A wide variety of commercial insecticides base their effectiveness on the extracts of some plants, especially on a group of substances known as pyrethrum. The dried flowers of the pyrethrum daisy (Chrysanthemun cinerariaefolium) contain active components such as pyrethrins, cinchines and jasmolines.
On the other hand, there are multiple references that place tobacco (powdered leaves and extract) within the field of natural insecticides. It has been proven by experimentation that its active substance, nicotine, is toxic to many insects, which it kills by contact. It should be noted that nicotine is also present in a considerable number of different plants, sometimes in significant quantities. Other insecticides of plant origin are retenoids, which are found in plants of the genus Denis (leguminosae).
Recently a series of phototoxic metabolites from plants such as Astereaceae and Rutáceas have been discovered. To activate their toxicity, which affects insects, nematodes and different kinds of pathogenic microorganisms, these compounds must first undergo a photochemical process that begins with the presence of sunlight.
The following describes some plants with insecticidal properties that have been identified and used by man:
Crotalaria (Crotalaria agatiflora) . In addition to repelling cockroaches, it is toxic to pests that infest stored grains.
Curcuma (Domestic Curcuma) . It is native to India and southwest Asia. Its rhizomes, apart from being used as a condiment, are used for crop protection. It is insecticide and repellent against pests such as weevils, caterpillars and worms, which is why it has great value in storage. The insecticide is obtained by spraying the rhizomes.
Neem (Azadirachta Indica) . It is native to India and Pakistan, and is currently found in some regions of Central America, the west coast of Africa and the Subsahel, Southeast Asia and the Fiji archipelago, among other places. It is insecticide, fungicide, nematicide and repellent. It also inhibits the growth of other plants. It is a sclerofolia species that grows rapidly in both semi-arid and semi-humid areas. It does not require much water (it can develop even with rains of less than 500 mm) and it does not require too much of the soil (it is found in sandy, stony and poor places).
Its fruits contain approximately 40% oil, and the first harvests occur around the fourth year, with an average of 40 kg per tree. Although the active substance is present in all parts of the plant, the highest concentrations of the insecticide are located at the level of the seeds.
The neem acts mainly against aphids, nematodes, cob worms and foliage eaters, spiders, borers, choppers, the Mediterranean fly and the cabbage butterfly. Because its active substance is rapidly decomposed by ultraviolet rays, it is recommended to apply it overnight. Finally, it is good to note that neem has little or no effect on beneficial insects for crops.
Pyrethrum (Chrysanthemum cinariaefolium).It is a perennial plant similar to chrysanthemums or daisies. In its flowers there is an insecticidal substance that is toxic to contact and, in addition, has a repellent effect. The toxin manifests itself on the nervous system, causing incoherent movements and paralysis in the final stage. When the doses received do not reach a lethal level, the affected insects usually recover within a day. The pyrethrum is effective against aphids, spiders, thrips, worms, beetles and palomillas, among others. As with the neem, the active substance of the natural pyrethrum is easily broken down by the sun’s rays. On the other hand, a permanent contact between the pyrethrum and the human skin can cause excesses to form on it, while its inhalation causes headache and nausea. When applied as a powder, the pyrethrum can be used pure or mixed with other substances such as plaster, which increase its adhesion power. When used in liquid form, it can be diluted in soapy water or simply water. The solution with soapy water allows a greater humidification of the treated plants, also improving the effectiveness of the pyrethrins. It is recommended that this insecticide be applied immediately after its preparation.
Tree of paradise (Melia azedarach). It is native to India, and is currently found in a large part of the tropics and subtropics, where it is frequently used as an ornamental or shade tree. Like the neem, of which it is a close relative, its seeds have an insecticidal effect. It is also repellent and growth inhibitor, and its toxin acts by contact or ingestion. Its sheets and parts are used for the protection of leather goods, books, fabrics or other objects susceptible to being damaged by insects. It has an effect on spiders, borers, cogolleros, ticks, weevils (except the cereal weevil), leaf-eating worms, various kinds of aphids and migratory lobster.
Although there is not much data on its management and application, its worldwide distribution is a good incentive – and at the same time a challenge – to get to know it better and take advantage of its great protective properties in crops and storage.
Quassia (Quassia amara). It is a small tree (about 5 m high) that is found in Central America and Brazil mainly. It is insecticide, nematicide and larvicide, and its toxins act by contact and ingestion. The active substance of Quassia amara is especially concentrated in its wood, although it is also present in the leaves, bark and roots. It is commonly used as a solution against sucking insects and pests such as aphids, worms, spiders, miners, borers and the potato beetle; however, it has no effect on the green moth and aphid, nor on the beneficial insects for crops. There are other plants such as Aeschrion excelsa and Picrasma excelsa that belong to the same family and have similar characteristics.
Aromatic calamus (Acorus calamus).It is native to India, although, because it is quite prized for its medicinal value, it has spread almost all over the world. It is found in stagnant yaguas marshes, and its growth range extends to 2,000 meters above sea level, altitude at which it produces the greatest amount of etheric oil. The calamus is insecticide, repellent and inhibitor of reproduction. Its potential for pest control in crops is yet to be developed, since its use has traditionally been extended to storage-related applications. Many years of research have shown, however, that the calamus is harmless to treated crops, which are not affected by its germination power or nutritional qualities.
Among the pests that can be controlled with the calamus are several types of weevils, cogolleros, fleas, flies and the cabbage butterfly. It is an easy to grow plant that does not compete with its neighbors.