|Controlling Insect Pests of Stored Products Using Insect Growth Regulators and Insecticides of Microbial Origin (NRI, 1994, 58 p.)|
|Section 2: Insect growth regulators: general account|
Insect growth regulators (IGRs) are compounds which interfere with insect metabolism in a manner which affects growth. During their development from egg to adult, insects go through larval or nymphal stages and gain biomass by feeding. Insects have an exoskeleton which cannot expand sufficiently to allow for growth. During pre-adult life, the exoskeleton is therefore renewed a number of times by the process of moulting or ecdysis. The formation of new cuticle at each moult and the shedding of the old exoskeleton, which are critical periods in the development of an insect, are under the control of a number of hormones. IGRs act in several ways. They may inhibit the formation of the chitin required to make a new cuticle at each moult, or they may replace or disrupt the production of the juvenile hormone (JH) which controls the moulting process. Short reviews of the effects of IGRs on storage insects have been published by various authors including Bengston (1987) and Mian et al.(1990).
Chitin inhibitors disrupt the synthesis of chitin. They act against the larval stages which, when affected, usually fail to survive the next moult. Death is caused by incomplete ecdysis and cuticle malformation. There are several known groups of substances which act as chitin inhibitors. The best known are probably the benzoylphenylureas which include diflubenzuron and its analogues teflubenzuron, flufenoxuron and triflumuron.
Insect juvenile hormone (JH) controls metamorphosis and development. Studies have shown that maintenance of JH at a high level prevents the development of larvae and nymphs into adults. They remain as juveniles, often continuing to grow, and sometimes producing what are known as super-larvae.
Once the structure and function of juvenile hormone has been established analogues were synthesized, of which several were exploited commercially. The best known of these juvenile hormone analogues (JHAs) are methoprene, hydroprene and fenoxycarb. These substances do not kill adult insects but they prevent juvenile stages from completing their development. Control of an insect population is therefore a gradual process.
Once the effects of JH and JHAs had been demonstrated, research
was directed towards determining the effect of their absence prior to the final
Bowers (1976) discovered that extracts from the plant Ageratum houstonianum cause premature metamorphosis in some Hemiptera. It was subsequently shown that the active molecules, or precocenes, destroy the glands (corpora allata) which produce JH after first being converted to highly reactive metabolites by tissue-specific enzymes within the glands (Menn et al., 1989).
Screening led to the discovery of anti-JHs such as fluoromevalonate (FMev). FMev showed activity against virtually all the Lepidoptera in which it was tested but little in other insect orders.
Although anti-JHs have become useful research tools, none have shown sufficient promise to be developed as practical pest control agents. Also, precocenes have been found to be toxic to the liver and kidneys of vertebrates (Steal, 1 986).
IGRs are arthropod-specific and as they are not neurotoxic, they are potentially less harmful to man and other vertebrates than conventional insecticides. The agrochemical industry has screened many compounds for potential IGR activity and isolated a number of active compounds.
If IGR activity is identified, a compound is given a company registration number and is then tested to determine its specificity against a range of economically important insect pests. For registration purposes, promising materials are also evaluated for their physico-chemical properties, mammalian and avian toxicity, effects, if any on the environment, and toxicity to other wildlife.
Before they can be used as grain protectants, compounds must first gain approval by the Joint FAO/WHO Meeting on Pesticide Residues (JMPR) which establishes the acceptable daily intake (ADI) levels and maximum residue limits (MRL) on produce. Once these levels have been approved, protectants are submitted to the Codex Committee on Pesticide Residues (CCPR). The CCPR carries out an 1 1 -step procedure to establish internationalIy acceptable MRLs for the food commodities moving in international trade. These standards are then recommended to governments by the Codex Alimentarius Commission.
Where appropriate, the various methodologies used in the evaluation of each IGR are described. Most trials have been laboratory-based. The methods used to apply IGRs can be classified into the following three groups: topical application, admixture to food media and application to surfaces.
For topical application, 1-100 µg of IGR are applied in solvent to the insect body. Various solvent carriers in volumes between 0.03 µl to 2 µl are used. For the admixture of IGRs to insect food media, widely differing volumes of various solvents have been used. Generally, the treatment is applied directly to the food medium, although the compound is occasionally applied, in solvent, to the glass above the commodity in the treatment vessel. Distribution of active ingredient (a.i.) is achieved either by mechanical tumbling followed by controlled solvent evaporation, or by manual mixing followed by natural evaporation over 18 hours. An IGR in solvent may also be directly applied to paper or cardboard, but this is a method which is rarely used.
Many of the IGRs examined in this review have remained at the preliminary evaluation stage, whilst others have been approved and used commercially against insect pests. Cross-referencing of data for a particular compound is often difficult because various registration numbers have been issued by different chemical companies. It is possible that some of the registration numbers quoted may have been superseded as a result of a substance having being withdrawn by its manufacturer. As they form only a small fraction of the insecticide market, it is unlikely that substances will be evaluated solely as grain protectants.
IGRs have generally been regarded as harmless to beneficial insects, but their effects on the parasitoid and predatory insects which occur in storage situations have not yet been investigated. These effects will need to be determined before IGRs are widely used as grain protectants.