8. Thesis structure
2.4 Discussion and conclusion
The results have shown clearly that bruchids as storage pests seriously affect smallholder farmers in Malawi. However, farmers from the three extension planning areas gave varied responses regarding the proportion of harvested beans affected by bruchids in storage. Their overall perception was that more than 50% of stored beans may be lost to bruchids if no control measures are put in place. The stated difficulties associated with storage problems, as mentioned in this study, confirm those previously reported by Scott and Maideni (1998), who conducted a similar study which focused on general constraints to bean production.
The average gross yield based on secondary data presented is ± 0.45 t ha-1. Farmers’
perceive that 50% of this, as opposed to the 38% reported in literature (Chirwa, 2001), may be lost in storage to bruchids in the absence of proper control measures. The net yield, after these storage losses, is therefore very low. This has serious implications for food security and potential income for Malawian farmers. This analysis of the severity of crop damage by bruchids for smallholder farmers is more serious than initially thought and calls for immediate intervention strategies. Clearly, there is a need to come up with effective control measures to reduce yield losses.
According to farmers, almost all of the improved bean varieties succumb to bruchid infestation, which may explain the huge losses caused by bean weevils. This impacts negatively on the adoption of improved bean varieties. This study has shown that farmers grow 65% of bean landraces and only 35% of the improved varieties. These findings support the hypothesis that there is low adoption of the improved bean varieties in Malawi. These results further explain why bean production figures are generally low.
Farmers predominantly grow landraces, which sometimes are more resistant to insect but they have a lower yield potential than the improved varieties. Improved bean varieties, under good crop management on -station, have a yield potential of up to 2.0t ha-1 compared to the 0.45t ha-1 that smallholder farmers obtain from landraces (Malawi Government, 2003). Inspite of the low yield, farmers grow landraces because they combine all the other preferred traits such as medium to large seed size, red to red speckled seed colour, good flavour, high storability and adaptability to the local environments.
It is apparent that screening for bruchid resistance has not been a major focus of the breeding and bean research programme in Malawi. The challenge for breeders is that potential bean lines should be screened for resistance to bruchids before release. It is possible that failure to engage farmers in breeding programmes could have resulted in breeders overlooking the importance of bruchid resistance in varieties already deployed in the smallholder sector. Acceptance and adoption of any new agricultural technology can be increased if participation becomes an ingredient of technology development (Monyo et al., 2001). The PRA can be used to improve communication between farmers and breeders so that farmers’ concerns and preferences are incorporated into the research objectives (Sperling et al., 1993; Eyzaguirre and Iwanaga, 1996; Snapp et al., 2003). In future, farmers in Malawi should be involved in testing for resistance to storage pests, especially at the resistance validation stage.
Realising the extent of the damage that bruchids inflict on stored beans, farmers have taken deliberate steps to develop and use various indigenous control methods, such as vegetable oil, ash from various plant parts, neem leaves and kernels, tobacco powder and paraffin, to minimize this damage. Farmers, however, noted that most of these indigenous control methods were not totally effective. This could be attributed to variability in quantities applied and the time of application. In addition, quantities applied were dependent on the level of observed damage and the quantity of beans stored.
Clearly, there are no recommended application rates for most of these indigenous bruchid control methods. In this study, ash from the bean plant was very effective in controlling bruchids, particularly against A. obtectus, even when applied after the bruchids had laid their eggs. Tete, on the other hand, was effective against Z.
subfasciatus. As a single treatment, it reduced adult bruchid emergence. When used in combination with Neem or Mphanjobvu, it effectively killed the emerged adults of Z.
subfasciatus (Table 2.9). This presumably could be attributed to the dosage rate used.
It may be that the dosages used were slightly low as a combination of botanicals proved more effective, and compared well with super actellic dust (an inorganic pesticide), than when applied as a single treatment. However, this occurred under laboratory conditions whereas it would be more appropriate to prove this under farm conditions.
The appraisal study has shown that some of the control methods that farmers use are more effective against Z. subfasciatus than A. obtectus. It is not clear why this should be the case. However, one reason could be the oviposition behaviour of the two insects.
The eggs of Z. subfasciatus are glued onto the seed and hence are in continous contact with the botanicals, whereas the eggs of A. obtectus are scattered around the seeds (Schoonhoven and Cardona, 1982) and this could reduce contact with the botanicals.
However, one would argue that A. obtectus larvae were even in more contact with the botanicals since the larvae move around. The results of the current study on the effectiveness of some of the indigenous bruchid control methods, such as using plant ash, agree with what was reported by Rahman and Talukder (2006) on black gram seeds. In their study, Bablah ash was very effective and significantly reduced the adult emergence of Callosobruchus maculatus (F.). Neem, though reportedly known to be effective against storage pests, was not effective in the appraisal study to control bruchids, when used as a single treatment. Sunita (2006) also reported that neem was only effective when used in combination with other botanicals to control the two bean bruchid species.
Some of the botanicals were effective in killing adult larvae of Z. subfasciatus (Table 2.9) when used in combination. For example, Mphanjobvu + Tete; Neem + Tete were more effective in combination than when used as single treatments. The standard practice is that farmers use only one control method. These results suggest that indigenous bruchid control methods would be more useful if applied timely (before egg laying) and used in combination.
Farmers’ variety and trait preferences were evident in the study. Farmers preferred large-seeded bean varieties with good flavour. In Malawi, before the inception of the Bean Improvement Programme, most of the varieties released were the small-seeded (18-25g/100 seeds) (Mesoamerican) type. Most of them were not popular among smallholder farmers in spite of their high yield potential. Three new bean varieties released in 1993 were medium (26-35g/100 seeds) to large-seeded (>35g/100 seeds) (Andean) types and were therefore preferred by many farmers, and are still popular, e.g., Chimbamba (25-2x 8-7) and Kalima (PVA 692) (Masangano, 2000). Farmers still prefer local landraces over new improved bean varieties. Seed colour preferences were also observed among some farmers. Dark red and red speckled (calima type) beans were the most prefered. Consumers in Malawi’s central region prefer khaki or “sugar beans” (tan with brown, black or red speckles). Those in southern Malawi tend to prefer red beans. There is evidence of a premium price for red beans in some markets because of the demand. The preference for seed colour implies that breeders cannot overlook its importance in developing new cultivars. Again, this underlines the importance of conducting a participatory rural appraisal to assist the breeder to set relevant breeding objectives to develp cultivars that are area - or region-specific.
In general, discussions with farmers and traders revealed their concerns about risk avoidance and yields of good quality beans. Bean cultivars should be well adapted over a wide range of environmental conditions. Results of this study have shown that both traders and producers were concerned about culinary quality, taste and selected traits such as seed size, colour and plant growth habits. It is important for a breeder to be aware that apart from agronomic traits such as growth habit and yield, culinary characteristics such as cooking time and flavour are also important and are used by farmers in selecting bean varieties. This further underscores the need to involve farmers in variety development.
The level of bruchid infestation, according to farmers, has changed over time. Bruchid damage is recently rated more severe than has been the case in the past, suggesting that breeder-farmer interaction should be encouraged. It may be necessary to explore the factors that may have contributed to this change in order to develop practical intervention strategies. Although the adoption rate for improved varieties is low, a few varieties have been adopted and are widely grown by farmers in the two study areas,
and elsewhere. As most improved varieties succumb to bruchid infestation in storage, their widespread production may exacerbate storage losses. Some respondents indicated that the weather pattern has changed. Rainfall is now erratic and mean temperatures are higher. High temperatures and humid conditions favour rapid multiplication of the insects.
In conclusion, the results of this study show that the two bruchid species are very important storage pests for smallholder farmers in Malawi, inflicting severe damage on stored beans in the absence of any effective control measures. The perceived 50%
damage in storage and the ineffectiveness of some of the indigenous bruchid control methods call for immediate action. The high levels of damage caused by bruchids compromise the food security and nutrititive quality of the beans. Apart from agronomic traits such as high yield and disease resistance, breeders must also take into consideration varietal preferences such seed colour, seed size and the flavour of bean varieties. Screening and selecting for bruchid resistance should be an essential part of the breeding strategy; farmers must be involved at all stages of the breeding programme and crop development to ensure uptake and adoption of new varieties.
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Chapter Three
Resistance of Malawian Dry Bean (Phaseolus Vulgaris L.) Landraces to Acanthoscelides Obtectus Say and Zabrotes
Subfasciatus Boheman
Abstract
Seed damage by storage pests cause substantial losses in Malawi and most of Africa.
The existing genetic diversity of dry beans has not been exploited fully in terms of the sources of resistance to bruchid strains found in Malawi. In a search for adaptable and effective sources of resistance to the two economically important bruchid species in Malawi, Acanthoscelides obtectus and Zabrotes subfasciatus, Malawian bean landraces were screened for both laboratory and field infestations. Due to limited seed numbers, the 135 genotypes examined were randomly divided into two sets. The first set comprised 42 and 56 genotypes which were tested separately for resistance to A.
obtectus. The second set comprising of 37 genotypes, mostly improved varieties, were tested for resistance to Z. subfasciatus. Two SMARC bean lines, SMARC 2 and SMARC 4, containing arcelin, which confer resistance to Z. subfasciatus, were included as resistant checks. First generation insects from a laboratory culture of both species were used to infest beans in the laboratory at 27-30oC and 66-70% relative humidity (RH) using a no-choice test. In other tests, insects collected from the farmers’ fields were used to infest genotypes under field conditions at 100% flowering, full podding and 95% physiological maturity. Field infestation was via a free-choice test whereby the insects had some room to choose the bean genotypes they preferred. At harvest, the pods were brought to the laboratory and were incubated for 120 d at 27-30oC and 66- 70% RH. The emerging adult insects were counted daily and grain weight loss measured. Bean resistance was measured using the Dobie susceptibility indices, which combine the number of bruchid emergence and the development period. Results of the study showed that there was wide variation among the bean genotypes for resistance/susceptibility to both insects, thus providing an opportunity for selection. A few genotypes displayed high levels of resistance but 83% of the tested lines were highly susceptible to Z. subfasciatus, while 50% of the tested lines were highly susceptible to A. obtectus. None of the improved cultivars were resistant. However, a landrace, KK35 consistently showed high resistance to the two bruchid species. Two
other landraces, KK73 and KK90, displayed stable resistance to A. obtectus under both laboratory and field infestations. Performance of most genotypes was not consistent between laboratory and field tests, suggesting that resistance mechanisms could be different and that laboratory screening should always be validated by field screening.
The seed coat played a significant role in conferring resistance to both bruchid species in this study.