Insect growth regulators

Insect growth regulators (IGRs) interfere with embryonic, larval and nymphal development, disrupt metamorphosis and reproduction. They are highly selective to insects and arthropods but because they kill through disruption of growth and development they take more time to reduce insect populations than conventional insecticides. IGRs can be classified as juvenile hormones, chitin synthesis inhibitors and triazine derivatives.

Two hormones are involved in the control of larval and nymphal moulting: the moulting hormones or ecdysones and the juvenile hormones. The ecdysones are necessary for the resorption of the old cuticle, deposition, hardening and tanning of the new cuticle while the juvenile hormones are present during the larval stage at each moult to prevent the insect maturing (Bowers, 1971). The ecdysones have generally proved to be too expensive to synthesize and use as control agents. However, recently a new class of insect growth regulators which are non-steroidal ecdysone antagonists have proven effective against Lepidoptera (Chandler et al, 1992; Cadogan et al, 1997). The product tebufenozide demonstrates activity on neonate larvae of Helicoverpa zea, Spodoptera frugiperda (Valentine et al., 1996) and Choristonsura fumiferaria (Cadogan et al., 1997).

The role of juvenile hormones in metamorphosis is to perpetuate immature growth and development when they are present and permit maturation when they are not (Bowers, 1971). A number of juvenile hormones have been identified and structures established. Derivatives of these hormones have also been produced, known as juvenile hormone analogues or mimics (sometimes referred to as juvenoids). Potential utilization of the juvenile hormone or its analogues is dependent upon application at a late larval or early pupal stage when it can induce morphogenetic damage, resulting in development of intermediate larval/pupal or pupal/adult stages or 'monster' individuals that are unable to mature, but take some time to die. Herein lies a disadvantage of juvenile hormones. In pest species in which the larval stage is the most destructive, hormonal extension of the feeding period may reduce any value from control even if the insect does not manage to reproduce. This has restricted the use of these IGRs to situations where only adult stages are pests, e.g. mosquitoes, ants and fleas, or where it is appropriate to prevent the build-up of small populations causing negligible damage to levels causing economic loss e.g. stored products, long term control of cockroaches (Menn et al., 1989).

A number of juvenile hormone mimics are available, the oldest, methoprene, is used for control of flies (particularly of livestock), fleas, mosquitoes, stored food and tobacco pests and pharaohs ants. A commonly used newer IGR is fenoxycarb which can be used to control Cydia pomonella and Epiphyas postvittana (Valentine et al., 1996) and termites (Su, 1994). Pyriproxifen, another mimic, is active against fleas at very small doses and has recently been released in the USA in spray, collar and wash formulations (Wall and Shearer, 1997).

The insect cuticle presents a potentially vulnerable and specific target for the disruption of its chemistry, structure and function by insecticides (Reynolds, 1989). The amino sugar polysaccharide chitin is a particularly important component of the insect cuticle. If synthesis of chitin is disrupted at crucial times, such as egg hatch or moult, then the insect will die. Among the substances known to inhibit chitin synthesis are the benzoylphenylureas, e.g. diflubenzuron, hexaflumuron and triflumuron. These IGRs have been found to be effective against a range of pest species including termites, lep-idopteran, mites and scarid flies.

The substituted melamine, cryomazine, has IGR effects causing reduced growth and eventually death from integumental lesions. However, it is considered to be in its own class of triazine insect larvicides (Kotze and Reynolds, 1989) since it does not act directly on chitin synthesis. Cryomazine death of insects is characterized by a rapid stiffening of the cuticle, and it seems to have more specificity than the benzoylphenylureas, affecting mostly larvae of Diptera.

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