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Saving cotton from the pink bollworm
By Dr Zahoor Ahmad

THE cotton crop is hit by many pests but pink bollworm is its worst enemy which not only causes loss to crop but also affects the lint quality. The economic loss it inflicts is heavier than the impact of any other worm. Depending on the extent of infestation and weather conditions, this pest can cause about 20 to 30 per cent crop loss. It is estimated that the yearly loss from pink bollworm in Pakistan is about one million bales.

Saving cotton from the pink bollwormPink bollworm (Pectinophora gossypiella-Saunders) is an important pest of cotton throughout the world, though it is more serious in Asia and Africa than in America. Along with the yield reduction, it also damages the lint quality by causing yellow spots in its fibre thus discounting the cost in the international market.

In 2001, Pakistan faced difficulty in disposing off its lint in the international market as the problem had struck more than once. The government and growers must join hands to eliminate or at least minimize its impact on the crop.

During active season, pink bollworm completes four generations on cotton and the larvae of the fifth generation start entering diapauses when temperatures start falling. Diapause is a state in which the insect remains inactive for months due to the climatic conditions or the lack of food. Its activity is a complex phenomena affected by oil contents, temperature and the day’s length.

Most of the leftover bolls on cotton sticks have diapausing larvae at the end of cotton season. Such bolls do not open and stay on cotton sticks. They are the main source of carry-over of pink bollworm from one season to another. Pink bollworm larvae hatched from the eggs take 30-60 minutes to enter the fruit. Most of it enters through the base of fruits, leaving no mark of entry.

Larvae do not survive if an infested fruit sheds at an early stage. It continues to feed on internal parts of fruit and later stitches the petals together, preventing the flower from opening, a condition known as “rosette flower”.

In green cotton bolls, larvae generally enter through the tip of 14-28-day old bolls) leaving a yellow spot on the lint at entry point. It feeds on the boll contents, including the lint near the entry point at least for 24 hours. It feeds on lint, seed coat and kernel completely before attacking the second seed.

Two seeds are sufficient to complete larval development but some of the population also feeds on more seeds within the same boll. A week old infested boll remains attached to the plant and dries up. If the boll is attacked when it is half-grown, it does not shed but exhibits partial damage and early opening. The rate of infestation varies considerably and depends mainly on climatic factors such as the temperature and rainfall.

Frequent rains during August and September provide favourable conditions for pink bollworm infestation. Its larvae leave the bolls for pupation on soil during different times of the day. Maximum number leaves the bolls for pupation during midday, around 1200-1300 hours and almost 69-75 per cent larvae came out of bolls between 1100 to 1500 hours when the temperatures are at their highest. The rate of dropping to the ground decrease gradually after 1500 hours and the decrease is at its minimum at night. Pupation occurs either in the top 5cm of soil or in the soil crevices, mostly under the plant periphery. Pupation period is 6-10 days during the summer season. In November, larvae enter diapauses in the bolls or in the soil until conditions become suitable for their emergence and for starting a new life cycle.

Control strategy: Pink bollworm is a mono-host pest and hence, can be easily managed provided all necessary precautions are taken. Chemical control is difficult because the insect spends most of its time within the fruiting bodies and is not exposed to direct chemical spray. Only pesticides with good residue can help in controlling the pest at egg-hatching stage. This means 4-6 sprays at weekly intervals. Excessive use of pesticides always results in a flare up of secondary pests like army worm, American bollworm and spider mites because sprays perish most of their natural enemies.

The best control of pink bollworm has been demonstrated with the use of pheromone. Fields treated with pheromone also have a higher population of predators and parasites that are an important component of integrated pest management. Fields where pheromone products were applied had negligible moth activity compared to chemically controlled cotton fields. The yield of seed cotton improved in pheromone treated fields. A new formulation of pheromone has recently been developed.

Experiments carried out at the Central Cotton Research Institute, Multan over the past 20 years have proved that this method is environmentally safer and helps in controlling pink bollworm. Trials conducted in Pakistan since 1974 have proved that pink bollworm can be controlled through the communication disruption technique by releasing pheromones in the cotton agro eco system and disrupting inter-moth communication.

The population of natural enemies was also observed in the pheromone and chemical treated fields. It was found that various pests that counter pink bollworm were in abundance throughout the cotton season in the blocks treated with pheromone whereas in chemical treated, the population of natural predators was negligible.

These natural enemies also help in controlling the secondary pests like American bollworm, spider mites and army worm because they are exposed to the attack of natural enemies. The control of pink bollworm in this way not only improves the yield but also increases the quality of lint.

The pink bollworm larvae feed on the seed and affect the germination quality of the seed. The loss in weight of seed cotton will be there. The oil content is also affected. In fact, pink bollworm also reduces the oil content and quality of cake. In financial terms, pink bollworm is one of the most damaging pests of cotton. Moreover, the damaged seed does not germinate.

Courtesy: The DAWN
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