L.E.N. Jackai

Calendar and monitored insecticide application for the control of cowpea pests

Abstract ‘Monitored’ sprays are applied when the action threshold (AT) has been reached or exceeded, based on monitoring of crop damage or pest infestation, whereas ‘calendar’ sprays are applied at specific days after planting without taking into account continued presence or absence of the pest. In this study a comparison was made between monitored and calendar spray applications to determine whether monitored spraying would reduce the number of insecticide applications and still produce yields comparable to calendar spraying. The study focused on four insect pests — the cowpea aphid (Aphis craccivora Koch), legume bud thrips (Megalurothrips sjostedti Tryb.), legume pod borer (Maruca testulalis Geyer) and pod-sucking bugs. These pests damage cowpea at various stages of growth. The trials were carried out at three locations in Nigeria — Ibadan (forest transition zone), Mokwa and Bida (in the southern Guinea savanna zone). At all locations the two calendar schedules used were 7- and 10-day spray intervals. Differences in their effect on insect pest numbers were not significant, neither were there differences in grain yield (p ≥ 0.05). In general, the calendar schedules recorded lower infestation/damage by aphids, flower thrips and pod borers than monitored spraying but grain yields did not differ between them. Monitored spraying required only two sprays at Ibadan and Mokwa and three at Bida. This was half the number used for calendar spraying. Monitored spraying therefore looks quite attractive as a component in the overall management of cowpea pests.

A field screening technique for resistance of cowpea ( Vigna unguiculata) to the pod-borer Maruca testulalis (Geyer) (Lepidoptera: Pyralidae)

A method for screening resistance of cowpea ( Vigna unguiculata ) to the legume pod-borer Maruca testulalis (Geyer) was developed. Several damage parameters were measured, including damage to stem, flowers, pods and seeds. Some of these parameters were assessed in more than one way. A decision was made at the end of the study as to which parameters were most appropriate for a field screening methodology. A plant resistance index, I pr , was generated to aid in this decision. Based on this, it was concluded that flower, pod, and seed damage measurements provided the most important assessment of resistance to the borer. The larval population in the flowers showed marked differences in infestation levels between cultivars and is suggested as a means of comparing results in the future. Based on the methodology described, TVu 946 was the most resistant cowpea cultivar in every category of assessment. Five other cultivars were also selected for further evaluation as probable resistance donors: Kamboinse Local, TVu 1, VITA-5, TVx 3890–010F and VICAM-1/SP. The rationale behind the development of a field-screening methodology and some of its shortfalls are discussed.

The African yam bean seed lectin affects the development of the cowpea weevil but does not affect the development of larvae of the legume pod borer

Artificial feeding assays were used to study the effect of purified galactose-specific lectins from African yam beans (Sphenostylis stenocarpa) on development of larvae of the cowpea weevil, Callosobruchus maculatus (Coleoptera : Bruchidae) and the legume pod-borer, Maruca vitrata (Lepidoptera : Pyrialidae). Inhibition of development of C. maculatus was observed when larvae were fed on artificial cowpea seeds containing 0.2%, 2.0% and 5.0% (wt/wt) of dietary lectin. Larval mortality was between 30% and 88%, while delays in total developmental time ranged between 7 and 13 days. The lectin had no effect on development of larvae of M. vitrala, when tested through topical artificial diet incorporation assays, except at the extremely high dose of 35% dietary level.

Effect of plant lectins on larval development of the legume pod borer, Maruca vitrata

The legume pod‐borer Maruca vitrata (Fabricius), [Lepidoptera: Pyralidae] is a major constraint restricting increased cowpea production in tropical Africa and Asia. Since lectins are known to have insecticidal properties against several pests, a survey was undertaken to screen for the effects of 25 lectins from 15 plant families on the development of Maruca pod borer (MPB) larvae. The list included 8 galactose/N‐acetylgalactosamine‐, 7 mannose‐, 5 complex glycan‐, 2 sialic acid‐ and 3, N‐acetylglucosamine‐specific lectins. Feeding bioassays using artificial diet were carried out at 2% (w/w) topical levels. Although a total of 16 lectins had detrimental effects pertaining either to larval survival, weight, feeding inhibition, pupation, adult emergence and/or fecundity, only the Listera ovata agglutinin (LOA) (Orchidaceae) and Galanthus nivalis (Amaryllidaceae) agglutinin were effective against MPB larvae for all six parameters examined. Larval mortality and feeding inhibition caused by the most active lectin (LOA) was above 60%.

Field evaluation of cowpea cultivars (Vigna unguiculata [L.] Walp.) for resistance to flower bud thrips (Megalurothrips sjostedti Trybom) (Thysanoptera: Thripidae)

Ten cowpea cultivars were screened for resistance to flower bud thrips, Megalurothrips sjostedti, in replicated field trials. Resistance was evaluated by measuring differences in populations and visual damage on the cultivars. Statistical correlation was employed to assess the degree of association between damage indices and thrips population in the cultivars for each season. In the first season cultivars were conveniently grouped into three, using damage indices and thrips populations size. In the second season, due to the high thrips population, division into groups was not entirely achieveable; nevertheless the cultivars exhibited different resistance qualities. Sanzibanili, Moussa local, Sewe and IT90K-277-2 had consistently low damage indices and thrips population numbers, hence they performed better than the resistant control (TVu1509). TV × 3236 also performed well in the two seasons, but with slightly higher damage index and thrips numbers than the cultivars aforementioned. IT91K-180 and Kpodjiguegue, while supporting a higher number of thrips than the other cultivars, also had larger number of pods, suggesting tolerance as their mechanism of resistance.

The Potential of Neem Azadirachta Indica A. Juss. for Controlling Post-Flowering Pests of Cowpea, Vigna Unguiculata Walp—I. The Pod Borer, Maruca Testulalis

Neem oil (NO), and different formulations of neem oil in a solution of Tetrapleura tetraptera as emulsifier, including neem oil emulsifiable concentrate (NOEC), neem oil slurry emulsifiable concentrate (NOSEC) and 5% NOEC obtained from the seeds of the neem plant, were tested for their efficacy in the control of Maruca testulalis Geyer, under laboratory conditions using flowers of Ife Brown, cowpea variety.The different concentrations of NOEC (5, 10 and 20%) exhibited a high degree of insecticidal activity on M. testulalis larvae. All the treated flowers were protected from larval damage 2 days after treatment as compared to the 100% damage recorded on untreated flowers and on the flowers treated with T. tetraptera solution alone.The NOSEC and the 5% NOEC prepared using different volumes of the emulsifier also exhibited some insecticidal activity which decreased with a decrease in concentration. NO/NOEC were superior to NOSEC and the 5% NOEC prepared in different volumes of the emulsifier.RésuméL’effet insecticide ainsi que les méchanismes inhibitifs de l’alimentation des insectes présentés par des produits naturels, dérivés essentiellement d’huile de nim (HN), on été testés au laboratoire sur Maruca testulalis, un ravageur important de la post-floraison du nièbé. Il s’agit de concentrés émulsifiables d’huile de nim (CEHN), de concentrés emulsifiables à base de bouillie d’huile de nim (CEBHN) et de CEHN à 5%, préparé dans différentes concentrations d’une solution de Tetrapleura tetraptera (émulsifiant). Les fleurs de la variété améliorée Ife Brown ont été utilisée pour cette étude.Les différentes concentrations de CEHN testées (5, 10 et 20%) ont eu un effet inhibitif prononcé sur les larves de M. testulalis. Deux jours après le traitement, toutes les feuilles traitées étaient exemptes de dégâts par Maruca testulalis contre 100% de déprédation chez le témoin (fleurs non traitées) et les fleurs traitées avec la solution de T. tetraptera.Un certain degré d’activité insecticide a également été observé après la pulvérisation de concentrés émulsifiables à base de bouillie d’huile de nim (CEBHN) et de CEHN à 5% dans différentes concentrations d’émulsifiant.Toutefois, l’activitié insecticide de ces deux produits faiblissait au fur et à mesure que leur concentration diminuait.Cette étude a mis en lumière la supériorité de HN/CEHN par rapport au CEBHN et au CEHN à 5% préparé dans différentes concentrations d’émulsifiant.

Morphology, distribution, and the role of trichomes in cowpea ( Vigna unguiculata ) resistance to the legume pod borer, Maruca testulalis (Lepidoptera: Pyralidae)

Studies on the morphology and distribution, and the role of trichomes in damage to cowpea ( Vigna unguiculata ) by the legume pod borer Maruca testulalis (Geyer) showed that trichome cover on individual cultivars varied in trichome length and density, but not in trichome type from different plant parts. Trichome density on different parts decreased with increasing plant age. Significant ( P M. testulalis in cultivated cowpeas. It is, therefore, concluded that breeding for a higher level of trichomes in high yielding and agronomically desirable cowpea cultivars, will serve as an important component in the integrated management of M. testulalis .

The Legume Pod-Borer, Maruca Testulalis (Geyer): Past, Present and Future Research

The legume pod-borer, Maruca testulalis, continues to be a major limitation to successful cowpea cultivation in many countries. During the past decade, research on this insect has increased in scope and intensity at international agricultural research centres and a number of universities. The work that has been done on the biology and control of this pest at the International Institute of Tropical Agriculture (IITA) is briefly discussed with emphasis on the development of a management strategy centred around host plant resistance, and some suggestions for future research direction are given.RésuméLe rongeur de cosse de légume, Maruca testulalis, continue d’être une contrainte majeure de la culture de petit pois (cowpea) dans beaucoup de pays. Au cours de la dernière décennie, la recherche sur cet insecte ravageur a augmenté d’intensité dans les centres de recherche agricole internationaux et dans plusieurs universités. Le travail qui a été effectue sur la biologie et le contrôle de cet insecte ravageur à l’Institut International d’Agriculture Tropicale (IITA) est brièvement discuté, l’accent étant placé sur le développement d’une stratégie de contrôle basée sur la résistance de la plante hôte, et quelques suggestions sur l’orientation future de la recherche sont données.

Isolation of insecticidal lectin‐enriched extracts from African yam bean (Sphenostylis stenocarpa) and other legume species

Insecticidal lectins were isolated from 20 resistant Vigna and non‐Vigna legumes and tested againstn 3 pests of cowpea namely: Maruca vitrata, Callosobruchus maculatus and Clavigralla tomentosicollis. Crude lectins were separated from seeds using sodium chloride extraction, ammonium sulfate fractionation, and dialysis. SDS‐PAGE indicated the molecular size of ca. 30 kDa for the most intense (and presumably active) band. Haemagglutination assays using trypsin‐treated rabbit erythrocytes suggested that lectins were among the extracted proteins. Extracts from Lablab purpureus and Sphenostylis stenocarpa both non‐Vigna spp., caused greater agglutination than those from the wild Vigna species. Bioassays on all three insect species using the lectin extracts incorporated in either artificial cowpea seeds (5% w/w) or in modified Vanderzant legume pod borer diet (1% w/v) indicated that the non‐Vigna extracts were highly toxic to the insects. Mortality after 10 days was >80% in the most toxic extracts. The extract from one of the accessions of Sphenostylis stenocarpa, an edible legume, was singled out for lectin purification and future gene cloning with the view of using it for engineering resistance to cowpea pests.

Mating and oviposition behaviour in the legume pod borer, Maruca testulalis.

Experiments were conducted outdoors and in the insect rearing laboratory at the International Institute of Tropical Agriculture (IITA) in Nigeria. Four or five nights pairing duration gave the highest mating percentage and oviposition by the pod borer. Direct observations on adult mating revealed that some males mated more than once. The majority of females mated only once. A one‐to‐one ratio (with 10 males: 10 females) gave the best results for mating and oviposition. Mating took place between 21.00 h and 05.00 h when temperature ranged between 20–25 °C and rh over 80% with peak activity occurring between 02.00 and 03.00 h. Oviposition followed a similar trend to that observed for mating with an equal sex ratio (of 10 or more pairs per cage) giving the highest egg count. The average number of eggs laid was ca. 400 per female. These results show a much higher oviposition potential for Maruca testulalis than has been previously reported. The results are discussed with regard to the rearing of this species under laboratory conditions.