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African Crop Science Journal
African Crop Science Society
ISSN: 1021-9730 EISSN: 2072-6589
Vol. 6, Num. 4, 1998, pp. 417-421
African Crop Science Journal, Vol

African Crop Science Journal, Vol. 6. No. 4, pp. 417-421, 1998
Printed in Uganda. All rights reserved
Ó 1998, African Crop Science Society


Effect Of Solar Drying Period Of Beans On Seed Viability, Cooking Time And Injuriousness Of Acanthoscelides Obtectus Say


Kawanda Agricultural Research Institute, P.O. Box 7065, Kampala, Uganda

(Received 11 February, 1998; accepted 24 May, 1998)

Code Number:CS98044


The efficiency of solarisation using a low cost solar dryer, on control of Acanthoscelides obtectus Say (Coleoptera: Bruchidae) in sequentially artificially infested beans (Phaseolus vulgaris L.) was investigated. The effect of the heat treatment on seed viability and cooking time were also examined. Solarisation periods varied for 0, 1, 2, 3, 4, 5 and 6 hours and the minimum and maximum temperatures achieved were 44 and 75.5 º C, respectively. The beans were then stored under ambient room conditions for 3 months. Relative cooking time was determined by the Mattson Drop Bar Method during which the beans were cooked to the half-cooked point. Seed viability was determined by planting the non- and solarised beans in moistened lake sand. There was no single adult emergence on the beans when exposed in the solar dryer for any duration beginning from 1 hour and above. The non-treated beans were severely damaged. Seed viability was adversely affected by solarisation resulting into 0% emergence of beans treated for 2 hours and above; 35% for 1 hour exposure and 90% in the controls. The shortest cooking time was achieved in the non-solar exposed beans and the longest time was observed on beans exposed for more than 2 hours in the dryer. Solarisation is recommended for beans for consumption but not for seed, and exposure should not be for more than 2 hours in the dryer.

Key Words: Bruchidae, germination, Phaseolus vulgaris, solarisation


L=efficacité de solarisation sur le contrôle de l'Acanthoscelides obtectus Say des graines de haricot (Phaseolus vulgaris L.) artificiellement infestées en séquence était étudiée en utilisant un séchoir à bas prix. L'effet de traitement en chaleur sur la viabilité des semences et le temps de cuisson étaient établis. Le temps de solarisation variait pour 0, 1, 2, 3, 4, 5 et 6 heures, et les temperatures minimales et maximales accomplies étaient respectivement de 44 et 75.5 C . Les graines étaient stockées à la température ambiante de la chambre pour 3 mois. Le temps relatif de cuisson était déterminé par la méthode de Mattson Drop Bar pendant laquelle les haricots étaient à moitié cuits. La viabilité des graines était déterminée en plantant les haricots solarisés et non solarisés dans le sable du lac. Il n'y avait pas d'émergence d'insecte adulte sur les haricots exposés dans le séchoir solaire pour n'importe quelle durée à partir d'une heure ou plus d'exposition. Les haricots non-traités étaient sévèrement endommagés. La viabilité des graines était au contraire affectée par la solarisation résultant à 0 % d'émergence de haricots traités pour 2 heures et plus, 35 % pour une heure d'exposition et 90 % dans les contrôles. Le temps de cuisson le plus court était atteint pour les graines non exposées à la solarisation, et le temps le plus long était observé avec les haricots exposés pour plus de 2 heures dans le séchoir. La solarisation est recommandée pour les haricots de consommation et non pour les semences dont la durée d'exposition ne devrait pas dépasser 2 heures dans le séchoir solaire.

Mots Clés: Bruchidae, germination, Phaseolus vulgaris, solarisation


Various techniques have been used to control bruchid pests of beans (Phaseolus vulgaris L.) in storage. The techniques range from physical, tumbling (Quentin et al., 1991), sunning and sieving regimes (Silim and Agona, 1996); chemicals, contact insecticides and fumigants (Schoonhoven and Cardona, 1986; NRI, 1991), botanicals, tobacco, tephrosia, tagetes and neem (Silim and Agona, 1996), and vegetable oil (Pandey and Verma, 1979; Silim and Agona, 1996). Some of these control techniques have proven useful in enhancing the shelf life of beans in storage to more than 3 months (Silim and Agona, 1993). It is anticipated that, at subsistence farming level, the usage and/or adoption of the different pest management techniques would depend on availability, effectiveness, costs, ease of application, user and environmental friendliness (Baier and Webster, 1992).

Solarisation of infested beans for 6 hours (Silim et al., unpublished) showed high mortality at all the developmental stages of Zabrotes subfasciatus (Boheman) (Coleoptera: Bruchidae) but at the expense of seed viability. However, the effect of enhanced solar drying on cooking time was not evaluated. Likewise, the extended drying period was not justified and the critical lethal temperatures not established.

This study, therefore, focused on the establishment of the optimum exposure period of bruchid infested beans in a solar dryer, which can ensure 100% mortality of the pest, but without impairing the seed viability and cooking time.


Sequential infestation technique. A common bean variety, K20 was sequentially infested artificially with one day old adult A. obtectus, at a rate of 10 pairs per 200 g of beans. Sequential infestation was to ensure the presence of all the developmental stages of A. obtectus. The insects were obtained from routine stock cultures. The insects were allowed to mate and oviposit for three days, after which they were discarded. After two weeks of incubation, the beans were re-infested at the same rate and the adult insects discarded. Re-inoculation was again conducted after one week and the same procedures above were maintained, but the adults were not discarded. On the twenty fourth day, the cultures were transferred into cotton bags and disinfested using a solar dryer. The infested beans were kept in 250 ml polystyrene jars fitted with perforated lids to permit maximum aeration and at the same time prevent escape.

Investigative parameters included temperature changes within the solar dryer, mortality levels of the adult beetles, number of adults that emerged, incidence of damaged seed, germination percentage and bean cooking time.

Solar disinfestation technique. Prior to disinfesting the beans, the initial moisture content was determined, and this was established at 14%. A solar dryer was constructed by digging a shallow pit 30 cm deep into the ground. The pit, which measured 3 m long and 1 m wide, had its bottom lined with dry bean stubble 10-15 cm thick. The stubble was overlined with a black polyethylene sheet. The sheet was allowed to overlap at the edges. It acted as a solar energy trap and the stubble as heat insulator against rapid heat dissipation into the surrounding soil.

The bags and their contents were placed on top of the black sheet in a completely random manner. Two thermometers were placed among the bags and the pit was covered with a transparent polyethylene sheet. The overlapping ends of the two sheets, that is black and transparent, were folded against each other to facilitate heat retention. Both sheets were of gauge 1000. A high gauge was used to avoid the melting of the sheets at higher temperatures.

Temperature measurements were taken at intervals of 20 minutes and the experiment was run from 11:00 to 17:00 hours. The treatment which was replicated four times included solarisation for 0-(control), 1, 2, 3, 4, 5 and 6 hours, respectively. The beans were transferred into the culturing polystyrene jars and incubated under prevailing laboratory conditions for three months.

Germination test. Seed viability tests were conducted on the solar treated and non-treated bean seeds after 3 months of storage. Fifty seeds per replicate were isolated and transferred to germination plates containing moistened sand. The seeds were allowed 10 days to germinate.

Cooking time test. The cooking time of the non- treated and solar treated beans was determined by the Mattson Bar Drop Method (Jackson and Mattson, 1981). Twenty four seeds from each treatment were soaked overnight in distilled water and then placed in a bean cooker with a pointed metal rod resting on each bean seed. The bar bean cooker was placed in boiling distilled water. Cooking time was estimated by measuring the time required for 13 of the 24 rods to perforate and penetrate the seeds. This was repeated three times per treatment.


There were significant (P<0.05) temperature variations within the solar dryer (Table 1 and Fig. 1). The minimum and maximum temperatures recorded were 44 and 75.5 ºC, respectively. The maximum temperature mean was achieved after two hours of exposure. There was, however, no significant temperature rise in the solar dryer from three to six hours of exposure.

Figure 1: Temperature variation in a solar heater and its effect on bean cooking time.

TABLE 1. Temperatures attained in a solar dryer and its effects on the mortality on Acanthoscelides obtectus adults and other development stages

Exposure period (hr)

Temperature mean (° C)

Adult mortality(%)

Mean adult emergence

Incidence of damaged seed (%)




































CV (%) 12.09
SED (21 d.f.) 4.51

Additionally, all the adult beetles which were exposed in the solar heater, unlike in the control, were killed irrespective of the exposure period. Survival rate in the control was 100% (Table 1). Adult mortality was 100% in all solar treated beans and hence there was no damage. In the control treatment, 609 adults emerged and 12.8% of the seeds were damaged (Table 1). The infestation level in damaged seeds averaged 3.34 insects per seed.

The germination percentage was significantly affected by solarisation. Seed viability was 90%, 35%and and 0% for 0,1 and >2 hours of solarisation, respectively (Table 2).

TABLE 2. The effect of solansation on cooking time and viability of bean seeds

Exposure period (Hr)

Cooking time (Min)

Seed viability (%)






















CV (%)



SED (14 d.f.)


SED (21 d.f.) 1.12

Solarisation affected the cooking time of beans. The longest cooking time was observed on beans solarised for three hours and the shortest time was observed in the unexposed beans (Table 2). There was, however, no significant difference (P>0.05) in the cooking time between the beans treated for 1, 2, 4 and 6 hours.


The 100% mortality of adult beetles and the non- emergence of adults in solar treated cultures indicates that the low-cost solar dryer was effective in eliminating all the developmental stages of A. obtectus. Most stored product insect pests are known to succumb to death at about 45 ºC (Kitch et al., 1992). The duration of exposure and the stage of development, however, needs to be clearly defined. In this case one hour exposure at 45 ºC or more is sufficient to wipe out all the infestation of any developmental stage of the insect.

The extended cooking time of the beans dried in the solar dryer, especially from three hours and above, could possibly be linked to the physiology of the bean seed. After two hours of exposure, the temperature build up within the dryer reached its highest amplitude (75.5 ºC) and this was followed by a sudden but steep fall in temperature (46.5 ºC) (Fig. 1). The rapid temperature rise could have resulted in the expansion of the seed and seed coat pores, but the sudden reduction in temperature led to collapse of the pores due to contraction (Jackson and Mattson, 1981). Since cooking involves water imbibition through the pores, the collapsed pores did not permit fast cooking, and, hence, the prolonged time.

Solar disinfestation had a negative effect on seed viability. It is apparent that the germ tissues of the seeds succumbed to high temperatures and died. Recommended seed drying procedure requires that the drying temperature for bean seeds of 10-17% mc should not go beyond 35-38 ºC (Kelly, 1988) otherwise the germ gets destroyed. Additionally, fast drying due to high temperature induces seed cracking and splitting (including internal cracking) due to the trapped moisture (Kelly, 1988), case hardening, loss of germination and vigour (Hill, per. comm.).


The solar drier was 100% effective in reducing damage by A. obtectus to stored beans. All the life stages of the bruchid succumbed to death even at the lowest temperature achieved within a disinfestation period of one hour and above. It is, however, recommended that successful disinfestation should be followed by strict store hygiene to avoid re-infestation; solarisation of stored produce should be repeated at least once a month during storage.

Since the viability of the seeds was greatly affected, the technology is recommended for beans for cooking, but not for seed. If the grains are destined for seed, other techniques like the use of ash, oils and tobacco can be applied for prolonged storage against insect pests. Furthermore, it is recommended that the process of solarisation should not go beyond 1-2 hours, otherwise the cooking time of beans will be greatly extended.


The authors express sincere gratitude to the Steering Committee of ECABREN, CIAT and NARO for having commissioned and supported the study.


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Copyright 1998, African Crop Science Society

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