In virtually all of the 82 developing countries that grow sweetpotato, weevil pests are a major source of economic loss. Although the crop can be produced under difficult growing conditions with minimum inputs, weevils continue to plague production despite the use—and misuse—of insecticides.
In 1995, CIP researchers in Africa and the Caribbean reported significant progress in the testing of component technologies that farmers can use in integrated pest management (IPM) programs. In the Caribbean, where the predominant weevil species is Cylas formicarius, CIP-recommended IPM practices include the use of natural enemies such as predatory ants, the fungus Beauveria bassiana—which can be applied as a biologically safe insecticide—and mass trapping of adult male weevils using sex pheromones.
After two years of large-scale testing sponsored by the Organization of Petroleum Exporting Countries (OPEC) Fund, storage-root damage dropped from 25 percent to 5 percent in the Dominican Republic and from 44 percent to 8 percent in Cuba. In Cuba's case, this is about the same level of damage that farmers experienced when they applied Soviet-supplied insecticides 8 to 12 times per season. When Soviet insecticides became unavailable, Cuba was forced to seek alternatives and turned to IPM.
Large-scale trials in the Cienfuegos area show that sweetpotato grown with integrated weevil management practices produced average yields of 18 tons per hectare—roughly twice that of crops produced without IPM—with damage limited to just 8 percent of the crop. Sex pheromones, which were initially used to attract and monitor pest populations, proved to be effective in reducing pest numbers by luring adult males into trapping or killing devices. In 1995, approximately 40,000 such traps were tested successfully in farmers' fields.
Cuba Project
East Africa Project
Dominican Republic Project
Until 1990, Cuban farmers used 12-14 sprays to control the sweetpotato weevil. In 1991-92, when insecticides from the Soviet Union were no longer available, weevil damage affected 40 to 50 percent of production. But after two years of using IPM, damage dropped to 12 percent (1 to 18 percent) in the pilot and surrounding areas, without the use of insecticides. Although the adult weevil is able to fly long distances, studies in Cuba indicate that planting infested sweetpotato cuttings results in the insect spreading to new fields in the form of egg-larvae infested vines and free adult weevils. Research showed that more than 90% of the eggs are deposited in the basal part of the vine. Therefore, this part of the vine should not be used to produce cuttings for planting new fields. Cuttings selected for planting are submerged in a water suspension of the fungus Bacillus bassiana prior to planting. Any adult weevils present in the cuttings become infected by the fungus and die in two to three days.
The use of predatory ants, Pheidole megacephala and Tetramorium guineense, was also investigated. After studying the biology and behavior of the ants, including their efficiency as predators, researchers developed a technique to transfer the ants from their natural or artificial reservoirs to sweetpotato fields. Experiments in farmers' fields colonized with 100 nests per hectare showed reduced weevil damage at harvest (2.5 to 3.5 percent). Damage in the control fields with insecticide treatments was 6 to 10 percent.
Pilot areas for this program are located in Arimao, Cienfuegos Province, Cuba, and La Vega, Dominican Republic.
Encouraging results were reported in 1995 from East Africa, where sweetpotato is an important secondary crop and plays a critical role in rural diets during the "hungry months." Uganda alone produces 1.7 million tons, making it the fourth-largest sweetpotato producer in the world.
Sweetpotato production in the region is predominantly rainfed and the crop is planted and harvested piecemeal throughout the year. East African sweetpotato growers are usually small-scale, resource-poor female farmers. Production practices contrast with those used in the Caribbean as the predominant weevil species are different. In addition, East African farming traditions and the socioeconomics of production differ from those of other regions. In one district in central Uganda, for example, sweetpotato has replaced cassava as the predominant food staple because of severe losses from cassava mosaic virus. Although sweetpotato is far more nutritious than cassava, cassava can be left in the ground and harvested as needed. Sweetpotato roots are extremely vulnerable to weevils if left unharvested.
CIP's research agenda for sweetpotato in East Africa includes biological studies of the predominant weevil pests in the region, plus field experiments to test the effectiveness of the B. bassiana pathogenic fungus as an IPM component technology. In addition, work is under way to identify female sex pheromones for two African weevil species (C. brunneus and C. puncticollis). Sample lures developed were field tested successfully in Uganda using various trap designs to determine the most attractive pheromone blends for the two species and the most effective slow-release pheromone dispensers.
In the Dominican Republic, estimates from 31 sweetpotato fields sampled in 1991 indicated average losses of 39 percent from weevil infestation, a figure that translates into 40 percent decreases in gross income for farmers. Average estimated incidence and severity of damage were calculated at US$300 per hectare.
CIP's pilot IPM project in the Dominican Republic emphasized a variety of control measures, including crop rotation, deep plowing, timely weeding, and the use of sex pheromone traps. For farmers, pheromone traps proved to be an attractive option because of their effectiveness and low cost—each trap costs about $1, for a total cost of $80 per hectare.
Initial acceptance of pheromone traps was encouraging. By 1993, 15 percent of all sweetpotato fields in the Dominican Republic were protected by the traps. Reported thefts of pheromone dispensers attested to demand. Benefits were calculated from farmers' fields monitored in 1993 and 1994. On average, farmers who practiced IPM produced slightly higher yields, and experienced substantially less damage than other farmers. Without pheromone traps, crop damage was 1.8 tons per hectare; with the pheromones, damage totaled just 0.7 tons per hectare. The economic value of a 1.1 tons per hectare of gain was approximately $110 per hectare.
CIP scientists believe that half of all sweetpotato in the Dominican Republic will be grown under pheromone protection by 1997. In that event, the net benefit from research should reach $300,000 annually by the year 2000.
