Cover Story

Biological Control in the Nursery – Part 3: Effective Use of Microbial Control Products

By Kripa Dhakal, Alfred Daniel J, and Karla M. Addesso – Tennessee State University, Otis L. Floyd Nursery Research Center, 472 Cadillac Lane, McMinnville, Tennessee, 37110, USA

In Part 3 of this three-part series, we discuss the effective use of microbial control products in pest and disease management programs. These products can infect and kill both plant pests and pathogens. They can serve as alternatives to broad-spectrum chemical pesticides or work together with them as part of an integrated management plan.

Microbial Control Products

Microbial products are naturally occurring, or genetically modified microorganisms used to control plant pests and disease. They include specific species of fungi, protozoa, viruses, and bacteria. These products are most effective as preventative treatments or under conditions of low pest or disease pressure and are less toxic to the environment and nontarget organisms. A range of problems can be managed using microbial products and are available for various applications, from professional to homeowner use. It is important to follow product labels for safe and effective application.

Microbial pesticides function through various mechanisms:

• Direct infection of target pests or pathogens with fatal diseases (Fig.1).

• Release of toxic substances to kill competing plant pathogenic microbes or arthropods.

• Outcompeting pathogenic microbes for space in soil or plant tissue.

Figure 1. a – Ant infected with fungal pathogens.

Figure 1. b – Dipteran fly infected with fungal pathogens.

Figure 1. c – Caterpillar infected with nucleopolyhedrovirus.

The most well-known microbial product is Bacillus thuringiensis (Bt), which produces a protein toxin in the gut of infected insects. The toxin creates holes in the digestive tract, causing the insect to starve to death. Different Bt strains can target specific insect groups, such as those of mosquitoes, moths, flies, and beetles, while leaving other non-target insects, like pollinators, unaffected. In addition, different strains of naturally occurring nuclear polyhedrosis virus (NPV) are used against gypsy moth and helicoverpa larvae. This allows for a targeted treatment of the problem pest, while leaving beneficial insects unharmed.

Advantages and Challenges

Numerous advantages can be found in the utilization of microbial control products, including increased safety for humans and other nontarget organisms, reduction of pesticide residues in food, preservation of natural enemies, and increased biodiversity in managed ecosystems. However, live microbes need specific temperature and humidity conditions to thrive and can be vulnerable to sunlight degradation. Their performance varies based on environmental factors and narrow target ranges can limit their functionality against multiple pests. Continuous use can lead to resistance development as is seen with chemical pesticides, and some products have shorter residual activity, requiring repeated applications. Careful reading of all product labels is recommended to ensure that the product is properly stored, as some products require refrigeration. Microbial products generally have a shorter shelf life than conventional chemical pesticides.

Application Methods

Microbial pesticides, utilized mainly for prevention or when pest populations are low, are applied similarly to chemical pesticides. Depending on the specific formulation, these products can be used in conventional and organic production systems. They come in various formulations such as dusts, liquids, powders, or granules, and can be administered through spraying over foliage, soil drenching, or incorporation into media. Application methods include crop dusting, tractor-mounted sprayers, or backpack sprayers with droplet sizes adjustable to enhance efficacy. Soil applications are preferred for below-ground pests, aiming to transfer beneficial microbes to the root zone. Granular formulations may require activation through water. Importantly, equipment previously used for fungicides must be thoroughly cleaned to prevent harm to beneficial microbes since many beneficial microbes can also be killed by fungicides. Additionally, most biocontrol approaches benefit from being used in combination with cultural practices that optimize their success. Routine scouting can aid in timing re-applications of microbial products before stronger, curative measures are needed. These products can be used as the first line of defense to maintain pest populations below economic thresholds. Conventional pesticides can be used in rotation with microbial products or when curative control is necessary to bring population numbers down rapidly. Care must be taken to ensure that the chemical products are compatible with the microbial products in use. Table 1 provides examples of microbial products available for the control of arthropod pests and diseases in commercial nurseries.

Table 1. Microbial Products Available for Management of Pests and Diseases by Commercial Nursery and Landscape Professionals