1. Fungi. Dramatic epizootics (outbreaks) of pathogenic fungi have caused the complete collapse of pest outbreaks. An example is the gypsy moth populations in the northeastern USA which collapsed in 1995 and 1996 due to epizootics of a fungus (Slide). However, fungal epizootics are usually associated with very specific weather conditions, particularly warm, humid weather, so they cannot be relied upon to terminate outbreaks all of the time.
2. Bacteria are common parasites of forest insects. The best know of these is Bacillus thuringiensis, better known as Bt. Bt is one of the few lethal bacteria, killing its host by producing a toxin. It is often employed as a "biopesticide" sprayed on the forest by helicopters.
3. Protozoa infect many forest insects but rarely cause direst mortality. They may, however, lower the survival and/or the reproductive potential of infected individuals.
4. Viruses are by far the most virulent of the pathogenic microorganisms. Virus epizootics are commonly associated with the collapse of defoliator outbreaks. For example, gypsy moth and Douglas-fir tussock moth outbreaks in North America are frequently terminated by virus epizootics that kill almost all of the caterpillars (Slide).
5. Nematodes are associated with many insects but probably have their greatest impact on insects living within their plant hosts. Heavy nematode infections may kill bark beetle larvae (Slide) or sterilize females (Slide).

Unlike other natural enemies, pathogens are incapable of searching out and finding their hosts and must rely on contact between infected and susceptible individuals for transmission from one host to another. Some pathogens can be transmitted from mother to offspring through the ovary but, for obvious reasons, these pathogens rarely kill their hosts and probably have little effect on pest population dynamics. Because transmission of virulent pathogens depends upon host contact, epizootics can only get started in dense populations. In addition, insects living in dense populations are often more susceptible to pathogen infections because of stress due to overcrowding. Hence, although pathogens are often associated with the collapse of pest outbreaks, they are not expected to play an important role in the regulation of pest populations at sparse densities.

<Use BACK key to return to Sessions>

© 1998 Alan A. Berryman