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.     

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.     

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.     

Inhibition of Callosobruchus maculatus (F.) (Coleoptera: Bruchidae) development with essential oil extracted from Cymbopogon schoenanthus L. Spreng. (Poaceae), and the wasp Dinarmus basalis (Rondani) (Hymenoptera: Pteromalidae)

The control of the development of Callosobruchus maculatus was studied using a method that combined exposure to essential oil extracted from Cymbopogon schoenanthus and the introduction of a pteromalid natural enemy of the bruchid, Dinarmus basalis. The effect of the essential oil used was evaluated on all developmental stages of C. maculatus and on adults of D. basalis. At the highest concentration tested (33.3 μl/l) all adults of C. maculatus were killed within 24 h of exposure to the oil and the development of newly laid eggs and neonate larvae was also inhibited. However, the oil had variable efficacy against the bruchid instars developing inside the seeds: 5-day-old larvae (63% LI and 37% LII) of C. maculatus developing inside the seeds proving to be highly susceptible while 15-day-old insects (84% of pupae and 16% of larvae) were tolerant. Under the same conditions (33.3 μl/l), adults of D. basalis were very susceptible to oil vapours and to the residual activity of the oil after 3 or 6 days. However, the introduction of 10 pairs of adult D. basalis into a jar containing 100 hosts aged 10 days, 3 or 6 days before the oil application, gave respectively an emergence of 26 or 18 adults of the parasitoid compared to 28 in the control and there was no adult emergence of the host. The possibility of an integrated pest management strategy by using allelochemicals such as essential oils and indigenous natural enemies to control C. maculatus development in cowpea stocks is discussed.     

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.     

Environmental conditions affect the response of West African Callosobruchus maculatus (Coleoptera: Bruchidae) populations to susceptible and resistant cowpeas

Development and release of resistant cowpea varieties represents an attractive alternative to conventional chemical methods for control of Callosobruchus maculatus (F.), a major pest of stored cowpea, Vigna unguiculata (L.) Walpers, across sub-Saharan West Africa. However, laboratory studies on geographically distinct populations have repeatedly highlighted significant intraspecific variation in performance on resistant cowpeas, which could interfere with their effectiveness as a control method. In this study, the effect of different environmental conditions on population performance on susceptible and resistant cowpeas was investigated to determine whether they could also affect the degree of control achieved in different areas. Seven Nigerian populations were screened against cowpeas under two environmental regimes (Condition A—26°C, 75±5% relative humidity (r.h.), LD11.5:12.5 h, and Condition B—30:15°C (11.5:12.5 h), 30±5% r.h., LD11.5:12.5 h), designed to simulate field conditions during the storage period in different regions of Nigeria. Independent of environmental conditions and seed variety, significant differences among populations were expressed in terms of embryonic mortality, post-embryonic mortality, development time and adult weight. Resistant seeds delayed development, resulted in protracted adult emergences and increased post-embryonic mortality. Environmental conditions are shown for the first time to have a significant effect on population performance on resistant seeds, principally on development time and post-embryonic mortality. Effects on these parameters were not the same; rapid development was favoured under Condition A, although mortality was higher under this regime. The effects of resistant seeds and environmental conditions on performance could not be generalised across biological traits. Furthermore, significant interactions between environment and seed variety and between environment and population, were identified in certain parameters, suggesting performance on a resistant variety may vary significantly across populations, with seed variety and in response to differences in environmental conditions. It is suggested that the only reliable way of predicting the effectiveness of resistant varieties in the field would be to test them against local populations in the areas targeted for release, under local environmental conditions.     

Effects of cowpea varietal susceptibility and low pressure on the mortality of life stages of Callosobruchus maculatus (Coleoptera: Bruchidae)

Low pressure creates a low-oxygen controlled atmosphere that can kill all developmental stages of Callosobruchus maculatus (F.) (Coleoptera: Bruchidae). This study investigated the interaction of low pressure with seed variety to determine the susceptibility of different cowpea varieties to developmental stages of C. maculatus. Laboratory studies were performed to determine exposure periods to low pressure (32.5 ± 1.0 mm Hg) necessary to kill all life stages of C. maculatus on different cowpea varieties. The results indicated that with the exception of eggs, differences in exposure periods required to achieve 100% mortality varied with life stage and variety. The larval stages were the most susceptible, and larvae required shorter exposure periods to low pressure when reared on C. maculatus resistant varieties (24-125B-3 and IT89KD-288) than on susceptible varieties (CB-3, CB-5, CB-46 and UC-27). Analysis of variance (ANOVA) showed a significant interaction between the life stages of C. maculatus, exposure time and cowpea varieties on mortality. Integration of low pressure applications with storage of cowpea varieties resistant to C. maculatus has the potential of providing an alternative pest management tool to fumigants, which are sometimes used in protecting cowpea in storage from infestation by C. maculatus.     

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.     

Biological activity and persistence of four essential oils towards the main pest of stored cowpeas, Callosobruchus maculatus (F.) (Coleoptera: Bruchidae)

The use of essential oils extracted from native aromatic plants is perceived as a promising alternative to protect stored cowpeas in West Africa. However the optimal conditions for their efficiency remain to be determined. A study was therefore carried out to compare the biological activity and temperature-related persistence of four selected essential oils towards Callosobruchus maculatus, the main pest of stored cowpeas. Essential oil extracted from Ocimum americanum proved to be very toxic towards C. maculatus adults (LC₅₀ = 0.23 μl/l) while the oils from Hyptis suaveolens, Hyptis spicigera and Lippia multiflora exhibited higher LC₅₀ values (1.30 μl/l; 5.53 μl/l and 6.44 μl/l respectively). The persistence of the biological activity of the four oils was variable and that from O. americanum was most persistent. Fourteen days post-application, this oil was still as active on C. maculatus adults as it was immediately after its application. Exposure of this oil to a high temperature, close to temperatures occurring during storage in Burkina Faso in the dry season, however, resulted in a rapid decrease in its efficacy. Our results emphasize the need to take into account environmental factors such as temperature to optimize the use of promising essential oils for controlling stored-product pests in West Africa.     

Triple-bagging of cowpeas within high density polyethylene bags to control the cowpea beetle Callosobruchus maculatus F. (Coleoptera: Bruchidae)

Laboratory and on-farm trials were carried out to determine the effectiveness of cowpeas triple-bagging with heavy-grade polyethylene to control the cowpea weevil, Callosobruchus maculatus (F.), the main storage pest of cowpea, Vigna unguiculata, Walp, in West Africa. In the laboratory bruchids numbers and seed damage were significantly reduced when storing cowpeas within 2 layers High Density Polyethylene (HDPE) bags of at least 80 μm wall thicknesses. This thickness considerably reduced oxygen concentration in the bag after 5 days of storage and inhibited insect development. However late instar larvae and pupae were less affected by low oxygen concentration. On-farm storage trials with 2 layers HDPE 50 kg capacity bags tightly sealed and placed in an additional woven nylon bag (triple bag) was effective in controlling the bruchids for 7 months. Moreover, seed damage (<7%) and grain germination were not significantly affected (>89%). These findings allow optimizing the triple-bagging technology with readily local manufactured and affordable bags for long duration cowpea storage.     

Low and high temperatures for the control of cowpea beetle, callosobruchus maculatus (F.) (coleoptera: Bruchidae) in chickpeas

Chickpea (Cicer arietinum L.; Leguminasae) is an important pulse crop grown, around the world. The whole grain of chickpea is damaged by the cowpea seed beetle, Callosobruchus maculatus (Coleoptera: Bruchidae), which is the most important field-carry-over storage pest of pulses. The management of this insect in storage using chemicals leads to insecticide residues in grains and insecticide resistance development in insects. Thermal disinfestation is one of the means of physical insect control. Eggs, larvae, pupae and adults were held at 42 or 0 °C for varying durations. Pupae and adults were equally heat tolerant. The lethal time to reduce survival by 50% (LT₅₀) at 42 °C for eggs, larvae, pupae and adults were 18, 57, 78 and 71 h, respectively. Pupa was the most cold-tolerant stage. The LT₅₀ at 0 °C for eggs, larvae, pupae and adults were 3, 8, 10 and 4 d, respectively. The LT₅₀ for pupae were 4907, 4262, 336, 36 and 13 min at the grain temperature of 42, 45, 50, 55 and 60 °C, respectively. The LT₅₀ of pupae at 0,−5,−10 and −15 °C were 274, 122, 7 and 2 h, respectively.     

Deterrent and insecticidal properties of bean seed (Phaseolus vulgaris L.) whole meal or protein extract incorporated into the diet of Callosobruchus maculatus (F.) (Coleoptera: Bruchidae)

Callosobruchus maculatus, a pest that causes serious damage to chickpea Cicer arietinum, cannot develop in the seeds of Phaseolus or Vigna spp. which contain lectins. The insecticidal activity towards C. maculatus in these seeds is attributed both to lectins with specific affinity to N-acetylglucosamine, the major component of insect chitin, and to alpha-amylase inhibitors (lectin-like proteins). The insecticidal properties of bean meal or bean protein extracts from different sources towards different pest species are variable and need to be experimentally evaluated. The main objective of this study was to determine through a feeding trial on artificial chickpea seed the potential of bean seed meal from a wild bean Vigna caracalla, four varieties of Phaseolus vulgaris, and of a protein extract of P. vulgaris seed, to alter different life history traits of C. maculatus. The chickpea weevil was set up on artificial chickpea seeds containing different amounts of bean meal to observe the effects on female oviposition, percentage of development to adulthood and juvenile development time. These traits were combined in a composite index measuring the alteration of the multiplication rate of C. maculatus fed on artificial seed. The activity of lectin-like extracts was observed on chickpea artificial seed spiked with bean seed extract. Incorporation of bean flour at a rate of 10 and 20% in chickpea artificial seed significantly decreased C. maculatus female fecundity, percentage of adult emergence, and greatly increased the development time. Feeding trials with protein extracts of P. vulgaris reduced fecundity and survival of C. maculatus. Incorporation of 10% V. caracalla bean seed meal in chickpea artificial seed, reduced the multiplication potential of C. maculatus by over 90% showing that bean seed lectin extracts are worthy of further investigation for post-harvest infestation control.     

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.     

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.     

Variation in the response of Callosobruchus maculatus (F.) to a resistant veriety of cowpea

A Nigerian cultivar of the cowpea has previously been identified as being less suitable than conventional varieties for the development of a local strain of C. maculatus (F.). The survival and duration of development of the immature stages of three strains of C. maculatus in the dried seeds of this cultivar were compared with the performance of the same strains of beetle on a susceptible variety. In all beetle strains, fewer larvae survived to produce adults, and the duration of development was significantly extended, in the resistant cowpea cultivar. A Brazilian strain produced fewest adults and a Nigerian strain had the longest mean development period. Combining all the data, it appeared that a strain from the Yemen Arab Republic was least influenced and that from Brazil most adversely affected by the resistant cowpea. Evidence is presented indicating that factors in both the cowpea seeds and the beetles themselves may contribute to the poorer performance of bruchids in the resistant as compared to the susceptible variety.     

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.     

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.     

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.     

Comparative study of field and laboratory evaluations of the ethnobotanical Cassia sophera L. (Leguminosae) for bioactivity against the storage pests Callosobruchus maculatus (F.) (Coleoptera: Bruchidae) and Sitophilus oryzae (L.) (Coleoptera: Curculionidae)

The powdered leaves of Cassia sophera along with hot- and cold-water leaf extracts of this plant were tested in laboratory experiments in the UK and in field trials in Tamale, Northern Ghana, using traditional storage containers, to determine their inhibitory and toxic effects against Sitophilus oryzae and Callosobruchus maculatus infestation of stored rice and cowpea, respectively. Laboratory and field experiments with cowpea showed that the use of C. sophera hot-water extracts was more effective at reducing C. maculatus infestation and adult emergence on cowpea than the traditional leaf-powder application (1% and 5% w/w) or the use of a cold-water extract of C. sophera. Hot-water extracts of C. sophera might be a more effective technique of applying the plant material on to stored cowpea than using powdered C. sophera leaves, the currently used application by small-scale farmers. In contrast, experiments with S. oryzae on rice showed that C. sophera leaf powder (5% w/w) effectively reduced adult emergence in the laboratory, but this could not be confirmed under field conditions. The hot and dry climatic conditions in the field might impart a natural protection against rice infestation by S. oryzae, making the use of protectants and pesticides less necessary for farmers. This was supported by the negligible rice grain damage after 6 months of field storage and by the failure of the S. oryzae population to establish itself under field conditions. The implications of using botanicals in pest control are discussed.     

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.