An
effective host-defense mechanism capable of resisting, containing, and
eventually eliminating harmful foreign matter is necessary for insect
survival. Diseases in insects are produced by a variety of pathogens
including; viruses, bacteria, fungi, protozoa, rickettsiae, and nematodes.
Moreover, predatory and parasitic insects can also induce host-defense
responses in injured insects. Information about insect host-defense
mechanisms is primarily derived from laboratory investigation and little
is known about their effectiveness in natural settings.
Generally,
insects utilize three primary lines of defense to avoid infection. The
first is a physical barrier between the internal environment and the
external environment which includes the exterior cuticle and the lining
of the gut. Additionally, the gut of most insects is maintained at a
pH which inhibits the growth of most bacteria and incorporates a peritrophic
membrane to keep microbes that enter the digestive tract with food from
entering the hemocoel through the gut wall. The second and third lines,
humoral and cellular, are activated once a foreign invader infects the
hemocoel.
Humoral
responses require several hours for their full expression, and involve
induced synthesis of anti-bacterial proteins such as cecropins (4kDa),
attacins (21 kDa), diptericins (8 kDa) and defensins (4 kDa). The detergent
properties of these anti-bacteria proteins disrupt bacterial cell membranes.
Insects also synthesize lysozymes, enzymes that directly attack bacteria
by hydrolyzing their peptidoglycan cell walls.
Cellular
defense mechanisms, mediated by hemocytes, include phagocytosis, nodule
formation, and encapsulation. Hemocytic responses feature direct cellular
interactions between circulating hemocytes and the foreign material.
Typically, these interactions occur within minutes of infections.