Program 4

Integrated Pest Management for Sweetpotato in East Africa
N. Smit¹ and B. Odongo²

Pest Problem Assessment
Development of Management Components
Farmer Participatory Research in Pilot Area
Selected Reading

In East Africa, sweetpotato is mostly grown as a subsistence crop by resource-poor female farmers, who do not use inputs. Often, a few roots are dug up and sold to generate cash for household necessities. Production plots are small, rarely larger than 0.5 ha, although some larger-scale production exists. The crop is grown in two types of food systems. One is cereal-based, where sweetpotato is a food-security crop, as in Kenya. The other is nongrain, starchy staple-based. In this system, sweetpotato is one of several staple foods, and plays an important role in Uganda, Rwanda, Burundi, and adjacent parts of Zaire and Tanzania. Uganda alone produces 2.0 million t annually, making it the world’s fourth-largest sweetpotato producer.

Agroecological conditions under which sweetpotato is grown range from semiarid to high-altitude, temperate climates. There is almost no fresh storage of roots. Farmers practice in-ground storage and piecemeal harvesting. This means that crops are left in the ground for 7 mo to more than a year, and roots are removed as necessary for family meals. This practice guarantees that fresh roots are available for consumption during a large part of the year. But it also means that sweetpotato crops are in the field throughout the year in many areas where they are susceptible to pest infestation.

Since 1990, CIP researchers have collaborated with the national root crops programs of Kenya, Uganda, and Tanzania; the Crop Science Department of Makerere University; the Natural Resources Institute (NRI), United Kingdom; and the International Institute of Biological Control (IIBC) to develop integrated pest management (IPM) for sweetpotato in East Africa. The major pest in the region is the sweetpotato weevil Cylas spp.

IPM development can be divided into five distinct and consecutive phases, some of which may overlap. They are (1) pest problem assessment and characterization, (2) development of management components, (3) integration of key components, (4) implementation of IPM in pilot units, and (5) implementation of IPM on a large scale.

This CIP strategy has proven to be very successful in managing potato pests in the Andean region and North Africa, and managing sweetpotato weevil Cylas formicarius (Fabricius) in the Caribbean. However, these successes were obtained with commercial crops grown by relatively resource-rich farmers who commonly use insecticides to protect their crops. The situation of sweetpotato production in East Africa is very different and requires adjustments in strategy and expectations.

Pest Problem Assessment

Socioeconomic research into sweetpotato production and use pointed toward some important considerations relating to pest control.

First is farmers’ perceptions of pest problems. It makes little sense to help farmers solve problems that they do not consider important. During surveys throughout East Africa, farmers in areas with long dry seasons indicated that root-damaging sweetpotato weevils (Cylas spp.) were a major constraint to their sweetpotato production. An insect pest of regional importance is the sweetpotato butterfly Acraea acerata in the high-altitude regions of Uganda, Rwanda, and Burundi. Throughout these regions, most sweetpotato roots are destined for home consumption. Consequently, quality demand is low and what entomologists consider high pest levels are tolerated.

Biological research is one part of pest problem assessment. The sweetpotato weevil species found in East Africa, Cylas puncticollis (Boheman) and C. brunneus (Fabricius), are unique to the continent. Their counterpart C. formicarius (Fabricius) is a pest on sweetpotato in Asia, the United States, and the Caribbean. It has been widely studied, but little or no published information is available on the biology and ecology of the African sweetpotato weevil species or on the sweetpotato butterfly. Basic studies revealed differences in the biology of the two weevil species (Table 1) and the presence of species-specific pheromones for the weevils.

Table 1.

Life cycle data on two Cylas species taken in Kenya at 27+1oC and 45+5% relative humidity, 1992.

Cylas species	Developmental period (d)	Longevitya (d)	Number of eggs per female	Number of eggs per female per day
puncticollis	20-28	140+10 a	103+16 a	1.10+0.04 b
brunneus	32-41	92+12 b	100+18 a	1.53+0.06 a
Means within a column followed by different letters are significantly different by the t-test (P<0.05).

Knowledge of sweetpotato weevil ecology, biology, and behavior may indicate how to manage the pest. For instance, weevils cannot dig down through soil; the only roots females can lay their eggs in are those they locate through soil cracks or those exposed aboveground. Soil cracks are more common during the dry season, thus exposing roots for attack. Possibilities for control are the use of varieties that escape weevil damage by producing roots deep in the soil, covering exposed roots with soil and filling soil cracks to protect roots from weevil attack, and adjusting planting and harvesting times so that roots are not present in the dry season.

Development of Management Components

The technological options for an IPM program include host-plant resistance; biological control; cultural control; chemical control; and behavior-influencing techniques such as pheromones, sticky traps, and repellents. Methods described in the literature for controlling C. formicarius were validated under ambient conditions for the local species. Practices that farmers traditionally use to control pests were considered as a good basis for research and verification trials.

Host-plant resistance
So far, research has not identified germplasm immune to C. formicarius. But some studies suggest that sweetpotato clones differ in their levels of resistance. These levels are low and do not stand up under high weevil pressure. CIP and its collaborators have conducted several field and laboratory experiments with varieties from local germplasm collections to detect resistance to the African sweetpotato weevil species.

Although some varieties were clearly less susceptible than others, no reliable source of resistance has been identified. Conventional breeding techniques appear to have limited potential to incorporate weevil resistance into sweetpotato. An alternative approach under study is the development of transgenic sweet-potato with proteinase inhibitors for Cylas spp.

Some varieties are less susceptible to weevil damage than others due to an escape mechanism known as pseudoresistance. Short-season varieties can be harvested early before the weevil population builds up. Deep-rooted varieties escape weevil damage because their roots are less accessible for females to lay eggs.

Biological control    
Because several life stages of sweetpotato weevils are completed underground within the roots, it is difficult for parasites to locate them. Entomopathogenic fungi, bacteria, nematodes, and ground-dwelling insect predators appear to have greater potential as biological control agents of Cylas spp.

Of all known fungal pathogens reported to attack Cylas spp., Beauveria bassiana is the predominant species. In East Africa, scientists from IIBC isolated several strains from field-infested C. puncticollis and C. brunneus specimens. CIP collaborated in experiments to field-test strains that had proved to be the most pathogenic in the laboratory. Results were inconclusive.

Further field experimentation is planned in swampy areas, where farmers maintain their sweetpotato planting material during the dry season. Here environmental conditions might be more suitable for establishment of the fungus.

Cultural control
Recommended cultural practices that may help reduce C. formicarius damage include crop rotation, field sanitation, use of clean planting material, planting away from weevil-infested fields, hilling up to reduce soil cracking, adjusting planting time, and timely harvesting.

The ecology and biology of C. puncticollis and C. brunneus are similar to those of C. formicarius considering flight activity, host range, and mode of entry into the plant. In principle, the cultural practices advocated for reducing damage by C. formicarius also apply to the African sweetpotato weevil species.

Collaborative field experiments at research stations in Uganda and Kenya on some of the cultural practices confirmed this. However, the production systems of sweetpotato in Africa are very different from those of the United States and Asia, where the recommendations were developed. Suitable cultural control practices are site-specific and depend on agroecological and socioeconomic conditions. Some practices such as hilling up and rotation have become so common among farmers for agronomic reasons that they are not recognized as control techniques.

Two practices that are expected to have the most noticeable effect on weevil control—good field sanitation and planting away from weevil-infested fields—might require a community effort in densely populated areas. Practices that would require more labor from farm families might be a constraint, especially during peak periods of farm activity. An example is extra hilling of mounds and filling soil cracks.

The traditional practice of in-ground storage combined with piecemeal harvesting is contrary to the recommendation of prompt harvesting. On-station research, however, demonstrated that accumulated yield and yield loss under piecemeal harvesting compared favorably with once-over harvesting at the optimum harvesting time (Figure 1).

  

Knowledge of the biology and behavior of weevils will give farmers insight into the rationale behind recommended cultural practices. Site-specific, farmer-participatory research on cultural practices is presently needed, to verify which practices fit the farmers’ uses and customs.

Sex pheromones
The sex pheromone of C. formicarius, identified in 1985, proved to be an important component of an IPM program for this insect in its host range. C. puncticollis and C. brunneus appear to have their own species-specific, female-produced pheromones. In 1995, a holdback project funded by the Overseas Development Administration (ODA) began developing pheromones for monitoring and controlling the African Cylas spp. This is a collaborative project of the National Agricultural Research Organization (NARO) of Uganda, NRI, and CIP. NRI identified pheromone compounds that proved effective in catching male weevils under field conditions in Uganda. Exciting research results have been obtained on the most effective trap type (Figure 2), pheromone dose, lure and composition, male weevil diurnal activity (Figure 3), etc. A 5-L jerry can, filled with soapy water and tied to a pole so that it hangs slightly higher than the canopy of the crop, is presently the most effective and robust trapping system.

 

We are conducting on-station field experiments on the use of pheromone traps for mass trapping of males for weevil control. With participating farmers, mass trapping experiments have begun in small planting material nurseries, which are kept during the dry season. During this time of the year, the sweetpotato area is smaller and mass trapping might be more feasible. Sex pheromone traps might show potential as a component of IPM.

Chemical control
Most sweetpotato weevil life stages take place underground within plants. Therefore, postplant application of insecticide requires frequent applications to kill newly emerged adults. This is not cost-effective for subsistence farmers. Preplanting insecticide applications by dipping cuttings in systemic insecticide kill weevils within the vine and can protect it for at least 1 mo after planting. But the chemicals involved are highly toxic, and expensive. For these reasons, little attention is paid to chemical control of weevils.

Farmer Participatory Research in Pilot Area

The most promising IPM components at the moment—cultural control practices and sex pheromones—both require adaptation by farmers. The input of farmers at an early stage would alert researchers to any major unforeseen constraints to the eventual adoption of these management components. Research on other components, such as host-plant resistance and biological control, for which readily available technologies do not yet exist, can continue at the research stations.

IPM in complex, diverse, and risk-prone agriculture in sub-Saharan Africa has had few successes. Those reported were based on classical biological control or highly resistant varieties, or involved cash crops on which there was an overuse of insecticide. None of these conditions relate to IPM for sweetpotato, so a careful approach has to be taken. To increase the likelihood of practical success of the program, some modification of the CIP IPM model is required.

Selection of pilot area
Pests are often but one of many risks and sweetpotato but one of many crops. Farmers in East Africa often tolerate high yield losses. It is important to select a pilot area where pest management of sweetpotato might be a high priority for farmers.

A combination of specific socioeconomic and agroecological factors controls the selection of an IPM pilot site in East Africa. At the site, sweetpotato should be a basic staple food or a major cash crop, rainfall should be low or poorly spread seasonally with one or two long dry seasons, and sweetpotato production should have become increasingly important and intensive.

Part of northeastern Uganda forms a region within East Africa that fulfills all three conditions and is suitable as a pilot area for sweetpotato IPM. A suitable pilot area was found in Gweri subcounty in Soroti District.

Sweetpotato is the predominant staple crop, providing the majority of dietary starch throughout the year. And it is an important cash crop for the Kampala market during the major harvest months. Urban consumers prefer one variety, Tanzania, which is early maturing and highly susceptible to weevil attack due to its rooting characteristics. Farmers also like this variety for its taste and high yield.

The preferred staple and important cash crop cassava had virtually disappeared from the area due to African cassava mosaic virus disease (ACMVD) since 1986, but is slowly reappearing in the form of resistant varieties. Sweetpotato has taken over most of cassava’s role as a cash crop and for food security.

For similar reasons, the site was suitable for testing storage technology of fresh roots, which offered the opportunity to extend the period of availability of the crop. Before the onslaught of ACMVD, fresh cassava roots were available more or less throughout the year, as these can be stored in-ground on the plants. Sweetpotato plots, however, have to be dug up at the beginning or toward the middle of the dry season, because sweetpotato weevil damage increases rapidly to unacceptable levels.

At that time of the year, sweetpotato is dried and stored, to provide food during the long dry season. However, the product cannot be stored very long because of common storage pests. Sweetpotato therefore cannot completely replace cassava, and a period of insufficient food availability occurs nearly every year just before the earliest harvest of cereal crops in the rainy season. If final harvesting of sweetpotato could be delayed 1 mo, by reducing the incidence of sweetpotato weevil, this would be a large benefit for the farmers. IPM for sweetpotato weevils and storage of fresh roots are complementary in extending the period of availability of the fresh produce.

Integrated crop management (ICM) approach in pilot area
The pilot program began in July 1996. Enthusiastic extension staff and an NARO/CIP-sponsored local staff oversee the daily activities in the program. Basic information on impact of sweetpotato pests, farmers’ knowledge of pests, and current crop and pest management practices is obtained through participatory rural appraisal (PRA) methods, formal questionnaires, and year-long recordkeeping in farmers’ fields.

The research phase began in December 1996. During the PRA exercises, farmers expressed an interest in and need for management of sweetpotato weevils. In three target villages, farmer-experimenters, selected from and appointed by the farmers involved in the pest problem assessment, are trained on the biology and behavior of the insect pests, the ecological basis for pest outbreaks, and the rationale for the control measures to be tested. Mass trapping with pheromone traps, in combination with good field sanitation and planting away from infested fields, is being tested in the dry-season planting material nurseries in swampy areas.

Besides IPM components, farmers also expressed an interest in testing other technologies. In partnership with researchers from other disciplines, testing of new varieties, methods for storing fresh sweetpotato, and the use of sweetpotato in pig keeping takes place. Based on the needs and interests of the farmers, this integrated crop management approach was taken, as it became clear that yield loss due to sweetpotato weevil damage was but one of several related constraints that farmers face in sweetpotato production.

The pilot project will run for two years. If farmers are convinced of the practical value of an ICM program and take up crop protection practices, more pilot units will be considered, including extension of the program to neighboring countries. We cannot expect quick results from the development of an ICM program for the subsistence crop sweetpotato, but the present holistic approach with farmer participation holds promise to improve their living conditions.

Selected Reading

Smit, N.E.J.M. n.d. The effect of the indigenous practices of in-ground storage and piecemeal harvesting of sweetpotato on yield and quality losses caused by sweetpotato weevil in Uganda. Agric. Ecosyst. Environ. (In press.)

Smit, N.E.J.M. 1995. Farmers’ cultural practices and their effects on pest control in sweetpotato in South Nyanza, Kenya. Int. J. Pest Manage. 41:2-7.

Smit, N.E.J.M., M.C.A. Downham, B. Odongo, D.R. Hall, and P.O. Laboke. n.d. Development of pheromone traps for control and monitoring of sweetpotato weevils, Cylas puncticollis and C. brunneus, in Uganda. Entomol. Exp. Applic. (In press.)

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1 CIP-Uganda.
2 National Agricultural Research Organization (NARO), Uganda.