Insects and Humans

Abstract

Only avery small proportion ofinsect species interact directly or indirectly withhu- mans. Most of these are beneficial and relatively few insects (about 0.1% of all species) constitute pests, which alone or in combination with microorganisms causedamage, disease, or death to humans, crops, livestock,and manufactured goods. Among the insects that directly benefit humans are those whose products, notably honey and silk, are commerciallyvaluable, that serve as food,educational, research,and medical resources, or that simplygive pleasure. Indirectly, humans gain immensely from the activities of insects, especially as pollinators, as agents for biological control of insect pests and weeds, and as soil-dwellers and scavengers. Pest insects may beclassified according to their ecological strategies as r pests, K pests, or intermediate pests. The r pests have potentially high rates of population increase, well developed ability to disperse and find new food sources, and general food prefer- ences. Though they have a large number of natural enemies, biological control would not be effective against r pests because of their reproductive potential. The use of insecticides is the best method for the control of r pests. At the opposite end of the spectrum are K pests, whichhave relatively low fecundity, poor powers ofdispersal, specific food pref- erences, and few natural enemies. They occur in relatively stable habitats and are best controlled by methods that render their habitat less stable, for example, cultural prac- tices and by the breeding of resistant strains of the organisms they attack. Intermediate pests form a continuum between r and K pests and are normally held below the eco- nomic injury threshold by natural enemies. Biological controlistheprimary methodfor control of these pests, supplemented as appropriate by chemical, genetic, and/or cultural methods. Legal control aims to prevent or control pest damage through legislation. It includes the setting up of quarantine stations, systems for monitoring pest populations, and mechanisms for the certification ofdisease-free plants and animals. Chemical control traditionally has been the use of naturally occurring or synthetic chemicals to kill pests. It has been the major method of pest control for about 90 years but has created three serious problems: (1) a great increase in the resistance of pests to the chemicals, (2) the death of many beneficialinsects as a resultof thechemicals’ non-specific activity, and (3) pollution of theenvironment. Insectistatics (insect growth regulators) have found use in specificsituationswhere rapid “knockdown” is not critical,and sex attractants and aggregation pheromones now play major roles in monitoring systems for estimating pest populations. Biological controlistheregulation of pest populations by natural enemies. For insect pests, parasitoids, predaceous insects, and microorganisms are themajor control agents. Biological control (including natural control,augmentation, classical control,and neoclas- sical control) offers several advantages over control by insecticides: (1) the control agent is ready-made (does not have to be developed or go through an extensive registration pro- cess), cheap to produce and apply, and persistent (many microbialinsecticides are excep- tions to this generalization—and see point 3); (2) it does not endanger humans or wildlife through pollution of theenvironment; and (3) the methoddoes not stimulate rapid genetic counterattack by pests (though examples of developing resistance to microbial insecticides are known). Its main disadvantages are its slowness of effect and the fact that the final (equilibrium) pest population density is normally higher than that achieved after insecti- cide application. Other problems identifiedfrom some biological control projects include extinction (of both the original pest and non-pest species), enhancement of the target pest population as a result of secondary outbreaks, and change to pest status for the original control agent. Microbial control (the use of pathogenic bacteria, viruses, nematodes, fungi,and pro- tozoa) isplaying an increasingly important role inthe control ofinsect pests. Strategies for the use of microorganismal agents include introduction, augmentation, and conservation. The advantages of microbial control (safety to humans and wildlife, specificity, biodegrad- ability, and low registration costs) have been somewhat offset by the slow-acting nature, low powers ofdispersal,and mass-production problems of the agents. Genetic engineering should remove some of these disadvantages. Methodsofgenetic controlinclude (1) those that render pests less capableof repro- duction, for example, the sterile insect release method, use of chemosterilants, and sterility resulting from mating incompatibility or abnormal gametogenesis, and (2) those in which resistance is increasedinthe organisms attackedby pests. Genetic engineering is starting to playamajor role in improving plant resistance to insect pests. Cultural controlincludes the long-established agricultural practices that make habitats less suitable for pests. The methods used may either directly affect a pest, stimulate an increase in density of a pest’s natural enemies, or make the organisms on which a pest feeds more tolerant of attack. Integrated pest management (IPM) is a combination of methods for reducing and maintaining pest populations below the economic injury threshold.There are three phases in the development of anIPMstrategy: problem definition, research, and implementation, of which the first is the most important.To be most effective, IPM requires the input of as much information as possible, not onlyabout the agroecosystem, but also about the socioeconomic framework of the farming system inwhich the pest problem occurs. Thus, thecollaboration of experts fromawide range ofdisciplines is necessary. If conducted properly, IPM leads to considerable financial saving and a great improvement in environmental quality.