Mangoes are an important smallholder and commercial crop in Vietnam, Thailand and other Southeast Asian countries, and one of the most important commercial crops in northern Australia. To achieve good yields with top quality fruits, mango growers currently rely on regular pesticide applications. This leads to increased costs, the reduction of natural predators and parasitoids that help control the insect pests, increased pest resistance to insecticides, pesticide residues in the fruits and environmental pollution.
Green ants are efficient predators of a wide range of insect pests in many tropical fruit crops and they are abundant and widely distributed in Southeast Asia. Previous work in Indonesia and preliminary results from cashew crops in northern Australia indicate that green ants can control some of the main pests of mangoes.
Experiments in a mixed-cropping mango orchard using the ants together with limited applications of insecticides indicate that an integrated pest management (IPM) model for mango orchards can be constructed. Since Vietnam, Thailand and Australia all share similar insect pests of mangoes there exists common ground for research involving the three countries.
The project aimed to develop an IPM program that combined the use of green ants as a major biological control agent with other agricultural strategies and the selective use of insecticides - with the intention of reducing populations of homopteran pests (such as leafhoppers, aphids and scales) in mangoes in Vietnam, Thailand and Australia.
Green ants were studied for their potential as a major biological control agent against caterpillars, bugs and beetles. As well, certain farming strategies were introduced to increase population sizes of natural enemies, and judiciously selected insecticide chemicals were tested for their effectiveness in controlling scales, mealybugs and fruit flies without devastating green ants.
Surveys, observations and experiments were conducted in the field and the laboratory in Thailand and Vietnam to determine:
the effect of green ants on key insect pests in mango orchards;
the relationship between green ants, homopteran pests and their natural enemies;
the effect of mix-cropping on the insect pests and their natural enemies;
methods to reduce ant activity and aggressiveness to people during harvests;
a method to stabilise green ant populations in mango orchards, including moving them from mangoes to other trees and bringing them back from other trees to the mangoes, determining other suitable hosts for ants and controlling other competitive ant species;
how to best apply a range of pesticides suitable for reducing homopteran insect pests populations without devastating green ant populations.
In Vietnam and Thailand the yield and the fruit quality produced in the treatment with weaver ants plus environmentally-friendly soft chemicals (WPS) were similar to, or higher than, those with the chemical insecticide treatment (CI). In the Northern Territory, Australia, the profit from WPS, or from trees with abundant weaver ants, increased by more than 70% compared with profits using CI or in trees without weaver ants.
In comparison to chemical insecticides, weaver ants effectively controlled the major mango insect pests - including mango leafhoppers, thrips, fruit spotting bugs, flower caterpillars, seed weevils, fruit flies, elephant beetles, stem borers, planthoppers, leaf cutting weevils, web caterpillars and seed borers. Although weaver ants have a mutual relationship with scales and mealy bugs, the soft chemicals (such as white oil, D.C.Tron oil, Neem oil, potassium soap, Biobit, Beauveria or Applaud) effectively reduced the populations of these pests -and these soft chemicals are safe for weaver ants.
Weaver ant formic acid caused black spots on fruit skin, mainly due to ant fights between colonies. Separation of weaver ant colonies effectively reduced levels of fruit damaged by the formic acid. Weaver ants also annoy people working in the orchard during the harvest. Water spray greatly reduced the ant activity prior to harvest, and it did not cause fruit lenticels or postharvest diseases. Mango growers are pleased with this method in Australia.
In Thailand, farmers found a picking pole or net with a long handle plus shaking action after a few picks was useful in reducing the disturbance from the ants. In Vietnam, two methods were used to reduce ant disturbance: (1) to rub wood ash on the main branches of the relevant part of the tree to break ant trails when harvesting; and (2) to lure the ants away from the trees, which will then be harvested or pruned. Weaver ants do not have a detrimental impact on the level of parasitism against homopteran pests in mango orchards.
The mixed cropping orchards harbour more natural enemies of the insect pests compared with the monoculture orchards. To stabilise weaver ant populations in mango orchards, it was essential to isolate the ant colonies, transplant ant colonies with queens and mix mango trees with other tree crops such as citrus. If mango orchards are monoculture, ant food must be provided when trees are in dormancy.
Two types of IPM program, for both organic and conventional growers, have been successfully developed for mango growers in Vietnam, Thailand and Australia. These programs are friendly to the environment, and they allow mango growers to significantly cut chemical insecticide use and to produce insecticide-free or organic fruits. Step by step guides to using the IPM program can be obtained from booklets, video (DVD and tapes) and a series of posters (in either English, Vietnamese and Thai) developed through the project. They are easy for farmers to understand and to use.
Links:
[1] http://www.aciar.gov.au/country/Thailand
[2] http://www.aciar.gov.au/country/Vietnam
[3] http://www.aciar.gov.au/programarea/Crop Protection