0038 0.8092 0.1809 0.4573Y 6 P 6 V 0.0014 0.7265 0.0171 0.3077 50.0001 0.2153 0.0746 0.8672Y 6 S 6 V 0.6920 0.0358 0.1867 0.3544 0.0003 0.0797 0.5655 0.4914Y 6 P 6 S 6 V 0.0979 0.5615 0.5138 0.3187 0.0003 0.9134 0.7248 0.7580

Notes: Y, year; P, planting date; V, variety; S, insecticide spray regime; PSB, pod-sucking bugs.

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higher on IT89KD-391 than on IT89KD-288 whencowpea was planted on August 9th without insecticideapplication. The population on IT89KD-288 was morethan double that on IT89KD-391 when planted onAugust 21st without insecticide application (Table 3).The population on IT89KD-288 was 2 to 4 timeshigher than that on IT89KD-391 when cowpea wasplanted on August 21st in all insecticide-sprayedtreatments.

There was no significant interaction betweenplanting date and cowpea variety in Miringa. Thepopulation level decreased with the increase infrequency of spraying at all planting dates in 2006and 2007 (Table 2). In all insecticide treatments,infestation was significantly lower on cowpea plantedon August 21st in both locations and both years.Population levels did not differ significantly amongtreatments that received 2 and 3 insecticide sprayswhen cowpea was planted on August 15th or 21st.

3.1.2. Maruca

At both localities, year 6 planting date and year 6insecticide spraying regime were significant (Table 1)for the population level of Maruca. The interactionbetween planting date and insecticide spraying regimewas significant only at Miringa. Year 6 plantingdate 6 variety interaction was significant at Damboa.In 2006, the population level increased as planting datewas delayed but in 2007 the population level wasreduced by 67% when cowpea was planted on August21st compared with August 9th (Figure 2). Thepopulation level was generally higher in Miringa thanin Damboa (Figures 3a, b). At both locations, thepopulation level was higher in 2006 than 2007, except

in treatments sprayed three times at Damboa andunsprayed at Miringa. The population level wassignificantly higher in the unsprayed treatment thanin the sprayed treatments on all planting dates. AtMiringa, three sprays reduced the population levels by70% in 2006 and by 99% in 2007 compared with theunsprayed treatment (Figure 3b). Two sprays reducedpopulation levels by 54% in 2006 and by 97% in 2007.The population levels in insecticide treated plots didnot differ significantly with changes in planting date,but in the unsprayed treatment population level wasreduced by 41% when cowpea was planted on August21st compared with August 9th (Figure 4). Thepopulation was significantly higher on IT89KD-288than on IT89KD-391 when cowpea was planted onAugust 15th (Table 4).

3.1.3. Pod-feeding bugs

At both localities, year 6 planting date 6 insecticidespray interactions were significant for population levelsof pod-feeding bugs. Year 6 planting date 6 variety,year 6 insecticide spray 6 variety and plantingdate 6 insecticide spray 6 variety interactions weresignificant at Damboa (Table 1). The population levelsaveraged across insecticide spray applications washigher in Miringa than in Damboa in both years anddecreased by 41% at Damboa and by 15% at Miringawhen cowpea was planted on August 21st comparedwith cowpea planted on August 9th in 2007 (Table 5).In 2006, the population level on cowpea planted onAugust 21st was 44% lower than on cowpea planted onAugust 9th without insecticide application at bothlocations (Table 5). At Damboa, the population levelat all planting dates was lowest after 3 sprays in 2006.

Table 2. Interaction effects of year, planting date and spraying regime on number of thrips per 20 flowers at Damboa andMiringa.

2006 2007

Spraying regime

Mean

Spraying regime

MeanLocation Planting date Zero One Two Three Zero One Two Three

Damboa August 9 22.3 6.5 10.8 7.2 11.7 31.5 11.1 6.1 2.3 12.8August 15 20.2 33.2 21.3 16.7 22.8 12.6 4.2 1.7 2.0 5.1August 21 30.7 11.8 5.7 2.2 12.6 8.3 2.1 2.6 0.2 3.3Mean 24.4 17.2 12.6 8.7 17.4 5.8 3.5 1.5SED Y 6 P 2.99*SED Y 6 S 2.71 n.s.SED Y 6 P 6 S 4.69*

Miringa August 9 7.50 8.50 2.67 2.83 5.38 0.83 0.33 0.25 0.08 0.38August 15 4.33 3.83 3.44 3.50 3.78 0.42 0.08 0.25 0.42 0.29August 21 4.67 2.17 1.00 1.00 2.21 0.00 0.00 0.58 0.42 0.25Mean 5.50 4.83 2.37 2.44 0.42 0.14 0.36 0.31SED Y 6 P 0.48*SED Y 6 S 0.48*SED Y 6 P 6 S 0.95*

Notes: Y, year; P, planting date; V, variety; S, insecticide spray regime; SED, standard error of the difference between treatment mean.

*Significant at P 5 0.05; n.s., not significant at P 5 0.05.

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In 2007, the population level was highest in 1-spraytreatment at both locations (Table 5). In 2007, morepod-feeding bugs were observed on IT89KD-391 thanon IT89KD-288 on all planting dates but not in 2006(Table 3). The number on IT89KD-391 decreased asplanting dates were delayed in 2007 (Table 4).T

able

3.

Interactioneff

ects

ofplantingdate,sprayingregim

eandvarietyonnumber

ofthrips20flowers7

1,andpod-feedingbugsm

72atDamboa.

Insect

pest

Plantingdate

Zerospray

Onespray

Twosprays

Threesprays

IT89KD-288

IT89KD-391

IT89KD-288

IT89KD-391

IT89KD-288

IT89KD-391

IT89KD-288

IT89KD-391

Thrips/20

flower

August

921.9

31.9

9.6

8.0

7.9

9.0

5.1

4.4

August

15

16.8

16.0

28.0

9.3

18.6

4.4

12.1

6.6

August

21

26.9

12.0

6.8

7.2

6.5

1.8

0.8

1.6

Mean

21.9

20.0

14.8

8.2

11.0

5.1

6.0

4.2

SED

P6

S6

V5.98*

PSM/m

2August

96.0

10.7

15.2

50.0

15.5

18.7

6.3

16.3

August

15

4.7

6.0

19.8

33.1

16.2

17.8

6.8

10.8

August

21

6.0

5.0

20.6

22.2

15.2

15.7

4.0

7.0

Mean

3.87*

SED

P6

S6

V

Notes:P,plantingdate;V,variety;S,insecticidesprayregim

e;SED,standard

errorofthedifference

betweentreatm

entmean.

*SignificantatP5

0.05.

Figure 2. Interaction effects of year and planting date onnumber of Maruca/20 flowers at Miringa averaged overinsecticide spray applications.

Figure. 3. Interaction effects of year and insecticide sprayapplication on number of Maruca/20 flowers at Damboa (A)and Miringa (B) averaged over planting dates.

248 A.Y. Kamara et al.

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The population level on IT89KD-391 decreased withdelay in planting date at all insecticide spray regimes.This trend was not observed on IT89KD-288.

3.2. Grain yield

Cowpea grain yield was not influenced by year.However, year 6 planting date and year 6 insecticidespray interactions were significant at both locations.Year 6 variety interaction was significant only atMiringa. The three-way interaction of year, plantingdate, and insecticide spray was significant at Damboabut not at Miringa (Table 1). Average cowpea grainyield was significantly higher at Miringa than atDamboa in both years (Table 6). At both locationsand on all planting dates, grain yield increasedsignificantly with an increase in the number of sprays.At Damboa, the highest grain yield was obtained when

cowpea was planted on August 21st and sprayed thricein 2006 and when cowpea was planted on August 15thand sprayed thrice in 2007. At Miringa, grain yield washighest when cowpea was planted on August 15th andsprayed twice in 2006. In 2007, the highest grain yieldwas obtained when cowpea was planted on August 9thand sprayed thrice. At Damboa, averaged acrossvarieties and planting dates, insecticide applicationonce at flowering increased grain yield by 435%;application at flowering and podding increased grainyield by 1124% and by 1390% when applied at budinitiation, flowering, and podding compared to zerospray (Table 6). At Miringa, insecticide spraying onceat flowering increased grain yield by 192%. Applica-tion at flowering and podding increased grain yield by266%. Application at bud initiation, flowering, andpodding stages increased grain yield by 271% (Table 6).At Damboa, IT89KD-391 produced significantly high-er grain yield than IT89KD-288. This trend wasreversed in Miringa (Table 7) where IT89KD-288 hada significantly higher grain yield in 2007 than in 2006and IT89KD-391 produced similar grain yields in bothyears.

3.3. Economic analysis

For both cowpea varieties, a summary of the estima-tion of the returns is presented in Table 8 that showsfor each locality, planting date, and number of sprays,the revenue, the costs and gross margin. In Damboa(SS), for both varieties, for all planting dates, sprayingcowpea leads to higher returns than not spraying. Inaddition, spraying three times had a higher return thanspraying once or twice. Based on the planting date, theresults are different for the two varieties. For the brownvariety (IT89KD-391), if the cowpea is sprayed (for all

Figure 4. Interaction effects of planting date and insecticidespray applications on number of Maruca/20 flowers atMiringa averaged over years.

Table 4. Interaction effects of year, planting date and variety on number of thrips and Maruca 20 flowers71, and pod-feedingbugs m72 at Damboa.

Variable Planting date

2006 2007

IT89KD-288 IT89KD-391 IT89KD-288 IT89KD-391

Thrips/20 flower August 9 10.5 12.9 11.8 13.8August 15 33.8 11.8 3.9 6.3August 21 17.1 8.1 3.4 3.2Mean 20.5 10.9 6.3 7.8SED Y 6 P 6 V 3.32*

Maruca/20 flower August 9 1.1 1.3 2.2 1.3August 15 2.0 0.8 0.6 1.0August 21 1.0 1.4 0.2 0.4Mean 1.4 1.2 1.0 0.9SED Y 6 P 6 V 0.49*

PSM/m2 August 9 9.8 9.3 11.8 38.6August 15 8.7 10.5 15.0 23.4August 21 9.2 9.0 13.7 15.9Mean 9.3 9.6 13.5 26.0SED Y 6 P 6 V 2.73*

Notes: Y, year; P, planting date; V, variety; SED, standard error of the difference between treatment mean.

*Significant at P 5 0.05.

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numbers of sprays), the best returns are obtained byusing the second planting date (August 15th). How-ever, if cowpea is not sprayed, the third planting date(August 21st) gives the best return. For the whitevariety (IT89KD-288), results are a little mixed withthe second planting date having the highest return forzero, one and three sprays. For two sprays, the highestreturn was obtained at the third planting date(Table 8).

In Miringa (NGS), at zero spraying, the thirdplanting date gives the best return for both varieties.Here also, spraying has higher returns than zero sprayfor all planting dates and varieties. In fact, sprayingthree times has always led to higher returns thanspraying once or twice for all planting dates and

varieties. Again, planting date results are here differentfor the two varieties. For IT89KD-391 here also,results suggest that the second planting date (August15th) is always the best in terms of returns whencowpea is sprayed once, twice, or even three times. ForITKD89-288 also, the second planting dates gives thehighest return, when cowpea is sprayed once or twice.When a third spray is applied, the first planting date(August 9th) has the best return (Table 8).

4. Discussion

Thrips, pod-feeding bugs, andMaruca pod-borers werethe major insect pests of cowpea in the two localities.This was consistent with the findings of Alghali (1991),

Table 5. Interaction effects of year, planting date and spraying regime on pod-feeding bugs m72 at Damboa and Miringa.

2006 2007

Spraying regime

Mean

Spraying regime

MeanLocation Planting date Zero One Two Three Zero One Two Three

Damboa August 9 12.0 7.1 13.7 5.3 9.5 4.7 58.2 20.5 17.3 25.2August 15 9.0 9.2 14.2 6.0 9.6 1.7 43.7 19.8 11.7 19.2August 21 6.7 14.0 12.5 3.3 9.1 4.3 28.8 18.3 7.7 14.8Mean 9.2 10.1 13.4 4.9 3.6 43.6 19.6 12.2SED Y 6 P 1.93*SED Y 6 S 2.23*SED Y 6 P 6 S 3.87*

Miringa August 9 11.2 10.0 11.2 11.7 11.0 36.0 49.0 16.8 13.8 28.9August 15 12.0 9.7 12.8 12.0 11.6 27.7 32.3 17.5 22.2 24.9August 21 6.2 9.7 10.7 10.7 9.3 30.5 34.5 12.8 20.8 24.7Mean 9.8 9.8 11.6 11.4 31.4 38.6 15.7 18.9SED Y 6 P 1.82 n.s.SED Y 6 S 1.92*SED Y 6 P 6 S 3.64*

Notes: Y, year; P, planting date; V, variety; SED, standard error of the difference between treatment mean.

*Significant at P 5 0.05; n.s., not significant at P 5 0.05.

Table 6. Interaction effects of year, planting date and spraying regime on cowpea grain yield (kg ha71) at Damboa and Miringa.

2006 2007

Spraying regime

Mean

Spraying regime

Location Planting date Zero One Two Three Zero One Two Three Mean

Damboa August 9 119.8 180.1 690.4 1237.1 556.9 25.4 435.1 799.7 893.1 538.3August 15 107.7 67.6 1049.5 1331.0 638.9 43.4 815.6 902.2 968.5 682.4August 21 80.6 87.7 1189.6 1520.1 719.5 62.1 761.1 746.3 593.0 540.6Mean 102.7 111.8 976.5 1362.8 43.6 670.6 816.1 818.2SED Y 6 P 60.28*SED Y 6 S 69.61*SED Y 6 P 6 S 121.81*

Miringa August 9 26.9 829.0 1194.6 1138.6 797.3 362.2 945.9 1021.1 1153.7 870.7August 15 35.3 948.0 1309.8 1127.9 855.3 457.4 761.9 936.3 1055.9 802.9August 21 37.2 652.3 805.9 820.3 578.9 751.9 756.3 850.0 916.3 818.6Mean 33.1 809.8 1103.4 1028.9 523.8 821.4 935.8 1042.0SED Y 6 P 51.80*SED Y 6 S 61.98*SED Y 6 P 6 S 113.44 n.s

Notes: Y, year; P, planting date; S, insecticide spray regime; SED, standard error of the difference between treatment mean.

*Significant at P 5 0.05; n.s., not significant at P 5 0.05.

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Amatobi (1995) and Kamara et al. (2007). Pestpopulation levels were higher in 2006 than 2007because of higher rainfall in September 2006 whencowpea was flowering. High rainfall with associatedhigh humidity usually increases insect pest pressure incowpea in West Africa (Alghali 1991, 1992b).Although rainfall was higher at Miringa than atDamboa, infestation by thrips was higher in Damboaprobably because of the lower temperature and shorterday length there during the study period. Populations

of thrips are reported to crash during periods of hightemperature and long day length (Alghali 1991).Maruca population levels generally were higher in alltreatments at Miringa than at Damboa, probablybecause of the high rainfall at Miringa (Figure 1).

Averaged across insecticide application schedules,the lowest population of all insect pests was recordedwhen cowpea was planted on August 21st. Cowpeaplanted in late August will flower in late September; aperiod of low rainfall and high temperatures. Insectpopulation levels are generally low when cowpea isplanted in late August in the West African savanna.Oghiakhe et al. (1995) reported lower populations ofMaruca in a location that received less rainfall than inwhere rainfall was higher. Although delaying plantinggenerally reduced insect infestations of cowpea in bothlocalities, the results were not consistent for thrips. Theinconsistencies may be caused by the differences inrainfall between the 2 years. Our results suggest thatinsecticides are effective in controlling insect pests ofcowpea. This is confirmed by the high numbers ofthrips, Maruca, and pod-feeding bugs and the sig-nificant reduction in grain yield when cowpea was leftunsprayed. Other studies in West and East Africa havefound the application of insecticides to significantlyreduce insect pest populations and increase grain yieldsof cowpea (Alghali 1992b; Karungi et al. 2000b;Kamara et al. 2007). The effect of insecticides in ourstudy was, however, dependent on the date of plantingand the frequency of sprays, with a strong influence of

Table 7. Interaction effects of year and variety on cowpeagrain yield (kg ha71) at Damboa and Miringa.

Variety 2006 2007 Mean

DamboaIT89KD-288 579.7 505.1 542.4IT89KD-391 697.2 669.2 683.2Mean 638.44 587.12SED Y 34.8n.s.SED V 34.8*SED Y 6 V 49.2n.s.MiringaIT89KD-288 753.3 917.8 835.5IT89KD-391 734.3 743.7 739.0Mean 743.81 830.74SED Y 29.4*SED V 29.4*SED Y 6 V 41.0*

Notes: Y, year; V, variety; SED, standard error of the differencebetween treatment mean.

*Significant at P 5 0.05; n.s., not significant at P 5 0.05.

Table 8. Revenues, costs and gross margin (US $ ha71) of cowpea varieties under different spraying regimes and planting datesin northeast Nigeria.

LocationNumber ofSprayings

IT89KD-288 IT89KD-391

Plantingdate – D1

Plantingdate – D2

Plantingdate – D3

Plantingdate – D1

Plantingdate – D2

Plantingdate – D3

1. Damboa(SS)

0 Revenue 19.5 22.8 13.7 39.6 38.1 45.5Cost 15.9 15.9 15.9 17.5 17.5 17.5Return 3.6 6.9 72.2 22.1 20.6 28.1

1 Revenue 45.4 135.4 135.2 213.1 220.0 205.2Cost 34.7 34.7 34.7 36.3 36.3 36.3Return 10.7 100.8 100.5 176.8 183.7 169.0

2 Revenue 237.8 297.5 327.0 359.8 488.2 445.4Cost 53.5 53.5 53.5 55.1 55.1 55.1Return 184.3 244.1 273.5 304.8 433.2 390.3

3 Revenue 395.4 362.3 332.2 446.6 560.9 516.3Cost 72.3 72.3 72.3 73.9 73.9 73.9Return 323.1 290.1 260.0 327.8 487.1 442.4

2. Miringa(NGS)

0 Revenue 77.1 110.0 150.6 70.5 87.1 160.4Cost 15.9 15.9 15.9 17.5 17.5 17.5Return 61.2 94.1 134.7 53.0 69.6 143.5

1 Revenue 348.5 348.9 252.2 348.9 320.0 306.6Cost 34.7 34.7 34.7 36.3 36.3 36.3Return 313.8 314.2 217.5 312.6 283.7 270.4

2 Revenue 417.4 417.9 318.4 457.0 417.2 333.7Cost 53.5 53.5 53.5 55.1 55.1 55.1Return 364.0 364.0 264.9 401.9 416.1 278.6

3 Revenue 447.3 376.9 311.7 454.0 492.6 377.1Cost 72.3 72.3 72.3 73.9 73.9 73.9Return 375.0 304.7 239.4 380.1 418.7 303.2

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the cropping year. In a year with less rain, late plantingmay reduce the need for frequent applications ofinsecticides in Damboa. In Miringa, our results suggestthat a minimum of two sprays are required to controlinsect pests of cowpea irrespective of planting date.

In our study, spraying cowpea with insecticidesdramatically increased grain yield. However, yield afterspraying thrice did not differ significantly from yieldafter spraying twice, once at flowering and once atpodding stages. This suggests that a third spray maynot be necessary in this region. Because of environ-mental concerns and the cost of insecticides, carefulselection of spraying regimes is necessary in thechemical control of insect pests of cowpea.

Despite the reduction in insect infestation, delayedplanting beyond August 15th reduced cowpea grainyield by 12.3%. This may have been caused by moistureand heat stress when flowering occurred around lateSeptember. Rainfall was lower (Figure 1) and thetemperature was high in September in both years andlocalities. Cowpea planted in the last week of Augustwill not flower until the last week of September and thiswill subject the plants to moisture and heat stress. Manycowpea cultivars are susceptible to heat (Patel and Hall1990) and drought stress (Turk et al 1980).

Based on the estimated returns in this study, inboth Damboa and Miringa spraying cowpea threetimes is always best for both white and brown varieties.For those farmers who either cannot afford it or do nothave access to spraying materials, planting late in theMiringa seems to help increase their return. For thosefarmers who can spray their cowpea up to three times,planting on August 9th is their best option in bothlocalities. When they plan to spray once or twice,results are different for the two localities and varieties.If they are using the brown variety (IT89KD-391) inboth localities, planting on August 15th is the best forthem in terms of returns. At Damboa, planting onAugust 15th gives better results after one spray;planting on August 21st gives better results after twosprays. In Miringa, if farmers plant the white variety(ITKD89-288), planting on August 15 is best for themwhen they are planning to spray only once or twice.

Acknowledgement

We thank the Canadian International Development Agencyfor funding the project Promoting Sustainable Agriculture inBorno State, which financed this study and the support staffwho managed the trials and collected the data.

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