The sweetpotato weevil, Cylas formicarius Fabricius (SPW), is one of the most serious pests of the sweetpotato, Ipomoea batatas (L.) LAM., and is widely distributed throughout the world.
In the 1900's, the SPW invaded Okinawa and spread to the northern islands of Amami (1940) and Tokara (1950) (Setokuchi 1990). SPW then became endemic. At this time, it was confined to the south of Japan, to an area covering Okinawa Prefecture and the southern part of Kagoshima Prefecture ( Fig. 1). To prevent the spread of SPW, the domestic movement of host plants of SPW from infested areas to uninfested areas was prohibited or restricted, through the imple-mentation of Japan's Plant Protection Law in 1950.
In November 1995, however, SPW was found for the first time in Muroto City in Kochi Prefecture, located far to the north of the supposed boundary (Kochi Prefecture 1999).
This paper reports on the successful process of eradication, with an emphasis on the importance of collaboration between the organizations concerned and farmers.
Materials and Methods
The eradication project consisted of two stages. The first was the control project to eradicate SPW, which lasted from November 1995 to April 1998. The second was a survey to evaluate the efficiency of the eradication program. This lasted from May 1998 to December 1998. The eradication project used methods similar to those used in earlier successful projects carried out at Nishino-omote City, in Kagoshima Prefecture from 1990 (Kagoshima Prefecture 1998).
A council was established to ensure that the eradication program went smoothly. Members were from the Kochi Local Government, Muroto City Municipal Government and other related organizations ( Fig. 2). This council determined the protocol for the control project.
Monitoring of SPW Populations with Sex Pheromone Traps
The control project used the same sex pheromone trap which had been used in a previous project carried out in Nishino-omote City. This trap was fixed with a rubber septum impregnated with 100µg of synthetic sex pheromone, ( Z)-3-dodecen-1-o1 ( E)-2-butenoate, on a sticky plate ( Fig. 3).
The sex pheromone traps were classified into two types, according to their purpose. One was the "fixed point" traps, which were set out at permanent sites in the control area. The function of the fixed point traps was to monitor the seasonal occurrence of SPW, and to evaluate the efficiency of the eradication program. More traps were set up outside the control area to alert researchers to any spread of SPW. These "fixed point" traps were set out at two-week intervals at 191 points during the period of the control project ( Table 1).
The "supplementary traps" were additional traps set out around the fixed point traps, so that the number and density of traps in control areas would be increased. These included areas where sweet potato weevils had been caught in fixed point traps, and also areas where wild host plants had been found. The supplementary traps gave additional information about the presence of weevils on alternative host plants, and whether the eradication program had been effective. The supplementary traps were set up at intervals of 50 m once or twice a month at 630 points during the period of the control project ( Table 1).
Wild host plants growing in the control area included Calystegia soldanella, Ipomoea indica and Calystegia hederacea. These plants were often found growing along the beach, or in windbreaks and orchards. Poor-quality sweetpotatoes which had not been sold were sometimes dumped in these areas, and began to grow. Many of these discarded plants were infested with SPW. The wild host plants and discarded sweetpotatoes were removed 137 times during the control project. This task was carried out by 1,565 workers ( Table 2).
In cases where it was difficult to remove wild host plants because other weeds were growing over or around them, herbicides and insecticides were used. Chemical spraying was carried out 56 times by 575 workers between 1995 and 1998 ( Table 2).
Changing the Crop
To break the cycle of infestation of SPW in the control area, sweetpotato growers were advised to use substitute crops. They began to do this as soon as the eradication project started.
Agricultural extension agents suggested appropriate alternative crops to farmers, such as Yam ( Dioscorea japonica), "Senryo" ( Chloranthus glaber), an ornamental tree used for flower arrangements, and edible sunflower.
Cooperation and Publicity Activities for Eradication of SPW
Farmers and residents were asked to stop growing sweetpotato crops until the eradication project was completed, to prevent the spread of SPW to uninfested areas. Several notices for the eradication program were publicized by the organizations concerned to residents and farmers, using various media.
After the eradication program, the Kochi Local Government and the Kobe Plant Protection Station began a survey to evaluate the efficiency of the program. The survey was carried out for six months, from May to December 1998. During the first three months, the survey was carried out by the Kochi Local Government. It was completed by the Kobe Station for the final three months.
Pheromone Trap Survey
Pheromone traps were set up twelve times at 2-week intervals during the survey period. The pheromone traps were arranged at 790 points, as in the control area ( Fig. 4).
Survey of Wild Host Plants
Wild host plants were collected every month during the survey period. A total of 24,915 wild host plants were collected from the control area and checked for the presence of weevils.
Survey Using Sweetpotato Traps
Sweetpotato plants were placed in a plastic box after being dipped in insecticide, and put in the control area. Sweetpotato traps were set up every month at 250 points during the survey period ( Fig. 4).
Results and Discussion
At the beginning of the control project, the density of SPW was quite high. This made it difficult to determine the location of the host plants of SPW. Therefore, only fixed-point traps were used to determine the control area. As a result of this work, the control area was defined as within Muroto City (780ha) ( Fig. 5). The area contained 137 sweetpotato growers, with an estimated total farm area of 19 ha. The control area also contained a further 11 ha of wild host plants. Initially, the control program was applied to planted sweetpotato fields, and then extended to areas with several patches of wild host plants. After this, the supplementary traps were set up in the control area, and we began the monitoring of SPW in combination with fixed-point traps.
Replicated control work was carried out using several techniques. If weevils were captured in traps, we searched for host plants and eradicated them until no more weevils were found in the pheromone traps ( Fig. 6). At points where we captured many weevil, cylinder type traps ( Fig. 3) were used, because they had a larger capacity (Sugiyama 1998). The removal of dumped sweet potato and wild host plants was repeated as long as was necessary.
Weevil populations in the control area decreased rapidly after the start of the control project, November 1995 to December 1996 ( Fig. 7). The number of captured weevils was maintained at less than 10/trap from November 1996. In the summer of 1997, we encountered the first infestation of SPW in a Calystegia hederacea plant in Japan. This had happened because after the removal of Calystegia soldanella and Ipomoea indica during the control project, there were no proper wild host plants left for SPW to infest.
Results of the counter project indicated that no SPW were captured in pheromone traps after December 1997.
Therefore, we then began a survey to assess the effectiveness of the control project. The survey was carried out from May 1998 to December 1998, when there was no SPW in the control area. These periods corresponded to at least four generations of weevils in Kochi.
Together with the results of the survey and the above-mentioned results, SPW was found to have been completely eradicated from Muroto City. Following these successful results, a ban on the domestic movement of weevil host plants from the control area to uninfested areas was lifted in December 1998.
Since 1950, Kogoshima Prefecture in the southern part of Japan has been invaded by SPW several times, prompting immediate emergency control programs (Kami 1964, Setokuchi 1990, Kagoshima Prefecture 1998). Before the sex pheromone of SPW was discovered (Health 1986), there was no convenient monitoring system for SPW. Therefore it took a lot of time and money to implement an emergency control project.
After using sex pheromone traps came into use against SPW, the method of emergency control was changed, since the presence of SPW was easy to detect by sex pheromone traps (Yasuda et al. 1992, Sugimoto et al. 1994). Sex pheromone traps were used efficiently in an eradication project carried out in Nishino-omote City, Kagoshima from 1990 to 1998 (Kagoshima Prefecture 1998).
Sweetpotato weevils are difficult to control through the application of insecticides, because adults lay their eggs in the tissues, while larvae and pupae infiltrate the stems or tubers. The removal of host plants was an effective way of eradicating of SPW, especially in small control areas such as Muroto City. Fortunately, the area of wild host plants was restricted to small areas because of the unfavorable geographic features.
The main factors in the success in eradicating the weevil from Muroto City were accurate evaluation of the eradication program through the sex pheromone trap survey, the removal of host plants by stopping cultivation of sweet potato, and the removal of the wild host plants.
Following the success of the eradication project, a new production district was established in the control area for yam ( Dioscorea japonica) and "Senryo" ( Chloranthus glaber).
The success of eradication after only three years in Muroto City, Kochi, is entirely due to the cooperation of local people and related organizations. We are grateful to everyone who contributed to the success of this project. In particular, we would like to pay our deep respects to farmers who stopped cropping sweet potato in the control area for the eradication of sweet potato weevil.
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Index of Images
Figure 1 Spread of Sweetpotato Weevil in Japan (Based on Setokuchi 1990)
Figure 2 Organization of Emergency Control Project
Figure 3 Sex Pheromone Traps Used in Control Project.
Figure 4 Arrangement of Traps for Surveying the Effectiveness of the Control Project.
Figure 5 Location of Control Area
Table 1 Monitoring of Sweet Potato Weevil with Sex Pheromone Traps
Figure 6 Control Program of Sweetpotato Weevil
Figure 7 Capture of Sweetpotato Weevil (SPW) Males with Sex Pheromone Trap
Table 2 Labor Input of Control Project
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