In the broad sense (sensu lato) this can include all types of control involving the use of living organisms, so that, in addition to the use of predators, parasites, and disease causing pathogens (biological control – sensu stricta), one can include sterilization, genetic manipulation, use of pheromones, and the use of resistant varieties of crop plant.
As already indicated in this book, the use of resistant crop varieties is being dealt with separately as it is an aspect of control of such importance, and plant breeding is a very specialized subject in its own rights.
The main attraction of biological control is that it obviates the necessity (or at least reduces it) of using chemical poisons, and in its most successful cases gives long-term (permanent) control from one introduction.
This method of control is most effective against pests of exotic crops which often do not have their full complement of natural enemies in the introduced locality.
Then the most effective natural enemies usually come from their native locality, for the local predators/parasites/pathogens are usually in a state of delicate ecological balance in their own environment and cannot be expected to exercise much population control over the introduced pests.
On rare occasions a local predator or parasite will successfully control an introduced pest, but this is rare!
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This is the existing population control already being exerted by the naturally occurring predators and parasites (and diseases) in the local agro-ecosystem, and it is vitally important in agriculture not to upset this relationship.
Because it is not readily apparent, the extent of natural control in most cases is not appreciated.
It is only after careless use of very toxic, broad-spectrum, persistent insecticides which typically kill more predators and parasites than the less sensitive crop pests, and which is then followed by a new, more severe pest outbreak, that the extent of the previously existing natural control may be appreciated.
In summary, the importance of natural control of pests in most agro-ecosystems cannot be overemphasized.
The animals that prey and feed on insects are very varied, as are their effects on pest populations. The main groups of entomophagous predators are as follows:
Mammalia – (man), Insectivora, Rodentia Aves – Passeriformes (many families), many other groups, especially ducks, game birds, egrets and herons, hawks Reptilia – small snakes, lizards, geckos, chamaeleons Amphibia – most Anura (frogs and toads) Pisces – Gambusia etc. (control mosquito larvae) Arachnida – spiders, harvestmen, chelifers, scorpions, etc. Acarina – mostly family Phytoseiidae Insecta – Odonata (adults, and nymphs in water), Mantidae, Neuroptera, Heteroptera (some Miridae, Anthocoridae, Reduviidae, Pentatomidae), Diptera (Some Cecidomyiidae (larvae), Syrphidae (larvae), Asilidae, Therevidae, Conopidae, etc.), Hymenoptera (Vespidae, Scoliidae, Formicidae), Coleoptera (Cincindelidae, Carabidae, Staphylinidae, Histeridae, Lampyridae, Hydrophilidae, Cleridae, Meloidae and Coccinellidae).
A few predators are quite prey-specific; for example, the larvae of Meloidae feeding on the egg-pods of Acrididae in soil, and Scoliidae feeding on scarab larvae in soil and in rubbish dumps. But most predators are not particularly confined to any specific prey.
Some of the predators live in rather specialized habitats; for example, all the fish are aquatic, as are some insect larvae (e.g. Odonata), and so only prey on aquatic insects (such as mosquito larvae); some live in soil or leaf litter so their prey is restricted to certain types of insects.
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Control by pathogens is sometimes referred to as microbial control. There are three main groups concerned: bacteria, fungi and viruses, and some other groups of entomophagous micro-organisms which are rather obscure and little studied.
There are several types of Bacillus, which are specific to caterpillars or beetle larvae, responsible for natural epizootics, and several species are now commercially formulated and very important in pest control projects.
Fungi are responsible for producing antibiotics and apparently about 300 antibiotics do show some promise as pesticides; these act directly as killing agents or inhibitors of growth or reproduction.
Viruses are quite commonly found attacking insects in wild populations of caterpillars and beetle larvae, as well as some temperate sawfly larvae.
They have long been used as biological insecticides, by finding dead larvae in the field and making an aqueous suspension of their macerated bodies. But now a few commercial preparations are available.
Some of these new biological insecticides using insect pathogens are, however, only easily available in the USA as yet, although others are commercially available in Europe and parts of Asia.
This usually refers to the sterilization of males by Xrays or γ-rays and is called the sterile- male technique; control of a pest by this technique is termed autocide. Sterilization can be effected by exposure to various chemicals and this practice is called chemo-sterilization.
The rationale behind this method is that male sterilization is effective in species where females only mate once and are unable to distinguish or discriminate against sterilized males. The classical case was in about 1940 on the island of Curaçao against Screw-worm (Callitroga) on goats: the male flies were sterilized by exposure to γ-rays, and dropped from planes at a rate of 400/square mile/week.
The whole pest population was eradicated in 12 months. The life-cycle took only about four weeks to complete, and the females only mated once in their lifetime. Generally, autocide is most effective when applied to restricted populations (islands, etc.), but can be effective on parts of the continents.
The Screw-worm eradication campaign was extended to the southern part of the USA where the pest is very harmful to cattle. In Texas 99.9% control was achieved in only three years.
Male sterilization trials were effective against Mediterranean fruit fly (Ceratitis capitata) on part of the island of Hawaii in 1959 and 1960, but immigration from untreated parts of the island prevented control from being long-lived.
Chemosterilization has now advanced from a theoretical technique to a practical one, a variety of chemicals have been demonstrated to interrupt the reproductive cycles of a large number of insect species, see Curtis (1985).
In reality this method is an extension of the previous one, in that the electromagnetic radiations (X-rays, γ-rays) induce dominant lethal mutations in the germ cells of the insects. These mutations in insect sperm have been used successfully in several eradication programmes.
Lethal mutations are not lethal to the treated cell, they are lethal to its descendant in that the zygote fails to develop to maturity. These mutations arise as a result of chromosome breakages in the treated cells.
Potential uses for Pheromones in Pest Control
The two obvious ways in which pheromones may be used in a pest control programme are firstly, in pest population surveys or for population monitoring (for emergence warnings, and spray warnings), and secondly for direct behavioural modification control.
It is clear now from the work that has been done in recent years that pheromone traps are extremely useful in monitoring projects and this use is likely to be increased in the future.
But to date there has not yet been a good example of pheromone use actually achieving a significant level of population control in a pest management programme; though, as already mentioned there have been behaviour disruption trials on several different crops with very encouraging results.
Insect Population Monitoring
The presence or absence of a particular insect species in an area can be established through the use of attractant pheromones, so that control measures may then be exercised, if necessary, with precise timing.
Previously field population monitoring relied largely on either light-trapping, which requires a source of electricity, or the finding of eggs on the crop plants. The finding of the first eggs on a particular crop is a very tedious and time consuming process requiring a great deal of labour and is not particularly efficient.
The examination of a few pheromone traps for the presence of male insects is relatively very easy, and much more efficient. Alternatively, the pheromone traps can be used to monitor the effectiveness of a pest control programme, even though not directly employed in the programme themselves.
Emergence of male Codling Moth in apple orchards in the spring in Europe and N. America, and Pea Moth, is now regularly monitored by the use of small paper (waterproof) pheromone traps with a sticky interior.
Pink Bollworm, and other bollworms, on cotton crops in many parts of the tropics are likewise monitored with the use of these sticky pheromone traps, with considerable success.
Various species of fruit flies (Dacus spp.) are monitored, sometimes using sticky pheromone traps and sometimes in traps with insecticides inside, in citrus and peach orchards to determine whether or not insecticide spraying is required, and if so just when.
As more and more sex pheromones are being synthesized, and more chemical attractants are being discovered, it seems likely that the use of these chemicals in monitoring programmes will increase and will play a constant role in many pest management programmes.
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