Fungi as Agents of Biocontrol
Introduction
The use of fungi in biocontrol is not a new idea. The concept was developed in the late 1800’s to early 1900’s. Some attempts were made to utilize fungi in controlling insects and weeds, but with the development of insecticides and weed killers, the search for biocontrol agents waned since chemical sprays were relatively cheap and efficient. However, in the last several decades, problems such as carcinogenic compounds in many sprays, gradual resistance, in the case of insects and their slow biodegradability in the environment have brought about a renaissance in biocontrol agents.
Common sense should tell us that biocontrol is not going to be completely effective. As the term implies, this is a means of "controlling" the problem and does not eradicate the target organism. Since the organisms that we are using to control another organism would normally have encountered each other, it can only reduce the population size of the pest organism to an acceptable level. If it was possible for a biocontrol agent to completely eliminate an unwanted organism, it would most likely have driven it to extinction long ago. Thus, it is best to expose a biological control agent that has not been encountered by the pest organisms.
Bioinsecticides
The use of fungi in controlling insect is somewhat unique because we have a case here where the insects now don’t have to ingest anything in order for them to be eliminated. The fungi have the ability to penetrate the insect cuticle. Although there are many species of fungi that are pathogenic to insects, only a handful have been studied, only six have been registered for pest control. However, before fungi can be utilized on a large scale as insect controlling agents, a great deal of research must be carried out.
The number of species of fungi, pathogenic to insects, is not only large, but also represented in almost all taxonomic categories with the exception of some Basidiomycota and Deuteromycota. The virulence of these species also are quite variable, ranging from species which are obligate pathogens, to those that attack only weakened hosts, to some that are apparently commensal to symbiotic fungi. The species of fungi that have been targeted are those that are obligate parasites. One reason for this is host specificity. It would not do well to find a virulent pathogen that had the possibility of shifting to a different host, especially if that host would be of value, economically speaking. Also, flagellate fungi have been examined for possible use for insects that spend part of their life cycle in an aquatic environment. For example, the mosquito larval stage is found only in an aquatic environment.
Insect Biological Control Fungi
Most species of fungi that have been studied belong to the Entomophthorales (Zygomycota). This order consists of a large number of species of fungi that are parasitic on insect. In utilizing fungi as biological control agents against insects, the following categories of treatment are recognized:
Permanent introduction of a fungus to an area with a host population. This would involve establishing the fungus species at the site of the host population. This makes a great deal of sense since if the fungus was native to that area, the insect would either have adapted to the fungal parasite or that particular insect would not be a problem in that area because the fungus would have wiped it out. An introduction would be effective since the introduction of the fungus would expose the insect to a pathogen which it had not previously encountered. This method is one of the least costly and labor intensive methods, involving periodic release of fungal spores, in order to maintain a high density of the biocontrol fungus.
Inoculative augmentation involves releasing the pathogen in the field where it will control the insect pathogen. However, the inoculation of the fungus is not expected to carry on over to the following year. This method is used for those diseases that are an annual problem of crops.
Conservation or Environmental Manipulations involves the modification of the host environment to enhance the probability that the fungus will infect and eliminate the host. For example, the spraying of a mild chemical insecticide that would serve to only weaken the host, making it more likely that the fungus will infect and eliminate the host. Another common means by which the environment might be modified would be to maintain a constantly wet environment in order to favor fungal growth.
The means by which the hosts are infected is usually by spores, specifically conidia since they are produced in such large number than sexual spores and are continually produced by fungi as long as conditions for growth remain favorable. At this time, there is still not a great deal of research carried out with regards to optimizing the production of conidia in fungi. Most of the technology involved in mass producing fungi has been for the purpose of extracting metabolites produced by the fungi. Thus, in some cases, the mycelium is grown, in large fermentation vats, similar to those fungi grown for their metabolites. However, the mycelium produced is broken into pieces and lyophilized. The broken mycelium is then dispersed into the host environment where, upon rehydration, will grow and produce copious amounts of conidia.
There is also the prospect that since the fungi kill the insects after penetrating their cuticle, it is thought that perhaps these fungi also produce metabolites that have insecticidal properties. However, even less research has been carried out in this area.
Some Specific Examples of Biological Control From the Above Categories
Permanent Introductions: Control of Lymantria dispar, the Gypsy Moth
Entomophaga maimaiga, a species in the order Entomophthorales has been somewhat successful against the Gypsy Moth, or at least this is believed to be the case. The Gypsy Moth was introduced into Massachusetts, from France, in 1869, by Leopold Trouvelot in order to breed a better silkworm. However, the Gypsy Moth caterpillar soon escaped from his home and by 1889, the first major damage to trees occurred. With their voracious appetites, the Gypsy Moth can denude entire forests of all their leaves. Since the initial outbreak, the Gypsy Moth has spread approximately 15 miles each year.
The distribution, of this pest, has now spread north, into Quebec, west to Michigan, and south through Maryland into Virginia. Inadvertent transport of eggs by humans have also established isolated populations in California, Oregon and Washington. What is rather curious about the Gypsy Moth is that after severe damage has occurred, in an area, in a season, the population for some reason declines for a decade or so before another epidemic will occur. During these intervening years, caterpillars are so rare that they are almost never seen. Why was this?
In 1910 and 1911, Entomophaga maimaiga was introduced into the United States, from Japan, to control the Gypsy Moth. However, nothing more was reported of these introduction and it was assumed that E. maimaiga was ineffective as a biocontrol agent, until 1989 when there was a major die off of the Gypsy Moth in the northeastern United States of what was initially thought to be a viral epidemic. Ann Hajek (Research Associate, Boyce Thomson Institute for Plant Research at Cornell University) and her associates investigated this phenomenon in 1989 and in their examination of the cadavers of larvae of the Gypsy Moths, E. maimaiga was isolated. It was believed by Hajek that the die off was possibly due to the introduction of E. maimaiga early in the 20th Century, but if that was the case, why was this fungus not reported, again, until 1989? Looking back at Gypsy Moth populations, it appeared that there were population declines previous to 1989. However, the specific reasons for these declines were never reached. Hajek, et al (1995) have proposed several hypotheses on the decline of Gypsy Moth populations since 1989, but none of these hypotheses will probably ever be tested to a satisfactory conclusion. However, Hajek et al (1995) believe that it is most likely that the release of E. maimaiga in 1910 and 1911was a strain low in virulence that eventually mutated into the more aggressive strain discovered in 1989 or that a new aggressive strain was introduced into the United States at this time.
After two years of observing the Gypsy Moth die off, Hajek was able to obtain funds for release of E. maimaiga resting spores for the purpose of controlling the Gypsy Moth population. Hajek released the resting spores of the fungus in 1991 and 1992, in 50 sites, over 4 states. A year after the release, Gypsy Moth populations had not only declined in the areas of spore release, but cadavers of larvae could be found to in areas where release of spores did not occur. This was assumed to be due to the air-dispersal of the fungus . Follow up inspections of release areas, in 1994, found that the fungus had persisted well in seven of the sites. You can read the story as told by Ann Hajek here.
Inoculative Augmentation: Control of Pine Moth with Beauveria bassiana
The use of Beauveria bassiana in controlling species of Dendrolimus, the Pine Moth, in the People’s Republic of China, probably represents the largest program of biocontrol. At least one million hectares (1 hectare=2.47 acres) of pine forests are involved. The fungus is locally propagated, cheaply, on a bran or peat substrate, and is applied by air or ground equipment, as a spray or dust. Initially, during the 1970s, `mortar bombs' containing firecrackers were used dispersal of the fungus in tall pine plantations to control the moth, which proved to be effective. However, this technique was abandoned in the 1980s because the price of firecrackers made it
too expensive to use and the regulation of such goods as firecrackers and fireworks became stricter. It was also a potentially dangerous means of dispersing the fungus even though there were no reports of accidents involving this method. Applications are usually only needed at 3 year intervals.
Environmental Manipulation: Control of Hypera postica, the Alfalfa Weevil with Erynia sp.
Medicago sativa, alfalfa is mostly used for forage. Alfalfa fields often have a number of common pathogens, among them is the Alfalfa Weevil. Introduction of various species of Erynia (Entomophthorales) has led to significant control of this insect pest. By cutting alfalfa early and leaving it in piles or clumps, for several days, provides a moist and warm microclimate which encourages the development of Erynia sp. This, in addition to possible light spraying with insecticide during the early growing season has been projected to produce a significant net profit for Alfalfa growers.
Other Potential Fungi as Biocontrol Agents
The genus Cordyceps is a well known group of fungi that are usually parasitic to insects and arachnids. They are widespread, but apparently have not been tested as biocontrol agents. Cordyceps militaris, the Caterpillar Fungus is probably the most common species in North America. This genus does not occur in Hawai‘i. Cordyceps sinensis, which occurs in China, is used as a herbal remedy (this link leads to a description of its use, but in making this link I am in no way endorsing the product that is being described nor the company that is selling the product).
Coelomomyces is a genus of aquatic fungi that produce flagellated spores. The genus is mostly parasitic on mosquito larvae. The genus was first discovered in 1921, but as was the case with many aquatic fungi, the life cycle was incompletely known at that time. It was not until 1974 that Howard Whisler was able to observe the entire life cycle. It was only through his efforts that Coelomomyces was found to require a second host in order to complete its life cycle. This then is only the second group of fungi in which a second host is required to complete its life cycle (many species of rust, you should recall commonly require a second host). The second host is a copepod. With this discovery, it was now possible to induce the fungus to produce spores in the laboratory that can be used to inoculate mosquito-infested areas. Such a project would probably be useful, in Hawai‘i, where mosquitoes are present throughout the year. However, there are problems associated with using this fungus. An overview of biocontrol of mosquitoes as it relates to Dengue Fever can be found here. This overview describes the various organisms that are can be utilized for biological control of mosquitoes and the problems that are associated with them. It briefly describes Coelomomyces.
Major Advantages of Using Fungi for Biocontrol
Some of the obvious advantages of using fungi in biocontrol is the of the ubiquitous nature of fungi and their genetic diversity. This latter characteristic can provide a number of biocontrol agent from single species of fungi. Although there are other biocontrol agents, such as bacteria and viruses, there are some distinct advantages in utilizing fungi. Fungi have the ability to directly infect the host insect by penetrating the cuticle of the insects, something viruses and bacteria are unable to do. Instead these latter organisms must be ingestion in order to be effective as biocontrol agents. This is not possible with many insects, such as aphids, which feed exclusively by inserting their stylets into herbaceous plant organs in order to obtain plant sap. Also, there are few viral and bacterial diseases for a number of insects.
Finally, as mentioned at the beginning, fungi, in contrast to insecticides, do not pose a health hazard to people and domestic animals. Many of the more lethal insecticides have been banned for this reason. However, it should be pointed out that most fungi as biocontrol agents have, and will probably continue to have only limited success. One of the major problems that have caused negative reactions to fungi, as agents of biological control, is the overenthusiastic promises that some researchers have made in the past. A nice overview concerning the use of fungi as biocontrol for insects can be found here.
Bioherbicides
The use of fungi to control weedy plants should be a natural one, if you are in the right frame of mind. Recall the number of pathogens that we have discussed, earlier in the semester, in which various species have decimated certain species of plants throughout the United States, and in several species, throughout the world. Now think of what good these diseases would have done if they were only infecting weeds rather than prized lumber trees.
The idea of using biocontrol agents to control weeds is not a new one. The utilization of insects for this purpose have been attempted for quite sometime. The utilization of fungi for this purpose, on the other hand, has only been intensively studied during the last 20 years. Neither one, however, has really challenged the use of chemical herbicides which has been a thriving area of research, Weed Science.
In Hawai‘i, there are presently, several researchers that are using fungi as biocontrol agents against weeds: Dr. Donald Gardner, Plant Pathologist with the National Parks Service and Dr. Eduardo Trujillo, Professor of Plant Pathology, University of Hawai‘i. Dr. Gardner has more recently began carrying out research in this area and has not yet produced any significant results at this time. He has been working on biocontrol of such weeds as Passiflora tripartita, Banana Polka, utilizing a species of Fusarium as the biocontrol agent. However, Dr. Trujillo has been doing this research much longer and has a certain amount of success. Among some of the weeds that he has worked on include Ageratina riparia, Hamakua Pamakani, which has been controlled in some areas in Hawaii by the smut fungus, Entyloma compositarum, which was introduced from Jamaica. Because of the strict quarantine laws here in Hawaii, exhaustive tests had to be carried out in order to demonstrate that the pathogen would not spread to other species of plants. This was done. That’s another advantage that the use of fungi, as biocontrol agents, have. There are numerous species that are very specific as to what host they will attack. Release of this fungus was very successful in some areas. Under ideal conditions, i.e. high rainfall and temperatures between 10-18 C, the fungus would devastate the host plant.
One of Dr. Trujillo most successful biocontrol species is Colletotrichum gloeosporioides f.sp. clidemiae which was introduced from Panama. This species was introduced to control the noxious weed, Clidemia hirta, Koster’s curse. Colletotrichum was very successful in devastating Clidemia in areas where it was introduced. However, it appears to have a poor dispersal mechanism and the control of Clidemia is usually pretty localized. Other species that Dr. Trujillo has attempted to control include Lantana camara, an ornamental shrub that has escaped cultivation and also Banana Polka since the retirement of Dr. Gardner..
Not all species of fungi used as biocontrol were introduced from elsewhere. An unidentified species of Cephalosporium was isolated from a Kauai ranch in 1968, and caused a wilt on Cassia surattensis, Kolomona. Application of the fungus is by spore suspensions in water which is sprayed on man-made wounds on trunks of healthy trees.
Commercial Mycoherbicides and mycoinsecticides
There are presently only a few mycoherbicides and mycoinsecticides that are available and they have had limited success. However, a number of mycofungicides are available commericially. A list of products are given below (The links in most instances lead to commercial sites and does not indicate an endorsement for the product. These sights were the only sites that gave descriptions of the products and what fungi are contained in the product).
The most common species of fungi used in this type of biocontrol is Trichoderma harzianum. Other species of Trichoderma also have been used, as well as other genera of fungi.
This species was genetically engineered, at the Cornell Plant Pathology Department and is known to infect a number of different species of fungal plant pathogens, and can be used selectively on different parts of the plant body. In the product RootShield, it is disseminated in the soil as a wet able powder or as granules and can be applied with fertilizer, when fertilizing plants. Once released the fungus grows around the root system and protects it from various root diseases by parasitizing and killing root pathogenic fungi. However, it is compatible with mycorrhizal fungi and chemical fungicides. Some illustrations on plants with and without treatments of RootShield can be seen on this link.
Recently, tests have been carried out to study the use of honey bees in disseminating T. harzianum. Species of Botrytis are known to cause fruit rot in strawberries, as well as in grape plants. Previously, the fungus was sprayed on the flower and fruit by suspending the spores in water and spraying the plants. However, in using honey bees, a dual function is served. Not only are the honey bees disseminating the fungus, but in visiting the flowers, the bees also pollinate the flowers. It was concluded from this test that the strawberry plants produced more strawberry due to pollination by bees and there was significantly less fruit rot due to the biocontrol fungus.
Terms for Biocontrol
Biocontrol: The use of one organism or its resources to control another organism that is a pest.
Biofungicides: Refers to organisms or their resources that can be used to control unwanted fungi.
Bioherbicides: Refers to organisms or their resources that can be used to control unwanted plants, i.e., weeds.
Bioinsecticides: Refers to organisms or their resources that are used to control insect pests.
Coelomomyces: A genus of aquatic fungi that is known to be parasitic on the larvae of mosquitoes. Some research has been carried out investigating this organism as a means of controlling mosquito populations, beginning in the 1970s, but a practical means of utilizing this organism has yet to be achieved.
Conservation or Environmental Manipulations: The modification of the insect’s environment to enhance the probability that the fungus will infect and eliminate it.
Cordyceps: A genus of Ascomycota that is mostly parasitic on insects and arachnids. However, its potential as a means of biocontrol for insects has not been fully explored.
Inoculative augmentation: The release of a fungus in the field where it will control the insect pathogen. However, the inoculation of the fungus is not expected to carry on over to the following year. Thus, the fungus would be reinoculated each year.
Permanent introduction: The introduction and permanent establishment of a fungal species, to an area, in order to control an insect pest. Periodic release of fungal spores is still necessary in order to maintain a high density of the biocontrol fungus.
Trichoderma harzianum: Most common species of fungus used in various commercial biofungicides.
Questions of Interests (Questions 2 and 3 will definitely be on the final exam as one question and will be worth 7 points, please try to think about it and write a nice detailed answer ahead of time)
The concept of using biocontrol for management of pests came about during the late 1800s. However, the development of this concept stopped very soon after. Why?
Recently there has been a return to biocontrol for pest management. Why?
What are the advantages of using fungi as bioinsecticides?
In permanent introduction of a fungus species to control insect pests, why is it best to use species of fungi that are not normally found in the area where the fungus is to be introduced?
What part of the fungus life cycle is used to disperse the fungus in biocontrol of pests?
Cordyceps may seem to have tremendous potential as a Bioinsecticide, but it has already been used commercially for another market. How is Cordyceps currently being used, commercially?
Literature Cited
Hajek, A.E., R.A. Humber, and J.S. Elkinton. 1995. The mysterious origin of Entomophaga maimaiga in North America. Am. Entomol. 41:31-42.