Mii

712 (inherited abnormalities), and physiological, metabolic, or developmental disturbances.

Infectious diseases, which can be spread rapidly within a population of organisms, are caused by microorganisms, including viruses, rickettsias, spirochetes, bacteria, fungi, protozoa, and nematodes. Though infections of many of these pathogens may be directly fatal, other pathogens simply weaken an insect, rendering it more susceptible to predation, parasitism, or other pathogens, to chemical and other means of control, or altering its growth rate and reproductive capacity. Most of the time in natural populations, the effects of pathogens are not readily obvious. This is described as the enzootic stage. On occasion, however, conditions are such that the pathogens can reproduce and spread rapidly to decimate the host population. This is known as the epizootic phase, and the outbreak is described as an epizootic, comparable to an epidemic within a population of humans. Study of the factors that lead to epizootics, epizootiology, is of interest not only from a purely ecological perspective but also in light of the potential use of microorganisms in the biological control of insect pests.

This section will outline the important factors in outbreaks of infectious diseases and survey the major groups of insect pathogens.

5.1. Epizootics

Essentially, there are four primary components in the development of an epizootic: the pathogen population, the host population, an efficient means of pathogen transmission, and the environment, all of which are closely interrelated.

Key features of a pathogen are its virulence (disease-producing power), infectivity (capacity to spread among hosts), and ability to survive. Clearly, pathogens (or specific strains of a pathogen) that have both high virulence and high infectivity are the ones that most often cause epizootics, though the susceptibility of the host is also important. Some pathogens may be highly virulent but of low infectivity and, as a result, have a low potential for causing epizootics. Bacillus thuringiensis, for example, though pathogenic for many Lepidoptera, seldom causes an epizootic under natural conditions because of its poor powers of dispersal. Indeed, inability to disperse and limited capacity to survive outside a host are probably the main reasons why epizootics occur relatively rarely. Dispersal may be effected either by abiotic or biotic agents in the environment, including wind, rain, running water, snow, host organisms (both healthy and infected), and their predators (both vertebrate and invertebrate) or parasites. Host organisms may disperse the pathogen as a result of defecation, regurgitation, oviposition (i.e., the pathogen occurs either on or within the eggs), disintegration of the body after death, or cannibalism. Predators commonly distribute pathogens via their feces, though some insect parasitoids transfer the microorganisms via their ovipositor when they either sting the host or lay an egg on or in it. Pathogens may survive in either the host or the environment, sometimes for considerable periods. Those that survive in the environment typically have a highly resistant resting stage, such as spores (bacteria, fungi, and protozoa), inclusion bodies (viruses), or cysts (nematodes).

Members of the host population pick up pathogens as a result of physical contact with contaminated surfaces, or eating contaminated food (including cannibalism), or receive pathogens directly from the mother via transovarian transmission. Contact with a pathogen, however, does not necessarily result in ill effects for the host, as the latter has various means of defending itself (Chapter 17, Section 5). Further, even when some members of a population are susceptible to a pathogen, an epizootic does not always follow because of

Beekeeping for Beginners

Beekeeping for Beginners

The information in this book is useful to anyone wanting to start beekeeping as a hobby or a business. It was written for beginners. Those who have never looked into beekeeping, may not understand the meaning of the terminology used by people in the industry. We have tried to overcome the problem by giving explanations. We want you to be able to use this book as a guide in to beekeeping.

Get My Free Ebook


Post a comment