The Journal of Food Technology in Africa Vol. 9 No. 1, 2004, pp. 29-36
Effect of Insecticidal Plant Materials, Lantana camara L. and Tephrosia vogelii Hook, on the Quality Parameters of Stored Maize Grains
J. O. Ogendo*1, A. L. Deng1, S.R. Belmain2, D.J. Walker2, Musandu, A. A. O1
1Egerton University, P.O. Box 536, Njoro, Kenya
Code Number: ft04006
The effect of ground powders of two tropical plants, Lantana camara L. and Tephrosia vogelii Hook, on the level of insect damage and the grain quality parameters of stored maize were evaluated for five months. The evaluations were aimed at generating natural product treatments suitable for post-harvest grain protection and as sustainable alternatives to synthetic insecticides in the control of the maize weevil, Sitophilus zeamais Motschulsky. Three rates (1.0, 2.5 and 5.0% w/w) of each plant powder, a synthetic insecticide, Actellic Super 2% dust at 0.05% w/w and an untreated control were used as treatments. Results showed that the plant powders significantly minimised the magnitude of depression in percent grain moisture content albeit at a lesser rate with high concentration and had no effect on the percent germination of maize grains when compared to the controls. The botanical treatments and synthetic insecticide were equally effective in reducing insect damage by 25%, but the level of damage was independent of the concentration applied. Grain colour and odour were unaffected by the botanicals. Results are discussed with regard to the use of botanicals by small-scale farmers as cost-effective and sustainable alternatives to synthetic insecticides in maize grain storage.
Key words: botanicals, indigenous knowledge, grain storage, grain quality parameters, synthetic insecticides, insect damage, seed viability, food safety
Food security in sub-Saharan Africa largely depends upon improved food productivity through the use of sustainable good agricultural practices (GAPs) and the reduction of postharvest losses caused by pests and diseases. To ensure high food quality and safety standards, which are acceptable to the consumer, quality control, including good storage and handling practices, must be observed at all times (Kenny, 1998; Whitehead, 1998). For decades, the pest control policy in developing countries has been dependent upon the use of synthetic pesticides. Although synthetic pesticides are known to have undoubted benefits, their adoption rate and use for insect pest control in grain storage has remained remarkably low in the resource-poor farming environments. The subsistence nature of agriculture in developing countries coupled with the high cost, poor information and erratic supply of synthetic pesticides have emerged as reasons for farmers' reluctance to adopt pesticides (Tembo and Murfitt, 1995; Ogendo, 2000). Most farmers in these environments store their agricultural produce on the farm for short (less than three months) or long periods (³six months) (Wanjekeche, 1997) and use a variety of indigenous plant species with insecticidal properties for pest control (Bekele et al. 1996; Ogendo, 2000). Recent revelations that synthetic insecticides penetrate into stored grain and may be toxic (Lalah and Wandiga, 1996; El Sheamy et al., 1988), is worrisome. Moreover, Jood et al.(1993) reported that plant products and insect infestation adversely affected the taste, aroma and overall acceptability of ?chapatis? from treated maize rendering the grains unsuitable for human consumption. However, oils and crude powders of several plant species have been shown to have no adverse effects on the germination of maize, sorghum, pigeonpea and green gram (Pandey et al., 1986; Kasa and Tadese, 1995; Obeng-Ofori, 1995). This has stimulated scientists to search for natural pesticides that are environmentally friendly, safe to man and other non-target organisms and have no adverse effects on the organoleptic and market quality of stored grains.
Lantana camara L. and Tephrosia vogelii Hook have been shown to have toxic and repellent effects against certain insect pests of stored grains (Oliver-Bever, 1986; Sharma et al., 1992; Facknath, 1994; Ogendo, 2000) supporting indigenous farmer practice on their widespread use as grain protectants. However, conclusive recommendations on their use can only be made after the effects of these botanicals on the grain quality and safety are ascertained.
MATERIALS AND METHODS
Test plant materials
Mass rearing of test insects
Disinfesting of test maize grains
Grain quality evaluations
Grain moisture content
U = weight of undamaged grains,
Seed viability index
Viability index (%) = (NG*100)/TG
Where NG = number of seeds that germinated and TG = total number (=100) of test seeds in placed in each petri dish.
Grain colour and odour
Scoring for change in grain colour was done according to the following scale:
Scoring for the change in grain odour was done as follows:
To obtain unbiased scores, each grain sample was coded and presented in a well-lit and ventilated room for assessment. A panel consisting of six independent assessors (Kramer, 1956) scored for change in grain colour and odour. The assessors were allowed into the assessment room, one at a time, in rotation to ensure that their scores were independent from each other. This procedure was repeated on a monthly basis for five months and the same six panellists retained over the entire assessment period (Kramer, 1956). Blank scoring sheets were used for the different assessment dates to ensure that the previous data did not bias subsequent scores.
Experimental design and analysis
The effect of storage duration and botanical treatments on seed viability is presented in fig. 2a & b. Results indicated that the percent germination significantly (P=0.05) increased, except for 120 days, with storage duration for both untreated and treated maize grains. Germination was lowest after 120 days of grain storage. However, there was no significant treatment and storage duration by treatment interaction effects on the percent germination. There were no linear and quadratic relationships for either plant species treatment. Correlation analysis results showed that seed viability was weakly correlated with the grain MC, botanical treatments and the storage duration. All treatments, except the synthetic insecticide, recorded a decrease in percent germination for the first 30 days of grain storage. Untreated (control) grains had the lowest viability index which was judged significantly (no overlap of the standard error bars) different from the botanical treatments. Grains treated with Actellic super 2% dust registered increased percent germination during the first 30 days. L camara at 5% w/w and the control treatment had the lowest viability index at 120 days compared to its lower dosages (1-2.5% w/w) and Actellic Super. A similar response trend was observed with T. vogelii (all dosages) except grains treated with Actellic Super, which were significantly different from the botanical.
The results of the percent insect damage during 150 days of storage of untreated grains and those treated with different rates of L. camara and T. vogelii powders are presented in fig. 3. Results showed significant treatment (P=0.01), storage duration (P=0.01) and treatment by storage duration interaction (P=0.05) effects on the level of insect damage on stored maize grains. The untreated grains suffered significant (P=0.01) percent insect damage compared to grains treated with Actellic Super 2% dust and the botanical treatments. However, there were no significant differences between grains treated with Actellic Super 2% dust and the different rates of the two plant materials tested. Similarly, the concentration of botanical treatments had no linear and quadratic relationships with the level of insect damage recorded.
Figure 4 shows the mean evaluation scores for colour of untreated maize grains and those treated with Actellic Super 2% dust and the three rates each of the ground powders of L. camara and T. vogelii. Results indicated that there was significant (P=0.01) storage duration, quality assessors, botanical treatments and storage duration by botanical treatment interaction effects on the colour of stored maize grains. Highly significant (P=0.01) differences were recorded among the treatments. The Wilcoxon scores (rank sums) decreased with storage duration but increased with the dosage of the two plant powders applied.
The results of changes in odour during storage of untreated maize grains and those treated with Actellic super 2% dust and three rates each of ground powders of L. camara and T. vogelii are presented in fig.5a & b. Results showed highly significant (P=0.01) storage duration, treatment, storage duration by treatment, treatment by assessors and storage duration by assessors interaction effects on the odour of stored maize grains. The Wilcoxon scores (rank sums) increased with the dosage of ground powder of the two plant materials but decreased with duration of grain storage. The grains treated with Actellic Super 2% dust had the lowest rank scores followed by the untreated grains. Highest rank scores were recorded with T. vogelii at 5% w/w compared to L. camara at the same dosage. The change in grain odour was strongest early in storage but decreased with storage duration. Throughout the grain storage period, T. vogelii powder at 2.5 and 5% w/w imparted the strongest odour followed by L. camara powder at the same rates. However, the changes in grain odour, in all the cases, were mainly between odourless and less offensive. Except for T. vogelii powder at 5% w/w, all the other treatments had scored less than two on the five-point scale used.
Results showed that the moisture content and seed viability of treated and control grains were all significantly influenced by the duration of storage (Tables 2, 3; Figures 1 a, b & 2 a, b). Except for the periods 0-30 and 90-120 days, the results showed increased seed viability with duration of storage for all treatments. On the other hand, the effect of botanical concentration was insignificant. This concurs with Kasa and Tadese (1995) who reported that the use of crude powders of 17 botanical plant species for the control of S. zeamais on sorghum had no effect on seed germination. Similarly, Pandey et al. (1986) reported that petroleum ether extracts of L. camara and four other plant species had no adverse effects on the germination of green gram, Vigna radiata. However, storage duration, botanical treatments and their interaction effects influenced the grain MC. An increase in MC during the first 30 days was observed which was negatively correlated to the seed viability index. Thereafter, the grain MC decreased during storage for three months (30-120 days) and the magnitude of depression of MC decreased with concentration of plant powder. The grain MC and viability index were lowest after 120 days of storage. This extremely low MC and viability index observed after 120 days coincided with the time when the ambient relative humidity was only 38%. The changes in grain MC can be attributed to variations in ambient temperature and relative humidity during storage.
All durable stored products are hygroscopic and can therefore absorb or desorb (release) moisture from and to the surroundings. The moisture holding capacity of the air increases three times the amount of change in temperature (Haines, 1991). It follows from this argument that stored products release moisture to the surrounding air with a low relative humidity resulting in depressed grain MC. Where the air temperature and relative humidity are variable, the grain MC will also vary. This explanation seems plausible in this study where the ambient relative humidity and temperature were variable throughout the duration of storage. There was an inverse relationship between grain MC and the seed viability index. The seed viability index increased significantly (82 to 97%) for all treatments as the grain MC decreased (14.5 to 13.3%) during the period 30-90 days of grain storage. There was a sharp drop in seed viability index (97 to 78-85%) for all treatments when the grain MC fell below 13.25% between 90 and 120 days. This indicates that 13.3% could be the critical minimum moisture content of stored maize grains below which a marked decrease in seed viability index is realised. Haines (1991) reported that S. zeamais thrives best in grains with moisture contents of about 13%.
Statistical analysis showed significant treatment, storage duration and their interaction effects on the percent insect damage on stored maize grains (Figures 3 & 4). The level of damage was significantly higher in the untreated (control) grains than in those treated with the synthetic insecticide and different dosages of the ground powders of the two plant materials. However, the differences between the insecticide and plant materials were insignificant. No linear relationships were detected between the concentration of powder applied and the level of insect damage observed. The low moisture content of the grains, extremely dry weather conditions and lack of other grains stored in the neighbourhood could explain the exceptionally low damage levels (between 0.7 and 0.9% for all treatments). These results agree well with Schulten (1996) who worked with Prostephanus truncatus and observed that the progress of infestation in traditional maize varieties in large parts of eastern Africa was very slow during the dry season and that measurable losses only develop after the onset of rains. The level of insect damage recorded (0.7 to 0.9%) fits well into their findings of 0.5- 4% during the same months (November-April) of the year. Further research may be needed to establish the seasonality of S. zeamais infestation in relation to periods of bumper harvests and crop failures. Also it has been suggested that altitudes above 1500 m are likely to slow down insect breeding and hence cause less damage (Schulten, 1996).
The botanical treatments, storage duration, quality assessors and treatment-storage duration interaction significantly influenced the grain colour and odour evaluation scores (Figures 5a & 5b). For both quality parameters, the Wilcoxon scores increased with dosage of plant powder but decreased as storage progressed. This implies that concentration influences the amount of odour emitted by a plant material and that the concentration decreases with time. The decrease could possibly be due to the presence of volatile compounds in the plant tissues, which volatilise easily under ambient conditions. Grains admixed with synthetic insecticide had the lowest rank scores followed by the untreated control, but the differences were insignificant. This observation contradicts the general farmer perception that insecticides impart an offensive odour to maize grain and flour (Ogendo, 2000).
This study would appear to indicate that an insecticide treatment is likely to improve the grain colour and odour; however, further studies are planned to confirm this result. The evaluation results indicated that admixing maize with the botanicals marginally changed grain colour and odour over 5 months of storage and most treatments had mean assessment scores less than two on a scale of five. Jood et al. (1993) reported that chapatis (unleavened bread) prepared from sorghum grains treated with plant products were normal in colour, appearance and texture after 6 months but had their taste, aroma and overall acceptability adversely affected by either insect infestation or plant products. This means, therefore, that the colour and odour changes detected were expected to affect the market value of grains marginally.
From the results of these trials, it can be concluded that the use of L. camara and T. vogelii powders as natural product pesticides in maize storage significantly reduce grain damage with no adverse effects on seed germination. The botanical treatments caused only marginal changes in grain colour and odour, which should have virtually no impact on the local market value of treated grain. Further tests are planned to investigate the penetration of such botanical treatments into stored grain and their potential effects on the organoleptic content of the grain. Studies on the levels of residues remaining on treated food and their potential adverse effects when consumed should be evaluated before the use of these treatments are institutionally promoted as part of a sustainable insect pest management system for farm level storage.
It can be concluded that the botanical pesticides and storage duration influence the viability and moisture content of stored maize grains without affecting their colour and odour. There is, therefore, good promise to use the two botanical pesticides as alternatives to the synthetic pesticide, Actellic Super 2% dust.
This research was part of graduate studies funded by the Association of Commonwealth Universities and the University of Greenwich, UK, Shared Scholarship Scheme. The authors would like to thank the International Centre for Insect Physiology and Ecology (ICIPE), Nairobi, Kenya, for financial support under the DRIP Programme and Dr. A. Odulaja and Ms. Margaret N. Wabiri (Biostatistics Unit, ICIPE) for statistical advise.
Copyright 2004 - The Journal of Food Technology in Africa, Nairobi
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