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A team from CIRAD in Réunion set itself a target of zero pesticide use on vegetable crops. By planting maize all around market garden plots, there is no need to treat against vegetable flies, the main pests on such crops. This success story is the fruit of several years' research on agro-ecological management of horticultural crops in Réunion, and will serve to cut losses and costs and above all ensure healthier crops.
In Réunion, vegetable flies are seen as the main pests on horticultural crops. Three species from the family Tephritidae are the main culprits: Bactrocera cucurbitae, Dacus ciliatus and D. demmerezi. Females lay their eggs inside courgettes, chayotes, cucumbers, pumpkins, melons and other Cucurbitaceae, and the larvae develop in the flesh of the vegetables, resulting in their destruction.
Chemical control, which was widely used for years, involving massive insecticide applications, has shown its limitations: inefficacy, high cost, and environmental and health risks.
For several years now, a CIRAD team has been working with partners in Réunion on the possibilities managing fly populations agro-ecologically, notably using plants that attract and trap flies outside crop plots.
The plant trap technique is based on fly behaviour: vegetable flies spend 90% of their time on the surrounding vegetation, where they breed, find shelter and feed, and just 10% on vegetable crops, in the case of females, to lay their eggs. This prompted the idea of surrounding crop plots with plants that attract and trap flies.
Maize plants have proved wholly effective as traps: the flies concentrate on the plants and can be killed using a food bait mixed with a tiny quantity of bio-insecticide. There is no need to spray cultivated vegetables with insecticide and market garden crop harvests are preserved.
Using maize plants as fly traps has been adopted by producers in Réunion, who can now relaunch crops that they had all but abandoned, due to excessive losses.
Combined with other techniques such as fly population surveillance and collecting fallen fruits, using plant traps guarantees agro-ecological pest management. And the results are conclusive: minimal losses, reduced costs and healthier crops, some of which have already been awarded the "organic" label.
However, the technique has other interesting side-effects too. The maize plants do not just attract the flies that damage crops, they also harbour other, useful insects, such as syrphids. These insects are both pollinators and predators, and are also indicators of a more balanced ecosystem.
By removing the need for insecticides, the maize border technique also brings useful insects back into plots, particularly insects that prey on vegetable flies, thus supplementing the range of weapons available against their proliferation.
However, it is important to remain vigilant. Unless agro-ecological management measures are respected to the letter, flies could once again invade crop plots.