Edward A. Steinhaus.
Like human beings and other animals, insects are susceptible to a variety of infectious agents, which infect and kill hordes of them every year. Most of this mortality goes unnoticed, although at times outbreaks epizootics of disease are so spectacular as to claim considerable attention among growers and entomologists everywhere.
The possibilities of using disease agents to help control insect pests have excited entomologists and others periodically since infectious organisms were first detected in insects. As I explain in a later paragraph, however, that is only one of the applications that may be made of our knowledge of insect diseases, because insect pathology has already contributed greatly to other branches of entomology and to medicine, agriculture, and biology generally.
The infectious agents responsible for diseases in insects belong to the same major groups as those that cause diseases in other animals: Bacteria, fungi, viruses, protozoa, and nematodes. In general, however, insects are not very susceptible to those particular microorganisms that cause diseases of other animals and of plants. Furthermore, most of the micro-organisms that cause fatal diseases in insects are harmless to plants and higher animals.
The resistance shown by insects to pathogens of higher animals is largely a normal or innate one. Nevertheless insects often can ward off infection by virtue of mechanisms of immunity similar to those exhibited by other animals. Antibodies against foreign materials may be produced in the body fluids of the insects, thus giving a humoral immunity against infection. Cellular immunity frequently is evidenced by the activity of the hemocytes, or blood cells, which may engulf foreign particles that enter the body of the insect.
One of the most convenient ways in which to consider the various infections of insects is according to the nature of the etiologic agent, that is, whether it is a bacterial, fungus, virus, protozoan, or nematode infection. To be sure, insects, like other forms of life, are subject to definite metabolic and other noninfectious conditions. Here, however, we are concerned only with true infectious diseases of insects.
EVERY BEEKEEPER is familiar with diseases of the honey bee known under the general name of foulbroods. American foulbrood is caused by Bacillus larvae, European foulbrood by Bacillus alvei, and parafoulbrood by Bacillus Para-alvei. Those sporeforming bacteria are true insect pathogens and are not known to be infectious for other animals. Not until their true nature and etiologic role were discovered was it possible to accomplish much in the way of controlling the foulbroods. Although effective control procedures have been realized through strict sanitation and quarantine and such direct therapeutic measures as the use of sulfathiazole, the diseases in some areas still cause losses to beekeepers and agriculture. They and certain afflictions of silkworms were among the first insect maladies to be recorded.
The first widely publicized bacterial disease of a destructive insect was that of grasshoppers, caused by the small, non-sporeforming Coccobacillus acridiorum (now Aerobacter aerogenes var. acridiorum). The bacterium was observed first in Yucatan, Mexico, where it destroyed the hordes of locusts (Schistocerca) that were invading Mexico from Guatemala. The infected insects exhibited symptoms typical of dysentery and septicemia and died usually within a few hours.
Great hopes were held at first that the bacterium could be used in controlling grasshoppers. Attempts to accomplish that were made in several countries. Despite the apparent success of early trials, the method was abandoned in the light of present knowledge it is clear that some of the lack of general success can be ascribed to the failure of most of the users to adhere to the fundamental principles of bacteriology necessary to insure the identity of the cultures used, the maintenance of their virulence, and their application according to the epizootiological demands of the situation.
The most noteworthy and so far the most important of the bacterial diseases of destructive insects are the so-called milky diseases of the Japanese beetle, which are discussed in the next chapter.
Although the examples I have cited indicate to some degree the general types of bacterial infections in insects, they do not indicate the extent to which this group of diseases occurs in nature. Numerous other instances of bacterium-caused diseases have been observed and studied. Nearly all major systematic groups of insects have been recorded as hosts to one or more types of bacterial infection. Furthermore, insect pathogens have been identified as belonging to almost every major group of bacteria, although the small, gram-negative, rod-shaped bacteria and the large, gram-positive, spore-forming bacteria appear to be predominant.
The general characteristics of a bacterial disease of an insect are: As the disease develops, the animal usually becomes less active, has a smaller appetite, and discharges fluids from the mouth and anus. The infection may begin as a dysenteric condition with an accompanying diarrhea, but in most instances the invading bacterium eventually enters the body cavity of the insect and causes a septicemia that terminates in the death of the host. Following death, the insect's body usually darkens to brown or black. That especially is true of larvae and pupae in which the disintegration takes place rapidly, although adults may also show a rapid change in color. The freshly dead insect is usually soft and becomes shapeless. The internal tissues may disintegrate to a viscid consistency, sometimes accompanied by odor, but ordinarily they do not "melt" or liquefy as do insects dying of certain virus infections. The cadaver of the insect usually dries and becomes shriveled, the integument remaining intact. Microscopic examinations of smears or histological sections of an insect dead or dying of a bacterial disease usually show large numbers of the causative bacterium present. If the bacteriological examination is delayed too long, care must be taken to differentiate the true pathogens from similar-appearing saprophytes that may flourish in the tissues of the dead insect.
ONE OF THE FIRST DISEASES of animals to be recognized as being caused by a micro-organism was a fungus disease of the silkworm. As early as 1835 the mycelium and fruiting bodies seen on the cadavers of silkworms were recognized as being similar to those of the molds commonly found growing on bread and other food products. Today hundreds of species of fungi have been reported from insects. Many of them are known to be exclusively insect parasites.
Each of the four classes of fungi (Phycomycetes, Ascomycetes, Basidiomycetes, and Deuteromycetes, or Fungi Imperfecti) contain species capable of infecting insects. In any particular insect the appearance it assumes upon being infected with a fungus varies with the type of fungus. Some specimens dead of a fungus infection (mycosis) may show no marked external signs whatever. Others may be covered with mycelial growth, which makes it difficult to recognize the host insect. Infection usually begins with the penetration of the integument by the germinating hypha arising from a spore or conidium that has lodged on the animal's body. Once within the body cavity, the fungus multiplies rapidly and soon fills the insect. With proper conditions of temperature and moisture, conidiophores then make their way through the body wall of the host to the outside, where fruiting bodies are formed. By this time the insect is a hard, brittle, mummy-like object a condition unlike that seen in typical infections of any other type.
As I have indicated, the first fungus infection of an insect to be well studied was muscardine of the silkworm, caused by Beauveria bassiana, a disease that spells great loss to sericulturists. The fungus occurs wherever the silkworm is reared. It causes infection in a large number of insect pests, including the European corn borer and the codling moth.
A closely related species, Beauveria globulifera, also rather widespread, causes disease in many insects, including the chinch bug. At one time much thought was given to the possibility of using it to control the chinch bug, but investigation revealed that its spores were almost always present in the areas concerned and that artificial distribution of them would not affect materially the outbreak of disease, which would occur naturally if moisture and temperature were adequate. Green muscardine, caused by Metarrhizium anisopliae, is another important infection of insects. In some parts of the world it is responsible for a significant degree of natural control of certain insects.
In Florida, the Orient, and other places, scale insects are found to support the growth of certain fungi. Some of these fungi are true parasites. Others are apparently only secondary parasites, or even saprophytes, which live on the dead tissue of the scales. A great deal of attention has been given these entomogenous fungi in the citrus-growing sections of Florida, where attempts were made to use them in the biological control of the scale insects. Advocates of this method of control relied considerably on the effectiveness of scale fungi of the genera Sphaerostilbe, Nectria, Podonectria, Aschersonia, and others. Later work in Florida indicated that many of these "friendly fungi" do not actually parasitize the scale insect, but are in fact saprophytes or secondary parasites. On the other hand, it appears that considerable mortality of scale insects does result from the activities of an interesting endoparasitic fungus (Myiophagus). Whiteflies on citrus in Florida are hosts to fungi (e. g., Aschersonia, Aegerita, Fusarium), which are said by some investigators to be important in the control of those insects.
One of the most important groups of entomogenous fungi from the standpoint of their role in the natural control of insects is included in the order Entomophthorales. Members of the genera Empusa and Entomophthora are particularly noteworthy. Many insects are susceptible to them. The natural mortality they cause is tremendous. Among their hosts are aphids, leafhoppers, flies, grasshoppers, mealy-bugs, and various caterpillars, a commonly seen infection of this type is that of the house fly by Empusa muscae. The infected flies attach themselves to walls, ceilings, and window panes, where they die of the fungus but remain in a lifelike position, usually with a halo of discharged spores around them.