‘Biofertilizer’ is a substance which contains living microorganism which, when applied to seed, plant surfaces, or soil, colonizes the rhizosphere or the interior of the plant and promotes growth by increasing the supply or availability of primary nutrients to the host plant.Biofertilizers are not fertilizers.
Pesticides and Insecticides
Pesticides are chemicals that may be used to kill fungus, bacteria, insects, plant diseases, snails, slugs, or weeds among others. These chemicals can work by ingestion or by touch and death may occur immediately or over a long period of time. Insecticides are a type of pesticide that is used to specifically target and kill insects. Some insecticides include snail bait, ant killer, and wasp killer.
Types of Chemical Pesticides
- Organochlorines: Chlorinated hydrocarbon (organochlorine) pesticides, solvents, and fumigants are largely banned in North America and Europe, but are used extensively in many developing nations. In addition, these chemicals may still be found in storage in the United States; thus, exposure remains possible. CNS excitation and depression, typically abrupt in onset, are the primary clinical effects of acute organochlorine toxicity; therefore, patients may present with any of the following:
- Initial euphoria with auditory or visual hallucinations and perceptual disturbances
- Agitation, lethargy, or unconsciousness
Organophosphates are a group of chemicals that have many domestic and industrial uses, though they are most commonly used as Insecticides and are responsible for a number of poisonings. The main mechanism is blocking the enzyme acetylcholinesterase causing nervous and respiratory damages that result in the insects death, but they are also hazardous to humans. After the cessation of use of organochloride insecticides, they became the most commonly used Pesticides and are responsible for 70% of pesticidal use in the United States.
Carbamates include pesticides such as sevin, aldicarb and carbaryl.
They are widely used as:
- They are more degradable than organophosphates.
- They have lower dermal toxicities. Mostly absorbed via inhalation, ingestion.
- The toxicity due to inhibition of acetylcholinesterases – however unlike the organophosphates they do not need to be modified ( S –> O ) before becoming effective.
- They do not penetrate the CNS so most effects are respiratory in nature.
- Depression of respiration combined with pulmonary edema is the usual cause of death from poisoning by N-methyl carbamate compounds.
- The N-methyl carbamate esters cause reversible carbamylation of acetylcholinesterase enzyme, allowing accumulation of acetylcholine, the neuromediator substance, at parasympathetic neuroeffector junctions (muscarinic effects), at skeletal muscle myoneural junctions and autonomic ganglia (nicotinic effects), and in the brain (CNS effects).
Pyrethroid insecticides are a special chemical class of active ingredients found in many of the modern insecticides found on store shelves and used by pest management professionals. The name pyrethroid means “pyrethrum-like” and refers to the origin of this class of pesticides. Not all pesticides in the same class are equally toxic, or equally effective the same pests. Nevertheless, most pyrethroid insecticides share the following characteristics:
- Low in toxicity to mammals and birds;
- High in toxicity to fish if applied directly to water;
- Require very low doses to kill insects (high arthropod toxicity);
- Especially effective against chewing insects, though many pyrethroid insecticides can be absorbed by the insect pest when it merely walks over the dry residue;
- Bind tightly to soil and organic matter (therefore not as effective in penetrating soil to kill underground pests);
- Dissolve very poorly in water.
Triazines are a group of herbicides derived from urea. They are used for controlling weeds in tea, tobacco and cotton. Examples are simazine, atrazine etc.
Environmental Hazards of Pesticides
If the credits of pesticides include enhanced economic potential in terms of increased production of food and fibre, and amelioration of vector-borne diseases, then their debits have resulted in serious health implications to man and his environment.
There is now overwhelming evidence that some of these chemicals do pose a potential risk to humans and other life forms and unwanted side effects to the environment. No segment of the population is completely protected against exposure to pesticides and the potentially serious health effects, though a disproportionate burden, is shouldered by the people of developing countries and by high risk groups in each country. The world-wide deaths and chronic diseases due to pesticide poisoning number about 1 million per year.
Animals or plants used wilfully to destroy pests are called Biopesticides. For common knowledge, we can divide them into bioherbicides and bioinsecticide.
The use of bioherbicides is another way of controlling weeds without environmental hazards posed by synthetic herbicides. Bioherbicides are made up of microorganisms (e.g. bacteria, viruses, fungi) and certain insects (e.g. parasitic wasps, painted lady butterfly) that can target very specific weeds. The microbes possess invasive genes that can attack the defense genes of the weeds, thereby killing it.
The better understanding of the genes of both microorganisms and plants has allowed scientists to isolate microbes (pathogens) whose genes match particular weeds and are effective in causing a fatal disease in those weeds. Bioherbicides deliver more of these pathogens to the fields. They are sent when the weeds are most susceptible to illness.
The genes of disease-causing pathogens are very specific. The microbe’s genes give it particular techniques to overcome the unique defenses of one type of plant. They instruct the microbe to attack only the one plant species it can successfully infect. The invasion genes of the pathogen have to match the defense genes of the plant. Then the microbe knows it can successfully begin its attack on this one particular type of plant. The matching gene requirement means that a pathogen is harmless to all plants except the one weed identified by the microbe’s genetic code.
Bioinsecitides are animals (including insects) that kill other insects. For example, the praying mantis eats aphids. Similarly, Gambusia fish is used to feed on larvae of mosquitoes.
Examples of Biopesticides
Bacillus thuringiensis, a bacterial disease of Lepidoptera, Coleoptera and Diptera, is a well-known insecticide example. The toxin from B. thuringiensis (Bt toxin) has been incorporated directly into plants through the use of genetic engineering. The use of Bt Toxin is particularly controversial. Its manufacturers claim it has little effect on other organisms, and is more environmentally friendly than synthetic pesticides. However, at least one scientific study has suggested that it may lead to slight histopathological changes on the liver and kidneys of mammals with Bt toxin in their diet.
Other microbial control agents include products based on:
- Entomopathogenic fungi (e.g. Beauveria bassiana, Isaria umosorosea, Lecanicillium and Metarhizium spp.),
- Plant disease control agents: include Trichoderma spp. and Ampelomyces quisqualis (a hyper-parasite of grape powdery mildew); Bacillus subtilis is also used to control plant pathogens.
- Beneficial nematodes attacking insect (e.g. Steinernema feltiae) or slug (e.g. Phasmarhabditis hermaphrodita) pests
- Entomopathogenic viruses (e.g.. Cydia pomonella granulovirus).
- Weeds and rodents have also been controlled with microbial agents.
- Various naturally occurring materials, including fungal and plant extracts, have been described as biopesticides. Products in this category include:
- Insect pheromones and other semiochemicals
- Fermentation products such as Spinosad (a macro-cyclic lactone)
- Chitosan: a plant in the presence of this product will naturally induce systemic resistance (ISR) to allow the plant to defend itself against disease, pathogens and pests.
- Biopesticides may include natural plant-derived products, which include alkaloids, terpenoids, phenolics and other secondary chemicals. Certain vegetable oils such as canola oil are known to have pesticidal properties. Products based on extracts of plants such as garlic have now been registered in the EU and elsewhere.