1.4 Chemical pesticides and their properties

The many types of chemical pesticides can be very different and are used in different contexts. There is a sliding transition between the definitions of preservatives, medicines and pesticides. In this booklet, only pesticides distributed in the environment to control pests will be described.

Pesticides are organised according to what category of organisms they are used against. They can be categorised as follows:

Herbicides - against weeds.
Fungicides - against fungi.
Insecticides - against insects.
Acaricides- against mites.
Nematicides - against nematode.
Rodenticides - against rodents.

Although each category of pesticide is intended for a particular group of weeds or pests, one should be aware that most chemical pesticides can have a varying degree of poisonous effect on other types of organism than the one the pesticide was intended for. The significance of the natural enemies of insect pests has been stressed in integrated protection management (IPM) projects (see ch. 1.5). The most important natural enemies are other insects, and comprehensive insecticides will often have an equally strong effect on the utility insects as on the insect pest. Experience shows that fungicides and herbicides can have a negative effect on the natural enemies of insects. In Table 4 at the end of this booklet, an attempt has been made to show the toxicity of the most important groups of pesticides to the natural enemies of insect pests.

All chemical pesticides are more or less poisonous and can cause health damage. Depending how harmful they are to humans and farm animals, they have been classified in one of the follwing toxicity groups:

Toxicity group X = Highly toxic compound.
Toxicity group A =Toxic compound.
Toxicity group B = The compound can seriously damage your health.
Toxicity group C = The compound can cause some damage to your health.

Tables 1-3 at the end of this booklet offer an outline of the toxicity groups of various fungicides, insecticides and herbicides.
The chemical pesticides can be classified into several main groups, based on their chemical structure and biochemical reactivity mechanisms. Chlorinated hydrocarbons (DDT and others) were very common compounds in the earliest pesticides, but such compounds have now been banned in most countries because of slow degradation and a disruptive effect on the food chain. Chlorinated hydrocarbons were replaced by organic phosphor pesticides. Synthetic pyretroids are yet another, more recent, group of pesticides.

Pesticides can be used in the following situations:

· Cultivated plants:

- To spray cultivated plants to protect against attacks from fungi, insects and mites etc. This will be of particular importance in monocultures, and for export products (cotton, fruit etc.).

- To use herbicides to attenuate or kill rival plants (weeds) which inhibit the growth of the cultivated plants.

- Remove unwanted vegetation prior to bringing new crops into cultivation.

· Seed: Seed treatment to protect against insects, fungi in the soil or fungi which can be transmitted by seed.

· Accumulation of pest organisms:

Spraying of such organisms, for example to control migratory grasshoppers.

· Other situations: Spraying water sources to improve visibility, passability etc. Chemical control of water plants can be required to improve conditions for fishing or transport. Spraying to remove unwanted vegetation in connection with right-of-way (ROW) clearance for power lines etc. Routine control of the vegetation or weeds along roadsides etc.

Some chemical compounds are not used for plant protection, but in the following situations:

· Farm animals: To kill ectoparasites on farm animals by bathing them in so-called cattle dips in a pesticide solution (see booklet No.2, "Animal husbandry").

· Products in storage: To spray agricultural products, fish products etc. in order to protect them during storage.

· Living conditions for intermediate hosts:

Spraying water sources or vegetation areas which serve as propagation areas or habitats for intermediary hosts for diseases, such as anopheles, tsetse flies or bilharzia snails etc.