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Since the 1940s, insect pest control has depended mainly on the use of synthetic insecticides, most of which act as neurotoxins. In the storage of grain, such compounds have either been admixed with commodities or sprayed onto stack or store surfaces; they have often been used to supplement fumigation treatments.
High toxicity, persistence and a wide spectrum of activity, which were initially desirable features, have led to the withdrawal of many compounds in recent years. Highly persistent compounds were found to accumulate in food chains and affect a wide range of wildlife. Pesticide misuse, which still causes chronic and acute poisoning in humans and domestic animals, has also led to the development of resistance. The use of broad-spectrum insecticides has sometimes resulted in the elimination of beneficial predators and parasites without control of the pest species. Once these problems were recognized by governments and the agrochemical industry, a search was initiated for chemicals which were less persistent and more pest specific.
The development of any new pesticide is an increasingly expensive and time consuming exercise. Only a few compounds fulfil the stringent requirements necessary for registration and of these, few are able to satisfy the additional requirements for use as grain protectants. As the financial return to agrochemical companies from the grain protectant market is very small, compounds solely for use on stored food are unlikely to be developed.
The grain protectants currently approved are likely to be the main agents of insect pest control for the foreseeable future. The number of compounds in use may even be reduced as regulations change. However, in order to extend the options, alternative materials with desirable properties need to be evaluated for their potential as grain protectants before the need for new materials becomes critical.
Screening programmes, in addition to identifying new conventional synthetic insecticides, should consider substances with different modes of action, particularly those which control or interfere with biochemical processes found only in insects, such as metamorphosis. Other potential insecticides include toxic plant extracts, fungi and bacteria, and live microbial agents. However, the same stringent requirements in terms of efficacy and safety will be necessary for novel substances as for conventional grain protectants.
If candidate materials are to be utilized to their best effect, assuming that they ultimately obtain approval, careful consideration must be given to the role they will play in pest control strategies. Various approaches may be adopted, depending on the properties of the individual material. If the material proves effective against a broad spectrum of target pests, it may be introduced to replace conventional contact insecticides. Alternatively, it may be used together with contact insecticides applied at reduced dosages. In farm level storage, there may be scope for its application in association with traditional methods. However, whatever the proposed use of these materials, they will need to be competitively priced.
This bulletin reviews published work on the use of insect growth regulators (IGRs) and microbial insect control agents against stored product pests, and aims to identify promising substances and new areas of investigation. An extensive search was made of the databases held by the Natural Resources Institute, and those of Biosis Previews, CABI abstracts, Agris and Agricola for the period 1 977-1 992.
The text is presented in five sections. In Section 2, the different types of IGRs are described, methodologies utilized in trials are outlined, and the main procedures of insecticide registration are summarized. In Section 3, the modes of action of individual juvenile hormone analogues (JHAs) and chitin inhibitors are described, and data relating to their effects on specific insect species are collated; the merits of different methodologies and the potential impact of IGRs as grain protectants are discussed. Section 4 contains information on microbial agents isolated from insect pests and examines their potential for economic pest control. In Section 5, the problems associated with IGRs and microbial insecticides in the control of insect pests of stored products are summarized.
The appendices contain references and further reading. The compounds described in the review are listed with their IUPAC chemical names, code numbers, trade marks, manufacturers or distributors, and toxicological data. Toxicological data for the individual materials examined are compared with data for conventional grain protectants.
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