Biological larvicides: recommended applications and dosages

Biological larvicides represent an ecological alternative to chemical insecticides, offering a more sustainable and less toxic solution for controlling pest insects. They are used to control insect populations, including mosquitoes that carry diseases like malaria and dengue fever, nuisance flies in livestock farms and urban areas, and other insect pests in agriculture. The demand for more sustainable and ecological solutions is constantly growing, and biological larvicides are proving to be a promising alternative to chemical pesticides.

Understanding Biological Larvicides

Biological larvicides are natural agents that act by targeting insect larvae, thereby preventing their development and the spread of nuisances. They are often derived from bacteria, fungi, viruses, or other living organisms, and have a much better safety and environmental impact profile than traditional chemicals.

Mechanisms of Action

• Bacteria: Bacillus thuringiensis (Bt) and Bacillus sphaericus (Bs) are bacteria widely used as larvicides. They produce toxic proteins that paralyze the digestive system of insect larvae, killing them quickly. These bacteria are highly specific, mainly targeting the larvae of mosquitoes, flies, and butterflies. For example, Bacillus thuringiensis var. israelensis (Bti) is an effective biological agent for controlling the larvae of mosquitoes Aedes aegypti, Anopheles gambiae, and Culex quinquefasciatus. Studies have shown that the application of Bti in stagnant water areas can significantly reduce mosquito populations.
• Fungi: Fungi such as Beauveria bassiana and Metarhizium anisopliae are effective biocontrol agents against insect larvae. They infect the larvae and develop inside, killing them gradually. These fungi can act on a variety of insects, including beetles, flies, and butterflies. For example, the fungus Beauveria bassiana is successfully used to control the red palm weevil (Rhynchophorus ferrugineus), an important pest of palm trees.
• Viruses: Certain viruses can be used as biological larvicides, specifically targeting insect larvae. They infect the larvae and cause a disease that kills them. Nuclear polyhedrosis virus (NPV) is a common example, used against the larvae of butterflies and caterpillars. For example, NPV has been successfully used to control the pine processionary caterpillar (Thaumetopoea pityocampa), an important pest of pine forests.
• Other Biological Agents: Nematodes, plant extracts, and other biological agents can also be used as larvicides. Nematodes parasitize insect larvae, killing them by consuming them from the inside. Plant extracts, such as neem oil, can have a repellent or toxic effect on the larvae. For example, the nematode Steinernema carpocapsae is used to control the larvae of the European corn borer (Ostrinia nubilalis), an important pest of corn crops.

Advantages of Biological Larvicides

• Reduced Environmental Impact: Biological larvicides decompose rapidly in the environment and do not accumulate toxic residues. They are less harmful to non-target animals, plants, and aquatic organisms, helping to preserve biodiversity. For example, a study has shown that the use of Bacillus thuringiensis var. israelensis (Bti) to control mosquito larvae has a minimal impact on fish populations and other aquatic organisms.
• Limited Toxicity: Biological larvicides are generally considered to be of low toxicity to humans and domestic animals. However, it is important to respect the precautions for use and to follow the manufacturer's recommendations. For example, the U.S. Environmental Protection Agency (EPA) has classified Bacillus thuringiensis var. israelensis (Bti) as having very low toxicity to mammals and birds.
• Reduced Risk of Resistance: Insects are less likely to develop resistance to biological larvicides than to chemical insecticides, because the mechanisms of action are different and biological agents often have a variety of modes of action. Studies have shown that mosquito populations have developed resistance to certain chemical insecticides but remain sensitive to biological larvicides such as Bacillus thuringiensis var. israelensis (Bti).
• Usable in Organic Agriculture: Biological larvicides are compatible with organic farming practices, offering a more sustainable and environmentally friendly pest control solution. For example, the use of Bacillus thuringiensis var. kurstaki (Btk) is permitted in organic agriculture for the control of certain pests such as the European corn borer (Ostrinia nubilalis).

Applications of Biological Larvicides

Biological larvicides are used in a variety of applications, targeting different types of insects and nuisances. Here are some examples of main applications:

Mosquito Control

Biological larvicides are an essential tool for controlling mosquito populations, which are important vectors of diseases such as malaria, dengue fever, chikungunya, and yellow fever. Mosquito species targeted include Aedes aegypti, Anopheles gambiae, and Culex quinquefasciatus.

Biological larvicides are used in vector control programs in urban and rural areas, in stagnant water points, marshes, irrigation canals, and water storage areas. They are also used to prevent mosquito bites in gardens, parks, and waterways. In 2020, the World Health Organization (WHO) recommended the use of Bacillus thuringiensis var. israelensis (Bti) for the control of mosquito larvae, especially in areas where malaria is endemic.

Fly Control

Biological larvicides can also control fly populations, which are important pests in livestock farms, urban areas, and public places. Target species include Musca domestica, Stomoxys calcitrans, and Glossina spp. (tsetse flies).

Biological larvicides are applied in stables, poultry houses, farms, and garbage storage areas. They help reduce fly populations, thereby reducing the risk of disease transmission and improving sanitary conditions in these environments. For example, Bacillus thuringiensis var. israelensis (Bti) has also shown efficacy in controlling the larvae of house flies (Musca domestica).

Control of Other Pest Insects

Biological larvicides are used against a variety of other pest insects, including the larvae of beetles, butterflies, black flies, and other insect pests in agriculture.

They are applied in agricultural fields, forests, gardens, and green spaces to protect crops, trees, and plants from damage caused by insect larvae. For example, Bacillus thuringiensis var. kurstaki (Btk) is used to control the European corn borer (Ostrinia nubilalis) in corn crops, while Bacillus thuringiensis var. aizawai (Bta) is used to control the pine processionary caterpillar (Thaumetopoea pityocampa) in pine forests.

Recommended Dosages and Methods of Application

The dosage of biological larvicides varies depending on the target species, the larval stage of development, the environmental conditions, and the type of application. Here is general information on dosages and methods of application:

Factors Influencing the Dosage

• Target Species: The choice of larvicide and dosage will depend on the species of insect targeted, as biological larvicides have different levels of specificity. For example, Bacillus thuringiensis var. israelensis (Bti) is more effective in controlling mosquito larvae than fly larvae.
• Environmental Conditions: Temperature, humidity, and the presence of sunlight can affect the effectiveness of biological larvicides. It is important to choose a product suitable for local environmental conditions. For example, fungal-based biological larvicides are generally more effective in humid and warm conditions.
• Larval Stage of Development: Younger larvae are generally more susceptible to biological larvicides. It is therefore recommended to apply the product at an early larval stage. For example, mosquito larvae in the first larval stage are more sensitive to biological larvicides than larvae in the fourth stage.
• Concentration of the Biological Agent: The concentration of the biological agent in the product will affect the dosage. The manufacturer's recommendations should always be followed. For example, a product containing a concentration of 10,000 units of Bacillus thuringiensis var. israelensis (Bti) per gram will require a lower dosage than a product containing a concentration of 5,000 units per gram.
• Type of Application: The method of application, such as spraying, granulation, or diffusion, can affect the distribution and effectiveness of the product. For example, aerial spraying is more effective for treating large areas, while granulation is more suitable for treating stagnant water areas.

Recommended Dosages

It is important to consult the manufacturer's recommendations and regulatory agencies to find out the precise dosages recommended for each biological larvicide and each application.

Here are some concrete examples of recommended dosages for targeted species, but they may vary depending on the product and local conditions:

• Bacillus thuringiensis (Bt) for Mosquito Larvae: A typical dosage is 100 grams of Bt per hectare for stagnant water areas. Bacillus thuringiensis var. israelensis (Bti) is often used at a concentration of 1 to 5 grams per hectare to control mosquito larvae.
• Beauveria bassiana for Beetle Larvae: The dosage may vary from 1 to 5 grams of spores per liter of water depending on the beetle species targeted. For the control of the red palm weevil (Rhynchophorus ferrugineus), a dosage of 5 grams of spores per liter of water is generally recommended.
• Nuclear Polyhedrosis Virus (NPV) for Butterfly Larvae: A typical dosage is 1000 infectious units per larva in spraying. To control the pine processionary caterpillar (Thaumetopoea pityocampa), a dosage of 100 infectious units per larva is generally recommended.

Methods of Application

• Aerial and Ground Spraying: Biological larvicides can be applied by aerial or ground spraying, especially in stagnant water areas, agricultural fields, and forests. Aerial spraying is more effective for treating large areas, while ground spraying is more suitable for treating specific areas.
• Application by Granules: Granules of biological larvicides are applied in stagnant water areas and agricultural fields, providing a more controlled distribution of the product. The granules dissolve gradually in water, releasing the biological agent and killing the insect larvae.
• Targeted Treatments: Targeted treatments can be carried out in larval habitats, such as stagnant water points, gutters, used tires, and water containers. This type of treatment makes it possible to concentrate control efforts in areas where the larvae are most concentrated.

Challenges and Future Perspectives

Biological larvicides offer a promising solution for controlling pest insects, but challenges remain and advances are needed to optimize their use and effectiveness.

Application Difficulties

• Sensitivity to Environmental Conditions: Some biological agents are sensitive to environmental conditions, such as temperature, humidity, and sunlight. Their effectiveness may be reduced in unfavorable conditions. For example, Bacillus thuringiensis var. israelensis (Bti) is more effective in high temperature and humidity conditions, while fungi are generally more effective in humid and warm conditions.
• Limited Duration of Action: The duration of action of biological larvicides may be limited compared to chemical insecticides. Repeated applications may be necessary to achieve effective control. For example, Bacillus thuringiensis var. israelensis (Bti) has a duration of action of a few days to a few weeks, while some chemical insecticides have a duration of action of several weeks to several months.
• Production Cost: The cost of producing biological larvicides can be higher than that of chemical insecticides, but it is important to consider the overall cost, including environmental and health costs. Biological larvicides may require larger initial investments, but their reduced environmental impact and lack of toxicity to non-target animals can generate long-term savings.
• Lack of Knowledge: There are still gaps in knowledge regarding certain biological agents, their modes of action, and their effects on the environment. Research and development are needed to improve our understanding of biological larvicides and to develop new, more effective, and more specific agents.

Future Perspectives

• Research and Development: Research and development of new, more effective, more specific, and more resistant biological agents to environmental conditions are essential. The use of artificial intelligence and machine learning techniques can help accelerate the discovery and development of new biological larvicides.
• Optimization of Application Techniques: Efforts are needed to improve the application techniques of biological larvicides, maximizing their effectiveness and their impact on targeted insect populations. The development of more efficient formulations and application technologies is crucial for improving the effectiveness of biological larvicides.
• Promotion and Awareness: The promotion of the use of biological larvicides by public authorities and professionals, as well as raising public awareness of the benefits of ecological solutions, is important to encourage their widespread adoption. Communication campaigns and awareness programs can help change attitudes and behaviors, promoting the use of more sustainable insect control methods.