Seed coat has no value in protecting cowpea seed against attack by Callosobruchus maculatus (F.)

Experiments were conducted to compare the oviposition preference of the cowpea seed beetle Callosobruchus maculatus (F.) on 22 cowpea varieties, with and without seed coat. The cowpeas included five resistant, four moderately resistant and 13 susceptible varieties. Ten of the varieties had smooth seed coats while 12 were wrinkled. Mean numbers of eggs laid on smooth and wrinkled varieties were not significantly different. The survival of the bruchid was also assessed on intact and decorticated seeds of five varieties, comprising two resistant varieties “Kanannado” and “IT89KD-288”, a moderately resistant variety “IT93K-513-2” and two susceptible varieties “IT87K-941-1” and “IT89KD-374-57”. There was no correlation of resistance level with the number of eggs oviposited on cowpea varieties. Under limited and free-choice conditions, cowpea seeds with intact seed coats were preferred to decorticated seeds for oviposition. Adult emergence, mean development periods and pre-adult mortality were similar when seed coats were removed or left intact. It is concluded that the seed coat may not be a useful aspect to consider for breeding of bruchid resistance into cowpea varieties.     

Seed coat tannins and bruchid resistance in stored cowpea seeds

Seeds of wild species and varieties of Vigna were screened for their tannins and α‐amylase inhibitor contents as defensive compounds against cowpea weevil. Seed coats contained condensed tannins that were positively correlated to their colour but not to their resistance against the insect. The α‐amylase inhibitors were present in different amount in cotyledons of all species analysed. Amongst the cultivated lines assayed, Vigna unguiculata TVu 2027, an accession identified as moderately resistant, was found to contain the higher amount of α‐amylase inhibitor. When wild species were considered, V luteola and V vexillata (two resistant species) showed the highest content of α‐amylase inhibitors. In addition, two cultivated accessions (Vita 7 and IT 84E‐1‐108) of cowpea seeds, both classified as susceptible accessions, showing a different degree of bruchid damage in storage, were also analysed. No α‐amylase inhibitory activity was found in cotyledons of undamaged Vita 7 seeds, while the seed coat tannin content was found to be 13 times higher in undamaged Vita 7 seeds than in IT 84E‐1‐108 infested seeds. These latter results support the hypothesis that seed coat tannins must also be considered in biochemical defence mechanisms, which can deter, poison or starve bruchid larvae that feed on cowpea seeds. Copyright © 2004 Society of Chemical Industry     

Resistance of landrace and commercial cowpea genotypes to Callosobruchus maculatus attack

In the northern and north-eastern regions of Brazil, the cultivation of cowpea (Vigna unguiculata L.) is socially and economically important. However, attack by weevils makes storage difficult. Landrace genotypes exhibit high genetic variability, which can confer resistance to pest attacks. In this study, the susceptibility of landrace genotypes of cowpea to the attack of the weevil (Callosobruchus maculatus (Fabr., 1775)) was assessed. A total of 13 genotypes were used (11 landraces and two commercial genotypes). Biological parameters (number of eggs; daily, accumulated, and final emergence; the instantaneous rate of population increase) of C. maculatus, and loss of cowpea seed mass were analysed. The treatments were composed of 30g of each genotype and 20 adult insects in an entirely randomised design with ten replicates. A principal component analysis and grouping analysis were performed due to the similarity between the genotypes, and three groups were described. Group 3 (‘BRS Pujante’, ‘Ligeiro’, and ‘Tardão’ genotypes) had the highest oviposition/number of eggs, daily emergence, number of emerging insects, and instantaneous rate of population increase. Group 2 (‘Azulão’, ‘Costela de Vaca’, and ‘Rabo de Tatu’ genotypes) showed intermediate values, and group 1 (‘Bala’, ‘Canapú’, ‘Corujinha’, ‘Manteiguinha’, ‘Moita’, ‘Seridó’, and ‘Sempre Verde’ genotypes) had the lowest values for all those variables. The commercial genotypes were grouped in groups 1 (‘Sempre Verde’) and 3 (‘BRS Pujante’) and showed similar behaviour to the landrace genotypes of those groups. This study demonstrated that the genotypes in group 3 are susceptible to the attack of C. maculatus, while those in group 1 showed antibiosis resistance. Utilization of insect-resistant genotypes can reduce cowpea infestation and synthetic insecticide application, leading to a positive economic impact.     

Genetic analysis of seed resistance to Callosobruchus chinensis and Callosobruchus maculatus in cowpea

Callosobruchus chinensis (azuki bean weevil) and Callosobruchus maculatus (cowpea weevil) are the most destructive pests that cause significant losses to cowpea (Vigna unguiculata) seeds during storage. Development of cultivar(s) resistance to bruchids is a major goal in cowpea breeding program. Cowpea accession “TVu 2027” has been identified as moderately resistant to C. maculatus. Genetic studies of the seed resistance in this accession using strains of C. maculatus from Africa and America have demonstrated that the resistance is controlled by one or two recessive genes. However, there is no reports on genetics of the resistance to C. chinensis and to strain(s) of C. maculatus from Asia. Therefore, the objective of this study was to investigate genetics of the resistance to C. chinensis and C. maculatus in TVu 2027. TVu 2027 (P₂) and PK2015VTN001 (P₁; susceptible cowpea) were used to develop six basic generations (populations), viz. P₁, P₂, F₁ (P₁ × P₂), F₂ (P₁ × P₂), BC₁P₁ (P₁ × F₁) and BC₁P₂ (P₂ × F₁). The population were evaluated for percentage of damaged seeds (PDS) and area under the disease progress stairs (AUDPS; indicating infestation severity) by C. chinensis and C. maculatus. The results showed that TVu 2027 was moderately resistant to both bruchid species. Broad-sense heritability for PDS and AUDPS was moderate, being 70% and 73% for C. chinensis, respectively, and 64% and 61% for C. maculatus, respectively. Number of genes controlling C. chinensis resistance and C. maculatus resistance was two genes and one gene, respectively. Generation mean analysis revealed that genes with additive effect and additive × dominance gene interaction are involved in the resistance to both bruchid species. Correlation analysis suggested that the genes controlling resistance to C. chinensis and those conferring resistance to C. maculatus are unlinked.     

Cowpea resistance and bioactivity of two diatomaceous earths against Callosobruchus maculatus (F.) (Coleoptera: Chrysomelidae) on three cowpea varieties

The bruchid, Callosobruchus maculatus (F.) is the most important pest of stored cowpea in tropical climates. Experiments were conducted to evaluate the susceptibility of three cowpea varieties and the bioactivity of SilicoSec® and FossilShield® against this insect pest under ambient laboratory conditions at the contents 0, 0.5, 1, 1.5 and 2 g/kg of cowpea. The Dobie susceptibility index for cowpea genotypes was estimated to compare their relative tolerance to this insect pest. Adult mortality was assessed after 1, 2, 4 and 6 days of exposure followed by the evaluation of the F₁ progeny. Population increase, percentage seed damage and weight loss were assessed after three months of storage. The results showed that the CRPS and TN5-78 varieties were highly susceptible to C. maculatus, while Lori variety was moderately susceptible to the weevil. SilicoSec® and FossilShield® caused a significant and increasing bruchid mortality according to the dose rates and periods of exposure. Malathion 5% caused complete mortality after one day exposure time. SilicoSec® was more effective than FossilShield® on the three varieties of cowpea. The greatest dose of SilicoSec® and FossilShield® (2 g/kg) caused 100% mortality on all three varieties; 4 days of exposure in the Lori variety was the shortest exposure time leading to complete mortality. Regarding LD₅₀ and LD₉₅ values, it was observed that C. maculatus adults are more susceptible to SilicoSec® than to FossilShield®.The two formulations of diatomaceous earth (1.5 g/kg) reduced the F₁ progeny by more than 70%, and produced less than 10% of seed damage and weight loss for the three varieties of cowpea. The varietal resistance and the use of diatomaceous earth against C. maculatus in storage could represent an alternative to Malagrain® for the protection of the cowpea.     

Comparative evaluation of six storage methods for postharvest preservation of cowpea grain

Several technologies reputedly minimize losses of stored cowpea grain to bruchid beetles on low resource farms in Africa. Side by side comparison of these different postharvest storage methods can provide the basis for deciding which performs best. We compared six different technologies for cowpea storage: (1) grain mixed with ash; (2) mixed with sand; (3) fumigated with phostoxin; (4) admixed with the stems and leaves of Boscia senegalensis (Pers) Lam ex Poir, a potential botanical insecticide; (5) disinfested using a solar heater, and; (6) hermetically sealed in triple-layer plastic bags. Sampling was done at thirty-day intervals over five months of storage. Counts were made of (i) adult emergence holes, (ii) dead larvae and (iii) surviving bruchid larvae and adults. Controls, which consisted of infested cowpea grain stored in cloth bags, were damaged extensively. Boscia senegalensis-treated grain suffered similar severe damage. All other treatments suppressed bruchid population increases as was evident from the much lower counts of emergence holes and lower numbers of surviving or dead insects.     

Genetics of bruchid (Callosobruchus maculatus Fab.) resistance in cowpea (Vigna unguiculata (L.) Walp.)

Bruchid resistance has been measured using a variety of techniques. Mean development period (MDP) and percentage adult emergence (PAE) are two of the more important traits measuring bruchid (Callosobruchus maculatus Fab.) resistance in cowpea. The objective of the study was to evaluate the genetic relationship between MDP and PAE in C. maculatus resistant cowpea. Seventy-two F₂ population plants were developed from the cross between C. maculatus resistant TVu-11953 (with delayed MDP and low PAE) and C. maculatus susceptible Ife Brown (with early MDP and high PAE) which were evaluated for resistance to cowpea C. maculatus. MDP at 45 days after infestation (DAI) and PAE at 60 DAI were taken as analysed measures for resistance to C. maculatus infestation. Data generated were subjected to chi-square analysis. Varied resistant response to C. maculatus infestation was observed among the F₂ generation plants but their responses showed inclination to the susceptible parent. A 15:1 ratio for susceptible to resistant genotypes was observed with MDP among the F₂ genotypes evaluated which is an indication of the presence of two recessive genes that control of MDP. The segregation of PAE also showed a 15:1 ratio goodness of fit (P > 0.05) for susceptible to resistant genotypes. Test of independence between MDP and PAE was not significant (χ² = 2.19, P = 0.19). This study revealed that the recessive genes controlling these two traits (MDP and PAE) were independently assorted and showed no linkage. This was also evident in the observed F₂ genotypes with C. maculatus resistance expressing either delayed MDP or low PAE and not both in the study.     

Death by desiccation: Effects of hermetic storage on cowpea bruchids

When cowpea grain is stored in airtight containers, destructive populations of the cowpea bruchid (Callosobruchus maculatus) don’t develop even though the grain put into the store is already infested with sufficient C. maculatus to destroy the entire store within a few months. The surprising effectiveness of hermetic storage for preserving grain against insect pests has long been linked with the depletion of oxygen in the hermetic container and with the parallel rise in carbon dioxide. With C. maculatus, low oxygen (hypoxia) leads to cessation of larval feeding activity, whereas elevated levels of carbon dioxide (hypercarbia) have little or no effect on feeding. Cessation of feeding arrests the growth of the insects, which don’t mature and don’t reproduce. As a result, population growth ceases and damaging infestations don’t develop. C. maculatus eggs, larvae, and pupae subjected to hypoxia eventually die after exposures of various duration. The cause of death is desiccation resulting from an inadequate supply of water. We demonstrate that blocking the supply of oxygen interdicts the main supply of water for C. maculatus. This leads to inactivity, cessation of population growth, desiccation and eventual death.     

Physical and biochemical bases of resistance in groundnut against bruchid,Caryedon serratus (Olivier)

A laboratory experiment was taken to study the “physical and biochemical basis of resistance in groundnut against bruchid, Caryedon serratus (Olivier)” at department of entomology, college of agriculture, svpua&t, Meerut during August 2018 to February 2019. Varietal screening of groundnut pods against bruchid were carried out with ten varieties and the performance of the varieties was assessed based on the physical and biochemical parameters. These varieties showed significant differences in the physical parameters selected for study such as pod length × width, pod shell thickness, test weight and inter granular space. Shell thickness contributed to resistance, while other parameters, pods size, test weight and inter granular space to susceptibility. Chemical parameters of those varieties showed significant differences for pod sugars, pod proteins, pod phenols and pod tannins. Pod sugars and pod proteins content had a positive effect on the development of the bruchid. While, pod tannins and pod phenols exerted a negative effect. Thus, resistance to groundnut bruchid is a multifaceted mechanism governed by both physical and chemical characters of pods.     

Efficacy of ozone for Callosobruchus maculatus and Callosobruchus chinensis control in cowpea seeds and its impact on seed quality

The insecticidal efficacy of ozone was evaluated against the adults of Callosobruchus maculatus (F.) and C. chinensis L. on stored cowpea seeds under laboratory conditions. Ozone was assessed at concentrations of 0.25, 0.5, 1.0, 1.5, and 2.0 g/m³. The effect of ozone treatments on the adult mortality after 1, 3, 5 and 7 days of treatment, progeny production after 45 days, cowpea seed weight loss, seed germination and chemical constituents of cowpea seeds were determined. Adult mortality of C. maculatus and C. chinensis was improved with the increase in ozone concentration. Thus, all tested concentrations caused complete adult mortality of C. chinensis after 7 days of treatment, while the concentrations of 1.0, 1.5, and 2.0 g/m³ caused complete adult mortality of C. maculatus after the same periods. Progeny of both species was significantly decreased in all ozone concentrations after 45 days of treatment. Nevertheless, strong suppression in progeny production was achieved at the highest concentration of ozone (2.0 g/m³). Also, the ozone treatment at the highest concentration protected the cowpea seeds from damage caused by C. maculatus and C. chinensis for 45 days. In addition, there was no significant effect of ozone treatments on the cowpea seed germination compared with untreated seeds. Moreover, the chemical analysis of treated cowpea seeds showed a slight decrease in protein, fat, carbohydrate, moisture, total phenolics, total flavonoids and tannins contents, and a slight increase in fiber and ash contents compared with untreated cowpea seeds. Our findings suggest the ozone can be effectively used for the control C. maculatus and C. chinensis and can provide sufficient protection of stored cowpea seeds.     

Effects of chemical- and bio-fertilizers on cowpea resistance to cowpea weevil,Callosobruchus maculatus (F.) (Coleoptera: Chrysomelidae)

The cowpea weevil, Callosobruchus maculatus (F.) (Coleoptera: Chrysomelidae) is a destructive pest of cowpea grains in the field and storage. Effects of two chemical fertilizers (triple superphosphate (TSP) and urea) and three bio-fertilizers (Bradyrhizobium japonicum, Pseudomonas putida and mycorrhizal fungi) compared to control were studied on resistance of cowpea pod and grain to C. maculatus. Pod resistance was evaluated as oviposition preference, egg-to-adult survival and developmental time, and grain resistance was measured as life history and life table parameters of the pest. Cowpea plants treated with tested fertilizers were grown under field condition. Then, the full-size green pods and their immature grains were harvested and used in the experiments. The highest oviposition preference and percentage of adults emergence were on control (untreated) pods as compared to treated ones. The longest developmental times were recorded on the pods obtained from TSP and B. japonicum treatments. In most cases, significant differences were observed for the life history and life table parameters of C. maculatus on the grains treated with examined fertilizers. Delayed developmental time was recorded when C. maculatus was fed with P. putida- and B. japonicum-treated grains. The highest and lowest number of eggs laid by each female (fecundity) was on the grains obtained from TSP and control treatments, respectively. The net reproductive rate (R₀), intrinsic rate of increase (r_m) and finite rate of increase (λ) were the lowest on cowpea grains fertilized with B. japonicum and highest on the grains treated with urea. Our results showed that B. japonicum can be suggested to utilize, as an alternative for chemical fertilizers, to minimize cowpea infestation by C. maculatus. These findings could be helpful in developing integrated management of the pest on cowpea.     

Performance of triple bagging hermetic technology for postharvest storage of cowpea grain in Niger

Triple bagging technology for protecting postharvest cowpea grain from losses to the bruchid, Callosobruchus maculatus Fabricius (Coleoptera: Chrysomelidae: Bruchinae) is currently being adopted on a fairly large scale in ten West and Central African countries, including Niger. The triple bag consists of two inner high-density polyethylene bags acting as oxygen barriers, which in turn are encased in an outer woven polypropylene bag that serves primarily for mechanical strength. These hermetic bags, available in either 50 or 100 kg capacity, are called Purdue Improved Cowpea Storage (PICS) bags. Adoption of PICS technology in West and Central Africa has been driven by its effectiveness, simplicity, low cost, durability, and manufacture within the region. From surveys on adoption we discovered that farmers have begun to re-use bags they had used the previous year or even the previous two years. In the present study, we compared the performance of three different types of PICS bags: (1) new 50 kg (2) new 100 kg bags and (3) once-used 50 kg bags, all filled with naturally infested untreated cowpeas. In these PICS bags the O₂ levels within the bags initially fell to about 3 percent (v/v) while the CO₂ rose to nearly 5 percent (v/v). After five months of storage, new and used 50 kg bags and new 100 kg bags preserved the grain equally well. There were greatly reduced numbers of adults and larvae in the PICS bags versus the controls, which consisted of grain stored in single layer woven bags. The proportion of grain having C. maculatus emergence holes after five months of storage in PICS bags was little changed from that found when the grain was first put into the bags. The PICS technology is practical and useful in Sahelian conditions and can contribute to improved farmers' incomes as well as increase availability of high quality, insecticide-free cowpea grain as food.     

Varietal effects of cowpea, Vigna unguiculata, on tolerance to malathion in Callosobruchus maculatus (Coleoptera: Bruchidae)

The impact of cowpea variety on the response of cowpea bruchid, Callosobruchus maculatus, to malathion was investigated. The interaction of six cowpea varieties (Adamawa Brown, Ife BPC, Ife Brown, Lilongwe, Ntcheu and NCRI-L25) with the geographical strains of C. maculatus (Brazil and Cameroon), temperature (23, 25, 27 °C) and insecticide concentration were considered. Cowpea variety (V) had an unpredictable effect on C. maculatus response to malathion. Bruchid populations produced by Ife BPC were the most susceptible to malathion while those yielded by NCRI-L25 were the most tolerant. Regardless of the cowpea variety, the Brazil strain showed higher tolerance than the Cameroon strain. There was significant effect of temperature (T) and insecticide concentration (C) on malathion tolerance in both strains (S). Likewise, there was significant impact of all two-way interactions on cowpea bruchid tolerance except V x C. Significant three-way interactions on C. maculatus tolerance to malathion was only observed in S × T × V and S × T × C. The predictability of changing one of the factors on the susceptibility of C. maculatus to insecticide was very low. This study suggests a need to take the insecticide tolerance of insect populations produced by novel varieties into account during plant breeding in addition to factors such as yield and resistance to insect and disease attack.     

Correlation between biophysical seed characteristics of rice bean,Vigna umbellata (Fabaceae: Faboideae: Phaseoleae) and the development of Callosobruchus maculatus (Coleoptera: Chrysomelidae: Bruchinae)

The bruchid, Callosobruchus maculatus is a serious pest of stored pulses. Four biophysical seed characteristics viz. Seed size, texture of seed surface, thickness of seed coat and seed hardness of 10 genotypes of rice bean, a wild pulse crop were investigated for assessing mechanism of resistance against C. maculatus. Oviposition preference and larval penetration in seed coat did not differ significantly among different test genotypes. Highest number of larvae which developed up to last larval/pupal instar were observed in susceptible check variety of mungbean, PAU 911 (30.00) and lowest in rice bean genotype, LRB 535 (0.83). Highest and lowest seed weight was noticed in rice bean genotype, LRB 529 (6.81 g) and PAU 911 (3.88 g), respectively. Almost similar seed coat thickness was recorded among different test genotypes. Maximum seed hardness was noted in genotype LRB 507 (6.45 kg) whereas minimum in PAU 911 (2.00 kg) among all test genotypes. Any remarkable correlation of biophysical seed characteristics of test genotypes with ovipositon, larval penetration and development of C. maculatus was not observed. Thus, biophysical seed characteristics of rice bean did not suggest to play a vital role in imparting resistance against C. maculatus.     

Changes in the response of Callosobruchus maculatus (Coleoptera: Bruchidae) to a resistant variety of cowpea

Three strains of Callosobruchus maculatus (F.), from Brazil, Nigeria and the Yemen Arab Republic, do not survive equally well or develop at the same rate in a resistant cultivar of cowpea, TVu 2027. First generation hybrids between them survive and develop at rates intermediate between those of their parents but generally nearer the level attained by the more successful or tolerant parental strain. The indication is therefore that resistance to antimetabolites in TVu 2027 in inherited as a dominant characteristic. Breeding of the Yemen and Nigerian strains on TVu 2027 for 3 generations demonstrated that the performance of the beetles rapidly improves in terms of both survival and developmental rate. It is concluded that the useful lifespan of TVu 2027 and its hybrid progeny, in respect of bruchid resistance, is probably very limited. The selection pressure imposed by its use would probably lead rather rapidly to the evolution of beetle populations capable of surviving in large numbers and spreading their resistant capacities through hybridization to other adjoining populations.     

Insecticidal potential of a synthetic zeolite against the cowpea weevil,Callosobruchus maculatus (Fabricius) (Coleoptera: Bruchidae)

The insecticidal potential of a synthetic zeolite was evaluated against the cowpea weevil, Callosobruchus maculatus (Fabricius). The synthetic zeolite was applied to concrete surfaces at 0, 2.5, 5, 10 and 20 g/m², and the mortality of adults was assessed after 1, 3, 6, 9, 12, 24, 36, 48, and 72 h at 28 °C and 65% r.h. The residual efficacy of zeolite applied at 5 g/m² was evaluated on concrete surfaces at 0, 1, 2, 3, and 4 months post-treatment. Adults of C. maculatus were exposed to cowpeas treated with zeolite at 0, 0.1, 0.5, 1, 2, 3, 4, and 5 g/kg. The number of eggs deposited on cowpea kernels and their hatchability were determined after 7 and 14 d, respectively. The cumulative adult mortality was recorded after 1, 2, 3, and 4 d, and adult progeny production after 42 d. The residual efficacy of zeolite on cowpeas was tested at 0, 1, 2 and 3 months after treatment. On concrete, zeolite applied at ≥ 5 g/m² resulted in 100% mortality of C. maculatus adults after 24–72 h of exposure. Zeolite at 5 g/m² exhibited high residual efficacy with 100% mortality of C. maculatus adults after 36 h exposure on treated surfaces for up to 4 months. The mortality of C. maculatus adults exposed to zeolite-treated cowpeas increased with increasing concentration and exposure time. The number of eggs laid by C. maculatus, number of kernels with eggs, and adult progeny production decreased with increasing zeolite concentration. Zeolite applied to cowpeas at 1 g/kg produced 100, 99 and 77% adult mortality at 1, 2, and 3 months post-treatment. Progeny production on zeolite-treated cowpeas was significantly lower than that on untreated cowpeas. These results show that the synthetic zeolite is effective on concrete surfaces and on cowpeas in controlling C. maculatus.     

Physico-chemical properties of the flours and starches of two cowpea varieties (Vigna unguiculata (L.) Walp)

The physical and chemical properties of the starches and flours of whole grain and decorticated two cowpea varieties (Vigna unguiculata (L.) Walp) were investigated. The two cowpea varieties were: C-152-White, having big grain and S-1552-White, having small grain with black eye. Results showed that starch yields were 19.2 and 16.4 g/100 g grain for C-152 and S-1552, respectively. Statistical analysis shows that the total amylose contents of the starch of the two cowpea varieties were significantly higher (P ≤ 0.05) than that of the whole grain and decorticated flours. The swelling power of C-152 starch was significantly higher than that of S-1552 starch. The water-binding capacity (WBC) for S-1552 starch was higher than for C-152 starch. The results of pasting profile of both the flour and the starch showed that the two cowpea varieties possess different properties in relation to gelatinization temperature(GT), peak viscosity (PV), hot paste viscosity (HPV), cold paste viscosity (CPV), break down (BD), set back (SB), total set back (SBt) and relative breakdown (BDr). The information generated in this study on the properties of the starch could provide guidance on possible industrial uses of starches of these two varieties.Industrial relevanceThis work showed the peculiar characteristics (solubility, water-binding capacity, amylose content, carbohydrate composition, granules characteristics and behaviour in Brabender viscoamlograph) of pure cowpea starch. The results that we obtained could be very valuable in decision making for industries that want to take advantage of cowpea starch as alternative or supplement to cereal/tuber starch. Cowpea starch could be useful in the manufacture of thickeners, gelling agent, extenders and texture modifiers in food formulation.     

Prebiotic effect of resistant starch from Vigna unguiculata (L.) Walp. (cowpea) using an in vitro simulated digestion model

There has been growing interest in the use of resistant starch (RS) as a prebiotic for physiological benefits and to reduce the risk of chronic diseases. In this study, RS was isolated from samples of five cowpea cultivars (DT129-4, TVU 11424, PAN 311, Fahari and Bechuana White) seeds to determine their potential prebiotic effects, and proximate and mineral composition. Results obtained showed samples of five cowpea cultivars to be high in protein (18.0–28.7%) and carbohydrates (54.9–66.0%) and low in fat (0.6–3.7%). The RS from cowpea samples ranged between 9.3 and 12.1% with cultivar DT129-4 having the highest amount. Fermentation of RS using mixed and pure cultures was done, and bacterial growth was monitored over a 24-h period. Resistant starch isolated from all cowpea samples fulfilled the criteria to be classified as a prebiotic which have a wide range of beneficial effects on the host, with cultivar DT129-4 showing the most prebiotic potential amongst cowpea samples tested.     

Comparative performance of five hermetic bag brands during on-farm smallholder cowpea (Vigna unguiculata L.Walp) storage

Cowpea (Vigna unguiculata L. Walp) grain is an important source of protein for smallholder farmers in developing countries. However, cowpea grain is highly susceptible to bruchid attack, resulting in high quantitative and qualitative postharvest losses (PHLs). We evaluated the performance of five different hermetic bag brands for cowpea grain storage in two contrasting agro-ecological zones of Zimbabwe (Guruve and Mbire districts) for an 8-month storage period during the 2017/18 and 2018/19 storage seasons. The hermetic bag treatments evaluated included: GrainPro Super Grain bags (SGB) IVR™; PICS bags; AgroZ® Ordinary bags; AgroZ® Plus bags; ZeroFly® hermetic bags. These were compared to untreated grain in a polypropylene bag (negative control) and Actellic Gold Dust® (positive chemical control). All treatments were housed in farmers’ stores and were subjected to natural insect infestation. Hermetic bag treatments were significantly superior (p < 0.001) to non-hermetic storage in limiting grain damage, weight loss and insect population development during storage. However, rodent control is recommended, as rodent attack rendered some hermetic bags less effective. Actellic Gold Dust® was as effective as the hermetic bags. Callosobruchus rhodesianus (Pic.) populations increased within eight weeks of storage commencement, causing high damage and losses in both quality and quantity, with highest losses recorded in the untreated control. Cowpea grain stored in Mbire district sustained significantly higher insect population and damage than Guruve district which is ascribed to differences in environmental conditions. The parasitic wasp, Dinarmus basalis (Rondani) was suppressed by Actellic Gold Dust® and all hermetic treatments. All the hermetic bag brands tested are recommended for smallholder farmer use in reducing PHLs while enhancing environmental and worker safety, and food and nutrition security.     

Biochemical basis of insect resistance in Vigna unguiculata

The bruchid beetle Callosobruchus maculatus (F.) causes extensive damage to seeds of the cowpea, Vigna unguiculata (L.) (Walp.), when this important tropical foodstuff is stored. A variety of cowpea resistant to attack by this pest has been described. In the present work seeds of a number of cowpea varieties, including the resistant one, were tested for the presence of a physical resistance to C. maculatus, in terms of repulsion of oviposition or of failure of larvae to enter the seeds. No evidence to suggest the presence of a physical resistance was found. When seeds of cowpea varieties were tested for the presence of various antimetabolic secondary compounds, only inhibitory activity against trypsin and, to a much lesser extent, chymotrypsin, could be detected. The resistant variety of cowpea contained a significantly higher level of inhibitors, about twice as much as any other variety. A proteinase inhibitor active against trypsin was purified from cowpea varieties by affinity chromatography on trypsin-Sepharose. The purified inhibitor was shown to inhibit chyraotrypsin also, in such proportions as to account for chymotrypsin inhibition by seed extracts. The inhibitor was shown to consist of a number of isoinhibitors by gel electrophoresis and isoelectric focusing, but no qualitative differences in the inhibitor between varieties could be detected. The antimetabolic nature of the cowpea trypsin inhibitor was confirmed by insect feeding trials in which various protein fractions were added to a basic meal and the effect on larval survival noted. The albumin proteins of cowpea (containing the trypsin inhibitors) at a level of 10% were toxic to larvae of C. maculatus whereas the globulin fractions were not. Further, if cowpea trypsin inhibitor was removed from the albumin proteins they ceased to be toxic. When purified cowpea trypsin inhibitor was added to the basic meal it was shown that a level slightly less than that found in the resistant variety of cowpea caused complete mortality of larvae, whereas lower levels had lesser or no effect. It is concluded that this example of insect resistance in the cowpea is due to an elevated level of trypsin inhibitor.