Biopesticides can be defined as pesticides that are derived from plants, animals, microbes, or any other biologically available source. Pests or insects are directly involved with crop loss. This problem was first encountered with chemical insecticides. But later on, several adverse effects of chemical insecticides were reported which makes it mandatory to search for an alternative. Biopesticides are then developed as an alternative to chemical insecticides to reduce the adverse effect. However, along with this advantage biopesticides also have some limitations which need to be controlled to make these efficient enough. Biopesticides are categorized under three different classes:
- Naturally occurring substances that can control certain pests by involving in nontoxic mechanisms.
- Microbial pesticides which include the involvement of several microbes to control plant pathogens and some pests.
- Plant incorporated protectants which are plant-derived pesticides mostly developed by genetically modified plants.
This article is mostly focused on microbial pesticides and describes different types of microbial pesticides.
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Herbicides are used to control unwanted plant growth, it is also known as weedkillers. The microbial origin of herbicides has developed drastically in the last few years and it is now contributing as an alternative to chemical herbicides. The microbial herbicides are effective enough to control weeds and it is applied in many countries. To illustrate, Puccinia chondrillina is a plant pathogen used to control Chondrilla juncea or skeleton weed in Australia, Cercosporella riparia controls the growth of Ageratina riparia in Hawaii, Phragmidium violaceum was reported for its activity against wild blackberry (Rubus spp.). Apart from these, several studies have supported the effectiveness of microbial herbicides. Such examples include control of water hyacinth by Cercospora rodmanii, the effectiveness of Colletotrichum gloesosporioides against Aeschynomene virginica etc.
Bacteria are mostly associated with causing disease in humans as well as plants. However, there are several bacteria that work as pathogens to several insects. These bacteria play an important role in the development of bacterial insecticides. According to several studies, bacteria that can control several insects are highly specific towards its hosts and it makes it efficient enough to develop as an alternative to chemical insecticides. There are different types of bacterial insecticides available commercially, some of those are described as following
I. Pseudomonas as Bacterial Insecticide
Along with plant growth promotion, several Pseudomonas species are reported to have pathogenicity against several insects. Pseudomonas aeruginosa is one of the species which is mostly reported as a bacterial insecticide. Apart from that P. taiwanensis was reported for its insecticidal activity against agricultural pests such as Plutella xylostella, Spodoptera exigua, Spodoptera litura, etc. These bacteria mostly carry toxin complex genes that have specificity towards insects. P. fluorescence is known for its plant growth-promoting activity. However, studies supported the fact that P. fluorescence involves bacterial antagonists to control fungal pathogens. Moreover, P. cepacia was also reported for its activity towards suppression of plant-pathogen by secretion of siderophores.
II. Bacillus as Bacterial Insecticide
a) Bacillus thuringiensis
B. thuringiensis (Bt) is widely used bacilli which can control insects such as moths, beetle, flies, aphids, butterflies, and even some pathogenic fungi like Pythium ultimum. The mechanism of Bt to control the insects is depending on its toxins. The endotoxin is a crystallized protein that is soluble in alkaline conditions. The pH in the gut of an insect is mostly alkaline and as it ingested the toxin, the toxin easily dissolves in the midgut region. Then proteases come into action by digesting the toxin which leads to the production of small active fragments. These active fragments then bind to the gut epithelial membrane and create pores which leads to disturbance in osmotic equilibrium. This results in the death of the insects. Several agricultural plants were genetically modified with the gene responsible for toxin production in Bt., such plants are Bt. Brinjal, Bt. Tomato etc.
b) Other Bacillus species
B. thuringiensis is one of the bacillus species which is used widely to control insects. However, there are other Bacillus species such as B. papilliae and B. lentimorbus which play an important role in controlling insects such as Japanese beetle.
Disadvantages of Bacterial Insecticides
- Many bacterial insecticides are specific for a species of pest. In many cases, these insecticides kill only one type of insects and the other type of insects survive and continue to cause damage to the crop.
- In the case of genetically modified plants such as Bt. the hypersensitive reaction occurred to the animals and humans consumed the plant.
- Environmental factors such as heat, desiccation, etc. can affect the viability of the bacteria. Therefore, it is difficult to maintain proper application procedures.
- Due to the same reason, proper preservation and storage are required for some bacterial insecticides.
Similar to bacterial insecticides, there are viruses specific for a type of insect. Baculoviridae family is a classic example of this kind of insecticides which are arthropods. However, baculoviruses infect only a few arthropod species. Apart from this, there are some viruses that can control insects such as sawflies and Lepidoptera such as NPV, cytoplasmic polyhedrosis virus, granulosis virus, entomopox virus, etc.
I. Metarhizium anisopliae
This fungus was reported for the production of toxins such as destruxin B and desmethyl destruxin B in silkworm larvae. Apart from the toxin, M. anisopliae culture filtrate is toxic to coleopteran hemocytes in vitro producing changes in organelles, the extract of mycelium is toxic to adult house fly.
II. Beauveria bassiana and B. brongriartii
These fungi were reported for the production of beauvercin which is a depsipeptide, comprises of a cyclic repeating sequence of 3 molecules of N- methyl phenylalanine alternating with three molecules of 2 hydroxyisovaleric acid. Apart from these beauverolides and bassianolide are other cyclopeptides produced by these fungi. These are also reported to attack insects.
III. Verticillium lecanii
The target hosts for this kind of fungus are scale insects and aphids. The control of aphids and seales is done by applying V. lecanii conidia suspended in phosphate buffer containing 0.2% Triton X-100 as a wetting agent by using sprays.
IV. Hirsutella thompsonii
It was reported as a potential killer of Phyllocoptrula oleivora also known as citrus rust mite, Eriophyes sheldone (citrus bud mite), and E. guerreronis (coconut flower mite). The formulation of conidia of H. thompsonii is commercially used in the USA.
R.C. Dubey and D.K. Maheswari, 2016, A Textbook of Microbiology, S Chand & Company Pvt. Ltd.
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