Pesticides for Stored Products
249
it may be desirable to increase the availability of new insecticides that act by
different modes of action. However, even this approach may be more optimistic
than can be justified by experience. An insecticide-resistant strain of Tribolium
castaneum showed greater resistance to a mixture of methyl (Z)- and (£)-10,11epoxy-7 -ethyl-3, II-dimethyl-2,6-tridecadienoates than normal strains of the species (Levinson, 1975; Dyte, 1972). Brown (1977) has given an account of the
problem and points out that multiresistant strains also show cross-tolerance to
juvenile hormone mimics and other insect growth regulators. He predicts that
resistance to insecticides will accumulate and spread in proportion to our use of
chemicals, and he recommends that standard tests for resistance be used systematically at regular intervals. Thus in our continued reliance on insecticides we
must be certain that they are used only when needed, and we must be aware of
the need to constantly seek alternatives.
Several such alternative approaches have been developed. Considerable
research experience has been gained in the potential applications of insect growth
regulators and pheromones. Both classes of chemicals are generally rapidly
degraded in the environment and are biologically metabolized to innocuous
compounds. In fact, maintaining their biological effectiveness over a prolonged
period often presents a problem, and special formulation techniques may be
required to protect them against oxidation or photochemical breakdown in sunlight.
9.4. 1. Compounds That Affect Insect Development
Insect growth regulators (IGRs) include a variety of compounds. Some,
like diftubenzuron, affect molt by interfering with the synthesis of chitin; others
mimic hormones, particularly the insect juvenile hormones. Siddall (1976) has
defined IGRs as "substances which act within an insect to accelerate or inhibit
a physiological regulatory process essential to normal development of an insect
or its progeny in such a way that the action of the substance is necessarily
dependent on the life stage of the insect." The use of IGRs would be insectistatic
rather than insecticidal. It has been suggested that the rapid rate of development
of stored-product pests renders them particularly suitable for control by IGRs
(Levinson, 1975). Some IGRs are very effective against Coleoptera, acting at
nanogram levels; sterility of emerging female kbapra beetles (Trogoderma granarium Everts) is induced by application of 10- 6 ~g of 3,4-methylenedioxyphenyl
6,7-epoxygeranyl ether to single pupae (Metwally et al., 1972). However, IGRs
are usually rapidly degraded in the environment and may need to be formulated
to provide controlled release. Methoprene [isopropyl (E,£)-11-methoxy-3,7,11trimethyl-2,4-dodecadienoate] provides an example. Metabolism in rice or alfalfa
plants gave 7-methoxycitronellal, an unexpected product, formed by oxidative
rupture of the 4-ene double bond (Quistad et al., 1974). In soil, breakdown was
249
it may be desirable to increase the availability of new insecticides that act by
different modes of action. However, even this approach may be more optimistic
than can be justified by experience. An insecticide-resistant strain of Tribolium
castaneum showed greater resistance to a mixture of methyl (Z)- and (£)-10,11epoxy-7 -ethyl-3, II-dimethyl-2,6-tridecadienoates than normal strains of the species (Levinson, 1975; Dyte, 1972). Brown (1977) has given an account of the
problem and points out that multiresistant strains also show cross-tolerance to
juvenile hormone mimics and other insect growth regulators. He predicts that
resistance to insecticides will accumulate and spread in proportion to our use of
chemicals, and he recommends that standard tests for resistance be used systematically at regular intervals. Thus in our continued reliance on insecticides we
must be certain that they are used only when needed, and we must be aware of
the need to constantly seek alternatives.
Several such alternative approaches have been developed. Considerable
research experience has been gained in the potential applications of insect growth
regulators and pheromones. Both classes of chemicals are generally rapidly
degraded in the environment and are biologically metabolized to innocuous
compounds. In fact, maintaining their biological effectiveness over a prolonged
period often presents a problem, and special formulation techniques may be
required to protect them against oxidation or photochemical breakdown in sunlight.
9.4. 1. Compounds That Affect Insect Development
Insect growth regulators (IGRs) include a variety of compounds. Some,
like diftubenzuron, affect molt by interfering with the synthesis of chitin; others
mimic hormones, particularly the insect juvenile hormones. Siddall (1976) has
defined IGRs as "substances which act within an insect to accelerate or inhibit
a physiological regulatory process essential to normal development of an insect
or its progeny in such a way that the action of the substance is necessarily
dependent on the life stage of the insect." The use of IGRs would be insectistatic
rather than insecticidal. It has been suggested that the rapid rate of development
of stored-product pests renders them particularly suitable for control by IGRs
(Levinson, 1975). Some IGRs are very effective against Coleoptera, acting at
nanogram levels; sterility of emerging female kbapra beetles (Trogoderma granarium Everts) is induced by application of 10- 6 ~g of 3,4-methylenedioxyphenyl
6,7-epoxygeranyl ether to single pupae (Metwally et al., 1972). However, IGRs
are usually rapidly degraded in the environment and may need to be formulated
to provide controlled release. Methoprene [isopropyl (E,£)-11-methoxy-3,7,11trimethyl-2,4-dodecadienoate] provides an example. Metabolism in rice or alfalfa
plants gave 7-methoxycitronellal, an unexpected product, formed by oxidative
rupture of the 4-ene double bond (Quistad et al., 1974). In soil, breakdown was