Many insecticides are short-lived or are metabolized by the animals that ingest them, but some are persistent, and when applied in large amounts they pervade the environment. When an insecticide is applied, much of it reaches the soil , and groundwater can become contaminated from direct application or runoff from treated areas.
ORIGIN Pyrethrum - LODI GROUP Hygiène Urbaine
Owing to repeated sprayings, these chemicals can accumulate in soils in surprisingly large amounts 10— kilograms per hectare [10— pounds per acre] , and their effect on wildlife is greatly increased as they become associated with food chains. The stability of DDT and its relatives leads to their accumulation in the bodily tissues of insects that constitute the diet of other animals higher up the food chain , with toxic effects on the latter. Birds of prey such as eagles , hawks , and falcons are usually most severely affected, and serious declines in their populations have been traced to the effects of DDT and its relatives.
Consequently, the use of such chemicals began to be restricted in the s and banned outright in the s in many countries. Cases of insecticide poisoning of humans also occur occasionally, and the use of one common organophosphate, parathion , was drastically curtailed in the United States in owing to its toxic effects on farm labourers who were directly exposed to it. Another problem with insecticides is the tendency of some target insect populations to develop resistance as their susceptible members are killed off and those resistant strains that survive multiply, eventually perhaps to form a majority of the population.
Resistance denotes a formerly susceptible insect population that can no longer be controlled by a pesticide at normally recommended rates. Hundreds of species of harmful insects have acquired resistance to different synthetic organic pesticides, and strains that become resistant to one insecticide may also be resistant to a second that has a similar mode of action to the first. Once resistance has developed, it tends to persist in the absence of the pesticide for varying amounts of time, depending on the type of resistance and the species of pest.
Insecticides may also encourage the growth of harmful insect populations by eliminating the natural enemies that previously held them in check. The nonspecific nature of broad-spectrum chemicals makes them more likely to have such unintended effects on the abundance of both harmful and beneficial insects.
Because of the problems associated with the heavy use of some chemical insecticides, current insect-control practice combines their use with biological methods in an approach called integrated control. Article Media. Info Print Print. Table Of Contents. Submit Feedback. Thank you for your feedback. Introduction Modes of penetration Synthetic insecticides Chlorinated hydrocarbons Organophosphates Carbamates Environmental contamination and resistance.
See Article History. Modes of penetration Stomach poisons are toxic only if ingested through the mouth and are most useful against those insects that have biting or chewing mouth parts, such as caterpillars , beetles, and grasshoppers. Start Your Free Trial Today. Learn More in these related Britannica articles:. It had long been realized that the only effective way of controlling malaria was to eradicate the anopheline mosquitoes that transmit the disease.
Tephrosia extracts were especially active and when applied to cabbageworm larvae caused pupal deformities. Substantial differences in the concentration of insecticidal principles was found in Chinese ecotypes ofMelia spp.. New potent insecticidal phytochemicals containing an agarofuran nucleus have been identified from species of Celastraceae. The advantages of pest control systems using botanical pesticides is discussed with reference to the problem of resistance and non-target insects.
African plants used in folk medicine and in traditional pest management practices continue to yield interesting and potentially useful research leads. The leaves of the wild shrub Ocimum suave and the flower buds cloves of Eugenia aromatica are traditionally used as effective stored grain protectants. Eugenol, a common constituent of the two, is repellant to the maize weevil, Sitophilus zeamais. The medicinal plants, Spilanthes mauritiana and Plumbago zeylanica contain insecticidal isobutyl amides and the naphthoquinone, plumbagin, respectively and may have potential in small-scale mosquito control programmes.
Hildecarpin, a pterocarpan from Tephrosia hildebrandtii is an antifeedant against the legume pod borer, Maruca testulalis, and the rotenoids tephrosin and rotenone are very potent antifeedants against a number of lepidopteran larvae. Comparison of the antifeedant activities of citrus limonoids and some of their modifications against the spotted stalk borer, Chilo partellus suggest that the units associated with antifeedant activity are spread out in the limonoid skeleton.
Limonoids, characteristic natural products of the Meliaceae, Rutaceae, and other Rutales, have marked biological activity against a variety of insects.
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In particular, the well-known compound azadirachtin is under development as a possible commercial insecticide owing to its potent antifeedant and growth-regulating properties. This compound inhibits the feeding, growth and survival of the variegated cutworm, Peridroma saucia, with an EC50 and LC50 of 0.
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In contrast, azadirachtin has no antifeedant activity against nymphs of the migratory grasshopper, Melanoplus sanguinipes, but does inhibit molting in a dose-dependant manner with an oral ED50 of The ED50 via intrahemocoelic injection is 3. The lesser toxicity via the oral route may be largely due to local metabolism, as the oral toxicity can be synergized by piperonyl butoxide, an inhibitor of mixed-function oxidases.
The topical ED50 is 4. Azadirachtin toxicity in this insect does not involve inhibition of sterol reductases or sterol transport. Of nine other limonoids tested, only cedrelone, anthothecol, and bussein inhibit larval growth of P. Foliar extracts of species in the subfamily Melioideae are on average more insecticidal than extracts from the Swietenioideae; some deserve further attention as sources of insecticidal phytochemicals. Derivatives of neem Azadirachta indica A.
Juss have traditionally been used by farmers in Asia and Africa to ward off insect pests of household, agricultural, and medical importance. Unlike ordinary insecticides based on single active ingredients, neem derivatives comprise a complex array of novel compounds which have diverse behavioral and physiological effects on insects.
Repellency, feeding and oviposition deterrence, growth and reproduction inhibition, and other effects have been attributed to neem compounds - azadirachtin, salannin, meliantriol, etc. However, the complexity of chemical structure of these compounds precludes their synthesis on a practical scale. Therefore, the use of simple formulations of neem derivatives such as leaf or kernel powder or extracts needs to be popularized. Their being safe to nontarget organisms, including humans, make them ideal insecticides.
Several azadirachtinrich formulations have already been commercialized for use on nonfood and food crops. Plants biosynthesize a dazzling array of structural types which exhibit an almost equally dazzling array of biological activities.http://bbmpay.veritrans.co.id/mujeres-solteras-manos-en-sant-quint-de-mediona.php
Insecticides of plant origin 
In insects, various plant Compounds affect nerve axons and synapses e. Some of these compounds have already been exploited in commercial insect control e. Still others may be important components of host plant resistance mechanisms. This chapter will focus on the activity of certain plant compounds as insect antifeedant, repellent, and growth inhibitory compounds.
Seven tetranortriterpenoids were isolated from neem Azadirachta indica A. Juss seed by use of the Epilachna varivestis bioassay. They were structurally elucidated and two of them chemically modified. All these compounds are similar to the main compound, azadirachtin A, as well in quality and quantity of their biological activity as in their chemical structure. Based on these data, a reduced chemical structure is proposed. Azadirachtins are insect growth inhibitors. They interfere with the neuroendocrine regulation of juvenile and molting hormone titers.
Main cellular targets are the Malpighian tubules and the corpus cardiacum CC of the insect. Consequently, levels of the morphogenetic juvenile and molting hormones are shifted and concomitantly decreased after azadirachtin injection. Although some rodent testing has shown that chronic dietary exposure to rotenone may induce tumor formation,the most recent US EPA registration standard considers rotenone to be noncarcinogenic. Rotenone is one of the more acutely toxic botanicals. Commercial rotenone products have presented little hazard to man over many decades.
Neither fatalities nor systemic poisonings in humans have been reported in relation to ordinary use. Sabadilla is derived from the ripe seeds os Schoenocaulon officinale , a tropical lily plant which grows in Central and South America. Sabadilla is also sometimes known as cevadilla or caustic barley. When sabadilla seeds are aged, heated, or treated with alkali, several insecticidal alkaloids are formed or activated. Alkaloids are physiologically active compounds that occur naturally in many plants.
In chemical terms they are a heterogeneous class of cyclic compounds that contain nitrogen in their ring structures.
Caffeine, nicotine, cocaine, quinine, and strychnine are some of the more familiar alkaloids. The alkaloids in sabadilla are known collectively as veratrine or as the veratrine alkaloids. Of these alkaloids, cevadine and veratridine are the most active insecticidally. European white hellebore Veratrum album also contains veratridine in its roots.
Hellebore was once commonly used in Europe and the U. Sabadilla kills insects of some species immediately, while others may survive in a state of paralysis for several days before dying. Sabadilla, in the form or dusts made from ground seeds, is the least toxic of the registered botanicals.
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Purified veratrine alkaloids are quite toxic, however, and are considered on a par with the most toxic synthetic insecticides. Sabadilla can be severely irritating to skin and mucous membranes, and has a powerful sneeze-inducing effect when inhaled. Ingestion of small amounts may cause headaches, severe nausea, vomiting, diarrhea, cramps and reduced circulation.