Credit:Infonet
Natural systems support a wide range of plants and animals. The plants provide food for plant-eating animals such as antelopes and many insects. The plant-eating animals are themselves eaten by a wide range of predators. Plant-eating animals that feed on a crop that we value (for example aphids that feed on cabbage), are considered pests. Plant-eating organisms are also eaten by organisms called natural enemies, which can be considered “farmer’s friends”.
Table: Plant-eating organisms and their predators.
| Type or organism | Example 1 | Example 2 |
| Plant | Tomato | Cabbage |
| Plant eating animal | Catterpillar (pest) | Aphid (Pest) |
| Predatory or parasitic animal | Parasitic wasp (natural enemy) | Hover fly larva (natural enemy) |
These feeding levels (see table above) exist in all stable natural and agricultural habitats. The number of animals from each feeding level is kept in check by complex interactions. In agricultural systems the balance between the four feeding levels is frequently upset and pest numbers increase rapidly, resulting in pest outbreaks. Reasons for this can be:
Monocultures: Planting of large tracts of land with one single crop type is referred to as monoculture. This makes it easy for pests to find the crop and once they have found it, to develop quickly because their food source is plentiful and closely spaced. Monocultures also contain few shelters and alternative food sources for natural enemies.
Poorly adapted crop cultivars: Many plant cultivars grown for food crops in tropical regions have originated from Latin America and Asia and are not always adapted to local growing conditions and thus have low levels of natural resistance to pests and diseases.
Use of pesticide: Use of pesticides can cause several problems. In many instances, natural enemies are more susceptible to pesticides than the pest itself and are thus harmed or killed while the pest is not much affected. This causes pests to multiply much more quickly.
Pests may also develop resistance to pesticides. This occurs in situations where pesticides are used frequently and the same type of product is used for long periods. A farmer said: “I’ve been spraying once a week for the last month and the crop is being destroyed by pests. What is going on?” This is because some of the pests are less susceptible to the pesticide and after a period of heavy pesticide use, they still persist because they have developed resistance to the pesticides. Increased frequency of spraying only affects natural enemies. Furthermore, other beneficial insects, such as pollinators, are also harmed.