Muhammad Ahsin Ayub*, Bilal Atta**
* Rice Research Station, Bahawalnagar, Punjab, Pakistan.
** Rice Research Institute, Kala Shah Kaku, Punjab, Pakistan.
Rice is an essential cereal crop, providing food for over half of the world's population. However, the yield of rice is frequently limited by various biotic factors, especially insect pests. Insects cause direct damage to rice plants by feeding on leaves, stems, and grains, leading to yield losses. The use of chemical pesticides has been the most prevalent means of controlling insect pests, but this approach is often costly, harmful to non-target organisms, and environmentally damaging. Hence, the utilization of entomopathogens as a natural means of pest management is gaining popularity. Entomopathogens are microorganisms that infect and kill insect pests. They are non-toxic to non-target organisms and environmentally friendly. The current article aims to assess the role of entomopathogens for the management of rice insect pests.
Entomopathogens:
Entomopathogens are microorganisms that infect and kill insects. They include viruses, bacteria, fungi, and protozoa. These organisms have evolved to live in the soil or on plants and have developed mechanisms to infect and colonize insect hosts. Entomopathogens can be classified as either generalists or specialists, depending on their ability to infect a wide range of insect species or a specific group of insects, respectively.
Baculoviruses:
Baculoviruses are DNA viruses that infect insects, including rice pests such as stem borers (Scirpophaga incertulas, Chilo suppressalis), leaf folder (Cnaphalocrocis medinalis), and the rice bug (Leptocorisa acuta). These viruses have been shown to have a high degree of specificity towards their hosts and are considered safe for non-target organisms. Baculoviruses are widely used as biopesticides for the control of insect pests. The baculovirus-based biopesticides have been commercialized and available in the market.
Fungi:
Fungi are an essential group of entomopathogens that infect insect pests. They are found in the soil, plant residues, and other organic matter. The most commonly used fungi for pest management include Metarhizium anisopliae, Beauveria bassiana, and Lecanicillium lecanii. These fungi have a broad range of insect hosts, including rice pests such as brown planthopper (BPH), green leafhopper (GLH), and the rice bug. The fungi spores attach to the insect cuticle and penetrate into the insect body, causing death. Fungi can be formulated as biopesticides and are considered safe for non-target organisms.
Bacteria:
Bacteria are also important entomopathogens that infect and kill insect pests. The most commonly used bacteria for pest management are Bacillus thuringiensis (Bt) and Serratia entomophila. Bt is a gram-positive bacterium that produces crystal toxins that are toxic to insect pests. Bt has been used for the control of stem borers, leaf folders, and BPH. S. entomophila is a gram-negative bacterium that infects BPH and is considered a promising biocontrol agent.
Protozoa:
Protozoa are unicellular organisms that infect and kill insect pests. The most commonly used protozoa for pest management is Nosema locustae. N. locustae is a microsporidian protozoan that infects BPH and is considered a promising biocontrol agent.
Application of entomopathogens for rice insect pest management:
Entomopathogens can be applied in different ways for rice insect pest management. These include direct application, application via bait, and application via host plant.
(1) Direct application:
Direct application involves applying the entomopathogens directly to the insect. This method is most effective when the target insect is in the early stages of development, and the infection can spread rapidly. The application can be in the form of spray, dust, or bait. For example, the fungus Metarhizium anisopliae can be sprayed onto rice plants, and the spores will attach to the insects as they feed on the plants. Baculoviruses can be sprayed onto rice fields, and the virus particles will be ingested by the target pests.
(2) Application via bait:
Application via bait involves applying the entomopathogens to a food source that attracts the target insects. This method is most effective for insects that are difficult to target using direct application. For example, the fungus Beauveria bassiana can be mixed with a sugar solution and applied to traps that attract and kill GLH.
(3) Application via host plant:
Application via host plant involves incorporating the entomopathogens into the host plant. This method is most effective when the target insect is a sap-sucking pest, such as BPH or GLH. The entomopathogens can be incorporated into the plant tissue or applied as a coating to the plant surface. For example, the bacterium Bacillus thuringiensis can be incorporated into the rice plant through genetic engineering, and the plant will produce the toxin, killing the target pests.
Advantages of using entomopathogens for rice insect pest management:
(1) Environmentally friendly:
Entomopathogens are considered safe for the environment and non-target organisms. Unlike chemical pesticides, they do not persist in the environment, and there is no accumulation of toxic residues in the soil, water, or air.
(2) Cost-effective:
Entomopathogens are cost-effective compared to chemical pesticides. They are easy to produce and can be mass-produced using fermentation technology. Moreover, entomopathogens can be produced locally, reducing transportation costs.
(3) High specificity:
Entomopathogens have a high degree of specificity towards their target pests. They do not harm beneficial insects, such as pollinators, and do not affect the quality of the produce.
(4) Sustainable:
Entomopathogens are sustainable and can be used as part of an integrated pest management strategy. They can be used in combination with other control measures, such as cultural practices, to manage pest populations effectively.
(5) Resistance management:
Entomopathogens can help to manage insecticide resistance in pest populations. As they have a different mode of action compared to chemical pesticides, they can be used to target insects that have developed resistance to chemical pesticides.
Challenges and limitations:
(1) Limited efficacy:
The efficacy of entomopathogens can be limited under certain conditions, such as high temperature, low humidity, and UV radiation. They also require specific environmental conditions to be effective.
(2) Limited shelf life:
Entomopathogens have a limited shelf life and require proper storage conditions to maintain their efficacy. They also have a short persistence in the environment, and repeated applications may be necessary to maintain control.
(3) Slow speed of action:
Entomopathogens can take longer to act compared to chemical pesticides. They require time to infect and kill the target pest, and the speed of action can be affected by environmental conditions.
(4) Limited availability:
Entomopathogens are not widely available, and their use is limited to certain regions. The lack of availability of entomopathogens can be a significant barrier to their adoption.
Conclusion:
The use of entomopathogens for the management of rice insect pests is an effective and sustainable approach. Entomopathogens have several advantages, such as their high specificity, cost-effectiveness, and environmental safety. However, there are also challenges and limitations associated with their use, such as their limited efficacy under certain conditions, limited shelf life, slow speed of action, and limited availability. Despite these challenges, the use of entomopathogens is an essential component of an integrated pest management strategy for rice insect pests.
To overcome these limitations, there is a need for further research on the development of more effective formulations, improving the storage stability, and exploring new delivery mechanisms. The integration of entomopathogens with other control measures, such as cultural practices and biological control agents, can also improve their efficacy and sustainability.
In conclusion, entomopathogens are a promising alternative to chemical pesticides for the management of rice insect pests. They are effective, sustainable, and environmentally safe, making them an ideal choice for farmers and consumers who prioritize sustainable agriculture practices. With further research and development, entomopathogens can become an integral part of pest management strategies for rice cultivation, ensuring a safe and sustainable food supply for future generations.