Use of Beauveria bassiana and Metarhizium anisopliae for the Treatment of Root Borer in Apple

Root borers, such as Dorysthenes hugelii, are destructive pests that attack apple orchards, primarily by feeding on the roots and lower trunks of apple trees. This can lead to severe damage, including reduced tree vigor, fruit yield, and even tree death. With concerns over chemical pesticide use, biological control using entomopathogenic fungi, such as Beauveria bassiana and Metarhizium anisopliae, offers an eco-friendly and sustainable alternative for managing root borer infestations.

Overview of Beauveria bassiana and Metarhizium anisopliae

Both Beauveria bassiana and Metarhizium anisopliae are naturally occurring fungi that specifically target insects. They are classified as entomopathogenic fungi, meaning they infect and eventually kill their host insects. These fungi are effective biocontrol agents against various insect pests and are particularly suitable for soil-borne pests like root borers.

Key Characteristics:

• Beauveria bassiana: Known for its wide host range, B. bassiana acts primarily by penetrating the insect’s cuticle (outer shell) and proliferating within the insect’s body, eventually killing it.
• Metarhizium anisopliae: This fungus is similarly effective against a broad range of insects. It invades the insect through the cuticle and releases toxins that contribute to the insect’s death.

Both fungi are applied directly to the soil or plant surface, where they come into contact with the root borer larvae or adults, infecting and killing them.

Root Borer Infestation in Apple Trees

Root borers, particularly the larvae, attack apple trees by boring into the roots and trunk, leading to:

• Damage to the roots: This disrupts the uptake of water and nutrients, weakening the tree.
• Tunneling in the lower trunk: This reduces structural stability and can result in girdling, leading to tree death.
• Symptoms: Wilting, yellowing of leaves, stunted growth, poor fruit production, and in severe cases, the collapse of the tree.

The larvae stage of root borers is the most damaging, and traditional control methods include chemical insecticides, which are often non-selective and can harm beneficial organisms.

Mode of Action of Beauveria bassiana and Metarhizium anisopliae against Root Borers

The fungi work by a series of infection steps that ultimately result in the death of the root borer:

• Spore Attachment: When fungal spores come into contact with the insect, they adhere to the insect’s cuticle. Moisture and temperature play a critical role in successful attachment.
• Cuticle Penetration: The spores germinate and produce germ tubes that penetrate the insect’s cuticle using mechanical pressure and enzymatic degradation. B. bassiana and M. anisopliae secrete enzymes like chitinase and proteases, which break down the cuticle.
• Infection and Proliferation: Once inside the insect’s body, the fungus spreads throughout the hemocoel (the insect’s body cavity), releasing toxins that disrupt the insect’s physiological processes, causing death. This usually takes several days, depending on environmental conditions.
• Sporulation: After the insect dies, the fungus emerges from the insect’s body, producing new spores that spread in the environment and infect other root borers.

This natural cycle allows for ongoing suppression of root borer populations.

Application Methods in Apple Orchards

Effective control of root borers using Beauveria bassiana and Metarhizium anisopliae depends on the method of application. The most common techniques include:

• Soil Drenching: This method involves mixing fungal spores in water and applying them to the soil around the base of the apple tree. The goal is to target root borer larvae feeding in the root zone. This ensures that the fungal spores reach the pests directly in their habitat.
• Trunk Spraying: For adult root borers or larvae that feed near the base of the tree, spraying fungal spores onto the trunk can be effective. Spores come into direct contact with insects as they emerge or move up the tree.
• Root Zone Inoculation: Inoculating the soil directly around the tree’s root zone with B. bassiana or M. anisopliae ensures that the fungi are present in the area where root borer larvae are actively feeding. This can be particularly effective in orchards with heavy infestations.

Environmental Conditions for Optimal Fungal Activity

The effectiveness of Beauveria bassiana and Metarhizium anisopliae is influenced by several environmental factors, which must be considered to optimize their performance:

• Soil Moisture: These fungi thrive in moist conditions. Adequate irrigation or application following rainfall can help improve spore germination and insect infection.
• Temperature: Optimal temperature ranges for fungal growth and infection are generally between 20°C and 30°C. Extremely high or low temperatures can inhibit the fungi’s ability to infect root borers.
• pH Levels: Both fungi prefer slightly acidic to neutral soil conditions, typically with a pH between 5.5 and 7.5. Soil that is too alkaline or acidic may reduce fungal efficacy.

Managing these environmental conditions through proper orchard management practices, such as irrigation and mulching, can enhance the effectiveness of fungal applications.

Integration into Integrated Pest Management (IPM)

Beauveria bassiana and Metarhizium anisopliae are well-suited to integrated pest management (IPM) programs in apple orchards. By combining biological control agents with cultural practices, mechanical controls, and selective use of chemical pesticides, growers can reduce root borer populations while minimizing environmental impact.

Some important IPM strategies include:

• Cultural Practices: Maintain orchard hygiene by removing infested trees and pruning damaged roots. Mulching and maintaining healthy soil conditions can help reduce root borer infestations.
• Monitoring and Trapping: Use pheromone traps to monitor adult borer populations and time fungal applications for maximum impact on emerging adults and larvae.

Advantages of Using Beauveria bassiana and Metarhizium anisopliae

• Eco-Friendly: These fungi are natural biocontrol agents that reduce reliance on chemical pesticides, promoting environmental sustainability.
• Specific Targeting: The fungi selectively infect root borers and other harmful pests, leaving beneficial insects like pollinators largely unaffected.
• Reduced Pesticide Resistance: Biological control agents such as these fungi provide a multi-mechanism mode of action, reducing the risk of pests developing resistance, a common issue with chemical pesticides.
• Sustainable Control: The fungi continue to reproduce and spread in the soil, providing long-term suppression of root borer populations.

Limitations and Considerations

Despite the many benefits of Beauveria bassiana and Metarhizium anisopliae, there are some limitations to their use:

• Slow Action: These fungi may take several days to kill root borers, whereas chemical insecticides typically act faster. This makes them less effective for acute infestations requiring immediate control.
• Environmental Sensitivity: The efficacy of these fungi depends heavily on environmental conditions, such as moisture and temperature. Poor conditions may reduce their effectiveness.
• Cost: The initial cost of biological control agents may be higher than chemical pesticides, although the long-term benefits and sustainability often outweigh the cost.

The use of Beauveria bassiana and Metarhizium anisopliae offers an effective and environmentally friendly approach to controlling root borers in apple orchards. These fungi provide sustainable pest control by infecting and killing root borers, reducing the reliance on harmful chemical pesticides. To maximize their effectiveness, growers must carefully manage environmental conditions and integrate these fungi into a comprehensive pest management strategy. By doing so, they can protect their apple trees from root borer damage and promote healthier, more productive orchards in the long term.