Abstract: Mixtures of aqueous extracts of various plants were tested for their efficacy in the control of Maruca vitrata Fab. and Clavigralla tomentosicollis Stal under field conditions using cowpea variety SAMPEA 7. The materials involved included leaf extracts of Tagetes erecta L., in mixtures with Eucalyptus citriodora Denn, Azadirachta indica A. Juss and Gmelina arborea L; stem bark extract of G. arborea in mixtures with E. citriodora, A. indica stem bark extracts; leaf extract of Hyptis suaveolens Poit. mixed with Vernonia amygdelina L. and Cymbopogon citratus Staph leaves. These plant materials were mixed in 10:10% w/w. The stem bark mixtures of G. arborea + A. indica extracts and the leaf extract mixtures of T. erecta + A. indica caused significantly higher reduction of M. vitrata larvae and C. tomentosicollis (adults/nymphs) compared with the other extracts mixture treatments and the untreated control. Cowpea pods were better protected from pests damage on plots treated with these two extract mixtures leading to increased grain yields. This pest management strategy is suitable for low-income agriculture practiced in the developing countries.
Introduction
Cowpea, Vigna unguiculata (L.) Walp is a popular and nutritionally important grain legume in tropical countries where it supplies the bulk of the protein intakes in the menu of most people. Cowpea yields are generally low in Africa due to heavy infestation by insect pests and diseases. The most damaging of all pests are those occurring during the flowering and podding periods of which the legume pod borer, Maruca vitrata Fab. (Lepidoptera: Pyralidae) and a complex of pod and seed suckers predominated mainly by Clavigralla tomentosicollis Stal. (Hemiptera: Coreidae) and the flower bud thrips, Megalurothrips sjostedti Trybom (Thysanoptera: Thripidae) are the most important ones (Jackai and Daoust, 1986; Singh et al., 1990). Synthetic insecticides are the most effective, economical and acceptable control measures on cowpea in spite of their serious environmental consequences (Booker, 1965; Jackai et al., 1985). In addition to its environmental concerns, the current economic crisis in Africa accentuated by high exchange rate of the local currencies and low purchasing power of the farmers have influenced research into alternative pests control strategies of which botanicals are a component part.
Many plants in the African landscape are being investigated for insecticidal attributes for control of pests of food crops (Schmutterer, 1990; Saxena, 1989) and some successes have been reported. Although, most of these trials were based on storage protection of cowpea (Ofuya, 1986; Ogunwolu and Idowu, 1994; Oparaeke, 1997; Dike and Mshelia, 1997) and maize (Kossou, 1989), more interests have been generated in the application of Plant-Based Insecticides (PBIs) against field pests. For instance, neem based preparations have been extensively studied to control field pests of crops such as cassava (Olaifa and Adenuga, 1988), cowpea (Jackai et al., 1992; Tanzubil, 1991; Emosairue and Ubana, 1998) and okra (Emosairue and Uguru, 1998). Since PBIs are slow acting mortality agents and the application of extracts from a single plant source may not likely produce satisfactory results, the “best mix” approach using insecticidal plants of various species is advocated. This involves the most logical combination of different plant species that might be compatible in mixture formulations for management of these pests. A limited database is available on the insecticidal activities of mixtures of naturally growing plants in Africa for control of crop pests. This study intends to fill this vacuum as it reports the results of part of an ongoing research work on cowpea protection in the Institute for Agricultural Research, using botanical mixtures.
Materials and Methods
The investigation was conducted at the Research Farm of the Institute for Agricultural Research, Ahmadu Bello University, Zaria located in Northern Guinea savanna of Nigeria, for the periods 2000 and 2001 under rain fed conditions. The plant materials used for the study included leaves of Tagetes erecta, L. (Compositae), Vernonia amygdelina L. (Compositae), Hyptis suaveolens Poit (Lamiaceae), Cymbopogon citratus Staph (Graminae), leaves and stem barks of Gmelina arborea L (Verbanaceae), Eucalyptus citriodora Denn (Myrtaceae) and Azadirachta indica A. Juss (Meliaceae). These materials were collected around the institute’s Head Office and were dried under the shade for 72 h. Each plant material weighing 500 g was pounded in a mortar with pestle and the appropriate combinations of plant powders (Table 1) were poured into plastic buckets containing 3.5 l hot water (70%) and soaked overnight. The extract mixtures were then filtered the next day with 1.0 l tap water using a double folded muslin cloth. Each of the extract mixtures received 250 mL of 20% w/v of soap and starch solutions to improve their rain fastness and spread on plant surfaces. The solutions were labeled and taken to the field for spraying.
The experimental site had an area of about 0.45 ha. The field was prepared by spraying glyphosate at the rate of 5.0 l ha-1 and allowed to stand for 21 days before harrowing and ridging at 0.75 m apart. Nine plots measuring 6.0x5.0 m were marked out in three replicates using randomized block design. Each plot consisting of five ridges (three inner ridges and two discards) was separated by a 1.5 m wide border along the ridge and two unplanted ridges. Cowpea variety SAMPEA 7, an improved, semi determinate, medium duration maturing crop (75-85 days) with a semi erect growth habit was used as the test material. SAMPEA 7 is reported to be highly susceptible to all major insect pests of cowpea in this ecological zone. The growth period synchronizes with the peak populations of these noxious pests (Amatobi, 1994). Three seeds of this cowpea variety were sown at an intra-row spacing of 0.25 m in the first week of August 2000 and 2001. The plots were immediately sprayed with Galex (Metalachlor 250 g a.i + Metabromuron 250 g a.i) at 2.5 kg a.i ha-1 to control volunteer weeds. The seedlings were thinned to two plants per planting hole 2 to 3 Weeks after Sowing (WAS). Compound fertilizer NPK (15:15:15) was top dressed at 37.5 kg a.i ha-1 two WAS. At four WAS, a mixture of benomyl and mancozeb was applied weekly for four weeks at 0.33 kg a.i ha-1 to control fungal diseases such as scabs, brown blotch and septoria leaf spots The field was manually weeded once with a hoe at 5 WAS. Spraying of both extracts mixtures and synthetic insecticide commenced at seven WAS (at flower bud initiation or at the onset of flowering) when the peduncles were 2-10 cm long. CP 3 Knapsack sprayers were used during insecticide application and four weekly schedules were conducted. A single cowpea row was sprayed per pass or trip for good coverage. There were some unsprayed plots used as the control check. If rain fell within 2 h of application of extract mixtures a repeat spraying was conducted the next day.
Ten plants were randomly inspected for signs of phytotoxicity two days after each spraying. M. vitrata larvae were sampled before each spraying from 7.00-9.00 am by random picking of 20 flowers from plants located within the three inner ridges per plot. The flowers were placed in vials containing 30% alcohol and taken to the laboratory where they were dissected the next day and the number of Maruca pod borers (MPBs) found was counted. Adults and nymphs of C. tomentosicollis were assessed by visual counting of the insect found on plants located in three 1.0x1.0 m2 quadrants, which were randomly placed within the inner ridges per plot. Cowpea pod damage was also assessed at 10 WAS when the pods have attained between 75-85% physiological maturity using the formula below:
Cowpea pods were harvested dry within the three inner ridges. The pods were threshed, winnowed and the grains weighed. Data collected were subjected to appropriate transformation (square root or arcsine for percentage data) before Analysis of Variance was done and means were separated by SAS-SNK (p<0.05) test (SAS, 1989).
Results
Extracts from different plants mixed at 10:10% w/w exerted different levels of efficacy on Maruca Pod Borers (MPBs) and Pod Sucking Bugs (PSBs) pre dominated by C. tomentosicollis. The stem bark mixtures of G. arborea + A. indica and the leaf extract mixtures of T. erecta + A. indica were most effective in reducing the numbers of MPBs (<1.0 larva/flower and/or pod) and PSBs (<1.5 bugs/plant) in the two years of investigation. These extract mixtures significantly (p<0.05) lowered the numbers of the tested insect pests compared with the untreated control and some other plant extract mixtures. Although, all the plant extracts mixture treatments were significantly superior to the untreated control (Table 1), the leaf mixtures of T. erecta + G. arborea, T. erecta+E. citriodora, H. suaveolens+V. amygdelina and stem bark mixtures of G. arborea + E. citriodora were not effective against the two pests. The synthetic insecticide treated plots caused the greatest reduction (p<0.05) in the population of MPBs and PSBs and was superior to all the plant extracts mixture treatments.
Similar trends were observed for pod damage and grain yields assessments where plots treated with the stem bark mixtures of G. arborea + A. indica and the leaf extract mixtures of T. erecta + A. indica had lower pod damage (p<0.05) and higher grain yields (p<0.05) compared with plots treated with other plant extract mixtures and the untreated control. The greatest pod damage and lowest grain yield were found in plots sprayed with leaf extract mixtures of T. erecta + G. arborea, T. erecta + E. citriodora and H. suaveolens + V. amygdelina. Although, all the plant extracts mixture treatments were superior to the untreated check they were inferior to the synthetic insecticide treatment (Table 2).
| Table 1: | Mean number of M. vitrata and C. tomentosicollis on cowpea treated with aqueous plant extracts mixtures in 2000 and 2001 seasons in Northern Guinea savanna of Nigeria |
Means in a column followed by similar superscript (s) are not significantly different by SAS - SNK, (p<0.0) test | |
| Table 2: | Mean number of pods damaged and grain yields of cowpea treated with aqueous plant extracts mixtures in 2000 and 2001 seasons in Northern Guinea savanna of Nigeria |
| Means in a column followed by similar superscript (s) are not significantly different by SAS - SNK, (p<0.0) test | |
Field observation also indicated that none of the plant extracts mixtures exhibited signs of phytotoxicity or discolouration of the cowpea plants throughout the periods.
Discussion
The effects of different plant extract mixtures on M. vitrata and C. tomentosicollis have been shown in this study. The greatest reduction in MPBs and PSBs numbers was found in plots treated with stem bark mixtures of G. arborea + A. indica and leaf extracts of T. erecta + A. indica. The same extract mixtures caused the greatest reduction in pod damage leading to higher grain yields on treated plots. Although, their values were significantly (p<0.05) inferior to the synthetic insecticide treatment, they were however significantly superior (p<0.05) to the untreated check in pod damage reduction and higher grain yields.
These findings were similar to earlier work conducted by some researchers (Jung, 1938; Allen et al., 1944; Yepsen, 1976; Snoek, 1984), which showed that plant extract mixtures possess higher insecticidal efficacy (where compatible) than a single plant material. In this study, the differences between the untreated check, which harboured more pests than the extract mixtures treatments, were significant. This could be ascribed to the persistence of the extract mixtures sprays which minimized latter reinfestation on the treated plots.
Phylogenically, plants belonging to the families Myrtaceae (for example, Eucalyptus sp.), Meliaceae (e.g., neem), Compositae (bitter leaf and African marigold), Lamiaceae (e.g., African bush tea), Graminae (lemon grass) and Verbanaceae (Gmelina sp.) possess the highest bioactivity against insect pests (Grainge et al., 1986). The poor performance of some of the plant species belonging to these families was unexpected. The reason might be due to the pattern of distribution of bioactive compounds among some genera and species in the plant kingdom as well as the geographical location of such plants (Grainge et al., 1986). Secondly, some parts of plants contain more concentrations of the active principles than others, for example, the seed and stem bark of neem possess more Azadirachtin, Nimbin and Salannin than the leaf portion (Saxena, 1989; Kossou, 1989; Schmutterer, 1990). This explains why the stem bark mixtures of G. arborea + A. indica was more effective in controlling MPBs and PSBs than their leaf equivalents in mixtures with leaf materials from other plant species. The issue of active principles compatibility can not be ignored since the stem bark mixtures of G. arborea + E. citriodora did not follow this trend possibly due to incompatibility factor in bioactive materials present in the two plants species. However, the poor performance of some extract mixtures against the two insect pests may not be the true reflection of the insecticidal potentials of the plants. The screening criteria adopted for the two pests might not permit recording the effect of semiochemical of chronic effects of the plant extract mixtures on developmental and reproductive physiology of these pests (Williams and Mansingh, 1993). For instance, A. indica and some other plants possess growth regulatory, antifeedant, repellent and semiochemical activities on different insects pests (Schmutterer et al., 1980; Schmutterer and Ascher, 1984; Grainge et al., 1986). The present investigation indicates that stem bark mixtures of G. arborea + A. indica and leaf extract mixtures of T. erecta + A. indica can significantly reduce MPBs and PSBs on cowpea plants.
In Nigeria where this trial was conducted, all the plants are readily available, cheap, safe and technologically friendly for farmers’ use to protect their crops. The possession of highly lethal compounds by G. arborea + A. indica offers good opportunity for developing the mixture as an alternative to the synthetic insecticides which are environmentally unfriendly and expensive for limited resource farmers in the country. Further reserch is needed to assess the efficacy of different proportional mixtures of these plant materials and their spraying schedules to ensure increased grain yields and better quality grains for farmers.
Acknowledgement
The authors thank Messres Gideon Gbilin, Emeka, Nnamdi, Obiora and Obumneme for assistance in data collection on the field. Finally, the author is grateful to the Director, Institute for Agricultural Research, Ahmadu Bello University, Zaria for permission to publish this report.