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Biological control of weeds, pests and diseases
By Malik Asmat Ullah Awan

THE biological 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’.

The 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 sugarcane.

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 single release.

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
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