Introduction

Agricultural production in the tropics is seriously affected by pests, i.e. insects, weeds, disease and rodents in particular.  These pests are responsible for substantial crop losses during crop production and in storage.  The pest problem is more severe in the tropics because of the year-round favourable climatic conditions that prevail in this region.

The pest problem has been aggravated by the intensive crop production practices now being adopted.  These practices include:

	monoculture
	increased cropping frequency
	use of high yielding but pest-susceptible varieties

The increasing difficulties experienced with pest attack have lead to a concomitant increase in use of chemical pesticides.

These pesticides are "chemicals which kill pests" thus

	a pesticide which kills insects is an insecticide
	a pesticide which controls fungal diseases is a fungicide
	a pesticide which controls weeds is a weedicide/herbicide
	a pesticide which controls rodents (rats) is a rodenticide

Pesticides are being widely used because they have been found to

	be convenient, and
	provide quick control by reducing pest populations to extremely low levels.

A pesticide is comprised of two components

	an active ingredient (a.i) - which is responsible for the chemical activity of the pesticide, and
	a carrier, an inert material, in which the a.i is impregnated.

Toxicity of Pesticides

Pesticides are poisons and can become more hazardous by improper use.  It is important that the user, whether farmer or spray operator, is fully aware of the potential hazards of products.

The commonly accepted measure of toxicity for any chemical is the LD₅₀ (Lethal Dose₅₀).  This is the dose of chemical required to kill 50% of the animals under test - usually rats.

The lower the value of the LD₅₀ the more toxic the chemical, since it requires less chemical to kill.

Based on the LD₅₀, pesticides are placed in HAZARD CATEGORIES.  The system also recognizes that toxicity alone is an insufficient indicator of hazard; formulation type is also important since, in general, a solid formulation e.g. wettable powder, granules, is safer to handle than a liquid formulation.

Routes of Pesticide Entry into the Body

Pesticides can enter the body by three routes

	Orally; through the mouth
	Dermally; through the skin - by far the commonest route of entry for pesticide users.
	Through breathing and inhalation.  When this happens it results in rapid poisoning.

Oral Ingestion

This is usually an accidental route of entry.  The consequences of oral ingestion are usually more serious than dermal absorption.

In practice, the majority of classification will be made on the acute oral LD₅₀ value.  However, dermal toxicity must be considered since it has been found that, under most conditions of handling pesticides, a high proportion of the total exposure is dermal.  Classification based on dermal data in a class indicating a great risk is necessary when the dermal LD₅₀ values indicate greater hazard than oral LD₅₀ values.

Generally, insecticides usually fall into the more hazardous categories while most herbicides and fungicides are safer to apply, being mainly Class III or unclassified.

Dermal Absorption

This is a route of entry that most farmers and pesticide users are unaware of, thus due care and attention is not paid to contamination of clothing and the body.

Poisoning by this route can occur through

	spillages onto skin during mixing
	wearing contaminated clothing
	continuous exposure to spray

Extremely vulnerable parts of the body are

	the face, especially eyes
	mouth, and
	genital area

Extra care needs to be taken during hot weather because sweating increases skin absorption.

Inhalation

Inhalation of pesticide vapour occurs

	when chemicals are used in enclosed spaces
	improper spray techniques
	failure to use respirators

Effects of the use of pesticides in agriculture 

Insect pests as a group seem to be most difficult to control, as a result of which farmers seek to apply more an more powerful pesticides with increasing frequency in order to prevent devastating crop losses - a process described as the "pesticide treadmill". 

The hazards posed by indiscriminate use of pesticides are related to the environment, and human health.  These chemicals pose wide spread risks because of

	their acute and chronic toxicity to living organisms, and
	their persistence in the environment, or cumulative properties.

The environmental hazards can be

	pollution of waterways and aquatic organisms
	the destruction of non-target organisms, e.g., bees and spiders.
	the accumulation of toxicants in flora and fauna.

The hazards posed to humans take the form of

	residues in food due to inappropriate use of pesticides
	contamination of the body due to exposure

Hints to Decrease the Hazards Caused by Pesticides

	Always read the label.
	Use the recommended dosage.  Do not apply excessive dosages since this can leave high residues in the food material.
	Avoid the use of banned pesticides.  They are banned for good reasons.
	Do not wash spilled pesticides into waterways.
	Wear protective clothing and observe all safety measures when handling and applying pesticides.
	Observe the withdrawal period i.e., the period between pesticide application and harvesting.
	Avoid pesticide application during the hot part of the day.
	Never eat, drink or smoke following spraying without first washing hands well.
	Never store pesticides in drink bottles or food containers.  Keep them in their original containers in locked stores.
	Do not transport pesticides with food in order to prevent cross contamination.

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