Infectious Diseases of Insects

By definition, infectious diseases are those caused by infectious agents. Infectious agents are living (or at least nucleic acid encoding) units that must invade the insect host in order to initiate an infection. Infection usually involves reproduction of the agent. Depending on the specific pathogen involved, invasion of the host may be dermal (contact with the body of the insect), body opening, oral by feeding/drinking, or introduced by stings of contaminated or infected parasites. Direct transmission, from infected insects to uninfected insects, usually in the same generation, is called horizontal transmission. Some groups of pathogens may also exhibit vertical transmission, generally from an infected female to the offspring.

As physical living particles, infectious agents have specific behaviors in air, water, etc. The spores of some bacteria, protozoans and microsporidia are very dense and settle to the bottom of a water suspension very quickly. In contrast, the conidial spores of some fungi are small light particles that may be airborne for long distances. Nematodes actively search for and enter the host. The specific characteristics of the infective stages of pathogens greatly influence how they contact and infect their hosts.

The infectious agents responsible for transmission of the pathogen are susceptible to many environmental factors. Few can survive more than a few hours of direct sunlight. Others may be particularly susceptible to dry conditions, high temperatures, freezing, and many chemicals. Survival of the infective stage of insect pathogens outside the host is a major factor in the development of microbial insecticides.

All insect pathogens have a physiological host range- hosts in which they can potentially survive and reproduce. Some pathogen species may be very host specific and able to infect only a few or possibly a single host species, while others may be able to infect a wide range of insect species. The host range of a pathogen is especially important when considering a nonindigenous pathogen for introduction into a new habitat. The important factor is the ecological host range (the host range of the pathogen in nature) but most of our estimates and predictions of host range must be determined in artificial laboratory settings by evaluating physiological host range.

The nature of an infectious disease is greatly influenced by the dosage of the infectious agent. The pathogen/host dose relationships are expressed as LDx, the dosage required to kill a certain portion, x%, of the population; IDx, the dosage required to infect a certain portion of the population; and LTx, time required for a certain portion of the population to die. These relationships have not been determined for most species of pathogens, but they have a major influence on how the pathogen interacts with the insect host population.

Unlike parasites and predators, pathogens do not always kill their hosts. Sublethal infections are not uncommon and these may include behavioral and developmental changes as well as a decrease in the fecundity of infected adults.

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