Essay on a leap Towards Safer Pesticides

It has become essential for mankind to curb the ever-growing losses caused by insects, pests and unwanted herbs to feed an ever-growing human population. The chemical methods of insect pest and weed control appear to be a convenient solution to the problem.

However most of the pesticides and related chemicals are violent poisons which in addition to suppress the pestilent organisms also harm non-target organisms. Apart from toxicity and threat to the environment the unequivocal demonstration of carcinogenic, teratogenic and mutagenic action of many pesticides adds a frightful dimension to their continued use.

Needless to say, all of the pesticide formulations in use these days are not so dangerous. A large number of pesticides are rapidly degraded in the environment or in the living organisms to yield simple non-toxic constituents. When used judiciously much of the harmful consequences of application can be avoided.

However, due to ignorance or economic considerations, a large number and quantity of such pesticides are freely used which produce an array of immediate and delayed tragic consequences. For example, compounds like DDT, Gamexene, Toxaphene etc. and the whole assortment of organochlorine derivatives are highly persistent chemicals which stay in toxic state for long duration of time and are shown to cause carcinogenic, teratogenic or mutagenic effects.

But their long lasting action and low cost are the chief reasons for their continued use. Organophosphate derivatives and carbamates are costly as they decomposed rather faster than organochlorine pesticides and due to their nominal residual action have to be applied more frequently.

The chemical methods of insect, pest and weed control have other drawbacks also. Repeated application of these chemicals results in the development of resistance and cross-resistance among the target organisms. Population resistant to one pesticide or one type of pesticide may also develop a certain degree of resistance to other pesticides as well.

The development of resistant populations’ necessitates increasingly larger subsequent doses and ultimately a point may be reached when the effective control becomes costly, potentially injurious to non-target organisms and is no longer of any practical utility. A satisfactory solution to the problem, therefore, can no longer be found in chemical methods.

A number of methods are available with which an efficient insect, pest and weed control can be achieved. Often a single method is not enough and an integrated approach which involves simultaneous application of more than one method is required. These methods can be grouped into the following major categories:

(1) Mechanical Control:

Hand picking, capturing and killing insects and pest by hand nets, shaking the plants to dislodge the organisms, sieving to remove the insect in case of grains or excluding the pest from plants by screens etc. are some of the common means of mechanical control. These methods, are however, labour intensive devices and hence are often too expensive for large establishments.

(2) Physical Control:

Physical control involves manipulation of physical factors like temperature, moisture, light, radiations etc. to curb the population of pestilent insects. These methods are feasible in case of food grains only.

(3) Cultural Control:

Cultural control involves manipulations in regular agricultural practices to minimize the level of economic injury. These methods require such adjustments as ploughing, sowing, irrigation and harvesting time, crop rotation using resistant varieties, pruning, stimulating vigorous growth, mixed cropping etc. Though cheapest of all methods, cultural control is often insufficient against a number of pestilent insects.

(4) Biological Control:

Insects and pest may also be controlled by introduction, encouragement and strengthening of biological agencies which normally restrict a population within reasonable limits. Just as the nuisance caused by rats in a house-hold can be checked by introducing a few cats, pestilent insects can be kept under control by introducing or encouraging their natural enemies.

Bacillus thuringiensis has successfully been used to suppress the population of many coleopterous and lepidopterist larvae in Russia, USA and many European countries. The spores of B. thuringiensis are dusted or their suspension in water is sprayed over the plants. This bacterium parasitizes a number of insects. It produces inclusions which are crystalline proteins, termed delta endotoxins when forming spores inside the body of host organisms. These crystals are toxic to a variety of insects and pests.

Similarly, many viruses such as cytoplasmic polyhedrosis virus, nuclear polyhedrosis virus, entomopox virus etc. have also been tested as insecticide. A nuclear polyhedrosis virus which parasitizes Heliothis species has been extensively studied and is now the world’s first viral insecticide being successfully used against two serious pests of cotton. Nuclear polyhedrosis virus is also very effective in checking the out-break of douglas fir tussock moth.

Likewise a number of predatory insects, birds, frogs and reptiles play a significant role in curbing the population of pestilent insects. Introduction of Vedalia lady beetle, Rudolia cardinalis in California as far back as 1890, effectively controlled the population of fluted scales which were almost completely eradicated within five years from the area.

The block-headed caterpiller, Nepentis serinopa a serious pest of coconut palm has been successfully controlled by Perisierola nephantidis which eliminates its grown up caterpillers. Another organism which can be used to control the population of coconut insects is Trichospilus pupivora which feeds on pupae of Nepentis serinopa. In India sugar cane borer, Chilo sacchariphagus is effectively controlled by Trichograma australicum which destroys eggs of C. sacchariphagus.

Similarly insectivorous birds and other such animals which feed on pestilent insects could be reared, protected and introduced into the fields to prey on insects. In South India ducks are allowed in paddy fields to feed on the stripped bug. Tetroda histories. Giant Mexican toads were introduced in Hawaiian Island to reduce the population of the white grubs of sugar cane.

Biological control, therefore, offers a promising means of combating the problem of insect and pests. We do not have to apply any poisonous or toxic substances on the insects and as such there is no danger of environmental contamination or causing harms to non-target organisms. However, this method requires a lot of skill and understanding of the behaviour and prey-predator relationship of the organisms concerned which is often not available to a common farmer.

(5) Hormonal Control:

The complicated changes which occur during the development of an insect are controlled by hormones secreted from neuro-endocrine system of the insects. An important enzyme present in the younger stages of many insects is Juvenile hormone which prevents metamorphosis of the organism to adult reproductive stage.

Juvenile hormone interferes with embryogenesis hence their applications disrupts reproductive process of the insects concerned and reduce their population. There are a number of synthetic chemicals which have the same action, as Juvenile hormone, on number insects.

These are called juvenile-hormone-analogues which can be conveniently prepared in laboratory or commercially manufactured. One such analogue is Altocid or Methoprene which is very effective against mosquitoes, hornflies, stable flies etc. and has been approved by the Environmental Protection Agency on an experimental basis. The half life of altocid is only two hours and within 24 hours its chemical residue disappears from the waters treated while its activity persists for many days.

The prothoracic gland of insects secretes moulting hormones or ecdysone which is responsible for moulting, pupation and maturation of insects. This hormone can be used to bring the insect in developing conditions during the wrong seasons thus forcing them to commit ecological suicide. One such compound has recently been isolated from Eichornia crassipes (Jalakumbhi) by scientists of Regional Research Laboratory, Hyderbad, which is very effective against the rice moth Corcyra cephalonica.

(6) Pheromonal Control:

The group of chemicals secreted by insects to attract their mate during reproduction is termed as pheromones. A large number of pheromones have been identified and their usefulness as bait or an attractant to collect a large number of insects to be killed or herded away from the opposite sex has been proved. Some of these compounds are simple chemicals like phenols which attract the grass grubs while others have a very complicated structure. Synthetic methyl eugenol has practically been used for controlling fruitflies.

(7) Control by Sterilization :

Control of insect’s populations by sterilization was first introduced by Knipling in USA in 1930. It involves collection, sterilization and release of sterilized male insects. These sterilized individuals mate with fertile females and result in sterile eggs. In Alabama, Florida and Gerorgia screw worm population has been successfully reduced in the way.