Many natural controls act to maintain a balance in insect populations:
• Environmental factors such as temperature and rainfall can restrict the distribution of insect species. For example, mites are usually more prevalent under dry conditions.
• Geographic barriers such as large bodies of water, mountain ranges, and deserts can also limit the distribution of insects.
• Frogs, toads, lizards, moles, and birds are some of the many natural enemies of insects.
• Beneficial enemy insects such as ladybugs feed on aphids while others such as the Braconidae wasp or Tachina fly lay their eggs on or inside certain insects, causing the developing larvae to kill the host insect. Some predatory insects like the rezadora also eat beneficial insects.
Insects are also attacked by viruses, fungi, and bacteria, all of which contribute to population control.
With the increase in agricultural activities, many of these natural balances have become unbalanced and are no longer safe measures to control harmful insects. Monoculture and the existence of extensive cultivated areas have caused an increase in the number of pests. The promiscuous use of pesticides has resulted in an increase in harmful insects in some cases. Many of the traditional crop varieties, despite their lower productivity, have better insect resistance than some of the improved varieties.
Biological control is the calculated introduction of natural enemies, parasites or diseases to combat a species of harmful insects. As 120 different insects have been partially or completely controlled by this method in various parts of the world. Microbial insecticides such as Bacillus thuringiensis (cash against some kinds of crustaceans) are currently commonly used by farmers and gardeners in many areas. Unfortunately, the biological control measures that currently exist are effective over a very small portion of the harmful insect species.
Cultural controls such as crop rotation, intercropping, burying waste, calculating the crop calendar to avoid certain insects, and controlling weeds and local herbs that are insect hosts are examples of effective control methods of certain insects. Even so, cultural controls need to be supplemented by other methods.
The Resistances of Varieties
Varieties of crops vary considerably in their resistance to certain insects. For example, maize varieties with long and narrow pearls show good resistance to heloteros (belloteros) and weevils. CIAT researchers have found that some varieties are relatively unaffected by leafhopper damage during the rainy season, while others suffer losses in yields up to 40 percent. Testing for insect resistance is an important part of crop breeding programs.
“Organic” control refers to all non-chemical methods in general. These include the application of “natural” household sprays made from garlic, pepper, onions, soap, salt, etc., and the use of materials such as beer to kill slugs, and ashes to kill tracers and other insects. Some of these “alternative” methods vary from slightly to fairly effective over small areas or where insect populations are relatively low. They are rarely practicable in the larger fields, especially under tropical conditions that favor the growth of pests.
Chemical control refers to the use of commercial insecticides in the form of sprays, powders, granulates, baits, fumigants, and seed treatments. While some of these insecticides like rotenone and pyrethrin are naturally derived, most are synthetic compounds that have been developed by research.