The Sweetpotato Weevil is native to the Indian subcontinent and eastwards to Malaysia. It can fly, but expansion of its range occurs mainly when infested roots are transported by people. The distribution range of the West Indian Sweetpotato Weevil is rather limited. It overlaps with the natural distribution area of sweetpotato itself (i.e. the Caribbean and South America). This weevil cannot fly. Like the Sweetpotato Weevil, it is usually spread by people transporting infested tubers.
Most of the Pacific islands, including Okinawa in southern Japan, have been invaded by both species of weevil.
Sweetpotato is a very important crop, especially for low-income farmers. In many parts of the world it serves as a staple food. It is also grown throughout the Asian and Pacific region as a livestock feed, and as a source of sugars and starches for industrial processing.
Sweetpotato weevils are a major pest of this crop. They damage stored tubers as well as crops growing in the field. Not only do they spoil the appearance and marketability of sweetpotato, but infested tubers taste so bitter that they cannot even be used as livestock feed. Since the weevils lay eggs in the soil, and hide inside the stems and roots, it is difficult to control them with chemical sprays.
New control methods use biological resources and improved cultural practices, sometimes supplemented by chemical pesticides. Since these methods tend to be low-cost, they are well suited to the small-scale farmers of Asia and the Pacific.
Sex Pheromone Traps
Adult female Sweetpotato Weevils emit a chemical known as a sex pheromone into the air to attract males. Recently, the sex pheromones of Sweetpotato Weevil have been isolated, identified, and mass produced synthetically. These pheromones can now be used in traps to attract sweet potato weevils. Pheromone traps may also contain the conidia of Beaveria bassiana. They infect weevils with fungus disease (see page 3).
Pheromone traps are also useful as a way of locating and monitoring the occurrence of sweet potato weevils. They are the best way of checking whether a control program having any result.
Unfortunately, the West Indian Sweetpotato Weevil uses sound, not pheromones, to attract mates. The male attracts the female with a courtship song. This means that pheromone traps cannot be used against this pest. There is an urgent need for some alternative system of monitoring.
The Fungal Pathogen Beaveria Bassiana
This fungus occurs naturally in many parts of the world. It causes a deadly disease in insects, but is completely safe for human beings and livestock. It has proved an effective control against weevils, as well as against other insect pests (e.g. diamondback moth and green rice leafhopper). Sometimes B. bassiana is sprayed in a formulation, but when it is used against the sweetpotato weevil, it is put inside sex pheromone traps (see page 2). The traps attract male weevils, which become infected with the fungus disease. They then leave the trap and move out to infect female sweetpotato weevils with the disease.
Successful Ipm Program in Cuba
Sweetpotato is one of the most important staple foods in Cuba. Each year, farmers plant around 60,000 ha of this crop. Weevils are a major pest, causing damage to 45% or more of the roots unless control measures are applied.
When chemical sprays were readily available in Cuba, farmers used them 10-12 times a year. After the supply of subsidized pesticides from the Soviet Union came to an end, Cuba was forced to find a new way of controlling sweet potato weevils. The result was a very successful strategy based on IPM, using biological control and improved cultural practices. Because this approach does not rely on purchased inputs, it is well suited to low-income farmers. It is also friendly to the environment.
Early in the weevil control program, it was noted that infested planting materials were a major source of infection. Many of the cuttings were found to contain weevils or weevil eggs. The first step was to ensure that cuttings used in planting were free of weevils.
Research showed that nearly all weevil eggs are laid in the lower 35 cm of the sweet potato stems. From then on, farmers were recommended to take cuttings only from the upper shoots of sweetpotato plants. As an extra precaution, cuttings were disinfected in a solution of insecticide.
The fungus Beauveria bassiana was an important part of the control program. As well as being an effective and low-cost control, it had the benefit of providing many farmers with income. The production of B. bassiania has become a cottage industry in Cuba.
Predatory ants which prey on weevils and other insects as food were also used against the weevil. Two species of predatory ants, Pheidole megacephala and Tetramorium guineense, are common inhabitants of banana plantations. A method was devised whereby ant nests were carried in a rolled-up banana leaf into sweet potato fields. The ants soon multiplied and began new colonies, preying on weevils all the while, as well as on other insect pests. Farmers who set out ant nests at a rate of around 60-100 nests/ha were able to keep weevil infestation down to only 3-5%.
Large-scale trials in various parts of Cuba have shown that sweetpotato grown with integrated weevil management practices give an average yield of 18 mt per hectare - roughly twice the yield of crops produced without IPM. Damage is limited to just 8% of the sweetpotato crop.
Eradication of Sweetpotato Weevils
Eradication is the ultimate success of any weevil control program. It is usually possible only on small, isolated islands. However, one outstanding program has eradicated Sweetpotato Weevil from Muroto City, Kochi Prefecture, Japan.
The Sweetpotato Weevil has been present in Japan for a long time, but was limited to south of latitude 30 oN. It was a shock to find it in 1995 in Muroto City, far to the north of the supposed boundary. A program to eradicate the weevil was launched immediately. The prefectural and municipal government, farmers' organizations and the prefectural research center all worked together.
Farmers agreed to stop growing sweet potato until the program was over. Agricultural extension staff recommended suitable alternative crops. Hundreds of pheromone traps were set up. Wild sweetpotato plants and plant species which are alternative hosts of the weevil were eradicated by more than 500 municipal workers using herbicide sprays. Harvested sweetpotato tops and roots were buried. A ban was placed on taking sweetpotato plants or other host plants to the rest of Japan.
After only three years, the Sweetpotato Weevil had been completely eradicated from Muroto city. This has been confirmed by intensive monitoring using pheromone traps. The key to the program's success was the close cooperation between farmers and the various organizations concerned.
While control programs may succeed in eradicating Sweetpotato Weevils, especially if these are small and isolated populations, they are not likely to succeed at eradicating the West Indian Sweetpotato Weevil. This is because pheromone traps, an essential component, cannot be used against this weevil species.
Low-Cost Control Methods
For resource poor farmers, some technologies based on cultural practices are recommended.
- - Maintain adequate soil moisture so that the soil does not crack. Such cracks are a favorable habitat where weevils can breed.
- - Practice good field sanitation and use clean planting materials. Other cultural practices such as crop rotation, deep planting, hilling and prompt harvesting are also recommended.
- - Local ant species may help in controlling sweet potato pests, although farmers need advice from extension staff on how to begin using them.
Integrated pest management (IPM) is the best way of controlling weevils. However, in spite of several successful IPM programs, this approach is seldom followed. There are two main reasons for this: the low prices paid for sweetpotato, and the fact that IPM needs the participation of many farmers.
There is not yet any sweetpotato variety that is highly resistant to weevil attack, in spite of many years of screening and breeding for resistance. If such a variety can be achieved, it will be an enormous benefit, especially if it also has other good qualities which appeal to consumers.
Index of Images
Figure 1 Fig. 1. Sex Pheromone Trap Used against Sweetpotato Weevil
Figure 2 Fig. 2. Dying Weevil Infected with B. Bassiana
Figure 3 Fig. 3. Sweetpotato Root Infested by Weevils
Figure 4 Fig. 4. Spraying Host Plants of the Weevil with Herbicide, Muroto City Eradication Program