Fungi used as a Insecticide

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[edit] Introduction

Certain species of fungi can act as parasites of insect. When a fungus is used as an insecticide, it is called mycoinsecticide. In recent years, crop protection has been trending towards integrated pest management (IPM) using bacteria and fungi as insecticides. Approximately 750 species of fungi are pathogenic to insects [1] Only 12 have been utilized for use as insecticides. Two prominent species of fungi used as insecticides are Beauveria bassiana and Metarhizium anisopliae[1]. Mycoinsecticides function by first being applied to the insects in spray form. The fungi then use their hyphae to burrow into the insects. The hyphae spread the insectotoxins throughout the insect to activate them, eventually leading to the the insect’s death [1].

[edit] General Process

Insecticides act differently from bacterial and viral pathogens of insects in terms of infection by contact and not consumption. Adsorption serves as the major rate limiting step for pest control in terms of insecticidal usage [2]. Entomopathogenic fungi require a multi-step process that includes various different physical interactions, germination, and penetration which varies by species[2]. Each of these different processes also goes on to serve different physiological, chemical and physical specificities[2]. Changes in normal physiology as well as the development of alterations including but not limited to an eventual reduction in both feeding and mobility as well as behavioural fever responses and alterations in typical migration patterns are all results of infection by Entomopathogenic fungi [2].

[edit] Mycoinsecticide Strength

Metarhizium anisopliae is considered to be a low power insecticide[3]. Studies have shown that the scorpion species Androctonus australis can be used to increase M. anisopliae’s insecticide strength by 22-fold [3].
Entomology professor Raymond St. Leger from the University of Maryland was the first to discover how to create hypervirulent fungi that can be used as insecticides [3]. From the scorpion, Androtonus australis insect neurotoxin (AaIT) is extracted and with it a recombinant DNA is formed [3]. This recombinant DNA is inserted into the fungi species, creating a bioengineered clone of M. anisopliae named ARSEF 549 [3]. AaIT is activated once the fungus has transferred it into the bloodstream of the insect. AaIT is an insectotoxin that works by attacking insect motor nerves, paralyzing them. Hypervirulent mycoinsecticides are beneficial because they fix one of the major problems with using fungi as insecticides, which is that the process of killing the insects can take a lot of time.

There have been other studies done in hopes of trying to determine ways to increase mycoinsecticide strength. One specific study tested the effects of M. anisopliae and B. Bassiana in combinations with common chemical insecticides against the Bihar hairy caterpillar (Spilarctia oblique)[1]. B. Bassiana and M. anisopliae were tested with common chemical insecticides Endosulfan, Imidacloprid, Lufenuron, Diflubenzuron, Dimethoate and Oxydemeton methyl in varying ppm concentrations based on recommended field-dose values (5 different trials of B. Bassiana with each insecticide)[1]. The results showed that the anti-insect strength all of insecticides mixed with either mycoinsecticide increased. [1]

[edit] Mycoinsecticide Accuracy

Insects, especially at the larva stage that have embed themselves into plant tissues or soil, do not feed on the crop [4]. They will not be in one specific location on the crop or in the soil and therefore cannot be accurately targeted. Spray applications designed to deposit mycoinsecticides over a field or crop row are not efficient. In many mycoinsecticide cases, a granular solution that is capable of penetrating the crop canopy gravitating into target sites (such as soil, leaves or nests) can be considerably more effective. Granular formulations for mycoinsecticides are produced in a range of ways. Some include coating dry spores onto bran or grain, another involves drying & fragmentation of mycelium to hold spores in starch [4]. A last method is the production of fungus on whole kernels of grain, followed by drying and pulverization of the kernel produces a good granular substance [4]. Including granular substances in mycoinsecticides increase the accuracy and efficiency of the spray.

[edit] Specific Examples

Beauveria bassiana

causes what is most commonly known as white muscadine disease. There are over 200 species, in 9 different orders that serve as hosts. The mode of action includes the invasion of the haemocel by spores and proliferation of the fungus[5]. Germ tubes develop and mycelia from the elongated germ tubes are septate and release blastospores. Cell death occurs due nutrient depletion of the haemolymph or toxaemia by fungal toxic metabolites[5]. Successful infection is dictated predominantly by different enzymatic activities for degrading proteins, lipids and chitin[5].

[edit] Conclusion

Mycoinsecticides cannot compete with the extremely effective and cheap chemical pesticides in the market at this time. Fungus based insecticides are environmentally sensitive. Rapid pest increases would not be tamed with the use of mycoinsecticides. Mycoinsecticides are slow, moderately effective and rather inconsistent. High replication rates (1013 to 1014 spore/ha) are often required to prove acceptable levels of control [4]. Spore based mycoinsecticide products are not mass produced because efficient production technologies exist only for select strains of a few pathogen species [4]. The most successful productions have been with B. bassiana due to the small size of conidia with respect to those of other entomopathogenic fungi. Commercial-scale production capacity necessary to support multiple applications to field crops at the rate of 1013 spores / ha at costs competitive with synthetic chemical insecticides has long represented a major production barrier [4] . Problems of efficacy are linked to production economics. Enhanced virulence is needed in mycoinsecticides to move forward with marketing them. Labour costs and decreasing efficiency with increases in scale are significant constraints to continued use of these low-technology production systems. The future looks promising, with scientists increasing pathogenic strength of entomopathogenic fungi to the verge of commercial success with genetic and physiological engineering.

[edit] Notes and References

  1. 1.0 1.1 1.2 1.3 1.4 1.5 Purwar, Sachan. (2006). Synergistic effect of entomogenous fungi on some insecticides against Bihar hairy caterpillar Spilarctia obliqua (Lepidoptera: Arctiidae). Microbiological Research 161, 38-42.
  2. 2.0 2.1 2.2 2.3 Khachatourians, G. G., Valencia, E., & Miranpuri, G. S. (2002). Beauveria bassiana and other Entomopathogenic Fungi in the Management of Insect Pests. Microbial biopesticides, 2, 239-275.
  3. 3.0 3.1 3.2 3.3 3.4 St. Leger, Raymond J., Wang, Chengshu. (2007). A Scorpion neurotoxin increases the potency of a fungal insecticide. Nature Biotechnology 25.12, 1455-1456
  4. 4.0 4.1 4.2 4.3 4.4 4.5 Hall, Franklin R. Biopesticides: Use and Delivery. (1998). Humana Press Inc. 233-270
  5. 5.0 5.1 5.2 Feng, M.G., Poprawski, T.J. and Khachatourians, G.G. (1994) Production, formulation and application of the entomopathogenic fungus Beaveria bassiana for inset control: current status. Biocontrol Science and Technology. 4:1,3-24
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