Fungi for the biological control of insect pests

Introduction

Fungi are a diverse group of organisms with close ties to agriculture. Some fungi create devastating diseases in crops, while others are crops themselves (mushrooms). Other fungi are used successfully to protect crops from a variety of pests. Among the most prominent of these are the fungi that are used against insects and other related pests.

Most fungi used for the control of insect pests belong to the group hyphomycetes. Some species have been developed as commercial products because of their ability to be mass produced. Most fungi in this group are usually found in the soil and can cause natural outbreaks on their own when environmental conditions are favorable. They can infect a wide range of insect hosts. Specific fungal strains in commercial products target thrips, whiteflies, aphids, caterpillars, weevils, grasshoppers, ants, Colorado potato beetle, and mealybugs. Currently (2008) allowable products containing the hyphomycete fungus, Beauveria bassiana, that are commercially available include Mycotrol O (Emerald BioAgriculture), Naturalis Home and Garden (H&G) and Naturalis L (Troy BioSciences, Inc.). Before applying any pest control product, make sure to include what you might want to use and how you intend to use it in your organic system plan and get your certifier's approval. (Caution: the use of an unapproved material can result in the loss of certification. Always check with your certifier before purchasing or using a new product or material to ensure that it is permitted for use in your organic farming system. For more information, read the related article, Can I Use This Input On My Organic Farm?)

There is another commonly encountered group of fungi called the entomophthorales. Fungi in this group can cause natural outbreaks in the populations of their insect hosts, but they are difficult to mass produce and as yet are not in commercial production. They tend to be much more host specific; one well known species only infects aphids. Despite the difficulties in producing them commercially, they can still have a large impact on the pest populations they infect. There are currently no commercially-based products available for organic vegetable production.

Life cycle of fungi infecting insects

Fungi that infect insects are found in the environment as spores. Insects can become infected when they come into contact with spores on the surface of plants, in the soil, in the air as windborne particles, or on the bodies of already dead insects (Figure 1). Spores attach to the surface of the insect and infect by penetrating through the insect cuticle, often at joints or creases where the insect’s protective covering is thinner. Once inside, the fungus grows throughout the body of the insect. Many fungi also produce toxins in the host that increase the speed of kill or prevent competition from other microbes.

Figure 1. Generalized life cycle of fungi infecting insects (spores not to scale). Figure credit: Jim McNeil, Department of Entomology, Penn State University.

Usually after the insect has died, the fungus grows out through the outer covering (exoskeleton) of the insect, usually at thinner areas like joints or creases, and begins to produce spores. The spores of commercially developed fungi in the group hyphomycetes are spread passively by the action of the wind, rain, or contact with other hosts or animals in the environment. The spores of fungi that create natural outbreaks, in the group entomophthorales, are often actively ejected from the dead insect. Since many species in this group of fungi infect insects which cluster together, like aphids, this tactic can drastically increase the spread of the infection. Insects killed by fungi often have a “fuzzy” appearance, caused by the growth of the fungus out of the exoskeleton to produce spores. Most commercial strains of fungi produce spores that are either white or green in color, although the color of the fungi can change over time as the fungus grows and ages.

Spores that do not encounter a host either die or persist in sheltered areas of crop plants or in the soil. Although some species of fungi can produce spores which can persist for years in the soil, most spores are only viable for a growing season or a year at most.

Advantages and disadvantages of fungi for controlling insect pests

Advantages

Fungi make good biological control agents for a variety of reasons. They generally do not affect people or other mammals, making them extremely safe to use. It is relatively easy to mass produce spores of insect-parasitic fungi in the hyphomycete group, so they are comparably priced with other biological control agents, such as bacteria. Most commercial fungal products are formulated as spores, which are easily adapted to existing application technology, such as spray rigs. The relatively broad host range of many fungi means one can often achieve control of multiple pests with the same product. Finally, successful infections can spread to other hosts and lead to high rates of persistence within a growing season, even if between season persistence tends to be low for most types of fungi.

Disadvantages

High concentrations of spores are often needed to get adequate control of pests in a crop, which can cut down on the cost effectiveness of fungal products. The kill time is relatively long (~1 week for most fungi), although strains used for commercial products are chosen to kill as fast as possible. Their broad host range can sometimes be a problem, especially if beneficial insects (i.e. predators, parasitoids, and pollinators) are present in a crop; non-target mortality in these populations of beneficial insects can negatively impact the success of the overall biological control program. Environmental factors can also play an important role in the success of fungi. Moist conditions or high relative humidity in the canopy of the crop are often necessary for control to be effective. Prolonged exposure to sunlight can also inactivate spores, reducing persistence in the crop. Owing to these environmental limitations, natural outbreaks of fungi tend to be sporadic and very patchy in the environment, which can limit their effectiveness in controlling pests.

Suggestions for Application

As with all biological control agents, fungi work best as one component of a comprehensive integrated pest management program. There are several tactics growers can us to increase their effectiveness, though, especially for applied commercial products:

• Scout consistently and often. Apply only when the target pest is seen, not as a preventive application as residues are not long-lasting. The best time to apply fungi is before pest populations reach their peak, so early application can increase their effectiveness. Also, scouting can help determine the population levels of beneficial insects and pollinators so the timing of fungal applications will not impact them as strongly. Finally, scouting can help discover natural outbreaks of fungi (e.g. aphid fungi) in time to influence control decisions.
• Time applications of fungi to coincide with host life stages that are more likely to come in contact with the spores. In general, B. bassiana products are more effective against earlier compared with later stages of insects. For example, applying a fungal product for grasshoppers will be most effective when there are active nymphs present that have not grown into winged adults.
• Do not apply fungal products during droughts or dry spells since the environmental conditions will decrease their effectiveness.
• Be aware of fungicide applications in the area. Even if fungicides are not directly applied to the crop, drift from nearby fields could impact the success of a fungal biological control agent.
• Apply fungal inoculum carefully to get effective coverage. Cover all plants thoroughly. Also try to reduce of spill-over into refuge areas where natural enemies may be present.
• Do not apply fungal products during the heat of the day since this will diminish the potency of the spores. There have been some reports of phytotoxicity to young vegetable transplants with products formulated as an emulsifiable suspension. Also, do not apply on rainy days, when spores will be washed off of plant surfaces and may not come into contact with the target pest.
• Use cropping practices which encourage a diverse understory and soil surface, such as cover cropping or conservation tillage. These practices will help maintain fungi in the field and could increase persistence within and between seasons.

Additional Resources

• Lacey, L. A., and H. K. Kaya. 2007. Field manual of techniques in invertebrate pathology. 2nd ed. Springer. Dordrecht, The Netherlands.
• Mahr, D.L., P. Whitaker, and N. M. Ridgway. 2008. Biological control of insects and mites: An introduction to beneficial natural enemies and their use in pest management. University of Wisconsin Cooperative Extension, No. A3842, Madison, WI.