248
J. R. Plimmer
often specific as to the species and functions that are affected. Affected
functions include larval development, molting ability, egg mortality, and
female sterility. (Neurohormones are currently a subject of interest; however, when Levinson's review was published, little was known of their
structure, and the topic was not discussed.)
3. Nutrients and regulators of metabolism. Pests of stored products require
a number of essential nutrients, including amino acids, carbohydrates,
B vitamins, sterols, carotenoids, inorganic salts, and water. Antagonists
of these nutrients can suppress growth or affect development, and overdoses of some may have similar adverse affects. Insect steroid metabolism presents an opportunity for design of molecules that may interfere with normal metabolic routes, and azasterols and other cholesterol
derivatives may block normal biosynthetic routes. For example, the
biosynthesis of ecdysone is important for the regulation of insect molt:
the biological precursor of ecdysone is cholesterol, which must be derived from dietary sources, e.g., through degradation of phytosterols.
The utilization of energy resources may be affected by a number of
substances termed hypolipidemic compounds. These cause rapid depletion of insect lipid content and may increase larval mortality or cause
other insectistatic effects.
The absence of nutrients produced by symbiotic microorganisms may
be lethal to insect species. Vitamin-producing symbionts such as yeast
organisms may be inhibited by low concentrations of chemical antagonists.
4. Sex attractants and mating inhibitors. Sex and aggregation pheromones
may be used for: (a) detection of sites of infestation; (b) estimation of
population density; (c) luring of insects into traps where they may be
killed, sterilized, or infected with pathogens; and (d) air permeation to
disrupt mating (Stockel, 1975).
In this brief summary it is impossible to do full justice to Levinson's
comprehensive review, which outlines many possible strategies for control based
on consideration of insect physiology and biochemistry. The value of such an
approach becomes more apparent as we recognize the continued appearance of
insecticide resistance among major insect pests. This is not surprising when we
consider many of the insecticides in current use and realize that they are designed
to act on a limited number of target sites. For example, organophosphates,
carbamates, pyrethroids, and organochlorine compounds act by affecting neural
transmission. Cholinesterase inhibitors predominate among insecticides frequently used; it does not seem unreasonable that the onset of resistance in an
insect species is often also accompanied by the appearance of cross-resistance
to other insecticides that have similar modes of action. To avoid this situation,
J. R. Plimmer
often specific as to the species and functions that are affected. Affected
functions include larval development, molting ability, egg mortality, and
female sterility. (Neurohormones are currently a subject of interest; however, when Levinson's review was published, little was known of their
structure, and the topic was not discussed.)
3. Nutrients and regulators of metabolism. Pests of stored products require
a number of essential nutrients, including amino acids, carbohydrates,
B vitamins, sterols, carotenoids, inorganic salts, and water. Antagonists
of these nutrients can suppress growth or affect development, and overdoses of some may have similar adverse affects. Insect steroid metabolism presents an opportunity for design of molecules that may interfere with normal metabolic routes, and azasterols and other cholesterol
derivatives may block normal biosynthetic routes. For example, the
biosynthesis of ecdysone is important for the regulation of insect molt:
the biological precursor of ecdysone is cholesterol, which must be derived from dietary sources, e.g., through degradation of phytosterols.
The utilization of energy resources may be affected by a number of
substances termed hypolipidemic compounds. These cause rapid depletion of insect lipid content and may increase larval mortality or cause
other insectistatic effects.
The absence of nutrients produced by symbiotic microorganisms may
be lethal to insect species. Vitamin-producing symbionts such as yeast
organisms may be inhibited by low concentrations of chemical antagonists.
4. Sex attractants and mating inhibitors. Sex and aggregation pheromones
may be used for: (a) detection of sites of infestation; (b) estimation of
population density; (c) luring of insects into traps where they may be
killed, sterilized, or infected with pathogens; and (d) air permeation to
disrupt mating (Stockel, 1975).
In this brief summary it is impossible to do full justice to Levinson's
comprehensive review, which outlines many possible strategies for control based
on consideration of insect physiology and biochemistry. The value of such an
approach becomes more apparent as we recognize the continued appearance of
insecticide resistance among major insect pests. This is not surprising when we
consider many of the insecticides in current use and realize that they are designed
to act on a limited number of target sites. For example, organophosphates,
carbamates, pyrethroids, and organochlorine compounds act by affecting neural
transmission. Cholinesterase inhibitors predominate among insecticides frequently used; it does not seem unreasonable that the onset of resistance in an
insect species is often also accompanied by the appearance of cross-resistance
to other insecticides that have similar modes of action. To avoid this situation,