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No place to hide for BFSB

Vegetables that look and taste good to consumers are invariably also attractive to pests. Enhanced aroma and flavour, together with softer flesh, also make pest damage more likely and more damaging. This is especially true of pest damage that gives rise to serious blemishes, which result in downgrading of produce for sale, or even make it worthless. A good example is the eggplant, aubergine or brinjal (Solanum melangena), now popular all over the world but especially important as an ingredient or accompaniment of curries in the countries of South Asia. Indeed, for the poor, brinjal (the preferred name in the region) is the only affordable vegetable available during the hot, rainy season. But it is at this time of the year when brinjal is most susceptible to the very damaging Brinjal Fruit and Stem Borer-BFSB (Leucinoides orbonalis).

Packing brinjal for market
Credit: Alan Cork, NRI

In the absence of control measures infestation of plants has been shown to be as high as 80-90 percent in Bangladesh and 100 percent in India. In desperation, growers have come to spray daily, some using a cocktail of insecticides, and they spend almost 30 percent of their total production costs on purchase of chemicals alone. If application costs were included the proportion of plant protection in total production would be even greater. Despite this expenditure and labour, the results are not satisfactory, not least because the pesticides are not proving effective and the significant outlay on pesticides is wasted. In addition, the frequent spraying is exposing growers and their families to sustained exposure to skin-absorbed chemicals and, since picking continues over a long season, spraying overlaps with harvesting and high levels of residual spray deposits in fruit are a serious hazard to consumers.

When spraying reduces crop yield

The gravity, not to say irony, of the situation was shown by field trials in Bangladesh in 1998, which revealed that intensively sprayed plots of brinjal yielded less than plots that were left completely unsprayed. What this demonstrated was that naturally occurring parasitoids can have a significant role in reducing BFSB. However, intensive spraying kills the parasitoids and, in their absence, pesticides now fail to control the pests, leaving a clear field for rampant pest population increase.

In studying the options for BFSB management, Dr Syed Nurul Alam of the Bangladesh Agricultural Research Institute (BARI), and Dr Alan Cork of NRI, UK, have concluded that an integrated approach to pest management offers the best option for this pest, with potential lessons for other vegetable pests including diamond back moth in brassicas. They have noted several good parasitoids for BFSB in Asia, the best being Trathala flavoorbitalis. While the pest is now resistant to systemic pesticides, which fail to kill the pest larvae deep in the plant tissues, these parasitoids locate and parasitise the BFSB larvae unerringly and to devastating effect.

However, parasitoids are only one element of this IPM approach: also important are the use of pheromones and strict crop hygiene. The pheromone for BFSB was identified more than two decades ago but it was more recently that Dr Cork and others, including Dr Alam, tried it in the field. Initially used for trapping to indicate pest numbers, the pheromone traps caught so many males that it was realised that such trapping could itself reduce the pest population very significantly. And, this has proved to be the case in practice. But, while pheromone traps can reduce pest numbers significantly, (down to 10 percent of plants infested, estimates Dr Alam) reduction is further enhanced if growers regularly inspect their crops and remove and destroy all tissues that are seen to have been attacked by BFSB.

The benefits of this IPM programme have been quickly appreciated by growers. The cost saving is dramatic in itself, down from Taka 56,000 (US$ 1,200) per hectare per season to Taka 6 to7000 (US$ 100). But the farmers also recognise that the time required for servicing traps and crop inspection is far less than that for daily spraying, and there is no risk of contamination with hazardous chemicals. Funded by the UK's Department for International Development, and managed through the Asian Vegetable Research and Development Center (AVRDC), the development of this IPM strategy, in particular refining the use of pheromones, promises economic and health benefits to growers and consumers alike. Simultaneously, plant breeders at BARI have been developing BFSB resistant brinjals and three varieties show promise. So, whatever one chooses to call this vegetable, and that largely depends on regional preference, its major pest appears to have succumbed to science-and good husbandry.

For further information, contact: Dr Syed Nurul Alam at entoipm@bdcom.com and Dr Alan Cork at a.cork@greenwich.ac.uk

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