Weaver Ants - Traditional pest management method under threat
The Vietnamese tradition of using weaver ants to protect citrus crops is in danger of being lost, according to a farmer participatory specialist having studied the practice over the past six years.
Intensification of horticulture and subsequent pressure from chemical companies are to blame, says Dr Paul Van Mele of CABI Bioscience, but encouraging farmers to reintroduce the method could offer an economically and environmentally viable way to sustain fruit production systems in Vietnam and beyond.
Natural pest management
Traditionally, Vietnamese farmers had a broad experience of natural pest management. Before the 1990s, fruit growers controlled insect pests by conserving a multitude of locally occurring natural enemies, including insect predators and parasitoids (small wasps that lay their eggs on or in a particular pest). This helped to keep pest numbers within acceptable limits and there was little neeed for use of pesticides.
In particular, use of the Asian weaver ant or Oecophylla smaragdina as a pest management method - a practice originating from China - has been used to protect citrus crops in Vietnam for centuries. Weaver ants get their name from their habit of binding living leaves with silk to form communal nests in trees. They feed on a variety of insects including the citrus stinkbug, leaf-feeding caterpillars, aphids and the citrus leafminer, which attack orange, tangerine, lemon and pomelo trees and their fruit. Use of the weaver ant as a pest management method is reported to protect both fruit and nut crops from pests and can also deter small rats. Other benefits include increases in cashew yield, increases in mango fruit sets and an improvement in citrus fruit quality and yield. Some farmers also use the ants as a form of weather forecasting, as a change in the insects' behaviour can denote an impending storm.
Increased pesticide use
Vietnamese fruit production intensified in the 1990s, when farmers converted paddy field into orchards to benefit from the significantly higher profits. . Newcomer fruit farmers lacked any knowledge of natural pest management, which led to a dramatic increase in pesticide use and a decline in the use of traditional methods. The weaver ant is now mainly confined to more extensively managed orange orchards.
With the increased use of broad-spectrum pesticides the number of pollinators and other natural enemies such as ladybirds, spiders and parasitoids have been affected. In some cases, over-use of chemicals has induced new pest problems by killing beneficial organisms and producing chemical resistance in pests - a phenomenon called 'pest resurgence'. This has, for instance, led to an increase in citrus leafminer and mites.
Farmers are responding with increased spraying, even some traditional citrus growers are resorting to chemicals to keep on top of pests coming in from surrounding areas. Investigations suggest that in the Red River Delta in northern Vietnam, none of the citrus farmers practise weaver ant husbandry anymore, while in the Mekong Delta it is mainly older citrus farmers who keep the tradition going.
However, attitudes are beginning to change. Rising chemical application costs, pest resistance and the imposition by many industrialised countries of strict limits for pesticide residues are causing some farmers to question their intensive farming practices. The premium on organic fruit in the West is also a powerful incentive to convert to greener farming methods.
"Vietnam is witnessing a growing interest in a return to cheaper, more environmentally friendly means of growing fruit," says Van Mele. "We hope that by exploring and building on local weaver ant husbandry knowledge through consultations and workshops in other provinces and countries, and by linking to organic traders in the West, these ancient practices can be revived and safeguarded for future generations."To contribute your experiences of weaver ant husbandry to this study, contact Dr Paul Van Mele, CABI Bioscience, Bakeham Lane, Egham, Surrey, TW20 9TY, UK Fax: + 44 1491 829100
Further information is available in a practical, illustrated manual "Ants as Friends: Improving your Tree Crops with Weaver Ants" (shown above).
To order copies, please contact Dr. Paul Van Mele.