The impact of weevil predation on seed viability and agronomic performance of cowpeas, Vigna unguiculata L. (Walp)

Thesis (MScAgric)--Stellenbosch University, 2023.

ENGLISH ABSTRACT: Cowpea, Vigna unguiculata, is a nutritious and high value legume that is often susceptible to weevil damage in storage. It is largely grown in Sub-Saharan Africa and a primary source of income for resource-poor farmers. Callosobruchus maculatus is an invasive pest of cowpea seeds in storage, better known as the cowpea weevil. In the larval stage, cowpea weevils consume food reserves from the seed endosperm. Unsuitable storage structures, where no methods of weevil control are applied, may increase weevil infestation of cowpea seeds in storage which hampers successful cowpea production. While attention is dedicated to understanding the role of generic cowpea seed damage to the legume’s production, there is a dearth of evidence on the impacts of the damage to different sites of the seeds. This study therefore aimed to quantify the germination rate of cowpea seeds damaged on different sites and assess the effect of the damage on cowpea seedling growth and yield. Undamaged cowpea seeds, also known as “black-eye beans”, were infested with cowpea weevils, then kept in storage for three months. The stored seeds were damaged on the top, bottom, side, back or hilum. Seeds with a single hole were selected from the infested seed and sorted according to the site of damage on the seeds. The positions of holes on seeds were used as treatments for the experiments and undamaged seeds served as the control group. Major findings were seed germination at day two showed a significantly poorer germination of top damaged seeds, relative to both undamaged seeds and the alternative damage sites. This result remained consistent for germination at day seven as well as for seedling height at two weeks. Seeds with a single site of damage at the top of the seed performed significantly worse than seeds where the damage was on other sites. At physiological maturity, plants grown from undamaged seed had the highest 100-seed weight. The seed weight obtained from seeds damaged at the top was significantly less than those obtained from plants grown from undamaged seed. These results suggest that farmers, to ensure seedling emergence, should refrain from planting cowpea damaged at the top, but they can still grow seeds with alternate holes. The results of the study will help farmers manage seed losses after storage, as some damaged seed can still be sown and will produce a yield. This is the first study of its kind, and it gives insight into the site of damage on seed and cowpea production. The findings could encourage further investigation into other legume crops that also suffer from weevil damage. In addition, conclusions from the study may be of practical value for smallholder farmers to support nutrition and food security in Africa.

AFRIKAANSE OPSOMMING: Swartbekboontjie, Vigna unguiculata, is 'n voedsame en hoëwaarde peulgewas wat vatbaar vir kalanderskade in opberging is. Dit word grootliks in Afrika, suid van die Sahara, verbou waar die gewas ‘n primêre bron van voedsel en inkomste vir kleinboere boere is. Callosobruchus maculatus, beter bekend as die swarbekboontjie-kalander, is 'n indringerplaag van swartbekboontjie-sade in opberging. In die larfstadium verbruik swartbekboontjie-kalanders voedselreserwes van die saad-endosperm. Ongeskikte opbergingstrukture, waar geen metodes van kalanderbeheer toegepas word nie, kan kalanderbesmetting van swartbekboontjies in die stoor tydperk verhoog, wat suksesvolle swartbekboontjie-produksie in die opvolgende seisoen kan belemmer. Terwyl aandag aan die begrip van die rol van generiese swartbekboontjie saadskade aan die peulgewasproduksie gewy word, is daar 'n tekort van bewyse oor die impak van die spesifiek geposisioneerde skade aan die sade. Hierdie studie het dus ten doel gehad om die ontkiemingstempo van swartbekboontjie-sade wat op verskillende plekke op die saad beskadig is, te kwantifiseer en die effek van die enkel-punt skade op swartbekboontjie saailinggroei en opbrengs te bepaal. Onbeskadigde swartbekboontjie saad, ook is besmet met swartbekboontjie-kalanders en dan vir drie maande in die stoor gehou. Die gestoorde sade is aan die bokant, onderkant, kant, rug of hilum beskadig. Saad met slegs 'n enkele gaatjie is uit die besmette saad gekies en gesorteer volgens die plek van skade aan die sade. Die posisies van gaatjies op sade is gebruik as behandelings vir die eksperimente en onbeskadigde sade het as die kontrolegroep gedien. Belangrike bevindings was dat saadontkieming op dag twee beduidend swakker was vir sade met skade aan die bokant, relatief tot beide onbeskadigde sade en die alternatiewe plekke van skade. Dié resultate was konsekwent vir persentasie saadontkieming op dag sewe en saailinghoogte op twee weke. Saad met 'n enkele plek van skade aan die bokant van die saad het aansienlik slegter gegroei as sade waar die skade op ander plekke was. By fisiologiese volwassenheid het plante wat uit onbeskadigde saad gekweek is, die hoogste gewig van 100-saadgewig getoon. Die saadgewig wat verkry is van sade wat aan die bokant beskadig is, was aansienlik minder as dié wat verkry is van plante wat van onbeskadigde saad gekweek is. Hierdie resultate dui daarop dat boere, om saailingopkoms te verseker, hulle daarvan moet weerhou om swartbekboontjie te plant wat aan die bokant beskadig is, maar hulle kan steeds sade met alternatiewe gaatjies verbou. Die resultate van die studie sal boere help om saadverliese na opberging te bestuur, aangesien sommige beskadigde saad steeds gesaai kan word en 'n opbrengs sal lewer. Dit is die eerste studie van sy soort, en dit gee insig in die plek van skade op saad and swartbekboontjie-produksie. Die bevindings kan verdere ondersoek na ander peulgewasgewasse aanmoedig wat ook aan kalanderskade ly. Daarbenewens kan gevolgtrekkings van die studie van praktiese waarde wees vir kleinboere om voeding en voedselsekerheid in Afrika te ondersteun.

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