|
|
Noninfectious Diseases of Insects
As indicated in the definitions
of terms used in insect pathology, “disease” literally
means any departure from the normal state. Thus, a disease in an insect may
be produced by any circumstance that causes such an abnormal state. Physical
injuries,
chemical injuries, nutritional deficiencies, genetic abnormalities, and neoplasms
may result in a disease state (Steinhaus,
1963).
With the exception of sterility, noninfectious diseases are seldom considered
a component of a biological control program, but when evaluating individual
insects or a population of insects for the underlying causes of disease,
researchers
should be aware of several possibilities other than infectious diseases.
Physical injuries
Insects are naturally somewhat
protected from injury by the cuticle, tracheal system, and physiological
adaptations. Nevertheless, injuries occur and include
distention (blockage), trauma such as abrasions/contusions/ concussions/crushing,
and open wounds. These injuries may include blood loss, tissue changes,
and exposure to pathogens. Extreme cold and heat, as well as solar radiation
and drought,
can harm insects, often figuring strongly in the increase or decline of
insect
populations.
Responses to physical injury include
release of haemokinin (injury factor) into the hemolymph, which speeds the
heart rate, mobilizes hemocytes, and stimulates
RNA synthesis. It also causes hemocytes to increase in motility and adhesiveness.
In addition, there may be accumulation of hemocytes at wound sites, coagulation,
protein synthesis, increased O2 consumption, and increased DNA synthesis.
Regeneration may occur; lost appendages are often regenerated or partially
so at the next
molt (Day and Oster, 1963). Insects have many adaptations to accommodate
temperature extremes. There is a large literature available on the cold-hardiness
of insects
(see Lee and Denlinger, 1991).
Chemical injuries
Chemically poisoned insects can
have the appearance of pathogen-infected insects. It is important to recognize
the symptoms of chemical poisoning, both in laboratory
colonies and in the field. Plant secondary compounds can poison nonadapted
hosts, as can toxins produced by microorganisms. Toxins are sometimes a factor
in the
virulence of entomopathogens.
Insecticides may be physical poisons,
protoplasmic poisons, metabolic inhibitors, hormone mimics, stomach poisons,
or neuroactive agents. Most synthetic insecticides
are neurotoxins. Genetic diseases
"All hereditarily transmitted
biochemical, physiological, and morphological characters which are harmful
for the organism are genetic diseases" (Benz,
1963).
Genetic diseases may be classified as 1) Lethal factors (mutants or deficiencies)
which,
as for other organisms, occur at a frequency of about 5% in each generation.
Some mutants that are viable in the laboratory would not be viable in the
field. 2) Sterility factors, usually chemical or irradiation mutations of
males, have
been used successfully in biological control programs. 3) Structural alterations
include any malformations such as winglessness, deformed body parts, supernumerary
appendages, etc. 4) Tumors, both malignant and nonmalignant occur in insects
and 5) Gynandromorphs are intersex mutants that are usually sterile.
Neoplasms
Neoplasms are abnormal masses
of tissue, the growth of which exceeds and is uncoordinated with that of
normal tissue and persists in the same excessive manner after cessation
of the stimulus that evoked the change (Harshbarger,
1968). Etiologies may
include carcinogens, inflammation, trauma, and viruses.
Biological agents Predators
that don't kill insect prey outright may leave wounds for which the same
responses occur as for
other physical injuries. Parasites may cause irritation
and destruction of tissues, resulting in mechanical injury, for example,
piercing of the host integument by female
parasitoids
,
and surface feeding or feeding on (usually nonvital) tissues. Emergence of
endoparasites usually causes death of the host. Parasitoids may also cause
paralyzation and
other physiological injury due to competition for nutrients, or parasite
castration.
Nutritional diseases
Deviations from basic conditions
needed for attraction to and survival on a food source can lead to nutritional
disease. It may be very difficult to determine
whether a disease is due to a genetic metabolic disease, or if unbalanced
metabolism is the result of nutritional disease.
Improper balance of nutrients,
amino acids, proteins, fats, carbohydrates, vitamins, inorganic ions, etc.
can cause various symptoms of nutritional disease. Limited
feeding reduces growth, development, and reproduction. When insect populations
outbreak, overutilization of the food source can result in starvation of
not only the outbreak species but also of other species utilizing the same
food sources.
In laboratory populations, lack of specific nutrients in laboratory diet
can cause rapid or delayed nutritional disease. Symptoms in larvae include
severely
affected growth rates and development. Often adults have deformed wings,
and mating and egg production may be seriously affected.
top | Infectious Diseases of Insects |
