Bacillus thuringiensis

Bacillus thuringiensis (or Bt) is a Gram-positive, soil-dwelling bacterium, commonly used as a biological pesticide. B. thuringiensis also occurs naturally in the gut of caterpillars of various types of moths and butterflies, as well on leaf surfaces, aquatic environments, animal feces, insect-rich environments, and flour mills and grain-storage facilities.[1][2] It has also been observed to parasitize other moths such as Cadra calidella—in laboratory experiments working with C. calidella, many of the moths were diseased due to this parasite.[3]

During sporulation, many Bt strains produce crystal proteins (proteinaceous inclusions), called δ-endotoxins, that have insecticidal action. This has led to their use as insecticides, and more recently to genetically modified crops using Bt genes, such as Bt corn.[4] Many crystal-producing Bt strains, though, do not have insecticidal properties.[5] The subspecies israelensis is commonly used for control of mosquitoes[6] and of fungus gnats.[7]

Bacillus thuringiensis
Bt-toxin-crystals
Spores and bipyramidal crystals of Bacillus thuringiensis morrisoni strain T08025
Scientific classification
Domain: Bacteria
Phylum: Firmicutes
Class: Bacilli
Order: Bacillales
Family: Bacillaceae
Genus: Bacillus
Species:
B. thuringiensis
Binomial name
Bacillus thuringiensis
Berliner 1915
Bacillus thuringiensis
Gram stain of Bacillus thuringiensis under 1000 × magnification

Taxonomy and discovery

B. thuringiensis was first discovered in 1901 by Japanese biologist Ishiwatari Shigetane (石渡 繁胤) in silkworms. He named it Bacillus sotto,[8] using the Japanese word sottō (卒倒, ‘collapse’), here referring to bacillary paralysis.[9] In 1911, German microbiologist Ernst Berliner independently rediscovered it when he isolated it as the cause of a disease called Schlaffsucht in flour moth caterpillars in Thuringia (hence the specific name thuringiensis, "Thuringian").[10] B. sotto would later be reassigned as B. thuringiensis var. sotto.[11]

In 1976, Robert A. Zakharyan reported the presence of a plasmid in a strain of B. thuringiensis and suggested the plasmid's involvement in endospore and crystal formation.[12][13] B. thuringiensis is closely related to B. cereus, a soil bacterium, and B. anthracis, the cause of anthrax; the three organisms differ mainly in their plasmids.[14]:34–35 Like other members of the genus, all three are aerobes capable of producing endospores.[1]

Tubulin was long thought to be specific to eukaryotes. More recently, however, several prokaryotic proteins have been shown to be related to tubulin.[15][16][17][18]

Subspecies

There are several dozen recognized subspecies of Bacillus thuringiensis. Subspecies commonly used as insecticides include Bacillus thuringiensis subspecies kurstaki (Btk), subspecies israelensis (Bti) and subspecies aizawa.

Mechanism of insecticidal action

Upon sporulation, B. thuringiensis forms crystals of proteinaceous insecticidal δ-endotoxins (called crystal proteins or Cry proteins), which are encoded by cry genes.[19] In most strains of B. thuringiensis, the cry genes are located on a plasmid (cry is not a chromosomal gene in most strains).[20][21][22]

Cry toxins have specific activities against insect species of the orders Lepidoptera (moths and butterflies), Diptera (flies and mosquitoes), Coleoptera (beetles), Hymenoptera (wasps, bees, ants and sawflies) and against nematodes.[23][24] Thus, B. thuringiensis serves as an important reservoir of Cry toxins for production of biological insecticides and insect-resistant genetically modified crops. When insects ingest toxin crystals, their alkaline digestive tracts denature the insoluble crystals, making them soluble and thus amenable to being cut with proteases found in the insect gut, which liberate the toxin from the crystal.[20] The Cry toxin is then inserted into the insect gut cell membrane, paralyzing the digestive tract and forming a pore.[25] The insect stops eating and starves to death; live Bt bacteria may also colonize the insect which can contribute to death.[20][25][26] The midgut bacteria of susceptible larvae may be required for B. thuringiensis insecticidal activity.[27]

It has been shown that a small RNA called BtsR1 can silence the Cry toxin when outside the host by binding to the RBS site of the Cry5Ba toxin transcript and inhibiting its expression. The silencing results in increased ingestion by C. elegans and is relieved inside the host, resulting in host death.[28]

In 1996 another class of insecticidal proteins in Bt was discovered; the vegetative insecticidal proteins (Vip; InterProIPR022180).[29][30] Vip proteins do not share sequence homology with Cry proteins, in general do not compete for the same receptors, and some kill different insects than do Cry proteins.[29]

In 2000, a novel subgroup of Cry protein, designated parasporin, was discovered from noninsecticidal B. thuringiensis isolates.[31] The proteins of parasporin group are defined as B. thuringiensis and related bacterial parasporal proteins that are not hemolytic, but capable of preferentially killing cancer cells.[32] As of January 2013, parasporins comprise six subfamilies: PS1 to PS6.[33]

Use of spores and proteins in pest control

Spores and crystalline insecticidal proteins produced by B. thuringiensis have been used to control insect pests since the 1920s and are often applied as liquid sprays.[34] They are now used as specific insecticides under trade names such as DiPel and Thuricide. Because of their specificity, these pesticides are regarded as environmentally friendly, with little or no effect on humans, wildlife, pollinators, and most other beneficial insects, and are used in organic farming;[24] however, the manuals for these products do contain many environmental and human health warnings,[35][36] and a 2012 European regulatory peer review of five approved strains found, while data exist to support some claims of low toxicity to humans and the environment, the data are insufficient to justify many of these claims.[37]

New strains of Bt are developed and introduced over time[38] as insects develop resistance to Bt,[39] or the desire occurs to force mutations to modify organism characteristics[40] or to use homologous recombinant genetic engineering to improve crystal size and increase pesticidal activity[41] or broaden the host range of Bt and obtain more effective formulations.[42] Each new strain is given a unique number and registered with the U.S. EPA[43] and allowances may be given for genetic modification depending on "its parental strains, the proposed pesticide use pattern, and the manner and extent to which the organism has been genetically modified".[44] Formulations of Bt that are approved for organic farming in the US are listed at the website of the Organic Materials Review Institute (OMRI)[45] and several university extension websites offer advice on how to use Bt spore or protein preparations in organic farming.[46][47]

Use of Bt genes in genetic engineering of plants for pest control

The Belgian company Plant Genetic Systems (now part of Bayer CropScience) was the first company (in 1985) to develop genetically modified crops (tobacco) with insect tolerance by expressing cry genes from B. thuringiensis; the resulting crops contain delta endotoxin.[48][49] The Bt tobacco was never commercialized; tobacco plants are used to test genetic modifications since they are easy to manipulate genetically and are not part of the food supply.[50][51]

Bt plants
Bt toxins present in peanut leaves (bottom dish) protect it from extensive damage caused to unprotected peanut leaves by lesser cornstalk borer larvae (top dish).[52]

Usage

In 1995, potato plants producing CRY 3A Bt toxin were approved safe by the Environmental Protection Agency, making it the first human-modified pesticide-producing crop to be approved in the USA,[53][54] though many plants produce pesticides naturally, including tobacco, coffee plants, cocoa, and black walnut. This was the 'New Leaf' potato, and it was removed from the market in 2001 due to lack of interest.[55] For current crops and their acreage under cultivation, see genetically modified crops.

In 1996, genetically modified maize producing Bt Cry protein was approved, which killed the European corn borer and related species; subsequent Bt genes were introduced that killed corn rootworm larvae.[56]

The Bt genes engineered into crops and approved for release include, singly and stacked: Cry1A.105, CryIAb, CryIF, Cry2Ab, Cry3Bb1, Cry34Ab1, Cry35Ab1, mCry3A, and VIP, and the engineered crops include corn and cotton.[57][58]:285ff

Corn genetically modified to produce VIP was first approved in the US in 2010.[59]

In India, by 2014, more than seven million cotton farmers, occupying twenty-six million acres, had adopted Bt cotton.[60]

Monsanto developed a soybean expressing Cry1Ac and the glyphosate-resistance gene for the Brazilian market, which completed the Brazilian regulatory process in 2010.[61][62]

Btcornafrica
Kenyans examining insect-resistant transgenic Bt corn

Safety studies

The use of Bt toxins as plant-incorporated protectants prompted the need for extensive evaluation of their safety for use in foods and potential unintended impacts on the environment.

Dietary risk assessment

Concerns over the safety of consumption of genetically-modified plant materials that contain Cry proteins have been addressed in extensive dietary risk assessment studies. While the target pests are exposed to the toxins primarily through leaf and stalk material, Cry proteins are also expressed in other parts of the plant, including trace amounts in maize kernels which are ultimately consumed by both humans and animals.[63]

Animal models have been used to assess human health risk from consumption of products containing Cry proteins. The United States Environmental Protection Agency recognizes mouse acute oral feeding studies where doses as high as 5,000 mg/kg body weight resulted in no observed adverse effects.[64] Research on other known toxic proteins suggests that toxicity occurs at much lower doses, further suggesting that Bt toxins are not toxic to mammals.[65] The results of toxicology studies are further strengthened by the lack of observed toxicity from decades of use of B. thuringiensis and its crystalline proteins as an insecticidal spray.[66]

Introduction of a new protein raised concerns regarding the potential for allergic responses in sensitive individuals. Bioinformatic analysis of known allergens has indicated there is no concern of allergic reactions as a result of consumption of Bt toxins.[67] Additionally, skin prick testing using purified Bt protein resulted in no detectable production of toxin-specific IgE antibodies, even in atopic patients.[68]

Studies have been conducted to evaluate the fate of Bt toxins that are ingested in foods. Bt toxin proteins have been shown to digest within minutes of exposure to simulated gastric fluids.[69] The instability of the proteins in digestive fluids is an additional indication that Cry proteins are unlikely to be allergenic, since most known food allergens resist degradation and are ultimately absorbed in the small intestine.[70]

Ecological risk assessment

Ecological risk assessment aims to ensure there is no unintended impact on non-target organisms and no contamination of natural resources as a result of the use of a new substance, such as the use of Bt in genetically-modified crops. The impact of Bt toxins on the environments where transgenic plants are grown has been evaluated to ensure no adverse effects outside of targeted crop pests.[71]

Concerns over possible environmental impact from accumulation of Bt toxins from plant tissues, pollen dispersal, and direct secretion from roots have been investigated. Bt toxins may persist in soil for over 200 days, with half-lives between 1.6 and 22 days. Much of the toxin is initially degraded rapidly by microorganisms in the environment, while some is adsorbed by organic matter and persists longer.[72] Some studies, in contrast, claim that the toxins do not persist in the soil.[72][73][74] Bt toxins are less likely to accumulate in bodies of water, but pollen shed or soil runoff may deposit them in an aquatic ecosystem. Fish species are not susceptible to Bt toxins if exposed.[75]

The toxic nature of Bt proteins has an adverse impact on many major crop pests, but ecological risk assessments have been conducted to ensure safety of beneficial non-target organisms that may come into contact with the toxins. Widespread concerns over toxicity in non-target lepidopterans, such as the monarch butterfly, have been disproved through proper exposure characterization, where it was determined that non-target organisms are not exposed to high enough amounts of the Bt toxins to have an adverse effect on the population.[76] Soil-dwelling organisms, potentially exposed to Bt toxins through root exudates, are not impacted by the growth of Bt crops.[77]

Insect resistance

Multiple insects have developed a resistance to B. thuringiensis. In November 2009, Monsanto scientists found the pink bollworm had become resistant to the first-generation Bt cotton in parts of Gujarat, India - that generation expresses one Bt gene, Cry1Ac. This was the first instance of Bt resistance confirmed by Monsanto anywhere in the world.[78][79] Monsanto responded by introducing a second-generation cotton with multiple Bt proteins, which was rapidly adopted.[78] Bollworm resistance to first-generation Bt cotton was also identified in Australia, China, Spain, and the United States.[80] Additionally, the Indian mealmoth, a common grain pest, is also developing a resistance since B. thuringiensis has been extensively used as a biological control agent against the moth.[81] Studies in the cabbage looper have suggested that a mutation in the membrane transporter ABCC2 can confer resistance to B. thuringiensis.[82]

Secondary pests

Several studies have documented surges in "sucking pests" (which are not affected by Bt toxins) within a few years of adoption of Bt cotton. In China, the main problem has been with mirids,[83][84] which have in some cases "completely eroded all benefits from Bt cotton cultivation".[85] The increase in sucking pests depended on local temperature and rainfall conditions and increased in half the villages studied. The increase in insecticide use for the control of these secondary insects was far smaller than the reduction in total insecticide use due to Bt cotton adoption.[86] Another study in five provinces in China found the reduction in pesticide use in Bt cotton cultivars is significantly lower than that reported in research elsewhere, consistent with the hypothesis suggested by recent studies that more pesticide sprayings are needed over time to control emerging secondary pests, such as aphids, spider mites, and lygus bugs.[87]

Similar problems have been reported in India, with both mealy bugs[88][89] and aphids[90] although a survey of small Indian farms between 2002 and 2008 concluded Bt cotton adoption has led to higher yields and lower pesticide use, decreasing over time.[91]

Controversies

There are controversies around GMOs on several levels, including whether making them is ethical, whether food produced with them is safe, whether such food should be labeled and if so how, whether agricultural biotech is needed to address world hunger now or in the future, and more specifically to GM crops—intellectual property and market dynamics; environmental effects of GM crops; and GM crops' role in industrial agricultural more generally.[92] There are also issues specific to Bt transgenic crops.

Lepidopteran toxicity

The most publicised problem associated with Bt crops is the claim that pollen from Bt maize could kill the monarch butterfly.[93] The paper produced a public uproar and demonstrations against Bt maize; however by 2001 several follow-up studies coordinated by the USDA had asserted that "the most common types of Bt maize pollen are not toxic to monarch larvae in concentrations the insects would encounter in the fields."[94][95][96][97] Similarly, Bacillus thuringiensis has been widely used for controlling Spodoptera littoralis larvae growth due to their detrimental pest activities in Africa and Southern Europe. However, S. littoralis showed resistance to many strains of B. thuriginesis and were only effectively controlled by few strains.[98]

Wild maize genetic mixing

A study published in Nature in 2001 reported Bt-containing maize genes were found in maize in its center of origin, Oaxaca, Mexico.[99] In 2002, paper concluded, "the evidence available is not sufficient to justify the publication of the original paper."[100] A significant controversy happened over the paper and Nature's unprecedented notice.[101]

A subsequent large-scale study, in 2005, failed to find any evidence of genetic mixing in Oaxaca.[102] A 2007 study found the "transgenic proteins expressed in maize were found in two (0.96%) of 208 samples from farmers' fields, located in two (8%) of 25 sampled communities." Mexico imports a substantial amount of maize from the US, and due to formal and informal seed networks among rural farmers, many potential routes are available for transgenic maize to enter into food and feed webs.[103] One study found small-scale (about 1%) introduction of transgenic sequences in sampled fields in Mexico; it did not find evidence for or against this introduced genetic material being inherited by the next generation of plants.[104][105] That study was immediately criticized, with the reviewer writing, "Genetically, any given plant should be either nontransgenic or transgenic, therefore for leaf tissue of a single transgenic plant, a GMO level close to 100% is expected. In their study, the authors chose to classify leaf samples as transgenic despite GMO levels of about 0.1%. We contend that results such as these are incorrectly interpreted as positive and are more likely to be indicative of contamination in the laboratory."[106]

Colony collapse disorder

As of 2007, a new phenomenon called colony collapse disorder (CCD) began affecting bee hives all over North America. Initial speculation on possible causes included new parasites, pesticide use,[107] and the use of Bt transgenic crops.[108] The Mid-Atlantic Apiculture Research and Extension Consortium found no evidence that pollen from Bt crops is adversely affecting bees.[94][109] According to the USDA, "Genetically modified (GM) crops, most commonly Bt corn, have been offered up as the cause of CCD. But there is no correlation between where GM crops are planted and the pattern of CCD incidents. Also, GM crops have been widely planted since the late 1990s, but CCD did not appear until 2006. In addition, CCD has been reported in countries that do not allow GM crops to be planted, such as Switzerland. German researchers have noted in one study a possible correlation between exposure to Bt pollen and compromised immunity to Nosema."[110] The actual cause of CCD was unknown in 2007, and scientists believe it may have multiple exacerbating causes.[111]

Beta-exotoxins

Some isolates of B. thuringiensis produce a class of insecticidal small molecules called beta-exotoxin, the common name for which is thuringiensin.[112] A consensus document produced by the OECD says: "Beta-exotoxins are known to be toxic to humans and almost all other forms of life and its presence is prohibited in B. thuringiensis microbial products"[113]

See also

Ovitrap-Ticino
An ovitrap collects eggs from mosquitoes. The brown granules in the water are a B. t. israelensis preparation that kills hatched larvae.

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Further reading

External links

Bacillus anthracis

Bacillus anthracis is the etiologic agent of anthrax—a common disease of livestock and, occasionally, of humans—and the only obligate pathogen within the genus Bacillus. B. anthracis is a Gram-positive, endospore-forming, rod-shaped bacterium, with a width of 1.0–1.2 µm and a length of 3–5 µm. It can be grown in an ordinary nutrient medium under aerobic or anaerobic conditions.

It is one of few bacteria known to synthesize a protein capsule (poly-D-gamma-glutamic acid). Like Bordetella pertussis, it forms a calmodulin-dependent adenylate cyclase exotoxin known as anthrax edema factor, along with anthrax lethal factor. It bears close genotypical and phenotypical resemblance to Bacillus cereus and Bacillus thuringiensis. All three species share cellular dimensions and morphology. All form oval spores located centrally in an unswollen sporangium. B. anthracis endospores, in particular, are highly resilient, surviving extremes of temperature, low-nutrient environments, and harsh chemical treatment over decades or centuries.

The endospore is a dehydrated cell with thick walls and additional layers that form inside the cell membrane. It can remain inactive for many years, but if it comes into a favorable environment, it begins to grow again. It initially develops inside the rod-shaped form. Features such as the location within the rod, the size and shape of the endospore, and whether or not it causes the wall of the rod to bulge out are characteristic of particular species of Bacillus. Depending upon the species, the endospores are round, oval, or occasionally cylindrical. They are highly refractile and contain dipicolinic acid. Electron micrograph sections show they have a thin outer endospore coat, a thick spore cortex, and an inner spore membrane surrounding the endospore contents. The endospores resist heat, drying, and many disinfectants (including 95% ethanol). Because of these attributes, B. anthracis endospores are extraordinarily well-suited to use (in powdered and aerosol form) as biological weapons. Such weaponization has been accomplished in the past by at least five state bioweapons programs—those of the United Kingdom, Japan, the United States, Russia, and Iraq—and has been attempted by several others.

Bacillus thuringiensis israelensis

Bacillus thuringiensis serotype israelensis (Bti) is a group of bacteria used as biological control agents for larvae stages of certain dipterans. Bti produces toxins which are effective in killing various species of mosquitoes, fungus gnats, and blackflies, while having almost no effect on other organisms. Indeed, this is one of the major advantages of B. thuringiensis products in general is that they are thought to affect few nontarget species.

Bti strains possess the pBtoxis plasmid which encodes numerous Cry and Cyt toxins, including Cry4, Cry10, Cry11, Cyt1, and Cyt2. The crystal aggregation which these toxins form contains at least four major toxic components, but the extent to which each Cry and Cyt protein is represented is not known and likely to vary with strain and formulation. Both Cry and Cyt proteins are pore-forming toxins; they lyse midgut epithelial cells by inserting into the target cell membrane and forming pores.Commercial formulations include "Mosquito Dunks"/"Mosquito Bits". It is also available in bulk liquid or granular formulations for commercial and public agency use.

Bacillus thuringiensis kurstaki

Bacillus thuringiensis serotype kurstaki (Btk) is a group of bacteria used as biological control agents against lepidopterans. Btk, along with other B. thuringiensis products, is one of the most widely used biological pesticides due to its high specificity; it is effective against lepidopterans, and it has little to no effect on nontarget species.

During sporulation, Btk produces a crystal protein that is lethal to lepidopteran larvae. Once ingested by the insect, the dissolution of the crystal allows the protoxin to be released. The toxin is then activated by the insect gut juice, and it begins to break down the gut.Btk is available commercially and is commonly known as "Garden Dust" or "Caterpillar Killer", both of which are produced by Safer Brand. Other Btk-producing companies include Bonide and Monterey.

Cry1Ac

Cry1Ac protoxin is a crystal protein produced by the bacterium Bacillus thuringiensis (Bt) during sporulation. Cry1Ac is one of the delta endotoxins produced by this bacterium which act as insecticides. Because of this, the genes for these have been introduced into commercially important crops by genetic engineering (e.g. cotton and corn) in order to confer pest resistance on those plants.Transgenic Bt cotton initially expressed a single Bt gene, which codes for Cry1Ac. Subsequently, Bt cotton has added other delta endotoxins. Products such as Bt cotton, Bt brinjal and genetically modified maize have received attention due to a number of issues, including genetically modified food controversies, and the Séralini affair.Cry1Ac is a mucosal adjuvant (an immune-response enhancer). It has been used in research to develop a vaccine against the amoeba Naegleria fowleri. This amoeba can invade and attack the human nervous system and brain, causing primary amoebic meningoencephalitis, which is nearly always fatal.

Cry34Ab1

Cry34Ab1 is one member of a binary Bacillus thuringiensis (Bt) crystal protein set isolated from Bt strain PS149B1. The protein exists as a 14 kDa delta endotoxin that, in presence of Cry35Ab1, exhibits insecticidal activity towards Western Corn Rootworm. The protein has been transformed into maize plants under the commercialized events 4114 (DP-ØØ4114-3) by Pioneer Hi-Bred and 59122 (DAS-59122-7) by Dow AgroSciences. These events have, in turn, been bred into multiple trait stacks in additional products.

Cry34Ab1/Cry35Ab1 binary toxins bind to the insect's brush border membrane vesicles (BBMVs) of cells in the epithelial lining of midgut, where they form pores; this leads to necrosis and, eventually, the insect's death. Most Bt delta endotoxins exist as a three-domain protein (around 70 kDa) in which BBMV specificity and pore-forming activity are conferred by the single protein. The Cry35Ab1 (45 kDa) protein does not convey specificity in the absence of Cry34Ab1, indicating that the smaller 14 kDa Cry34Ab1 protein is critical for BBMV binding and recruitment of Cry35Ab1 to induce insecticidal effect.Cry34Ab1 is composed of two beta sheets in a beta-sandwich structure; the total protein is composed of 117 amino acid residues and contains a hydrophobic core. As a member of the aerolysin family interaction with cell membranes is consistent with its role in the binary Cry34Ab1/Cry35Ab1 toxin complex.

Cry6Aa

Cry6Aa is a toxic crystal protein generated by the bacterial family Bacillus thuringiensis during sporulation. This protein is a member of the alpha pore forming toxins family, which gives it insecticidal qualities advantageous in agricultural pest control. Each Cry protein has some level of target specificity; Cry6Aa has specific toxic action against coleopteran insects and nematodes. The corresponding B. thuringiensis gene, cry6aa, is located on bacterial plasmids. Along with several other Cry protein genes, cry6aa can be genetically recombined in Bt corn and Bt cotton so the plants produce specific toxins. Insects are developing resistance to the most commonly inserted proteins like Cry1Ac. Since Cry6Aa proteins function differently than other Cry proteins, they are combined with other proteins to decrease the development of pest resistance. Recent studies suggest this protein functions better in combination with other virulence factors such as other Cry proteins and metalloproteinases.

Delta endotoxin

Delta endotoxins (δ-endotoxins) are pore-forming toxins produced by Bacillus thuringiensis species of bacteria. They are useful for their insecticidal action and are the primary toxin produced by Bt corn. During spore formation the bacteria produce crystals of such proteins (hence the name Cry toxins) that are also known as parasporal bodies, next to the endospores; as a result some members are known as a parasporin. The Cyt (cytolytic) toxin group is a group of delta-endotoxins different from the Cry group.

Host microbe interactions in Caenorhabditis elegans

Caenorhabditis elegans-microbe interactions are here broadly defined and encompass the associations with all microbes that are temporarily or permanently living in or on this nematode. The microbes might engage in a commensal, mutualistic or pathogenic interaction with the host and include bacteria, viruses, unicellular eukaryotes, and fungi. In nature C. elegans harbours a variety of different microbes. In contrast, C. elegans strains that are cultivated in laboratories for research purposes have lost their naturally associated microbial communities and are commonly maintained on a single bacterial strain, Escherichia coli OP50.

Insecticide

Insecticides are substances used to kill insects. They include ovicides and larvicides used against insect eggs and larvae, respectively. Insecticides are used in agriculture, medicine, industry and by consumers. Insecticides are claimed to be a major factor behind the increase in the 20th-century's agricultural productivity. Nearly all insecticides have the potential to significantly alter ecosystems; many are toxic to humans and/or animals; some become concentrated as they spread along the food chain.

Insecticides can be classified into two major groups: systemic insecticides, which have residual or long term activity; and contact insecticides, which have no residual activity.

Furthermore, one can distinguish three types of insecticide. 1. Natural insecticides, such as nicotine, pyrethrum and neem extracts, made by plants as defenses against insects. 2. Inorganic insecticides, which are metals. 3. Organic insecticides, which are organic chemical compounds, mostly working by contact.

The mode of action describes how the pesticide kills or inactivates a pest. It provides another way of classifying insecticides. Mode of action is important in understanding whether an insecticide will be toxic to unrelated species, such as fish, birds and mammals.

Insecticides may be repellent or non-repellent. Social insects such as ants cannot detect non-repellents and readily crawl through them. As they return to the nest they take insecticide with them and transfer it to their nestmates. Over time, this eliminates all of the ants including the queen. This is slower than some other methods, but usually completely eradicates the ant colony.Insecticides are distinct from non-insecticidal repellents, which repel but do not kill.

Larvicide

A larvicide (alternatively larvacide) is an insecticide that is specifically targeted against the larval life stage of an insect. Their most common use is against mosquitoes. Larvicides may be contact poisons, stomach poisons, growth regulators, or (increasingly) biological control agents.

Lesser wax moth

The lesser wax moth (Achroia grisella) is a small moth of the snout moth family (Pyralidae) that belongs to the subfamily Galleriinae. The species was first described by Johan Christian Fabricius in 1794. Adults are about 0.5 inches (13 mm) in length and have a distinct yellow head with a silver-grey or beige body. Lesser wax moths are common in most parts of the world, except in areas with cold climates. Their geographic spread was aided by humans who involuntarily introduced them to many countries.The mating systems of the lesser wax moth are well researched because they involve sound production. Lesser wax males produce ultrasonic pulses in order to attract females. Females seek the most attractive males and base their decisions on characteristics of the male sound. While sex pheromones are also emitted by the males, male calling is more effective in attracting mates.Because lesser wax moths eat unoccupied honey bee combs, they are considered pests to bees and beekeepers. However, unoccupied combs can harbor harmful pathogens that inflict damage to neighboring insects. By eating the combs, the moths can reduce the harm to insects of that region and provide a clean space for other organisms to inhabit.

List of Nepenthes endophyte species

This list of Nepenthes endophytes is a listing of endophytes recorded from the internal tissues of Nepenthes pitcher plants; that is, organisms that live within the plants for at least part of their life cycles without causing apparent disease.

The endophyte species are listed alphabetically and grouped by genus, family, and phylum. Additional information is included in brackets after the strain designation, namely: the host Nepenthes species from which the endophyte has been recorded; the geographical source of the record; and the type of tissue sampled.

Lysinibacillus sphaericus

Lysinibacillus sphaericus (reclassified - previously known as Bacillus sphaericus) is a Gram-positive, mesophilic, rod-shaped bacterium commonly found on soil. It can form resistant endospores that are tolerant to high temperatures, chemicals and ultraviolet light and can remain viable for long periods of time. It is of particular interest to the World Health Organization due to the larvicide effect of some strains against two mosquito genera (Culex and Anopheles), more effective than Bacillus thuringiensis, frequently used as a biological pest control. It is ineffective against Aedes aegypti, an important vector of yellow fever and dengue viruses.

L. sphaericus has five homology groups (I-V), with group II further dividing into subgroups IIA and IIB. Due to the low levels of homology between groups, it has been suggested that each might represent a distinct species, but owing to a lack of research on this topic, all remain designated as L. sphaericus.

MON 810

The MON 810 corn is a genetically modified maize used around the world. It is a Zea mays line known as YieldGard from the company Monsanto. This plant is a genetically modified organism (GMO) designed to combat crop loss due to insects. There is an inserted gene in the DNA of MON810 which allows the plant to make a protein that harms insects that try to eat it. The inserted gene is from the Bacillus thuringiensis which produces the Bt toxin that is poisonous to insects in the order Lepidoptera (butterflies and moths), including the European Corn Borer.

These genetically modified plants with Bt toxin are grown on a large scale around the world. Monsanto’s corn line MON810 is produced by ballistically transforming another corn line with a plasmid, PV-ZMCT10. This plasmid has a cauliflower mosaic virus 35S promoter and hsp70 maize intron sequences which drive the expression of the Cry1Ab gene. The gene then codes for delta endotoxins (Cry proteins) which are toxins that are very potent and provoke lesions in the cell membrane causing cell death These produced Bt toxins bind to certain localized sites on the epithelium of the midgut of insects. Proteins need specific receptors on cells in order to form the Cry proteins and become toxic, which is why the toxins are specific for the order Lepidoptera. The receptors are important for binding the toxic protein and starting the signal cascade, but the exact mechanism of these toxins is not well understood.

MON 863

MON 863 is a genetically engineered variety of maize produced by Monsanto. It is genetically altered to express a modified version of Cry3Bb1, a delta endotoxin which originates from Bacillus thuringiensis. This protects the plant from corn rootworm. Unlike MON 810, Bt 11, and Bt 176 which each produce a modified Cry1Ab, MON 863 instead produces a modified Cry3Bb1 toxin and contains nptII, a marker gene for antibiotic resistance.

Mesocyclops aspericornis

Mesocyclops aspericornis is a freshwater copepod species in the genus Mesocyclops found in the tropics. It was collected from Sumatra, Singapore and Hawaii.Together with the mosquito species Toxorhynchites speciosus, M. aspericornis forms a compatible predator pair for reduction of larval Aedes notoscriptus and Culex quinquefasciatus populations in tire habitats in Queensland. It has also been used in combination with Bacillus thuringiensis var. israelensis in controlling Aedes aegypti larvae in Thailand.

Pesticide resistance

Pesticide resistance describes the decreased susceptibility of a pest population to a pesticide that was previously effective at controlling the pest. Pest species evolve pesticide resistance via natural selection: the most resistant specimens survive and pass on their acquired heritable changes traits to their offspring.Cases of resistance have been reported in all classes of pests (i.e. crop diseases, weeds, rodents, etc.), with 'crises' in insect control occurring early-on after the introduction of pesticide use in the 20th century. The Insecticide Resistance Action Committee (IRAC) definition of insecticide resistance is 'a heritable change in the sensitivity of a pest population that is reflected in the repeated failure of a product to achieve the expected level of control when used according to the label recommendation for that pest species'.Pesticide resistance is increasing. Farmers in the US lost 7% of their crops to pests in the 1940s; over the 1980s and 1990s, the loss was 13%, even though more pesticides were being used. Over 500 species of pests have evolved a resistance to a pesticide. Other sources estimate the number to be around 1,000 species since 1945.Although the evolution of pesticide resistance is usually discussed as a result of pesticide use, it is important to keep in mind that pest populations can also adapt to non-chemical methods of control. For example, the northern corn rootworm (Diabrotica barberi) became adapted to a corn-soybean crop rotation by spending the year when field is planted to soybeans in a diapause.As of 2014, few new weed killers are near commercialization, and none with a novel, resistance-free mode of action.

Plant Genetic Systems

Plant Genetic Systems (PGS), since 2002 part of Bayer CropScience, is a biotech company located in Ghent, Belgium. The focus of its activities is the genetic engineering of plants. The company is best known for its work in the development of insect-resistant transgenic plants.

Its origin goes back to the work of Marc Van Montagu and Jeff Schell at the University of Ghent who were among the first to assemble a practical system for genetic engineering of plants. They developed a vector system for transferring foreign genes into the plant genome, by using the Ti plasmid of Agrobacterium tumefaciens. They also found a way to make plant cells resistant to the antibiotic kanamycin by transferring a bacterial neomycin phosphotransferase gene into the plant genome. PGS was the first company (in 1985) to develop genetically engineered (tobacco) plants with insect tolerance by expressing genes encoding for insecticidal proteins from Bacillus thuringiensis (Bt).

Pore-forming toxin

Pore-forming proteins (PFTs, also known as pore-forming toxins) are usually produced by bacteria, (and include a number of protein exotoxins but may also be produced by other organisms such as lysenin, produced by earthworms. They are frequently cytotoxic (i.e., they kill cells), as they create unregulated pores in the membrane of targeted cells.

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