Synthetic pesticides are the main method of pest management used by 92% of Thai and 100% of Vietnamese growers of yard-long bean. This confirms the findings of several previous studies showing the farmers’ reliance on chemical pesticides as a primary mode of pest control in Thailand and Vietnam.
Nine farmers (8%) in PathumThani Province in Thailand produced pesticide-free yard-long bean for a niche market; this shows that it is economically and technically possible to produce the crop without pesticides as long as there is a market for this.
Non-synthetic methods of pest management included mostly biopesticides and were used by 55% of Thai farmers and reportedly by 100% of the Vietnamese. However, further inspection and consultation with local experts in Vietnam showed that none of these were actually biopesticides.
According to local experts, pesticide traders sometimes give incorrect information to the farmers, telling them that the pesticides purchased are not harmful to their health because they are biopesticides. Very few farmers tried to control pests by removing infected plants from their field or through intercropping.
In Thailand, only one farmer had heard of trap crops and one other had heard of use of pheromones to manage arthropod pests. The average Thai growers of yard-long bean used 16.3 kg/ha of formulated pesticide products/cropping cycle. The most common pesticides used were abamectin, cypermethrin, methomyl, carbosulfan and EPN.
The latter three are highly hazardous chemicals and leave residues if the pre-harvest interval (the period between the last application of the pesticide and the safe harvesting of the crop) is not observed. Hence, it is not uncommon to have exceedingly high levels of pesticide residues.
For example, an earlier report had confirmed the presence of several toxic pesticides, including methomyl, triazophos, chlorpyrifos, methamidophos, omethoate, dicofol and dimethoate, to a maximum of 15-fold higher than the maximum residue limits (MRLs) in yard-long bean from Thailand.
In Vietnam, abamectin, fipronil, cypermethrin, l-cyhalothrin and cartap were the pesticides mostly commonly applied. The latter three belong to the synthetic pyrethroids and carbamate groups. It has already been reported that more than 75% of the pesticide products used in Vietnam against arthropod pests are organophosphates, pyrethroids and carbamates.
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Thus, the current survey also confirmed the high prevalence of carbamates and synthetic pyrethroids. Since yard-long bean is grown in smaller plots, growers usually buy smaller packs of pesticides. An earlier study has documented the discrepancy in the label information for large packages (intended for official inspection) and for small packages (intended for farmers), leading to pesticide misuse.
For instance, the pre-harvest interval for abamectin was 7 days according to the information on the large packages, but the label on the small packages mentioned only 3 days, a reduction which makes this product more attractive for farmers. Similarly, the pre-harvest interval for permethrin recommended on the large package was 12 days, but on the small package only 7 days. Other pesticides such as fipronil did not even have any pre-harvest interval information on the small packages, though on the large packages a period of 14 days (for rice and beans) was indicated.
Hence, it is apparent that pesticide use on yard-long bean in Vietnam may not be safe, since the harvest is done at least twice a week, and this may not leave sufficient time for residues to fall to levels below the MRL. In addition, pesticide use is currently higher on vegetables than on other crops in Vietnam. For instance, the pesticide use/ha in the Red River Delta is 5.52 kg/cropping season for vegetables compared with 3.34 kg/ha for rice and 0.88 kg/ha for other food crops.
Virtually all farmers in both countries felt that pesticides provided an effective means of pest and disease control, and were satisfied with their effectiveness to control pod borers. According to the farmers, the use of pesticides made their crops look healthier and more marketable, and helped them to get a better price for their produce. When asked whether their intensity of pesticide use had changed in the past 5 years, 23% of Thai farmers thought it had increased and 6% thought it had decreased.
Perceived reasons for the increase included a greater extent of pest damage and more difficulties in managing pests. These observations might point to the development of resistance against commonly used pesticides. Although resistance of legume pod borers to commonly used pesticides has not yet been documented for Thailand or Vietnam, resistance has been reported in other Southeast Asian countries.
Hence, it becomes imperative to assess the resistance of pod borers to commonly used pesticides in both countries. Of the Thai respondents, 41% indicated a willingness to try alternative methods of pest management if these were provided, but 59% responded that they were not interested as they thought these methods would not be effective. Some farmers mentioned that alternatives can be useful only if all farmers adopt them, as they were concerned that pests would otherwise concentrate in their fields.
Weekly spraying was the most common practice used by 64% of the Thai growers of yard-long bean who used pesticides; another 23% sprayed twice a week (Table 4). Of those growers using pesticides, 87% indicated that they did preventive (prophylactic) spraying; 37% indicated an increased frequency of spraying if they detected an increase in pest problems in their field (i.e. curative spraying). This is in line with earlier findings that also reported application as a prophylactic measure in Thailand.
In Vietnam, 80% of the respondents indicated that they sprayed once. a week; only one farmer sprayed twice a week. An earlier survey among the vegetable farmers in the same study sites in Vietnam also showed that about 50% of the respondents sprayed fewer than 11 times with 8–10 day intervals in one season (3–4 months) on average.
In Thailand, spraying was typically done using a power sprayer (54% of the respondents) or a mechanized pump sprayer (about 40% of the respondents). Manual pump sprayers are no longer commonly used. However in Vietnam, most farmers used either a portable manual pump sprayer or a mechanized pump sprayer (backpack type).