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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.
 Pink
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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Pakissan.com;
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