Contact and fumigant toxicity and repellency of Eucalyptus citriodora Hook., Eucalyptus staigeriana F., Cymbopogon winterianus Jowitt and Foeniculum vulgare Mill. essential oils in the management of Callosobruchus maculatus (FABR.) (Coleoptera: Chrysomelidae,Bruchinae)

The weevil Callosobruchus maculatus (Fabr.) is considered the most important pest of cowpea, Vigna unguiculata (L.) Walp., stored in tropical and subtropical countries. Secondary compounds extracted from plants can be used in its control, as a potential alternative to synthetic insecticides. Therefore, the present study aimed to: (a) make the chromatographic and mass spectrometry analyses of the essential oils of Eucalyptus citriodora Hook, Eucalyptus staigeriana F., Cymbopogon winterianus Jowitt and Foeniculum vulgare Mill; (b) to evaluate the contact and fumigant toxicity; (c) test the repellent effect. The oils' main compounds were: E. citriodora (citronellal 89.59%; citronellyl acetate 3.34%; 1,8-cineole 2.87%), E. staigeriana (limonene 28.75%; geranial 15.20%; neral 12.16%), C. winterianus (geranial 21.83%; citronellal 10.94%) and F. vulgare (limonene 41.82%; (E)-anethole 17.91%; α-pinene 11.13%). The LC_50s of F. vulgare, E. citriodora, C. winterianus and E. staigeriana in contact tests were estimated at 178.13, 298.17, 328.42 and 345.57 ppm cowpea grains, respectively. According to regression analyses, the higher the oil concentration, the lower the number of laid eggs and emerged insects. In fumigation tests with adults, LC_50s ranged from 2.58 to 7.85 μL/L of air, while the toxicity ratios ranged from 1.25 to 3.04. In all concentrations tested, the E. citriodora and C. winterianus oils were repellent to adult C. maculatus; F. vulgare was classified as neutral, while E. staigeriana was neutral at lower than 558 ppm concentrations and repellent at higher concentrations. Regarding the essential oils tested, the percentage of oviposition reduction varied from 6.3 to 100%, while emergence percentages varied from 0.9 to 100%.     

A laboratory assessment of the potential of different strains of the entomopathogenic fungi Beauveria bassiana (Balsamo) Vuillemin and Metarhizium anisopliae (Metschnikoff) to control Callosobruchus maculatus (F.) (Coleoptera: Bruchidae) in stored cowpea

Twelve indigenous and exotic isolates of Beauveria bassiana and Metarhizium anisopliae were evaluated for their virulence and their ability to suppress populations of Callosobruchus maculatus in stored cowpea. LT₅₀ values ranged from 3.11 to 6.12 days following immersion in aqueous suspensions containing 1×10⁸ conidia ml⁻¹. Indigenous isolates that had been recovered from C. maculatus were more virulent in laboratory bioassays than exotic isolates from other insects. The two isolates with the shortest LT₅₀ values were compared in dose-response assays by immersion and by exposure to cowpea grains treated with dry conidia. In both assays B. bassiana 0362 was consistently more virulent than M. anisopliae 0351. By immersion, LC₅₀ values on day 6 post-treatment were 9.10×10⁴ and 7.10×10⁵ conidia ml⁻¹ for B. bassiana 0362 and M. anisopliae 0351, respectively. Exposure to treated grains gave LC₅₀ values of 1.15×10⁷ and 4.44×10⁷ conidia g⁻¹ grain for B. bassiana 0362 and M. anisopliae 0351, respectively. In 1 kg batches of cowpea stocked with 50 adult C. maculatus, B. bassiana 0362 at both 1×10⁷ and 1×10⁸ conidia g⁻¹ grain led to significant adult mortality and reduced F₁ emergence relative to untreated populations. At 1×10⁸ conidia g⁻¹ the effect of the fungus persisted into the F₁ generation. The net reproductive rates, R₀, measured 26 days after insects were released were 5.16 and 7.32 for the high and low doses compared to 9.52 for the untreated control.     

Fungi and mycotoxins in cowpea (Vigna unguiculata L) on Nigerian markets

ABSTRACT

In this study, 81 samples of two cowpea varieties (brown: 54; white: 27) collected from various markets in southwestern Nigeria were examined for fungal and mycotoxin contamination. Moulds belonging to Aspergillus, Fusarium, and Penicillium were recovered from 99% of the samples. In both cowpea varieties, Aspergillus (52–53%) dominated Fusarium (29–30%) and Penicillium (17–20%). The interactive effect of cowpea variety and sampled location was significant (p = .013) on the occurrence of Fusarium species. Aflatoxins were detected in one brown and two white cowpea samples at concentrations reaching 209 and 84 µg/kg, respectively. Additionally, beauvericin was found in two samples of each cowpea variety, albeit at low concentrations. Cowpea presents as an alternative vegetable protein source to groundnuts in household nutrition with respect to mycotoxin contamination. Simple techniques to prevent mycotoxins in dry cowpeas are discussed.     

Physico-chemical and nutritional properties of cowpeas (Vigna unguiculata) heated under pressure

Cowpeas (Vigna unguiculata (L) Walp) at four stages of maturity were heated in water at 34.5 kPa for varying times up to 40 min. All measurements except ash were affected by the stage of maturity and time of heating. Changes occurred most rapidly during the first 10 min of heating. The greenness of the most immature cowpeas was lost. The degrees of firmness of all groups differed initially but decreased rapidly during the first 5 min and more slowly thereafter. Fresh cowpeas were softened sufficiently for eating after 10 min at which time pH increased 0.65 units to pH 6.75 and acidity decreased to 0.17%. In-vitro protein digestibility of the most mature, unheated cowpeas was 70.7% and increased to 83.4% after 10 min heating. Heating decreased the original trypsin inhibitor activity by 81.7% after 5 min and 85.9% after 10 min.     

The economic impact of cowpea research in West and Central Africa: A regional impact assessment of improved cowpea storage technologies

Researchers from the Bean/Cowpea Collaborative Research Support Program (CRSP), a program supported by the USAID, developed several non-chemical cowpea grain storage technologies in the 1980s. These included hermetic storage in airtight containers, improved ash storage, and the solar heater. Impact studies conducted at the country level showed that the research program was economically a good investment. As the CRSP new storage technologies spread throughout West and Central Africa, a regional assessment including spillover effects became necessary to fully comprehend the impact and guide future research investments. Therefore, this study sought to measure the economic impact of the new CRSP cowpea storage technologies at a regional level. Surveys in seven countries were used to estimate storage technology adoption. Economic surplus was used to estimate annual benefits and internal rate of return (IRR), and net present value (NPV) were calculated to summarize the net benefits. From the perspective of recipients' countries the project was a good investment. The IRR, is found to be much greater than the cost of capital. The regional IRR was found to be about 29%, much higher than the real interest rate on government bonds in West Africa at the time. For example the real interest rate on bonds issued by the government of Ghana in 2004 and 2005 was 8.9% and 5.4% respectively. The IRR is also higher than the private bank real lending rates in West Africa. From the perspective of the principal donor, the US government, the project was a good investment given that the average real interest rate on US government’s bonds was 4.8% during the period. The net present value of the investment amounts to more than 295 million US dollars which yields an annualized value of about 17 million.     

Automatic, rapid screening of seed resistance in cowpea, Vigna unguiculata (L.) Walpers, to the seed beetle Callosobruchus maculatus (F.) (Coleoptera: Bruchidae) using acoustic monitoring

The biomonitor technique was investigated as a rapid and automatic method for measuring the resistance of cowpea varieties to the seed beetle Callosobruchus maculatus. This technique measures the activity of internally feeding insect larvae by counting ultrasonic emissions produced as they feed. Activity throughout the development of C. maculatus larvae in known susceptible and resistant cowpea varieties was recorded. This showed details of the development of each larval instar, and showed clear differences between the resistant and susceptible cowpeas. A rapid method for comparing cowpeas was proposed in which the activity of larvae was recorded for 24 h starting 14 days after oviposition. Using this method, significant differences in activity were apparent between one susceptible and two resistant cowpea varieties. Further comparisons using a randomised block experimental design also showed a clear difference in activity between one susceptible and one known resistant cowpea variety. This experimental protocol took 21 days. It is suggested that the biomonitor offers a promising method for screening seeds for resistance to insect pests with a reduction in the time and effort required over conventional bioassay methods.     

Effect of various essential oils on Callosobruchus maculatus (F.) (Coleoptera: Bruchidae)

Essential oils were extracted from four West African plant species [Tagetes minuta (Family Compositae), Hyptis suaveolens (Family Labiatae), white basil Ocimum canum (Family Labiatae), and sweet basil O. basilicum (Family Labiatae)] by steam distillation. The oil of the pepper Piper guineense (Family Piperaceae), was extracted from the fruits by hydro distillation and ethanol extraction. Mixed essential oil and total ethanol extract was used. Kaolin powder (clay) was mixed (aromatized) with these different oils. Cowpea weevils were reared on chickpeas and newly emerged males and females were deposited on uninfested seeds. Bioassays, i.e. fumigation with pure essential oils and aromatized kaolin powders, were carried out on adults and eggs. Twenty four hours after fumigation, 99 and 0% adult mortality were observed, respectively, as the result of treatments with Ocimum basilicum and the control. The application of powders aromatized with the same oils to weevil pairs resulted in a complete lack of oviposition, whereas 31, 56 and 76 eggs were laid in the controls after 24, 48 and 72 h, respectively. Application of aromatized powders did not have a significant effect on egg hatching (50 out of 110 with O. canum, 100 out of 115 with O. basilicum and 100 out of 130 in the control sample) but did have a significant impact on adult emergence: 0% for the two treatments compared with 100% in the controls. Our results suggest that plants of the genus Ocimum can be used as an alternative to synthetic insecticides.     

Effect of saponins on the physical characteristics, composition and quality of akara (fried cowpea paste) made from non-decorticated cream cowpeas

Changes in the foaming capacity and viscosity of cowpea paste, affected by the addition of saponins, were investigated, as well as the subsequent effect on the texture and oil content of Akara (fried cowpea paste). Two saponins, Quillaja and Yucca, were used. Results showed a significantly greater decrease in the specific gravity after whipping (39.9-52.9%), of the paste prepared with saponins compared to that prepared without saponins (25.1-32.9%). This demonstrated the increase in foaming capacity of the paste prepared with saponins. The results also showed a higher foaming capacity for the Yucca saponin compared with the Quillaja saponin. Subsequently, there was a significant decrease in the apparent viscosity of the paste prepared with saponins and a decrease in the firmness (6.51-7.78 N) of the fried product compared to that prepared without saponins (10.89-11.58 N). These results suggest that saponins can be used to increase the foaming capacity of cowpea paste resulting in a more desirably soft, spongy-textured product. There was a significant increase in the fat content of the samples produced with saponins (25.1-42.0 g/100 g) compared to the control (18.8-19.6 g/100 g) due to the formation of more air spaces in the cowpea paste.     

Physiological and behavioral effects of different concentrations of diatomaceous earth on common stored product pest Callosobruchus maculatus

The presented research focuses on the assessment of the changes in the behavioral and physiological parameters of Callosobruchus maculatus (F.) (Coleoptera: Chrysomelidae), treated with amorphous diatomaceous earth (DE) (Perma- Guard™). During the experiments, parameters such as stimulated and spontaneous locomotor activity, oxygen consumption and fecundity were tested. Increasing DE concentration caused significant inhibition in both oxygen consumption and locomotor activity. Moreover, the treated insects had significantly lower fecundity, due to decreased egg laying, lower number of infested beans as well as lowered hatchability. By limiting the progeny number, DE possibly prevents or at least reduces the size of the C. maculatus outbreak. Additionally, scanning electron microscope (SEM) micrographs revealed that DE particles accumulate especially in joints or hairy regions of the insect body close to the sensilla. This, along with data on locomotor activity, indicates a possible presence of additional, previously undescribed, mode of DE action - as an irritant. In conclusion, the presented results contribute to the understanding of the insecticidal activity of DE and provide novel data on the potential application of oxygen consumption as a pesticide efficiency marker.