Nepal's citrus industry satisfies fruit demand from China besides fulfilling domestic requirements. The government of Nepal has signed a trade agreement with the Chinese government for the export of mandarin and sweet orange from Syangja and Sindhuli districts respectively.

Citrus accounts for about half of the overall fruit production, with mandarin making up nearly 30 percent of the citrus output. Besides mandarin, sweet orange, several varieties of lemon and lime are typically grown over 55 mid-hill districts. A few limes like sun kagati are grown in the plains. A healthy tree bears more than 150 to 400 kg of fruits that translates into 15-40 tonnes per hectare annually. But the question is this: Are we even ready to capitalise on the production potential of citrus?

A drop in production

Citrus is a significant cash-generating crop in the country, but it can be devastated by an invasive insect pest, the Chinese citrus fly. This fly has caused an unprecedented drop in production in the past few years. The infliction of the citrus greening pathogen from several developmental programmes wiped out the industry in citrus pockets in Kaski district. The Chinese citrus fly now poses a threat to citrus groves with up to 100 percent of the pre-mature fruits dropping before the harvest period which has sparked anxiety among growers.

We often find numerous dirty whitish maggots inside the fruit. Perhaps nothing has hit the farmers harder than this pest. The underlying cause that has exacerbated its spread pertains to ill-equipped control measures and poor research. Despite being first detected in the early 1980s, the species was recognised only a couple of decades later in 2007. Until then, it was wrongly identified as another species, and all governmental approaches went in vain.

Once prevalent only in tight-skinned citrus, such as sweet orange, acid lime and lemon, its invasion is increasingly seen in loose-skinned species like the mandarin. The Chinese citrus fly likely migrated from southern China and made its way towards eastern Nepal through north-eastern India and Sikkim. This year, the fly was even seen in western parts of Nepal like Syangja, Lamjung and Gulmi.

The Chinese citrus fly has a high rate of survival and successful infestation. The female, with its sharp ovipositor, deposits about 70-750 eggs by piercing the fruit up to the pulp, unlike other species that only penetrates the peel. Developing fruits that are about 11 millimetres in diameter are the fly's favourite target. This is the prime reason why almost the entire acid lime crop was destroyed, as these tight-skinned fruits develop into the critical size slightly before loose-skinned fruits.

Numerous dirty creamy-white maggots develop inside an attacked fruit that consume all of the pulp, consequently inducing early ripening. The fruits become yellowish in colour and eventually fall from the plants at the beginning of November. Sweet orange and mandarin, if not attacked, are harvested from mid-November. What makes it difficult to curb the invasion is the resting behaviour of the pest. Its pupae are enclosed in a hard covering, and they overwinter in the soil, sometimes as deep as 45 centimetres in the ground and for nearly five months. As a result, the pupae are protected from the cover sprays and other pesticides that are applied on the surface.

So, for the management of the pest, it’s crucial to know the time of oviposition and emergence of adults from the pupal covering. The period varies from one place to another. It is almost impossible to achieve eradication in a short span of time. But this challenge should only make our work more focused and solution-oriented. We need to devise a community implemented integrated pest management technique that counts on the contribution of all players in the citrus industry. Since the main drivers of the spread are probably the shipment of infested fruits and the excellent flying ability of the adult fly, a researcher should be careful to apply protein hydrolase bait to attract female flies and kill them after their emergence.

Chemical pesticides

Farmers should clear the fruits that have dropped by packing them in airtight plastic bags to kill the maggots inside within a day. Before the emergence of the fly from the pupae, cover sprays of chemical pesticides will be highly effective. Stakeholders need to feel a sense of urgency to help threatened farmers. If they fail to do so in time, the consequences could be even worse. The Area-Wide Control Programme adopted in Sindhuli by the Junar Superzone, which replicated the method followed in China, has stemmed the invasion and restricted infestation to the minimum possible.

One key step for the effective control of this pest would be shifting our extension paradigm. Rather than simply transferring knowledge from research stations to the farmers’ fields, an experiential learning method will enable both researchers and farmers to acknowledge site-specific problems and act on this basis. Coordination with the community will help researchers to learn about the local variation of pest phenology, which is a vital piece of information to control the Chinese citrus fly.

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