Biological control of weeds, pests and diseases
By Malik Asmat Ullah Awan
control of weeds, pests and diseases is cost effective,
environment- friendly and safe. It increases the yield
and productivity of crops. The traditional way to reduce
crop loss is through pesticides. Continuous pesticides
use develops resistance in pests, destructs natural pest
enemies, and leaves behind the residue in commodities.
This has given rise to a new technology the ‘Biological
control of pests through beneficial insects’.
natural pest enemies are either parasitoid or predators.
Both are important in the biological control of pests.
“Out of 1,193 species of parasites and predators about
907 are parasitoids. Of the total parasitiods 765 are
hymenoptera, 125 diptera, and 17 belong to other groups.
Parasites are the organisms that live on other organisms
or hosts. Each individual parasite completes all or a
major part of its lifecycle on the host. It provides no
benefit to host as it is gets damaged or destroyed thus
unable to reproduce. The behaviour of predators is like
a parasite with some variations in size and the mode of
damage. Trichogramma is one such parasite.
Trichogramma wasps attack the eggs of over 200 species
of moth and butterflies. These are important in
preventing crop damage as they kill their hosts before
the insects can cause plant damage.
The female wasp lays eggs within the recently laid host
egg, and as larva develops, the host egg turns black.
This process is repeated after 8-10 days allowing the
population to increase rapidly. The population of
Trichogramma is not enough to suppress pest population.
Various species and strains have varying ability to
control different insects and adapt to environmental
conditions and crops.
The frequent release of strains or species results in
better parasitism and control. It should begin with the
first moth flight, or before the pest population builds
up. Pheromones traps, black light traps, or visual
inspection are useful for monitoring the adult flight,
but regular scouting to determine the appearance of
caterpillar eggs is more accurate to establish the
presence of host for Trichogramma.
The immature eggs of wasps are glued to small cards
which are attached by the hand to infested plants. The
glued cards should be kept in warm, humid place out of
direct sunlight until adults emerge. The emerging wasps
parasitize eggs in moderate weather. The early morning
or evening is the best time to release. Avoid making
release under extremely hard, cold, rainy or windy
conditions. As regards their effects on living, these
wasps are harmless to animals, plants and people.
Adaptation of these wasps as a biological control
minimizes the use of pesticides.
Chrysoperla, commonly known as Green Lace Wing, belongs
to Neroptera (order) and Chrsopidae (family). The light
green adult has long slender antennae, golden eyes, and
large, veined, guaz like wings. It is slow flying,
nocturnal insect that feeds on nectar and pollen.
The female lays green eggs which after two days turn
into grey and is usually found in groups on leaves. Eggs
are held away from the leaf surface on the end of a
slender stalk. A female has the capacity to lay 300 eggs
over period of 3-4 weeks but often it does not survive
that long in the field.
The larva is called green grey alligator and has mouth
parts like ice tongs. It seizes and punctures the prey
with sickle-shaped jaws. It injects the venom and
paralyzes the host. When the size of larva grows to 1/2
inch, it spins spherical white silken coccon for
pupation. It has voracious nature and consumes up to 200
aphids and is called aphidlion.
In addition to aphids, it eats mites, along with soft
bodied insects, including insect eggs, thrips, immature
white flies, mealybugs and small caterpillars. It
consumes a wide variety of insect pests of cotton and
Care should be taken because aphidlion can also consume
each other on the unavailability of prey. The larva of
green lace wing has food searching nature and can travel
to 80-100 ft in search of prey. It leaves the area after
exhaustion of food source. It feeds for 2-3 weeks before
it becomes adult.
An adequate food supply such as nectar, pollen and honey
dews and suitable habitat can contribute to lace wings
remaining and reproducing in the crops. There is
additional supply of larvae if adults stay and
reproduce. For Trichogramma, two or more successive
releases made at two weeks interval are better than a
The effects of pesticides on natural enemies are direct
and indirect. Direct effects include short and long-term
due to a straight contact with the pesticides or its
residue. In indirect, the impact of pesticides is
mediated through the host or prey. The most immediate
effect of pesticide on natural enemies is short-term
mortality (up to 24 hours after contact).
The complete biological control of crop pests and
subsequent elimination of pesticide use is not always
achieved. An Integrated Pest Management (IPM) programmes
is required to control those pest species which are not
adequately checked by the natural enemies.
However, biological and chemical controls are often
incompatible because pesticides severely reduce natural
enemy populations. Thus insecticide applications can
disrupt this and may cause outbreaks of secondary pest
population previously suppressed by the natural enemies.
The detrimental effects are compounded when pest species
develop pesticide resistance but natural enemies do not.
Awareness to biological control is the main challenge.
Ad hoc attempts rarely prove successful as these
solutions require many years to be identified,
developed, and put into place. Success requires
commitment of scientists with adequate support
facilities. This requires an understanding of population
and ecological functions of each system. Long-term
commitments and stable institutions and laboratories are
vital to the success of most programmes.
Courtesy: The DAWN