Organisation: Centro Internacional de Agricultura Tropical (CIAT) (http://www.cgiar.org/ciat/)
4.1 Relative status of major pest species
4.2 Pest Control
4.1 Relative status of major pest species
Many species of insect pests attack beans both before and after harvest. The living bean plant can often recover after insect attack has been controlled, but stored dry beans cannot. There are two major post-harvest pests of dry beans world wide-the Mexican Dry Bean Weevil, Zabrotes subfasciatus (Boheman), and the Bean Weevil, Acanthoscelides obtectus (Say).
Both belong to the order Coleoptera and the family Bruchidae, commonly known as bruchids. Indirectly, these insects force the rapid sale of post-harvest grain and short storage periods in granaries, thus causing post-harvest price collapse and marked seasonal price fluctuation (32).
Most of the many other insect species that attack stored beans migrate from other products (e.g., maize, sorghum, or rice) stored in the same warehouse. They only cause minor damage to their secondary host, beans (4).
In general, when beans are stored at 14 percentage-15 percentage moisture content or less, mould is not a problem. Since farmers are aware of this, losses caused by mould are negligible. Damage caused by rodents has not been reported. This is probably because dry beans in their raw state are highly toxic to warm-blooded animals.
4.1.1. Details of each major pest
Z. subfasciatus predominates more in warmer areas and A. obtectus at higher altitudes in the tropics and throughout temperate climates in general (4). In Africa, however, this differentiation is less marked (1). Z. subfasciatus does not attack beans in the field. Fresh eggs are attached to the testa of bean seed. The adults exhibit strong sexual dimorphism. Females are large and have four characteristic cream-coloured spots on the elytra. The male is entirely brown. A. obtectus scatters its eggs among stored seed and oviposits in maturing bean pods in the field. It is difficult to distinguish between male and female as size and colouring are the same (32).
4.1.2 Life history
In storage, the life histories of Z. subfasciatus and A. obtectus are similar. Larvae of both species moult four times before pupating. During the last larval instar, the feeding and pupation cell becomes externally visible as a circular window in the seed where the larvae feed on the lower testa surface. After pupation, the adult may remain in the cell for several days before pushing or biting out the window with its mandibles. Adults are short-lived, and mate and oviposit soon after emergence (4).
For the Mexican bean weevil, the egg stage lasts 5-6 days, different larval instars 14 days, pupal stage 6-7 days; adults live 10-13 days; and females lay an average of 36 eggs. For the bean weevil, the egg stage lasts 6-7 days, combined larval and pupal stage 23 days; adults live 14 days; female lay about 45 eggs. Sex ratios tend to be 1:1 in both species (4).
Photo 9: Bean Weevil and characteristic "window" left in seed
4.1.3 Damage symptoms and levels of loss
Losses correlate directly with length of storage, the longer the beans are stored, the greater the loss. There are two types of losses: quantitative, or the number of seeds or parts of seeds eaten by insects; and qualitative, or the grains contaminated by excrement or insect bodies. These losses may be augmented by subsequent attacks from fungi or bacteria because larval stage completion elevates temperature and relative humidity, inviting secondary rotting by micro-organism attack.
Damage shows in the circular holes or "windows" left in the bean when bruchids emerge. Losses caused by bruchids are known to be substantial on all continents. In Ethiopia, stored bean damage by bruchids reached up to 38 percentage with a corresponding weight loss of about 3.2 percentage (20). Burundi and Rwanda losses to bruchids are commonly about 30 percentage (19). Estimates in Mexico and Central America have been as high as 35 percentage (32).
Unclean storage conditions are the main cause of bruchid infestation. Storing beans with other grains, or newly harvested beans with the infested residue from other harvests, encourages bruchids to flourish (33).
4.2 Pest Control
Plant resistance as a principal method of insect control is effective, practical, and of low cost to farmers. Cultivars with genetic resistance to the Mexican bean weevil have been identified (15). High levels of resistance to these weevils have recently become available in commercial bean types (5). The resistance is a simply inherited dominant gene that can be rapidly backcrossed into local varieties.
Strict cleanliness in storage sites should be maintained. Control may be effected at two levels: domestic and small-scale farmer level, and large commercial level.
Domestic and small farm level
Reducing storage temperature to < 10 º C significantly affects bruchid growth and reproduction because most are adapted to higher temperatures of 20-32 º C. So storing beans in the freezer compartment of a refrigerator completely eliminates the insect in any of its stages.
A mechanical control is to store the beans mixed with ashes, which fill the spaces between seeds making it hard for bruchids to infest. The optimal mix of ash is 20 percentage of the weight of bean seed being treated. This method only works before infestation. Sand, lime, or other fillers can be substituted for ash.
Coating with edible vegetable oils (e.g., peanut, maize, or soybean) is a relatively effective control permitting storage for at least 6 months without fear of insect damage. It also makes the seed look more attractive. The oil penetrates bruchid eggs and destroys them. It reduces oviposition and increases adult mortality. In general it should be applied at a rate of 5 mL per kg of seed. Control by oil is inexpensive and simple.
Some control of Z. subfasciatus is achieved by storing beans in their pods as Zabrotes prefer laying their eggs on shelled beans. For control of A. obtectus early harvest reduces exposure time to the insect in the field. Beans should then be shelled and cleaned immediately to eliminate eggs and insects coming from the field on pods.
Curative control in warehouses is possible by disinfestation and/or protection. Several chemical products are available for both.
Disinfestation eliminates at the time of treatment and leaves no residue so beans can be eaten immediately after. For the same reason, beans are liable to reinfestation. Phosphine (aluminum phosphide) and methyl bromide are the most used disinfestants. Phosphine eliminates all stages of the insect, including those within the seed. Methyl bromide leads to ozone depletion and its use will be phased out (35). It can affect germination when temperatures are high so is not recommended for use on seed to be planted. Both phosphine
and methyl bromide are very toxic to humans and should be applied only by experts. Bean sacks should be completely covered with a plastic sheet and the edges sealed against the ground to prevent gas escape. Beans should be kept covered thus for 1-2 days after fumigation for gas to penetrate seed.
Protection is done by mixing seed with an insecticide that has residual effects. This is suitable for seeds for planting only. They cannot be reinfested once treated. Malathion kills 85 percentage-99 percentage of Zabrotes adults in the first 24 hours after application. Lindane gives longer protection but is toxic to humans and must only be used for seeds for planting. Pyrethrins are a more promising product as they are not toxic.
Application of insecticides must always be carried out with adequate knowledge of the product and potential dangers. It is not the best means of control in many situations. Oil treatments are just as effective at the smaller scale level (32).
Photo 10: Disinfestation in warehouse
4.2.1 Residue problems
Residues are not a major problem in dry beans because the pods absorb insecticides. In the tropics, the sun's radiation is perpendicular therefore loss of residues is high. Bush-type beans have a growth period of about 90-95 days in the tropics and 100-120 days in cooler regions. Ideally, farmers fumigate 3 to 5 times beginning at 15 days and finishing at about 60 days. This would leave at least a 2-month margin between last application and consumption. Climbing beans have a longer growing period of 150-180 days, and in the Andes (2700 m) of 180-270 days. Again preferably farmers should fumigate about 8 times over this longer period still leaving a 2-month margin at the end. However, in some areas of Latin America, farmers may spray as late as one week before harvesting. Residue problems will still not be as serious for dry beans as for fruit and vegetables that are eaten raw. In storage, insecticides are not used on beans for consumption. Given the above factors, it is highly unlikely that residues remain in dry beans (César Cardona, 1997, personal communication). There is a need to research this aspect.
4.2.2 Discussion on pest control
In Latin America, small holders hardly used insecticides before 1985 but have since taken quickly to their use, despite the implied cost. All use pesticides, fungicides, and insecticides to some extent and cannot be prevented from using these products. Intercropping reduces the need for pesticides. Some research would be useful, not only for residue problems, but to determine the effect that using insecticides has on the farmer.
In Africa, pest control is left to Nature, as small-scale farmers cannot afford pesticides. Ideally, integrated pest management (IPM) principles can be applied and information is reaching the farmers. Research should focus on low-input IPM approaches that include current farming practices, host-plant resistance, and natural biological control (1).
Many pest control technologies have been developed over the years but few have reached the end-users. In collaboration with farmers, these techniques should be refined. Collaboration with social scientists is urged for this effort. Dissemination of resistant and/or high yielding varieties to small-scale farmers would increase yields.