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.     

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.     

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.     

The influence of geographic origin and food type on the susceptibility of Callosobruchus maculatus (Fabricius) to Piper guineense (Schum and Thonn)

The tolerance of Callosobruchus maculatus from different geographical locations reared on two cowpea varieties, pale brown Ife Brown (IFBV) and dark brown IAR48 (IAR48V), to seed powder of Piper guineense (Schum and Thonn) was investigated. C. maculatus populations were collected from nine different locations across Osun state in the South Western part of Nigeria. The main and interactive effects of cowpea variety, population origin and dose on C. maculatus tolerance to P. guineense were explored. It was observed that bruchids that emerged from IAR48V had greater tolerance of P. guineense than bruchids reared on IFBV. There were significant effects (P < 0.001) of cowpea variety, population and dose, and significant interactions among these factors (except variety × dose, P > 0.05) on the response of bruchids to P. guineense. When reared on IAR48V, bruchid populations from the North-Eastern part of the state show greater tolerance to P. guineense than their counterparts from the South–West. This study underscores the importance of knowledge of the origin of the population and the cowpea variety on which C. maculatus developed when managing bruchids damage using P. guineense.     

Legume type and temperature effects on the toxicity of insecticide to the genus Callosobruchus (Coleoptera: Bruchidae)

Three bruchid pest species, Callosobruchus maculatus, Callosobruchus chinensis and Callosobruchus rhodesianus, were studied for their response to insecticide toxicity taking into account the separate and interactive effects of temperature and pre-adult food. The food types used were cowpea (Vigna unguiculata) and mungbean (Vigna radiata). Callosobruchus maculatus was the most tolerant to malathion and the least affected by temperature change while C. rhodesianus was the least tolerant. Over a 4 °C range (23°, 25°, 27 °C), there was generally a significant impact of temperature on the tolerance of the three species to the insecticide. The food type on which the insects developed influenced considerably the degree of insecticide tolerance. Callosobruchus maculatus and C. chinensis populations reared on mungbean had higher tolerance to malathion than their counterparts reared on cowpea, but the opposite was observed in C. rhodesianus populations. The food influence in this study suggested an ancestral cause or fitness cost depending on the species. The interaction of food-by-temperature had no significant effect on malathion toxicity to this genus. Correlation analysis showed C. chinensis to be relatively less sensitive to insecticide concentration over the range studied compared with the other two species.     

Effect of different legume seeds on life table parameters of cowpea weevil,Callosobruchus maculatus (F.) (Coleoptera: Chrysomelidae)

The cowpea weevil, Callosobruchus maculatus (F.) (Coleoptera: Chrysomelidae) is one of the most important storage pests of legume seeds. The effect of various legume species including chickpea (Cicer arietinum L.) (cultivars Hashem and Mansour), cowpea (Vigna unguiculata L.) (cultivars Mashhad and 1057), green gram (Vigna radiata L.) (cultivar Parto), lentil (Lens culinaris Medikus) (cultivar Bilehsavar) was studied on the life history and life table parameters of C. maculatus at 30 ± 1 °C, relative humidity of 65 ± 5% and complete darkness. The developmental time was longest on lentil and shortest on cowpea 1057. The fecundity (number of eggs laid per reproductive period) of the pest was the lowest on lentil. The longest oviposition period was observed on chickpea Hashem. The gross and net reproductive rates were the highest on chickpea Mansour and lowest on lentil. The intrinsic rate of increase and finite rate of increase were the lowest when C. maculatus was reared on lentil and highest when it was reared on other hosts. The longest and shortest mean generation times were observed on lentil and cowpea 1057, respectively. According to the obtained results, lentil was relatively resistant and the other tested legumes were more susceptible hosts for feeding and population increase of C. maculatus. It is concluded that the resistant host cultivar can be proposed to be incorporated into breeding programs to minimize the economic losses incurred by C. maculatus.     

Temperature-dependent development and reproduction of the cowpea weevil,Callosobruchus maculatus F., in mungbean: Estimating a target temperature for its control using aeration cooling

The bruchid, Callosobruchus maculatus F., commonly known as the cowpea weevil, infests stored mungbean and other legumes. Aeration cooling has potential as a non-chemical means of managing this species in stored legumes. Population growth of C. maculatus in mungbean was investigated at nine constant temperatures (15, 17.5, 20, 22.5, 25, 27.5, 30, 32.5 and 35 °C) at 60% RH so that a target temperature for cooling could be estimated. We used two laboratory strains: Strain 1 and Strain 2 that had been in culture for 16–17 years and 1–2 years respectively. The results for the two strains were very similar. Egg to adult development occurred between 20 and 35 °C for Strain 1 and 17.5 and 35 °C for Strain 2. The optimal temperature for population growth was estimated to be 32.2 and 33.7 °C for Strains 1 and 2, respectively. The estimated lower threshold for population growth, i.e. the temperature at which population growth is zero, was 17.5 °C for Strain 1 compared with 17.1 °C for Strain 2. Based on our results, we recommend a target temperature of 17 °C for aeration cooling to manage C. maculatus infestations in mungbean during storage.     

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.     

Characterization of host response to bruchids (Callosobruchus chinensis and C. maculatus) in 39 genotypes belongs to 12 Cajanus spp. And assessment of molecular diversity inter se

Thirty nine genotypes belong to 12 Cajanus species were characterized for host response to bruchids (C. chinensis and C. maculatus) using no-choice based in vivo and in vitro antibiosis assay, and these assays revealed seven genotypes of primary gene pool (C. cajan) were more susceptible to bruchid infestation as compared to the genotypes of secondary and tertiary gene pools. Among all the genotypes, C. cajanifolius acc. ICPW-31 showed higher degree of resistance to both the bruchids. The molecular diversity using CDDP and SCoT markers showed wide range of genetic variations among the 12 species of Cajanus, and was supported by estimates of Nei's genetic distance and fixation index (F_ST) based haplotype matrix. The genetic structure showed clustering of 39 genotypes into eight distinct groups (K = 8) on the basis of their allelic composition, and among them C. cajanifolius acc. ICPW 31 and ICPW 30 showed close affinity with the cultivars of C. cajan by sharing several alleles. AMOVA analyses showed the existence of higher extent of genetic variation both at the genotype (37%) and species (63%) level in the genus Cajanus. The dendrogram and the principal coordinate analysis (PCoA) placed the 39 genotypes into six major clusters at par with their sectional classification. MCheza and ARLEQUIN based outlier analysis revealed 13 common loci under balancing selection, which are supposed to be involved in maintenance of genetic polymorphism either at species or genotype level, and are assigned to putative gene families such as KNOX, WRKY, ERF, MYB, ABP1 and MADS. The in vivo and in vitro bruchid assay vis-à-vis the DNA marker based molecular diversity analysis affirmed the possible use of C. cajanifolius acc. ICPW-31 as donor genotype for the introgression of bruchid resistance allele(s) into cultivated genetic background.     

Semiochemical modulation of host preference of Callosobruchus maculatus on legume seeds

Host preference of Callosobruchus maculatus (F.) on seeds of three legume cultivars, Ife-brown and black-eyed cowpeas [Vigna unguiculata L. (Walp)], and soybean (Glycine max L.), was investigated. Mated female C. maculatus showed high (90–95%) attraction to the three legume cultivars in Y-tube bioassays. However, the weevils discriminated among the cultivars in four-choice tests and showed greater attraction to Ife-brown cowpea (50%) than to soybean (30%) and black-eyed cowpea (15%). Coupled gas chromatography-electroantennography (GC-EAD) and GC–MS analyses of the headspace volatile organic compounds (VOCs) emitted by the legume seeds identified 2-ethyl hexanol as the principal EAD active component. Emission of 2-ethyl hexanol was two-fold greater in Ife-brown cowpea (∼0.54 μg g⁻¹ seeds) compared with black-eyed cowpea (∼0.23 μg g⁻¹ seeds) and soybean (∼0.21 μg g⁻¹ seeds). Synthetic 2-ethyl hexanol attracted 68% of female C. maculatus at 0.01 μg dose in Y-tube bioassays. These results demonstrated that host preference in C. maculatus is odor-mediated, and identified 2-ethyl hexanol as a potential attractant for C. maculatus.     

Effect of monoterpenoids on oviposition and mortality of Callosobruchus maculatus (F.) (Coleoptera: Bruchidae) under hermetic conditions

Monoterpeniods have been shown to cause mortality in certain stored-product insect pests. The current report investigated the prospects of using monoterpenoids as oviposition deterrents of the cowpea beetle, Callosobruchus maculatus (Fabricius), as well as in the management of populations of the beetle. The monoterpenoids investigated include E-anethole, estragole, S-carvone, linalool, L-fenchone, geraniol, γ-terpinene and DL-camphor, and at the concentrations of 66.7, 33.3, 16.7, 8.33 and 0 μL/L. Exposure of the life stages of the beetle, which included eggs, young larvae (first instar), 4th instar, pupae and adults to different concentrations of the monoterpenoids over 24 h period caused varying levels of mortality. The stages of the beetle that were the least susceptible to the monoterpenoids were the 4th instar, and the pupae, which required high concentrations of the monoterpenoids to achieve 99% mortality. The adults and the eggs exhibited the highest susceptibility to the monoterpenoids. Mated C. maculatus females that were offered cowpea seeds upon treatment with low doses (8.33 μL/L) of the monoterpenoids did not lay eggs, while control female beetles offered untreated seeds laid several eggs. However, mated C. maculatus females laid eggs on cowpea seeds treated with monoterpenoids 3 weeks before to the day of experimentation. The monoterpenoids did not exhibit residual toxicity to the cowpea beetles. These monoterpenoids could be further investigated for the postharvest management of seed beetles of grain legumes.     

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.     

A case for the standardization of protocols used in screening cowpea, Vigna unguiculata for resistance to Callosobruchus maculatus (Fabricius) (Coleoptera: Bruchidae)

Cowpea cultivars Ife Brown, Maiduguri-A, Maiduguri-B and TVu 2027, were screened for resistance to Callosobruchus maculatus (F.) attack using different seed and insect densities. The number of eggs laid on the seeds and most other demographic variables increased with increase in insect density (number of females per 40 seeds). Although the same trend was observed with increase in seed density (seeds per 8 females), this was less predictable. However, the number of eggs per seed decreased significantly as seed density increased, but increased with increase in female density. Adult emergence increased significantly with increase in both insect and seed density while the growth index did not vary. Significantly more eggs were laid on the seeds of TVu 2027 (the resistant control) than on all the other cultivars when 20 or 40 seeds were available to 8 females, or at the lowest insect density (2 females per 40 seeds). However, percentage adult emergence and growth were higher on Ife Brown (susceptible control) than on TVu 2027 and the two Maiduguri cultivars. Measurements of percentage adult emergence and growth index indicated clear cultivar differences at all seed and insect densities. Also, although the loss in weight of seed increased with increase in insect density and decreased significantly with increase in seed density, meaningful differences in resistance among test cultivars were not always apparent at all densities. Based on these findings, we propose using 40 seeds and 2 pairs of adult beetles in screening cowpea for resistance. In addition, we conclude that % adult emergence, growth index and % weight loss are the most reliable indicators for resistance of cowpea to damage by this insect. The need to standardize experimental procedures used for host plant resistance work on storage pests of cowpea is emphasized.     

Species diversity and relative abundance of Callosobruchus (Coleoptera: Chrysomelidae) in stored cowpea in four major agricultural produce markets in the central region,Ghana

A study was conducted to investigate species diversity and relative abundance of Callosobruchus (Coleoptera: Chrysomelidae), a major insect pest infesting stored cowpea in the central region of Ghana. Cowpea stocks from Burkina Faso, Ejura, Ghana and Niger were obtained from the markets, and held to collect adults emerging from the cowpeas. Adult Callosobruchus spp. were identified based on the pattern of their hind femora and elytra. Five species, Callosobruchus maculatus, Callosobruchus rhodesianus, Callosobruchus chinensis Callosobruchus analis and Callosobruchus subinnotatus, were identified, with C. maculatus being the most abundant.     

Pretreating Callosobruchus maculatus (F.) eggs in mung bean with modified atmosphere conditions influence its adult emergence and survival

Cowpea bruchid (Callosobruchus maculatus Fab.) is a cosmopolitan pest that causes economic losses to legumes during storage. The present study determined the post-effects of exposing the C. maculatus eggs in mung bean to modified atmosphere (MA) conditions on the emergence and development of adults at ambient conditions (28 ± 3 °C and 65 ± 5% RH). The 24 h old C. maculatus eggs on mung bean kernel were packed with 500 g of mung bean in nylon/linear low-density polyethylene (nylon/LLDPE) bags (12.5 × 30 cm; 80 μm thick) and sealed with air (control), 100% CO₂, 100% N₂, and vacuum for 48 h. The very low O₂ conditions in the MA package had significantly caused longer onset and developmental duration of egg to adult emergence and significantly reduced the adult emergence percentage of C. maculatus, especially those in the vacuum treatment. This was due to low pressure that disrupted the structure of the eggs as indicated by the scanning electron microscopy (SEM). In addition, the MA conditions shortened the adult emergence duration, longevity, and mortality duration. The number of females was also reduced by the low O₂ conditions, particularly in those treated with vacuum. Eggs pretreated with the MA conditions probably affected the biomolecules of the eggs that in turn have affected the growth and development of the insect, with the order of effectivity highest in vacuum followed by 100% N₂ then 100% CO₂.     

Trehalose and proline failed to enhance cold tolerance of the cowpea weevil,Callosobruchus maculatus (F.) (Col.: Bruchidae)

The cowpea weevil, Callosobruchus maculatus (F.) (Col.: Bruchidae) is a cosmopolitan field-to-store pest ranked as the major post-harvest pest of cowpea in tropical regions. The cold tolerance of an insect species can vary as a result of abiotic features including food resources. In this study, C. maculatus larvae were fed with proline and trehalose (10, 20, and 40 mmol) treated cowpea seeds to determine the effects of these potential cryoprotectants on the supercooling (SCP) and cold hardiness of the upcoming adult beetles. The SCPs of the control, proline-fed and trehalose-fed adults non-significantly changed from −18.2 °C for the control to −17.2 °C for trehalose-fed adults. The cold hardiness (24 h at 0, -5.0, −7.5, −10.0, and −12.5 °C) of the adults was almost the same for control and treatments. Median lethal times (LT₅₀; lethal time for 50% mortality) were 6.3, 6.0, and 5.4 h, respectively. Moreover, feeding the larvae with proline and trehalose-treated seeds did not affect the proline and trehalose contents of the adult beetles. Our results showed that C. maculatus could not tolerate subzero temperatures well above their SCP, indicating that this species might be a chill-susceptible insect.     

Possible effects of Ultraviolet ray (UV-C) on biological traits of Callosobruchus maculatus (Col.: Chrysomelidae)

Cowpea seed beetle, Callosobruchus maculatus (F.) (Col.: Chrysomelidae), is one the most important pests of stored products worldwide, which specially distributed in the tropical and subtropical regions. In the present study, effects of different irradiation times of Ultraviolet ray (0, 10, 20, 30, 40 and 50 min) on the most important biological traits of this pest were evaluated under laboratory conditions. Our findings revealed that different irradiation times significantly affect mortality of immature stages of C. maculatus. The UV-C irradiation had no significant effects on duration of different life stages of this pest. Population growth parameters of C. maculatus were significantly affected by different treatments. The highest and lowest recorded values for net reproductive rate (R₀) were calculated at Control (38.70 offspring) and 50 min of irradiation (10.36 offspring), respectively. The calculated values for intrinsic rate of increase (r) at different treatments were significantly decreased with increasing irradiation time. The lowest value of this parameter was obtained at 40 min of irradiation (0.058 day⁻¹) and followed by 50 min (0.061 day⁻¹), while the highest value was recorded at Control (0.096 day⁻¹). Sex ratio of C. maculatus was not significantly affected at different treatments. The results of the present study can be used for further assessment of irradiation as a means to control C. maculatus.     

Toxicological and physiological effects of allyl isothiocyanate upon Callosobruchus maculatus

This study aimed at evaluating the toxicity of allyl isothiocyanate (AITC) to adults of Callosobruchus maculatus (Fabricius) (Coleoptera: Chrysomelidae) and its effects upon oviposition rate, population development, respiration rate, and dry mass loss in cowpea beans. The toxicity of AITC was determined by applying lethal concentrations (LC) of the compound, whereas the mortality curves were drawn by utilizing increasing concentrations of AITC (from 5.0 to 13.33 mL m⁻³ of grains). Population development was assessed through oviposition, progeny and both daily and accumulated emergence at sublethal concentrations (LC₁ = 3.81 mL m⁻³, LC ₁₀ = 5.71 mL m⁻³, LC ₃₀ = 7.67 mL m⁻³, and LC ₅₀ = 9.41 mL m⁻³). Furthermore, the loss of dry mass in grains and the respiration rate of adults when exposed to AITC (LC₁, LC₁₀, LC₃₀, and LC₅₀) were withal appraised. The values of LC ₅₀ and LC ₉₅ to adults of C. maculatus were of 9.41 and 17.85 mL m⁻³ of grains, respectively. At sublethal concentrations of AITC, the respiration oviposition, and emergence rates declined. The fumigant agent also prevented mass loss in cowpea beans. On account of that, it is safe to say that the compound has significant potential for controlling C. maculatus, inhibiting its development in storage conditions.     

Characterization of new sources of mungbean (Vigna radiata (L.) Wilczek) resistance to bruchids, Callosobruchus spp. (Coleoptera: Bruchidae)

The bruchid beetles Callosobruchus chinensis (L.) and C. maculatus (F.) are destructive pests of stored mungbean (Vigna radiata (L.) Wilczek). The development of resistant mungbean varieties to manage bruchids is a major breeding objective. In the present study, we investigated the characteristics of resistance to C. chinensis and C. maculatus in two new resistant mungbean accessions V1128 and V2817, and two previously reported resistant accessions V2709 and V2802, compared to the susceptible mungbean cultivar, KPS1. Evaluation for resistance to both bruchid species using whole and decorticated seeds revealed that V1128 and V2817 were free from damage, V2709 and V2802 showed partial damage with low or moderate number of bruchids emerging from seeds, and KPS1 showed complete damage with the highest number of bruchids emerged. Comparison of the seeds harvested at seed filling, early maturity and full maturity stages revealed that the percentage of damaged seeds from resistant accessions was lower at all stages compared with KPS1. V1128 and V2817 showed complete resistance against both bruchids regardless of when their seeds were harvested, while resistance in V2709 and V2802 were most pronounced at full maturity, and KPS1 was totally damaged at all times tested. These results suggest that the chemical factor(s) conferring resistance is synthesized as early as the seed filling stage. Evaluation of resistance using artificial seeds showed that increasing the percentage of resistant seed powder adversely affected bruchid growth and development. The number of adults emerging from seeds and number of damaged seeds decreased while adult developmental period increased as the proportion of resistant seed powder increased. The weight of emerging male and female adults of C. maculatus was lighter than those from the seeds containing susceptible seed powder alone. However, C. chinensis adults were not affected by the same test. The results suggest that biochemical(s) in cotyledon tissue are responsible for the resistance and the seed coat had no protective role against the bruchids. Although all four resistant accessions evaluated are useful for mungbean breeding, V1128 and V2817 show complete resistance to both C. chinensis and C. maculatus. Thus, these two new resistant sources may be the most effective for breeding purposes.     

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.