Excessive use of products of chemical synthesis for pest and disease control in agricultural crops has caused damage to the environment and health; furthermore, it has promoted resistance in pests, for which reason it is more difficult to control them.
Bio-insecticides are substances of biological origin, which help to control insect pests; these products can be derived from natural materials such as animals, plants, and microorganisms. Neem extracts are a bio-insecticide, in particular a very efficient and successful phytoinsecticide for ecological pest control in crops, and very widely used in recent years due to the benefits and effects which will be described in this article.
Bio-insecticides are substances of biological origin, which help to control insect pests; these products can be derived from natural materials such as animals, plants,
and microorganisms. Neem extracts are a bio-insecticide, in particular a very efficient and successful phytoinsecticide for ecological pest control in crops, and very widely used in recent years due to the benefits and effects which will be described in this article.
Neem (Azadirachta indica A. Juss) is a plant from the Meliaceae family, originating from India and Burma. It is a tree that produces white flowers, and its fruits are drupes, which are produced in clusters and are yellow when they are ripe. It is used for environmental purposes, as an ornament, for making soaps, oil extraction, to produce methane gas, and mainly as a biological insecticide, which is the function that this article concerns.
Neem is highly resistant to drought and also, due to its properties, has few pests of significance; the main ones are Orthopteras and ants. The latter is due to the fact that they prefer to feed on young shoots, where there has yet to accumulate enough insecticide to kill such a large insect. Defoliator ants do not ingest it directly, but rather produce their food by fermenting the leaves in their nest.
Various terpene-type compounds (diterpenes, triterpenes, and limonoids) have been found in neem, primarily concentrated in the seed. Some of them are protomeliacines, meliantriol, azadirone, genudin, vilasinine, nimbin, salannin, and azadirachtins. Azadirachtin is normally found in the range of 0.1 to 0.9 % in the seed. There are at least nine amorphous isomers of azadirachtin; the most abundant of which (83% total) has been called type A.
Azadirachtin is the agent with the greatest effect in fighting insects among all the compounds in neem. It has been demonstrated that it is one of the most powerful growth regulators and feed inhibitors that has been used. The anti-feeding activity produces short life and is variable. The most important quality of neem is the hormonal blocking of the larva metamorphosis process. Azadirachtin does not kill insects immediately, but rather repels them and destroys their growth and reproduction. There are currently various basic processes for azadirachtin extraction from neem seeds. Among those most widely used, those that use ethanol, hexane, acetone, ethyl acetate, oils, and methanol as a solvent stand out.
The use of isopropanol with vacuum evaporation or liquid gasses has also been assessed. All of them cause certain active substances to appear, which vary in each one. Those that have been observed to have a higher concentration of azadirachtin use alcohols, particularly ethanol or methanol, at concentrations higher than 85%. Oily extracts have the disadvantage that they can cause phytotoxicity if care is not taken with the dose.
Effect of azadirachtin on pest insects
It has been documented that upon application, the active ingredient intervenes in the neuroendocrine system and reduces ecdysteroid synthesis, especially the ecdysome hormone, normally present in insects and the purpose of which is to control multiple processes in the insect, including its metamorphosis from larva to adult. The effects of inhibiting this vital hormone are numerous; there are malformations produced in any of the stages, morphogenetic damage to the wings of adults or badly developed mouthparts, destruction and inhibition of egg development, larvae or chrysalises, metamorphosis blocking, no mating and sexual communication, sterility, repellent, decrease or even total blocking of feeding upon reducing intestinal mobility and making it impossible to swallow, decrease in the levels of proteins and amino acids in the hemolymph, and impeding chitin synthesis.
Out of all of these effects, it can be said that the repellent probably has the weakest effect. The anti-feeding activity produces short life and is variable. The most important quality of neem is the blocking of the larva metamorphosis process. All the effects can be produced in combination and with different degrees of action, depending on the species, state of development, extraction process, and the concentration.
Plants treated with need insecticides can be eaten by those insects, but upon reaching a certain point of ingestion, the insect, still in its larva stage, starts to eat less and less, until it stops eating and dies without reaching sexual maturity. The damage caused to the crop by the insects that managed to eat it can be considered within a normal permissible threshold.
Azadirachtin use in crops
There is a large amount of studies with different formulations evaluating the effect of azadirachtin at different doses and in various crops to control a great number of pests. For beetles in been cultivation, for example it was observed that the females stopped ovipositing and even became irreversibly sterile.
The effectiveness of azadirachtin at a dose of only 10 ppm against more than 220 species of pest insects of the Blattodea, Caelifera, Coleoptera, Dermaptera, Diptera, Ensifera, Heteroptera, Isoptera, Lepidoptera, Phasmida, Phthiraptera, Siphonaptera and Thysanoptera groups.
For aphid control (homoptera), concentrations higher than 100 ppm are required to induce a primarily anti-feeding effect due to its scarce mobility in phloem. A general dose of 30 to 60 g/ha of azadirachtin is recommended as enough to control various types of sucking and chewing pests; however, the doses vary according to the amount of water. When using a commercial product, the concentration of azadirachtin should be reviewed, which as described, largely depends on the method of extraction.
The effects on growth lose their activity at 65% after 16 hours under field conditions due to rapid degradation due to ultraviolet rays; azadirachtin degrades 100% after 7 days, although it is possible to prolong the effect with conditioners (enhancers and protectants) in the formulation. It is preferable to apply it in the early morning or the last hours of the afternoon in order to delay decomposition due to the sun. The systemic effects remain for around 6 days, due to which it has been sufficient in practice to obtain good pest control.
The effects and control furthermore depend on the formulations, the pest, and the crop, as well as the environment. Higher temperatures increase the effect because the insects are more active under these conditions, and the effects are achieved more quickly than in temperate or cold climates. In areas with excess rainfall, the appetite-inhibiting effect disappear after a few hours and the results are diminished in general due to leaching. In these cases, instead of applying higher concentrations, it is preferable to apply treatments more frequently and with a smaller dose.
Advantages of azadirachtin use as insect pest control
An interesting advantage in using neem extracts, as opposed to other pest control strategies, is that in spite of the multiple studies at different doses and concentrations, toxicity for the crops has not been reported in any of them.
Considering its relatively weak contact effect in insects and through its special mode of action, neem extracts do not represent major damage to the natural enemies of the pests, nor for other useful organisms, which do not feed from the crop. Only occasionally, when the application of azadirachtin is increases, can there appear effects on the predators due to an accumulation of azadirachtin in the pest insects. Furthermore, it is harmless to bird and mammals.
Another notable characteristic is the difficult development of resistance due to being a mix of bioactive components,systemic through the roots when applied to the soil, higher biodegradability due to the action of the UV rays and with a persistence in the field of 4 to 8 days.
Final recommendations and conclusion
The method of population control for insect pests is not immediate; azadirachtin causes changes in the behavior of the animal, inhibiting their will to eat and reproduce, due to which high mortality of the insects within 24 hours should not be expected, but through consistent and periodic applications, excellent results are obtained.
It is important to mention that only planting neem trees around or within the area of cultivation has not demonstrated a considerable repellent effect. In order to obtain favorable results, a concentrated extract must be made of the seeds, or commercial neem-based products used, the active ingredient of which is azadirachtin.
Studies have found biological success rates for various vegetable pests of around 75% and 100%, due to which we can confirm the feasibility of the use of neem seed extract as a bio insecticide, as well as having a broad spectrum of action in agricultural production, and contributing to more integrated pest management, favoring a more environmentally-friendly and sustainable agricultural production. Neem extract use is an excellent alternative to synthetic products, as the added control for useful insects, which are not affected, making it possible to reduce the number of applications as proved in the studies of various crops in different countries.
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