SCREENINGTECHNIQUES FOR HOST PLANT RESISTANCETO COWPEAINSECT PESTS1

L.E.N.Jackai and S.R. Singh2

Abstract: As research on food crops inlensifiesin the tropics, the spotlightis being increasingly focused on the developmentof varietieswith resistanceto pests and diseases. Much of lhe time, mon€y and patience needed to conduct research on host plant resistance(HpR) is spent on the developmentof methodologiesthat are eventuallyused to screengermplasm and breedinglines. The productsof this screeningprovide the buildingblocks for the developmentof resistantvarieties.

Methodologiesdiffer from one place to anotherand from one crop to the other,and are usually tailored to suit local and/or individual circumstances. There is, however, a need to develop standardiz€dmethodology to make comparisonsacross locationseasier. Any methodologyis capable of improvement,but improvementscome only aflsr use and evaluation. In this pap€r we try to defin€the currsntstatus of HPR screeningmethodology in cowpeaentomology, concentrating mainly on techniques developedand used with considerablesuccess at llTA. In addition,the shortcomingsof some of these techniquesare pointedout and areasneeding improvement indicated.

An important component of crop 2. Non-preference when he plant improvementprograms the world over is possessesattributes that ad to the the development of pest resistant crop non-useor reduceduse o' ne plant by varieties. The ability of a plant to resist the for food, shelt , oviposilion attackby anotherorganism is knownas host or any combinationof ,1€s€. This plant resislance(HPR). The original modality has been r€ hristened by definitionof HPR (Painrer1951) is the Kogan and Ortman (1978) as "relative "antixenosis" "bad amounl of heritable plant meaning host" to qualitiesthat influencethe ultimatedegree reflect a characteristicof the plant, as of damage sufferedby the planl" under a in other modalities, rather than a known/giveninsect pest population. This responseby the pest, but there is still quality is expressed in three classical some debate on its acceptabilityfor modalities: commonusage (Wiseman 1995). 1. Antibiosis- when the plant possesses 3. Tolerance- when the plant possesses attribute(s) that produce adverse attributes which enable it to grow, effectson the insect'sbiology, behavior repairinjury and producean acceptable and/or physiology. yield despite supporting a pest populationthat would normallycause significant damage and/or kill a 1 Paperpesented at the RegionalGrain Legume susceptibleplant. Workshop,organized by IFS in Tannanarive, Madagascar,22-28 February 1988. Theseterminologies have undergone 2 Grain Legumelmprovement Program, various degrees of redefinitionover the Internationallnstitute of Tropical Agriculture years (Beck 1965) but the originaltheme (llTA),Oyo Road, PMB 5320, tbadan,Nigeria. has remained,by and large,unchanged. Host plant resistancemay also be rated as low, moderateor high (Horber1980; Wiseman 1985), based on its manifestationin a Where field screeningis used it is definedrest situation. necessaryto develop appropriatesampiing methodsfor plantsor plant parts,as weli as for the insectpest. Samplingis a critical DEVELOPMENTOF RESISTANTVARIETI ES componentof HPR work. Poorlyconceived sampling designs can result in highly Developmentof resistancecan be a misleadingconclusions. Rating scales are long and expensiveprocess, often taking usefulfor both field and greenhousework over 20 years to accomplish" However, (Davies1985), but even the developmentof there are many examplesin the literature such scales, which ate often based on where resistantcrop varietieshave been degreesof damage,depends on a proper developedin a much shortertime (Gould understandingof the biologyof bothcrop and 1983). Success in the developmentof pest, as well as their interaclions. resistant varieties is predicated on a Contrary to the opinion held by some numberof factors. To start with, a highly workers, rating scales can be analyzed diversifiedgermplasm resource is essential statisrically(Little 1985; Little and Hills to provide the genetic variabilityrequired 1e77)'. for such work. Once this is available sourcesof resistancehave to be identified,a Followingthe above two phases processthat requiresthe developmentand (collectionand screeningof germplasm), use of reliablescreening procedures. This hybridizationtechniques are used to is probablythe single most importantphase lransfe r resislance fro m the original of host plant resistancework, as it provides sources to other genetic backgrounds, the foundalionon whichthe developmentof usuallybreeding lines or other materials crop resistantvarieties for the future is with one or several desirable based. characteristics.This processinvolves close collaboration belween breeders and A wide range of biological entomologists. Becauseof the peculiar informationis usually requiredto develop nalure of segregatingbreeding material, appropriate screening procedures. screeningprocedures must be suchthat the Informationon the phenologicalrelationship transfer of resistancecan be monitored betweeninsect pest and targetcrop, as well eitherin singleplants or in progenyrows at as that of non-targetcrops or alternative the appropriatestage of the breedingcycle. hosts; plant stage(s) attackedand insect stage(s)causing damage; and developmental A study of the mechanismsof and behavioralprofiles of the insect pest resistanceis the final phase in a HPR with referenceto its host plant. This research program and may require the informationis important in determining expertiseof biochemistsand physiologists. how frequentlyand at what levelsof pest While this is imporlant in understanding infestation screening in the tield or the overall resistancephenomenon, and elsewhereshould be carriedout. There is a could indeed provide clues to the continuing need to develop methods of developmentof more rapid screening evaluating resislance in the field, procedures,it is not a prerequisitefor the screenhouseand laboratory. developmentof resistantvarieties. The need for highly specialized personnel and Dahms (1972) outlined 16 criteria sophisticatedinstrumentation in some cases that have been used to evaluateresistance of makes its pursuitby inadequatelystaffed or plantsto insectdamage. Theseinclude both poorlyequipped programs unadvisable. host plant and insect responses,most of them relativelysimple but others involving Most crop improvementprograms in the use of complicatedphysiological and developingcountries have a HPR component biochemicalstudies. and it has receivedincreasing emphasis in agriculturalresearch in Africa during the someof them havenot beenpublished) past but two decades. Most nationalprograms ratherpresent a selectionof thosemethods are involvedin a lot of screeningof all-kinds that are more commonlyused and have of crops for resistancelo variousdiseases shown a good level of precisionand and insectpests. A varietyof methodsare repeatability.Most of thesemethods were employedfor this purpose,many of which developedandlor are currentlyused at the were developed elsewhere, or ate Internationallnstitute of Tropical modificationsof methodsused on other Agriculture(llTA) in Nigeria. crops. The diversityof proceduresused in HPR screeningis clearlyevident from the llTA has a cowpea germplasm publications on crop/varietalresislance collectionof over 12,000accessions, lhe that appear in scientificjournals; the largestin the world,and a globalmandate number of methodsalmost equals the for the improvementof cowpea. lts Grain numberof authorsl An obviousoutcome of Legumelmprovement Program, which has this is that comparisonsof resultsfrom researchand trainingresponsibilities for several.locationsmay sometimesbe quite cowpea, has a multidisciplinaryleam inappropriate.Superimposed on this is the comprisedof plantbreeders, entomologisls, fact that the internationalagricultural pathologistsand agronomistswho work on researchcenlers (lARCs), which have a variousaspects of crop improvement,with majorresponsibility for the developmentof HPRas thecenterpiece. crop varieties for use by national agriculturalresearch systems (NARSs), No extensivedetails will be givenon are scalteredthroughout the African(or how to evaluatethe mechanismsof other) continent in widely differing resistanceand/or the basisof theseas they environments.Sometimes varieties or havebeen treated elsewhere (e.g. Maxweil cultivarsdeveloped by an IARC, say for and Jennings1980; Davies 1995; Dahms resistanceto a particularinsect pest, are 1972). In the long term it is hopedthat senl to NARSswithout complete details some form of standardizedprocedures or (includingpossible options) on how to techniquesfor screeningcowpeas for evalualethe resistance. Many research resistanceto insectpests can be established, stations do nol have professional with acceptablemodifications to suit local enlomologistsor well-trainedtechnical conditions.Such documents are available staff, with the result that other less for a numberof other crops, e.g. rice qualified personnel are given the (Heinrichset al. 19gS),alfatfa (USDA- responsibilityof screeningfor insectpest ARS, ARS-NC-19,19741 and cotton resistance.This underscoresthe need to (SouthernCooperative Series Bulletin 2gO, developand publishscreening procedures 1983)(the lattertwo are cited by Davies that are both practicaland scientifically 1s85). valid,without being complicated.

Even though local ecological THERAT{GE OF COWPEA INSECT PE,STS circumstancesand other factors (both socioeconomicand biological)may vary Insectpests damage cowpeas at all from one region, countryor localitylo growlhstages. Thesepests have beenthe another,there still appearsto be an urgent subjectof recentreviews (Singh and van needto put intoa singledocumenl the more Emden 1979; Singh and Jackai t98S; frequentlyused and time-testedtechniques Jackaiand Daoust1986). Eventhough the that are availablefor HPRwork on cowpeas. rangeof pestsand their relativeimportance That is the raisond'6tre of this paper. We may vaty slightly from one location or cannotpretend to give an exhaustivelist of regionto anotherthere are a numberthat all the techniquesthat have ever been used are a regularfeature of cowpea agro- in cowpearesistance work (particularlyas ecosystemsworldwide. These are generally dividedinto three groups accordingto the HPRscreening techniques have been growlhstage of the crop. lried for practicallyall cowpeapests, with varyingdegrees of success. lt is clearthat Seedling phase: Pestsattacking this someof thesemethods have been based on a stage consistof aphid (Aphis craccivoral, shotgunapproach devoid of the necessary beanffies(Ophiomyia spp.), leafhoppers backgroundknowledge of pest biologyand (mainfyEmpoasca spp.),foliage beetles behavior.Many such techniques have ended (Ootheca spp.,Medyfhra spp. and others) up in the pagesof journalsor in-house and the arctiid defoliator( publications.Some of theseare citedbelow moloneyi). as examplesof unsatisfactorytechniques, but the greaterpart of this paperis devoted Early reproductive phase: This to the descriptionof proceduresthat have consistsof the flowerbud stageplus early been used frequently and yielded floweringand is attackedmainly by the satisfactoryresults. flower bud thrips (Megalurothrips sjostedti) and the legume pod borer (Marucatestulalis). SCREENIiIGPROCEDURES

Late reproductive phase: This Seedlingpests covers the period from late flowering throughpodding and is attackedprimarily Aphids by the legumepod borer and the podsucking bug complex of which two coreids The majorspecies encountered on (Clavigrallaspp. andAnoplocnemis sp.), cowpea is Aphis craccivora Koch two alydids (Riptortus sp. and Mirperus (Homoptera:Aphididae). This is probably spp.) and a numberof pentatomids(e.9. oneof the mostwidely researched in Nezara viridula and Aspavia armigera) are agriculturalentomology. Aphids damage most important. Other insectspresent youngcowpea seedlings by suckingsap from duringlhis phaseare the cowpeaseed the youngleaves and stemtissues. Even (Cydiaptychora), the cowpeapod weevil though they also infest reproductive (Apion varius), the cowpea curculio structures(llTA 1985)the effectis not as (Chalcodermusaeneus) and flowerbeetles damagingat this stageas at the seedling (Mylabrisspp. and Corynaspp.). stage. Screeningfor resistanceto aphids can be conductedin field trialsas well as Other classificationsystems are undermore controlled screenhouse tests. foundin the literature(e.9. Singh and van Mostof the resistantlines developed at llTA Emden1979; Taylor 1964): lnsectpests (Table1) have been obtainedusing the arealso encountered in storageand only one latter. genus, Callosobruchus,is knownto be of economicimportance. The importance Field techniques. Cowpea aphid attachedto thesepests by differentworkers populationstend to be spottyearly in the varies, a fact that is reflectedin the season,but if plantingis delayedor madeto researchemphasis that has beenplaced on coincide with drier periods a good differentmembers of the pest complex. infestationis generallypossible. At llTA, Aphids,leafhoppers and the storagebeetle field screeningis generallyconducted have receiveda great deal of attention during the dry season (December- worldwide(partly because they are easier February),when the majorpest problem on to study)while flowerthrips, pod sucking cowpeais aphids. This is, however,only bugsand the legumepod borer have received possiblebecause irrigation facilities are much less attention. The lattertwo are available.Where such facilities do not exist am6nglhe mostdifficult pests to workwith a late plantingduring the main cropping in cowpeaHPR research. Table 1. Cowpea varieties/cultivars with reslstance to various insect pests

Insect@mmon name Resislancesource Advancedbreeding (species) lines lines

Aphid TVu 36, TVu 408-P2,TV 410, tT81D- 1020 (Aphis craccivora Koch) TVu 801, TVU2896, TVU 3000 rT82E-1-108

Leafhoppers TVu 59, TVu 123,TV 662, tr83s-742-1 1 (Empoasca spp.) TVu 1190,(VITA 3) 1T83S-742-13 1T83S-742-14 fT835-747-4

Beanfly TVu 1433,TVU 3192, lT81D -1205-174 (Ophiomyia phaseolil Farv 13 lT82D-644

Flowerbud thrips TVu 1509,TVu 2870 TVx 3236 (Megalurothrips sjostedti) tT82D-713 1T82D-71 6 tT84S-224 6-4 Legumepod borer TVU 946 (Maruca testulalis I Kamboinselocal

Podsucking bug TV 1, TVu 1890 (CI av ig ral I a tom e n to sicoI I i s)

Cowpeaslorage weevil TVU 625, TYU 2027,TVU 11952 lT81D-1007 (Callosobruchus maculatusl TVU 11953,TVu 4200 lT81D-1137 (pod wall) tT81D-1 157 tT84s-2246-4

season will produce the same effect. One methodof assessingthe incitlence Applicationof DDT (Don-Pedro1980) and of aphidsis to determinethe percentageof certain older generation synthetic plantsin each row that is infestedat 10, 20 pyrethroids(Matteson 1982) producesan and 30 days after seedling emergenceor, increasedaphid infestation. This is thought where largerplots are used,on the center to result from a drastic reductionin aphid two rows at the same time intervals. The parasitesand predators. The use of DDT, incidenceof aphids is importantbecause however,should not be encouraged.Single they act as vectors in the transmissionof row plots (3-4m long) or plots consisting cowpea aphid-bornemosaic virus. The of four rows each four meters long can be severityof lhe infestationcan be measured used,depending on the amountof materialto using a rating scale of 0-9 based on the be screened. In each case resislanland/or numberand size of colonies(Table 2). The susceptiblechecks are included,and tests levelof infestationis assessedon 30 stands are repeatedfor verification. per plot (five stands per row at fixed intervals to avoid bias) and lhe scores averaged. lt is usuallynot necessaryto use Table 2. Visual rating scale for aphids (Starksand Burton lgZZ). Infested cowpea resistanceto aphids (in part trays are transferred after Listsinger to cages with fine el af. 19771 saranmesh, small enough to allowpassage of air but not insects. Thesecages'are fe-pt in a greenhouse. About two weeks after RatingNumber of Appearance infestation,or as soon as the susceptible aphids check dies, plantsare rated on a 1_5scale for damage (manifestedas decreasedplant vigor). The susceptiblecheck is usually 0 0 no infestation killed around 10-15 days after infestation. 1 1-4 a few individualaphids The ratingis first done on a row basis, and 3 5-20 a few isolatedcolonies laler withinrows. The latteris more useful 5 21-100 severalsmall colonies when dealing with segregatingbreeding 7 101-500large isolatedcolonies lines. A score of 1 indicates high 9 > 500 largecontinuous colonies resistance,2 or 3 indicate moderate/low resistance, while 4 suggests low susceptibilityand 5 high susceptibility. This processcan be repeatedseveral times for lines with scores both assessmentmethods, the former of 1, 2 or 3, with as being many replications possible, adequaiefor most bulk screening as using trays work. as the replications. Other methods are available for A similar technique determiningthe mechanismsof resistance is used for screeningsorghum for resistance involved(e.9. Davies19g5) but these to the are green bug, Schzaphis graminium beyondthe scope of this paper. Many Rondani of (Starksand Burton 1977\. these use no-choicetechniques lo identity antibiosisand non-preference,as well as tolerance. Leafhoppers Most leafhoppersfound Screenhouse techniques. A method on cowpea belong genus Empoasca developedat llTA uses cowpeaplants grown lo lhe (Homoptera: Cicadellidae). In Asia genus, in wooden trays (54 x 80 x 11 cm) filled another Amrasca,is encountered.At llTA resistance with soil to about 8 cm depth (Singhand to.leafhoppershas been identified Jackai 1985). Tesl materialsare ptanteO mainlyby using field techniques. Leafhoppers,-like in single rows equidistantfrom each other. aphids, are a major problemduring periods Includedamong these are resistantand of low rainfall. Fieldscreening is susceptiblechecks (Singh 1977, 19gO) therefore conductedby plantingtest materials where these are available. In a situation in late September/earlyOctober, where no resislantcheck is available roughlya month test later than is customary(Singh lines.are plantedwith a known susceptible and Jackai 1985). About two prior check only. Eventuallya resistant weeks to 'atwayiplanting source the test materiats(which will be identifiedand includedin subsequent should include a resistant and/or susceptibie tests. When plantsare aboul 10 days old, check) a susceptible spreader each plant of every test row is infestedwith row is plantedalong the bordersof 5 fourth instar nymphs using a camel_hair the field. lt is alsocustomary to leaveany grassaround blusfr Aphids for use in this test may be the field uncutuntil afterthe obtained from the field, but preteraOty test materialsare 10 daysotd. Theseprecautions shouldcome from a,l existingaphid ib9!t hetpto cullure build up the leafhopperpopulation. carefullymaintained in a screenhouseaway test linesare then plantedin singlerows, fro;'nparasites and/or predators. Several each 4 or 5m long. Checksare included methodshave been describedfor rearing once in every10 entries. The susceptiblespreader Table 3. vlsuat rating scale lor leafhopperdamage to cowpea

Rating Appearance

1 no visibledamage 3 slightyellowing of the midrib;no cupping pronouncedyellowing at midriband sideveins; yellowing extends slightly between the veins;symptoms may or may not be associatedwirfmoderate.,ippiig--' severeyellowing of the leavesfollowed "hopper by dryingof teaf tips and i6irhargins; characteristic burn" observed severedefoliation, followed by wilting,resulting in deathof plants

row is cut, but left on the plot, about 10_ Since the insect is not presentlya 15 days after the emergence of the test seriouspest of cowpea in Africa little or no materials.This resultsin movementof the attentionhas been given to it on cowpea. leafhoppersto the test materials. However,because it is an importantpest on other legumessuch as beans Resistancerating is fhdseolus done on a 1_9 spp.), which are widely grown in the scale (Table 3) on a row basis around 25_ easternand southernparts of Africa,as well 30 days afler seedling emergence.Resistant as on soybean,we considerit a potentially lines ate later tested in replicatedfield importantpest on cowpea trials. and'therefore Several resistantlines have been usefulto discussthe methods ide.ntified developedfor _using the field screening beanflyresistance work elsewhere. technique(Tabte 1). Screeningof cowpea for resistanceto Screenhousetests have also been O. phaseoll has been conductedbv llTA used(Raman et al. 1979) and are basedon a during the past three years in collaboration proceduresimilar to that described for with the Asian Vegetable Research and aphids. However,more consistent results DevelopmentCenter in Taiwan using lhe have been obtainedfrom field evalualioni fleld.techniques developed by Chiang and suggestingthat further refinement is Talekar(1980) for use on soybean. required to Singte improve the screenhouse rows of test materialare grown techniques. with known susceptiblechecks in the fashiondescribed for other pests. Four weeks Beanfly after planting the entries are assessed for perientage infestation.The best entries The predominant 1i.e.mose with specieson cowpea the lowest infestation)are later subiected is Ophiomyia phaseoti Tryon (Diptera: to replicatedfield trials. In such trials Agromyzidae). This insect iunnels in tne (plot sizes vary but 4 rows x 4m shouldbe slems of cowpea seedlings causingthem to adequate)about 30 plants are sampled per swell and crack at ground levei. Badly plot and the number of beanfly larvae infestedplants wilt and off and then die. eggi pupae, and the extent of damage are laid in the arc leaves or hypocotylby ihe recorded. The level of infestationpei plot small, is shiny black adult flies. Most me.asuredprior to resistance the collectionof sarhples. screeningis thereforetargeted at Using both measurements the stem. as criteria,'five cullivars were selected as resistant (llTA 1s86) (Table 1). It is conceivable that the pigeonpeaflowers where their population screenhousecaging technique described for buildsup. In the plot itself,susceptible aphidscould be used, with appropriate spreaderrows, usually the cowpeacultivar modifications,as a procedurefor beanfly VITA7, are establishedin a checkerboard resistance screening. This has been design,2-3 weeksprior to the plantingof attemptedin the Philippinesbut requires testlines. Thripsmove from the pigeonpea f urther study (C. Adalla personal to the spreaderrows once the latterstart to communication). produceflower buds.

Otherseedling pests The test materialis plantedroughly three weeks after the spreaderrows and No systematiceffort has been made VITA7 is plantedonce in every10 entries; to developspecific screening procedures for 35 days later the spreader rows are foliage beetles partly because of the uprootedand plantslaid betweenrows of the sporadicnature of their attacksand the fact lest plants. This forcesthe thripsto move thal they are not considered urgent away from ths *ying plants to those of the problemi. Wherean attempthas been made, lest rows. In additiona low dosage plots of fiefd were rated on a scale of 1-5, endosulfan(ca 2@ g a.i./ha)is sprayedto representinga gradationin leaf area damage reduce infestationby {rttpB which might and the presence/absenceof adult beetles otherwiseinterfere with the assessment (Jackai of er at. 1989). thrips damage. Visual rating is first carriedout at 45 days and againat 55 days Pests of the early reproductive after seedlingemergence, or first phase wh6n racemesare about5 mm longand then 10 days later. In the past,a damagerating Two of the more importantpests of scaleof 1-5was used,but this has recentlt cowpea,the flowerbud thrips(FTh) and the beenreplaced with a 1-9 scale(Table ai. legumepod borer (MPB),belong to this Ratingis basedon a combination varying group. of However,since the MpB is also intensitiesof browningof the stipulesand importantduring the late reproductive flowerbuds, non-elongation of peduncles, phase,the screeningprocedures used for and flgwerbud abscission.These damage this pestare describedin the nextsection. symptomsare used to diagnosedamage causedby M. sjostedtibutwe believeother Flower bud thrips speciesof flowerthrips cause lhe sameor similardamage symptoms. Considerablework has beendone at llTA on the development,of screening The distinctionbetween susceptible techniquesfor cowpearesislance to flower cuflivarsand thosewith even low levelsof bld thrips, Megalurothripssjostedti Ttyb. resislanceis very clear in fiefd (Thysanoptera: trials,so Thripidae). Screenirqhas lhat the use of screenhousemethods is been highlysuccessful in the field, but a generally.unnecessary.Even segregating numberof screenhousetechniques have also breedinglines can be screenedusing this beenrried (ltTA 1983). fiefdtechnique. Lines with a scoreoi l, 2 or 3 are consideredresistant. Theseare To ensure high and uniform usuallytested again in larger,replicated populations, dwarf pigeonpea(Cajanus plots where rating is restrictedto the cajan) is establishedatong the bordersof cenlerlwo rowsin orderto avoidany edge the field. Sincethis is a perennialcrop, effect.Two germplasm lines, TVu 1509and there is a specialfield designatedfor tfris TVU 2870, were identifiedwith low to p_urposeat llTA. Plantingis doneto ensure moderatelevels of resistance(Singh thal the pigeonpeaflowers before the 1977). TVu 1509has the highertevel of cowpea. Thrips are attractedto the Table 4. visual rating scale for flower thrips damageto cowpea

Rating Appeararrce

no browning/drying(i.e. scaling)of stipules,leaf or flower buds; no bud abscission 3 initiationof browningof stipules,leaf or flowerbuds; no bud abscission 5 distinctbrowning/drying of stipulesand leaf or flowerbuds; some bud abscission 7 seriousbud abscissionaccompanied by browning/dryingof stipulesand buds;non- elongationof peduncles very severebud abscission,heavy browning/dryingof stipulesand buds; distinct non-elongationof (mostor all) peduncles

/Vole: Scoringshould be done at the slage when pedunclesare about2-3 cm tongrandthen onswee[to= 10 days later. resistanceand has been usedin the breeding Pesls of the late reproductive phase programat llTA to develop severaladvanced breedinglines with resistanceto thrips Legumepod borer (Table1). A higherlevel of resistancethan lhat in TVu 1509 is yet to be identified. The legumepod borer or marucapod borer (MPB), Maruca testulalis Geyer This techniquehas been criticizedin (: Pyralidae),is perhaps the the past on the erroneousassumption that most elusivepest in cowpeaentomology. lt test linesclosest to the pigeonpeaborders attacks a wide range of sites including will receivea heavierinfestation than those terminalshools, young succulentstems and farlher away. Carefully planned and peduncles,flower buds, flowers and pods. execuled studies, however, have Becauseof this multiplicityof feeding sites demonstratedthat this is not the case (llTA screeningprocedures have to be developed 1982). Anothercriticism has been that bud taking each site into considerationbut abscission and non-elongationof the weighting them according to their pedunclesare symptoms lhat can result importance(Jackai 1982). Thus, as from damageby other pests. Whilethis is damagelo someparts (e.9.flowers and pods) possible,the other pests that might cause is more critical than that to other parts similarsymptoms (the legume pod borer (e.9. stem) evaluationof resistanceis often and the pod suckingbugs) do not causelhe weighted towards the former. Field characteristicbrowning associatedwith screening has been the mainslay of thrips damage lo cowpea. Finally,field evaluationsfor resistanceto MPB. Several screeningcan, and has been, carriedout attempts have been made to develop withoutusing permanent pigeonpea borders. greenhousetechniques (Dabrowski et al. The main cowpeagrowing season in most 1983) but these have generallynot been localionsis generally suitable for field repeatableby other workers. screening.The susceptiblespreader rows, however,help lo create a uniform insect Field techniques. A systematic pressure.Their use shouldbe encouraged. methodfor screeningcowpeas for resislance to MPB in field plots was developedat llTA (Jackai1982). Over the years this method has undergone slight modifications to

t0 Table 5. Visual rating scales for legume pod borer damage to cowpea

Pod load (PL) Poddamage (PD) Rating Degreeof podding Rating "/"

most (>60%)peduncles 1 0-10 bare (i.e. no pods) 2 11-20

31-60% pedunclesbare 3 21-30

4 31-40

5 16-30%peduncles bare 5 41-50

6 51-60

7 up to 15%peduncles bare 7 61-70

8 71-80

occasionalbare peduncles 9 81-100

Note'. Pod evaluationindex (lpe) = PUPD. Highervalues of lpe indicate higherresistance (with PL > 5 and PD < 5). Even scores(2, 4, 6,8) are given for PL where no clear-cut(odd-numbered) score appearsto fit. increaseits precisionand efficiency(Jackai in single 3m-long rows, with two and Singh 1983;llTA 1986,1987) but susceptible(one early and the othermedium has, by and large, retainedthe basic maturing)and one resistantcheck included componentsof theoriginal method. once in every20 entries. Thesechecks servea dualpurpose. First, they provide a An early maturing susceptible means of assessingthe severityand cultivaris plantedalong the bordersof the distributionpattern of MPB throughlarval field, as well as perpendicularto every countsin flowersamples, or of assessing rangeof rowswithin the plot. Thisensures percentageflower infestation based on 20 that every test row has a spreaderrow flowersper row collectedand examinedon runningacross each end. Alternatively,two the spot. Second,and morecommonly, they rowsof the spreadercan be plantedat every provide a basis for comparisonand olherrange, in whichcase a test linehas a subsequentselection. spreaderrunning along only one end. The latterdesign is preferableas it producesthe For effectivefield screeningfor same pest augmentationand distribution resistanceto MPB, the earlieroccurring effect and allows adequate room for pests,flower thrips in particular,must be movemenlbetween plots without stepping on controlled.This is achievedby sprayingthe lhe crop. Two lo threeweeks after planting plotswilh monocrotophosat 250 g a.i./haat the spreaderrows the test linesare planted 10-14day intervals. lt has been shown

11 that weekly sprays of monocrotophos(as Laboratory and screenhouse Nuvacron@ 4OEC)at a dosageof 500 g techniques. The use of laboratory a.i./ha produce or a MpB larvai population screenhouseprocedures for evaluatiohof similar to that of unsprayed 'higheiplots (Jackai resistanceto the MpB hinges on 1983) the and sometimeseven lttfn availability of a rearing facility/insect 1979). This treatmentalso controls'pod culturewhich can supplyadequate numbers sucking bugs which would also interiere of insectsat the desired stages and times. with pod evaluationsfor resislanceto the Unfortunately,most national programs are MlB. When large amountsof germplasmor not equipped with such a facility and are other materialare being evaluatedrating is therefore unable to carry out proper lim.ited to pod measurements,i.e. pod load laboratory and/or screenhouse resistance (PL) and pod damage (pD). These evalualions. Even at the international evaluations are made using a 1-9 scalewith centers, where insecl rearing facilities lhe degreesof acceptabilityat oppositeends exist (Jackai and Raulston 19-gg), pL pD these of the scate for and (T;6b S). For proceduresare still in the developmental example; a s@re closer to 9 is desirablefor phase (llTA 1985, 19g6, 19g7). lnsects PL while pD. one closerto 1 is desirablefor producedat llTA's rearingfacility have also These evaluationsare best made when the been used in the developmentof pods artificial are mature but still green, as damage infestationtechniques to supplementnatural at this stage is much easierto observethin field populations of MpB, but these when pods the are dry. Where,unknown to lechniques are undergoing further the invesligator beforehand, materials of refinements(llTA 1995). For greatly now, . varying maturity ate being screeningcowpea for resistanceto the MpB screened,rating shouldbe done when about by national programs will continue to 50"/" of the lines are mature and startingto dependon existingfield techniques. So far senesce. lt shouldbe notedthat for this, as these have proved satisfactory. in many other cases, it is preferableto screen germplasmor olher genotypesin Pod sucking bugs groups with similar maturity. The most importantmembers pL of lhe Malerials with scores of 5 or pod.suckingbugs (pSBs) are the cowpea better and PD scores of 5 or less are coreids, Clavigralla tomentosicol/is Stai. in selected for a second screeningin replicated west and centralAfrica, and C. elongata in 2-row plots with unprotected(i.;. with east and southernAfrica. These and other sprays of monocrotophos) and protected fSBs belong to the order Hemipteraand lreatments. When lhe number of entries descriptionsof their biologiesmay be found has been reduced to less than 50. more i1 the lirerarure(Singh and Jaikai 1985; detailedmeasurements are taken on flower Jackaiand Daoust1986). All pSBs cause infestation pL pD. in additionto and ln due damageto young developingpods by sucking course pod a evaluationindex and seed sap from the seeds. This often results in damage are also computedusing previously aborted or badly deformed seeds and described methods (ilTA t g-86; Jackai s.hrivelledpods. A heavy attack by pSBs 1982). This techniqueand its earlier during late floweringor early poddingcan versions(Singh 1980) have been used to causeexlensive flower and pod abscission. identify resistance in TVu 946 and Kamboinse local (Table1). More recently, The only known research on a numberof crossesusing these two lines lechniques for screening cowpea for have shown distinctpromise (llTA 1996). resistanceto this group of pests is that at Breeding lines are usually scored on a llTA. At leasthalf a dozendifferent methods progeny row basis afler whichsingle plants havebeen tried,both in the fieldand under are selected from each of lhe setecledrows contrclledconditions in screenhouseand for further testing. laboratr;ry.Few havestood the test of time.

iL Are podspres€nt on peduncles?

Are podsnormal? (i.e. not shrivelled)

Areseeds formed? (i.e. not aborted)

Areseeds only slightlydamaged? (score< 6 out of 9)

Retainfor furtherscreening

Figure1. Schematicrepresenlation of selection procedure for massscreening of cowpeasfor resistanceto podsucking bugs. (Plantsshould be sprayedwith deltamethrin 2.5 EC at 12.5g a.i-lhaevery 10 days, startingat flowerbud formation,until pod borerinfestation is minimal.)

13 productionas well as an assessmentof pod Field screening. Test lines are and seed damage, the latter usuallyin the planted in single or double row plots with a laboratory. This procedurecan be carried known susceptible and/or resistant check out on a choiceor no-choicebasis for each includedonce in every 10 or 20 entries. replication (cage). lt is, however, The plantingis usuallyslightly delayed in cumbersomeand time consuming,and it is order to expose the plants at the podding not surprisingthat it has not been widely stage to a high PSB populationpressure. adopted. Modificationsto it are presently The plantsare sprayedwith deltamethrinat beingundertaken at llTA. 12.5 g a.i./ha every 10 days, starting at flower bud formation,lo control thrips and Other screenhousetechniques are MPB. This treatment is terminatedat being developed. These requirerelatively poddingor afler two sprays unlessthe MpB simpleequipment - mesh-sleevecages or infestationcontinues to be high. During plastic petri dishes and rubber bands. certainyears at some locationswe have had However,as in the abovetechnique, a source to spray.up to four times, but wilhout any of insectsis a prerequisite.Simple rearing discernibleetfect on the PSB population.At proceduresfor PSBs using dry seed have mid- to late-podding,or at full maturityif been developedrecently (Jackai, in press) the PSB populalion is low, the trial is but conventionalmethods using fresh scoredfor damage based on a sequenceof cowpeaor pigeonpeapods are still useful. eliminativesteps (Fig. 1). Laboratorytechniques consisting of a Entries with a low score for seed set of simple bioassayshave also been damageare selectedfor furtherscreening in developed recently (L.E.N. Jackai replicatedtrials with two levels of pSB unpublishedwork; llTA 1988). Theseneed populationdensity, obtained by spraying to be adequatelytested before they can be with deltamethrinas earlier describedto recommendedfor "high" use by nationalresearch maintainthe status quo (a density) programs. and using half the recommendeddosage of endosulfan(i.e. 250-300 g a.i./ha) + The field screening procedure deltamethrin(i.e. 6.5 g a.i./ha)at twice the describedhere has identifiedlow levels of recommendedinterval (i.e. every 14 days) resistance (Table "low" in TVu 1 and TVu 1890 to obtain a density. 1).

Normallypod samplesare takenback Storage pests to the laboratorywhere seed damage is assessed. A resistanceindex (percentage Speciesof the genus Callosobruchus seed damage in test line divided by (Coleoptera:Bruchidae) are the most percenlageseed damage ,in check) is importantpests attacking stored cowpeas. calculated and used for resislance Its cosmopolitannature has made it one of classification and making seleclionsfor the most widely studiedinsect pests. Adult furtherevaluation. beetleslay eggs on pods (in the field)or on seeds (in storage)and larvaedevelop within Screenhouse evaluation. Test lines seedscausing exlensive damage. are plantedin pots and placed in screen cages(1.20 x 1.25 x 1.30m)at the onsetof Screeningcowpea for resistanceto pSBs flowering. Ten to twelve pairs of are bruchids is conventionallycarried out in introducedinto each cage with five plants laboratories. The method used at llTA and allowedto feed for two weeks. Flower (Singh1977) is an adaptationof the method and pod abscissionis closelymonitored and used at KansasState University(Nwanze and any eggsare removeddaily. At the end of the Horber 1975). Twenty to 40 seeds (an exposureperiod, counts are taken of pod equal number for all test lines) of each

14 accession(ca 13% moistureconlenl) are In screeningfor resistanceto placed in small plastic boxes (5 x 5 x 2 storagepesls, lfe brown (a susceptible cm). Two pairs of day-old adults arc line) is often used as a checkbut other introduced into each box and allowed to checks can be used. TVu 2027 was oviposittor 24 hours. To ensurethat there identifiedas havingmoderate levels of seed is no hidden infestationprior to carrying resistance (Singh 19771 from over out this procedurethe test materialshould 10,000accessions that were screened. be fumigated (e.9. with aluminum Sincethen this linehas beenused as the phosphide)for one or two days and then resistantcheck at llTA. Severalbreeding aerated for about the same period to get rid lineshave now been developed with almost of the fumigantbefore screening. Storage of identicallevels of resistanceas TVU 2027 seeds at extremely low temperaturesis (Singhet al. 1985)(Table 1). Theseare another method of eliminatingconcealed availableupon request. The maintenanceof infestalion. In this case screeningcan be a continuousculture of C. maculatus(or carriedout as soon as seedshave returnedto other bruchidspecies) is importantfor a room temperature. successfulscreening program for resistance to this insect. Appropriatemethods have Boxes with infested seeds are beendescribed (Singh and Jackai 1985). generaflyletf at 28"C and 70-80% relative hu m idity, but where rooms with Techniquesused for other cowpea temperatureand humiditycontrol are not pests that are consideredof minor available an ordinary laboratoryroom will importancecan be foundin the reviewby suffice. Five days afler infestationthe Singhand Jackai (1985). lt is hopedthat numberof eggs laid per seed lot is counted. in due coursespecific screening procedures By this time the eggs are easilyobserved will be developed for pests with and larvae may have already hatchedand restricted/localizedimportance such as moved into the seed. Startingfrom 25 days Apionvarius, meloid beetles, the seedmolh after infestationthe adultsthat emergeeach (Ofuya and Akingbohungbe1986) and day are countedand removed. This is done defoliatorssuch as Amsacta moloneyi for up to four weeks, or until no further (N'Doye1978). Whatwe haveattempted to emergence occurs on the susceptible do in fhispaper is lo summarizethe various control. At the end of this period the methodsavailable for use in screening percentageadult amergenceis determined cowpeasfor resistanceto insect pests. usingthe numberof eggs laid as an estimate Whenfield tests are usedfor screeningit is of the expected number of adults. A importantthat the prevailinginsect suitabilityindex (= growth index, Gl) is populationbe monitored(by sampling)and calculatedusing the formulaGl = log S/T, reportedalong with the resullsof the whereS = percentageadult emergence and T screeningtests. This is importantin = ffi€Bndevelopment time (Howe1971). On comparingthe performanceof resistant the basis of this, as well as the level of varieties under different ecological oviposition,an estimateof the resistance conditionsor in differentlocations. statusof the test materialsis made. Samplingprocedures used for most Pod-wallresistance has been less cowpeapests are identical to thosedescribed well studied. To screen for this factor, for closelyrelated pests on soybean(Kogan differentcontainers (e.9. paper bags) are and Herzog1980). Thosefor flowerthrips used to hold 6-12 pods which are infested havebeen reportedrecently by Salifuand with 3-6 pairs of adult bruchids tor 24 Singh (1987) and for MPB by Jackai (1982) Lawson(1987), [ours. After this the insectsare removed and Jackaiand and lhe samedata collected as for seeds. while methodsused for samplingPSBs appearin variousllTA AnnualReports (e.9. llTA 1983,1984, 1985, 1987) as well as

15 in Todd and Hezog (19S0)and Suh et al. Dahms, R.G. 1972. Techniqrrs in the evaluatlon (1986). The method for sampting and developmont of host plant resistance. Bruchidaedescribed by Alzouma (19S8) Journal of EnvironmentalQuality 1: 254- can be used to determinefield infestation 259. fevefsof bolh C. maculatusand Bruchidius atrolineatus.These methods are also useful Davies, F.M. 1985. Entomologicaltechniques for screeninginsecticides. Even though and methodologiesused in research programs most methodsused for samplingcowpea on plant resislance to insects. Insect Science pests requirefurther researchto improve and its Application6: 391-400. lheirprecision and reduceoost, as has been recentlydone for FTh (Salifuand Singh Don-Pedro,K.N. 1980. A populationexplosion 1987;Salifu and Hodgson1987), they are of Aphis craccivora Koch following DDT quite adequale for use in resistance appfication in a cowp€a plot (V ig n a screeningand pest monitoring. unguiculata) cultivar (Prima) in Nigeria. Journalof NaturalHistory 14: 117-119. No research method is beyond improvemenl.Consequently, the various Gould, F. 1983. Genetics of plant-herbivore methodsdescribed here are constantlyunder systems: interactionsbetween applied and review. The authorswould be glad to basic sludy. Pages 599-653 in Variable receivesuggestions for improvementsor plants and herbivoresin natural and managed new proceduresfor host plant resislance systems, ediled by R.F. Denno and M.S. screening. McOlure.Academic Press, N.Y.

Heinrichs,E.A., F.G. Medrano and H.R.Rapusas. REFEREilCES 1985. Genetic evaluation for insect resistance in rice. lntgrnational Rice Alzouma, l. 1988. Cycle biologique, Researchlnstitute, Los Banos,Philipines. reproductionel developpementde Bruchidius atrolineatus Pis (ColeoptereBruchidae) en Horber, E. 1980. Types and classificationof zone Sahelienne. lmportance de leur resistance. Pages 15-21 in Breeding plants connaisancepour un controleefficace de ce resislantto insecls, edited by F.G. Maxwell ravageurdues graines de Vigna unguiculataL. and P.R.Jennings. John Wiley and Sons,N.Y. Walp. Paper presentedat the RegionalGrain Legume Workshop organized 6y lFS, 22-27 Howe, R.W. 1973. A parameterfor expressing February1988, Tannanarive,Madagascar. suitability of an environment for insect developmenl. Journal of Slored Product Beck, S.D. 1965. Resistanceof plants to Research7: 63-64. insects. Annual Review of Entomology10: 207-232. llTA (lnternational Institute of Tropical Agriculture).1979. Annual report for 1978. Chiang, H.S. and N.S. Talekar. 1980. lbadan,Nigeria. ldentification of sources of resistance to beanfly and two other agromyzid flies in llTA (lnternational Institut€ of Tropical soybeanand mungbean. Journal of Economic Agriculture).1982. Annual report for 1981. Entomology12: 260-265. lbadan,Nigeria.

Dabrowski,2.T., D.O.M. Bungu and R.S.Ochieng. llTA (lnternational Institute of Tropical 1983. Studies on the legume pod borer, Agriculture).1983. Annual report for 1982. Maruca testulalis (Geyer). 3. Methods used lbadan, Nigeria. in cowpea screeningfor resistance. Insect Scienceand its Application4: 141-145.

16 llTA (lnternational lnstitute of Tropical Jackai,L.E.N. and T.L. Lawson. 1987. Insect Agriculture).1984. Annual report for 1983. p€st surveys on cowp€a (Vigna unguiculata, lbadan,Nigeria. 218 pp. Walp.) and lhe possible effects of some climatic factors on the populationtrends of llTA (lnternational Institute of Tropical lhe legumepod borer and pod suckingbugs. Agriculture).1985. Annual rsport for 1984. Pages 245-256 in Proceedings, lbadan,Nigeria. Agrometeorologyand Crop Protectionin the LowlandHumid and Sub-humidTropics. WMO, llTA (lnternational Inslilute of Tropical Geneva. Agriculture). 1986. Grain Legume lmprovement Program annual report for Jackai, L.E.N. and J.R. Raulston. 1988. 1985. lbadan,Nigeria. 303 pp. Rearing the legume pod borer, Maruca testulalis Geyer (Lepidoptera: Pyralidae) on llTA (lnternational Institute of Tropical artificialdiet. TropicalPest Management34: Agriculture).1987. Annual rsport and 168-172. research highlights for 1986. lbadan, Nigeria. Jackai,L.E.N., J.M.F. Roberts and S.R. Singh. 1988. Cowpea seed treatment with llTA (lnternational lnstitute of Tropical carbosulfan: potentialfor control of seedling Agriculture). 1988. Annual report and p€sts. Crop Proteclion7: 384-390. research highlights 1987/88. lbadan, Nigeria. 161 pp. Kogan,M. and D.C. Herzog. 1980. Sampling methodsin soybeanentomology. Springer- Jackai, L.E.N. 1982. A field screening Verlag, New York. 487 pp. technique for resistance of cowpea (Vigna unguic,ulata) lo the pod borer, Maruca Kogan,M. and E.E.Ortman. 1978. Antixenosis testulalis (Geyer) (Lepidoptera: Pyralidae). - a new term proposedto replacePainter's Bulfetinof EntomologicalResearch 72: 145- "non-preference"modality of resistance. 156. EntomologicalSociety of AmericaBulletin 24.

Jackai L.E.N. 1983. Efficacy of insecticide Litsinger,J.A., C.B.Quirino, M.D. Lumabanand applicationat different times of day against J.P. Bandong. 1977. Grain legume pest the fegume pod borer, Maruca testulalis complex of three Philippine rice-based (Geyer)(Lepidoptera: Pyralidae),on cowpea cropping systems. Cropping Syslems in Nigeria. ProtectionEcology 5: 245-251. Program, lnternational Rice Research lnslitute,Los Banos,Philippines. 39 pp. Jackai,L.E.N. Laboratoryscreening of cowpeas for resislance to the cowpea coreid, Little,T.M. 1985. Analysisof percentageand Clavigralla tomentosicorlis Stal. (Hemiptera: rating scale data. Hortscience20: 642-644. Coreidae). Journal of EconomicEntomology (in press). Little,T.M.'and F.J. Hills. 1977. Agricultural experimentation- designand analysis. John Jackai,L.E.N. and S.R. Singh. 1983. Varietal Wileyand Sons,N.Y. 350 pp. resistancein integratedpest managementof cowpea (Vigna unguiculata) pests. Insect Matterson, P.C. 1982. The effects of Scienceand its Application4: 199-2O4. intercroppingwith cersal and minimal permethrinapplication on insect pests of Jackai,L.E.N. and R.A. Daoust. 1986. Insecl cowp€asand their naturalenemies in Nigeria. pssts of cowpeas. Annual Review of TropicalPest Management28: 373:380. 'E4tomology31 : 95-119.

17 Maxwell, F.G. and P.R. Jennings. 1980. Singh, S.R. '1980. Biologyof cowpea p€sts and Breeding planls resistant to insects. John potential for host plant resistance. Pages Wiley and Sons, N.Y. 683 pp. 398-421 in Biology and breeding for resistance to and pathogens in N'Doye,M. 1978. Donn6s nouvellessur la agriculturalplants, edited by M.K. Harris. biologieet l'ecologieau Senegalde la chinelle BulletinMP-1451, Texas A & M Univ. poilue du niebe, Amsacta moloneyi Drc. (Lepidoptera:Arctiidae). CahierORSTOM, Singh S.R. and L.E.N.Jackai. 1985. Insect S6rieBiologie 13: 321-331. pests of cowp€as in Africa: their life cycle, sconomic importance and potential for Nwanzs,K.F. and E. Horber. 1975. Laboratory controf. Pages 217-231 in Cowpea rssearch techniques for screening cowpea for productionand ulilization,edited by S.R. resistancelo Callosobruchus maculatus F. Singhand K.O. Rachie. John Wileyand Sons, EnvironmentalEntomology 4: 415-419. N.Y.

Ofuya,T.l. and A.E. Akingbohungbe.1986. Singh,S.R. and H.F.van Emden. 1979. Insect Aspects of varietal resislance in cowpea to pests of grain legumes. Annual Review of the black cowp€a moth, Cydia ptychora Entomology24: 255-278. (Meyrick)(Lepidoptera: Totricidae). lnsecl Scienceand its Application7: 777-78O. Suh,J.B., L.E.N. Jackai and W.N.O.Hammond. 1986. Observationson pod sucking bug Painter,R.H. 1951. Insectresistance in crop populationson cowpea al Mokwa, Nigeria. plants. McMillan,N.Y. TropicalGrain Legume Bulletin 33: 17-19.

Raman,K.V., S.R. Singh and H.F. van Emden. Starks, K.J. and R.L. Burton. 1977. 1978. Yield losses in cowpea flowering Greenbugs:determining biolypes, culturing leafhopperdamage. Journal of Economic and screeningfor plant resistancewith notes Entomology71: 936-938. on rearingparasitoids. USDA-ARSTechnical Bulletin1556. Salifu,A.B. and S.R. Singh. 1987. Evaluation of sampling mothods tor Megalurothrips Taylor,T.A. 1964. The field pest problemson sjostedti (Trybom) (Thysanoptera:Thripidae) cowpeas, Vigna sinensis L. in southern on cowpea. Bulletin of Entomological Nigeria. NigeriaGrower and Producer3: 1- Research77: 451-456. 4.

Salifu,A.B. and C.J. Hogson. 1987. Dispersion Todd,J.W. and D.C. Herzog. 1980. Sampling patternsand sequentialsampling plans for phytophagouspentatomidae. Pages 438-478 Megalurothrips sjostedti (Trybom) in Samplingmethods in soybeanontomology, (Thysanoptera:Thripidae) in cowpeas. edited by M. Kogan and D.C. Herzog. Bulletinof EnlomologicalResearch 77'. 441- Springer-Verlag,N.Y. 487 pp. 449. Wiseman,B.R. 1985. Typesand mechanismsof Singh,8.8., S.R. Singh and O. Ajadi. 1985. host plant resistanceto insect attack. Insect Bruchidresistance in cowpea. Crop Science Science and its Application6: 239-242. 25'. 736-739.

Singh,S.R. 1977. Cowpeacultivars resistant to insect pests in world germplasmcollection. TropicalGrain LegumeBulletin 9: 1-7.

18