Granulosis viruses- Biological agent for insect pest management
Environment friendly management of agricultural insect pest is major concerned and challenging subject for the agriculturist. To increase the agricultural production through pest suppression, numerous agro-chemicals were discovered and used and still going on. They have been found effective for controlling insect pest but in long term they have adverse side effects such as they disrupt the natural ecosystem, insect developed the resistance against them, many beneficial insects are erased from ecosystem, causes human and animal health hazard (cancer) etc. Because of this, environment friendly insect management method like use of biological agents (predators, parasites, parasitoids, microbes) are gaining attention on insect pest management even though their commercial production is quite tedious. Biological control is more effective and efficient, and may prove economical if employed after acquiring some knowledge of the life history and habits of a pest.
Virus is the effective microbial insect management biological agent that reproduces within the insect host causing disease and resulting death. Naturally, many viruses occur and have potential to infect insect without additional inputs. But nowadays, viruses are produced as commercial products. Several virus groups (baculoviruses, entomopoxviruses, and cypoviruses) exclusively infect arthropods. The most common and effective insect viruses are baculovirus which are known to infect more than 600 insect species throughout the world (McNeil, 2010). They are highly specific in their host range, usually limited to a single type of insect. These viruses are highly virulent, selective, stable and environment friendly. Among baculoviruses, granulosis virus (double- stranded DNA genome which produces small bodies called granules containing a single viron) is highly specific and safer to the non- targeted insect species (Sood and Choudary, 2019). But the use and availability of these viruses are still limiting, especially in developing countries. In 2008, a PhopGV was also isolated in Nepal and in vivo multiplied at NARC for further propagation as a biopesticide (Aryal, 2012).
Mode of action of Granulosis viruses
Typically, the initial infection occurs when a susceptible host insect feeds on plants that are contaminated with the occluded form of the virus. When the ingested OBs (occlusion body) reached to the insects’ midgut, the protein matrix gets dissolve in the alkaline environment (pH 8.0), releasing the infective particles (virions). Virions enter into the peritrophic membrane either by direct diffusion with microvilli on the brush border midgut columnar epithelial cells or by adsorptive endocytosis. This entry may also be receptor-mediated. In the next step, uncoating of the viron takes place before passing through the nuclear pores. The genome is then incorporated into the host genome and is transcribed by host machinery. They can produce either intracellular occlusion derived virus (ODV), or extracellular viral progeny (budded virus or BV). The ODV´s transmit infections from insect to insect, whereas the BV’s spread the infection from cell to cell within an infected insect (Granados, 1980). Budded viruses (BV) are formed using host machinery to infect more cells in the organism, as occluded viruses cannot be transmitted between cells due to presence of granulin, which cannot enter cells (Fields Virology, 2013). BVs are released into the haemolymph and undergo rounds of multiplication in the cells of susceptible tissues. BV synthesized following the secondary infection of haemocytes and tracheal matrix, initiates further infection of most other tissues of the lepidopteran host. Appearance of the virus within the fat body and epidermis indicates that invivo spread of the virus is almost complete and that the larva will soon succumb to infection.
During these late stages of replication before the larva die, the granulosis viruses begin to express a gene called ‘egt’ in gut cells which causes the larva to become agitated and move up the fruit/leaves they are on (Zimmer, 2014). This forced behavior ensures that the viral particles spread further upon liquification of the host (Fields Virology, 2013; Zimmer, 2014). Finally, the occlusion body protein (polyhedrin/granulin) crystallises to form the OBs, which are released into the environment. This occlusion body is efficient at protecting viral particles from heat and ultraviolet light protein denaturation while the budded virus membrane does not protect the virus from environmental damage (Fields Virology, 2013). Granuloviruses proteases and chitinase help to degrade host proteins, distrupt the chitinuous exoskeleton and liquify the host, which is the cause of insect death (Luque et al, 2001). Tissue susceptibility varies greatly between viruses with some NPVs being capable of infecting almost all tissue types and most GVs being tissue-specific replications (e.g. fat body cell only).
A single caterpillar at its death may contain over 109 OBs from an initial dose of 1000. The infected larvae exhibit negative geotropis, thereby facilitating widespread dissemination. The speed with which death occurs is determined by the environmental conditions. Under optimal conditions, the target pest may be killed in 3–7 days, but death may occur in 3–4 weeks when conditions are not ideal (Cunningham, 1995; Flexner and Belnavis, 2000).
Significance of Granulosis virus in insect pest management
Baculoviruses have been applied as targeted biocontrol agents against forestry and agriculture pests and this is due to the diversity of products already available, either developed or commercialized form in many countries of the world. Successful example of use of GV in agricultural pest control can found in many literature reviews. Soybean pest, the green cloverworm Plathypena scabra, (Lepidoptera: Noctuidae), has been efficiently controlled with GV (PsGV). In a test, PsGV @ 2.47 × 1011 OB ha–1 showed no significant differences in the control levels obtained when compared with a Bacillus thuringiensis product and a chemical insecticide, after six days of treatment. On the other hand, in Lima, Peru, the potato tuberworm, Phthorimaea operculella (Lepidoptera: Gelechiidae) has been efficiently controlled with its own GV (PoGV) (Sporleder et al, 2007). Producers dust the potatoes just after harvest and before storage with a dry formulate of PoGV, provided mostly by government agencies. This has proven to be a practical and easy method to control this pest. Similarly, the GV (PrGV) isolated from the imported cabbageworm Pieris (Artogeia) rapae (Lepidoptera: Pieridae) has been used for the control of this pest in broccoli and cauliflower, with good level of success. Experiment was carried out in the United States using this baculovirus in which control levels achieved ranged between 87 and 97% nine days after application at doses between 2.4 × 1011 and 2.4 × 1013 OB (granules) ha–1 (Huber, 1986). The codling moth Cydia pomonella (Lepidoptera: Tortricidae), is a pest on pear and apple crops. This insect has development resistance to several chemical insecticides. One baculvirus isolated of this pest CpGV, is highly virulent for codling moth with LD50’s as low as 1.2- 5 occlusion bodies per insect. CpGV was originally isolated from C. pomonella in Mexico, 40 years ago (Tanada, 1964). Preparations of OB’s of this virus are called Granusal™, Granupom™, Carpovirusine™, Virin-CyAP, Cyd-X, VirosoftCP4, and Madex™. The virus has been successfully used in the United States, Canada, and Europe as bio-insecticide on a hundred thousand or more hectares. Granulovirus also found effective against the Indian meal moth and helps to keep the damage below industrial standard up to 14-16 weeks. They are applied after disinfecting the commodity to protect them for longer storage (Scholler, 2006).
Conclusion
Entomopathogenic viruses are gaining popularity in controlling agriculture insect pest because of their high effectiveness and specificity on host insect. They even don’t have negative impacts on plants, mammals, birds, fish, or even on non-target insects. Among them GVs is more specific than other viruses but the research and commercial production on them are still limiting. Some of the developed countries have started their commercial production and researches have been done about their effectiveness at different doses. Granulosis virus shows its significance importance in controlling numerous agriculture insect pests like soybean pest (Plathypena scabra), potato tuberworm (Phthorimaea operculella), cabbageworm (Pieris (Artogeia) rapae), codling moth Cydia pomonella, Indian meal moth etc. It has potential to infect different orders of insect-pest including Lepidoptera, Diptera, Hymenoptera, Orthoptera, Isoptera and Neuroptera. So we should focus on research on granulosis virus to exploit its effectiviness and scope on insect-pest management.
