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 insect 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