Biological control practices involve three major techniques, viz., classical biological control, conservation and augmentation.
Technique # 1. Classical Biological Control:
The intentional introduction of an exotic, usually co-evolved, biological control agent for permanent establishment and long-term pest control is referred to as classical biological control. The basic approach in this technique is to identify natural enemies that control a pest in its home location and introduce these enemies in the pest’s new location.
It is usually used to control species that have attained pest status following invasion of a new geographical location. Importation of biological control agents has been widely practiced since the initial spectacular success of the Vedalia beetle in 1889-90. The approach has yielded some of the best known examples of biological control.
Procedures followed in natural enemy introduction programmes are fairly standard and generally include exploration for agents in areas of pest origin, pre-introduction studies (such as species identification, biological and ecological characterization, and rearing procedures), quarantine of agents for introduction, release and evaluation.
It is advisable to introduce an exotic species of a natural enemy either when there is an unoccupied niche in the life system of the pest which needs to be filled or when an inefficient natural enemy occupies -a niche and is required to be displaced by a more efficient exotic species that fulfills the conditions of an ecological homologue. The former is a common situation in newly introduced pests in a country.
Foreign explorations for parasitoids and natural enemies have been made primarily to introduce parasitoids from the place of origin of the pest and, sometimes also, to introduce exotic natural enemies of the indigenous pest species. In the latter case, attempts have not been very successful, because the indigenous parasitoids or predators which are already adapted to the host concerned are not easily replaced by the introduced species.
When an introduced pest is to be controlled naturally, the place of its origin should be determined first, because it would be best to find a suitable natural enemy only in its original homeland. Before on entomologist goes abroad, he should know the basic habitat, behaviour and biology of the host. His job abroad is to find the pest, to collect its natural enemies, if any, and to tranship them to the receiving country, in sufficient numbers, in viable condition.
The collector should also remember that the pest species concerned might be quite scarce in the country where he might be going. The duration for which search ought to be made should be extended to at least one full season of the pest’s activity, to search natural enemies in various stages of development or in different generations found in various seasons of the year.
The actual shipment of a parasitoid or a predator follows its collection and rearing. Since it is a very valuable and perishable commodity, all possible care should be taken so that it reaches its destination within the minimum possible time during the period of the year when temperature is favourable.
In the modern jet age of travel, it should not be necessary to send the consignment by steamers. Before a packing is prepared and handed to the airline concerned, definite instructions should be given that it is to be placed in a pressurized compartment at the desired range of temperature.
Personal contacts with the crew are important at both ends, from the origin of the consignment to its destination. As soon as the parcel has been handed to the airline staff, a fax message intimating the date and time of its arrival should be conveyed to the recipient.
At the receiving end, the parcel should be located in the mail bag, if sent by air parcel post. If the consignment is sent as an air-freight under special arrangements, it might be necessary to meet and contact the crew as soon as the aeroplane arrives.
Needless to say that the parcel should be made of light but strong material which can withstand handling and sufficient air holes should be provided, so that the insects do not die. Sometimes, it is necessary to make provision for maintaining humidity or to make emergence holes in case the adults are expected to come out of puparia in transit, so that they can move from the chamber containing the pupae to that of the adults, where food is kept.
The food consists of a cotton swab soaked in diluted honey. If the consignment is not very large, it is advisable to pack the material into a vacuum flask in which the temperature remains fairly constant even during transit. The export and import documents should invariably be attached to the waybill or stitched to the package.
The information that is generally provided along with the consignment includes the name of the parasite, the number of individuals, the host and plant from which collected, the name of the locality, the date of collection, and any other instruction that could be useful for rearing the organism.
When a live species is imported from abroad, it is kept in quarantine. Its biology and other attributes are studied to determine whether or not that species could be useful in the new country. Any accidental escape of the adults must not be allowed, because if the species turns out to be harmful, it will be impossible to stop its spread.
When it has been determined that the imported species is of the desirable type, free from any hyper parasitoid and is not by itself likely to become a pest of any economic plant or animal, it is then reared in large numbers. The laboratory rearing can sometimes be bypassed if the insects are imported in sufficiently large numbers and if, on their release, they easily get established in the field.
In laboratory rearing, the important studies to be made are: the ability of the imported species to adapt itself to alternative hosts, its tolerance to prevailing temperature and humidity, the factors affecting its mating, longevity, opposition and diapause. Some of this information may already be available from the collector. Sometimes, it is also desirable to try to rear the imported species on a new host, on an artificial medium or a storable plant product.
After the new species has been reared in sufficient numbers, it is released in the field during the season when the pest is to be controlled, and during the availability of the stage of development against which it is effective. The releases are made only in a few localities and are repeated a number of times.
The non-establishment or failure of the imported species will depend upon its inability to adjust itself to the extremes of heat, cold or aridity; the non-acceptance and incompatibility of the host; an accidental encounter with another undetected host which the introduced parasitoid might prefer; the lack of necessary alternative hosts when the pest is not available; the non-availability of suitable food for the adult parasitoid or predator; the non-synchronization of the life cycle with that of the host; and its inability to compete with the existing natural enemies. It has been observed that the colonization of the parasitoid takes place more readily inside the cages where there are no other parasitoids, and furthermore, its accidental dispersal is minimized.
Certain species, when studied in the laboratory, show promise but, on release in the field, they do not give the same performance and do not get established. The next step, if established, is to determine its success in reducing the pest population. The parasitized hosts are collected from the field and kept in the laboratory for the parasitoids to emerge.
Parasitization would indicate that the host has been accepted under field conditions, but it does not necessarily mean that the parasitoid has been established in that locality. Sometimes, it completes a number of generations, but as soon as it faces the extremes of weather, it perishes. This has been observed repeatedly in the case of parasitoids introduced into northern India for the control of tissue-borers of sugarcane, maize and sorghum.
Invariably, the parasitoid species have been brought from the tropical regions of the world and after getting an initial establishment, they perish during the winter, when most of these borers are either in hibernation or in diapause.
It is, therefore, essential that the species introduced must also have a synchronization of the life-cycles with that of the host. For northern India, the parasitoids collected from subtropical or near-temperate regions of the world are likely to be more successful whereas in the south these should be introduced from tropical areas.
Recently, many international organizations have been established to facilitate the movement of beneficial species from one country to another, and the largest of the organizations is the International Institute of Biological Control (previously called Commonwealth Institute of Biological Control), established in 1927, which also has laboratories in Switzerland, Trinidad, Malaysia and Pakistan.
About 40 per cent of the introduced natural enemies have established in the introduced countries and provided partial to complete control of important insect pests at the global level. In India, since the launching of All India Co-ordinated Research Project on Biological Control of Crop Pests and Weeds (AICRPBC) in 1977, 79 species of natural enemies have been imported, out of which 53 have successfully multiplied and 21 have been established in the field.
The most recent outstanding example of biological control by introduction is the control of cassava mealybug, Phenacoccus manihoti Matile-Ferrero by a tiny wasp, Epidinocarsis lopezi (De Santis) in Africa.
The mealybug was devastating the cassava plant in late 1970s, destroying as much as 80 per cent of the crop in some areas and widespread famine was a real possibility in Sub-Saharan Africa. E. lopezi was imported from Paraguay and released in Nigeria in 1981.
By mid 1980s, E. lopezi was multiplied in millions and released @ 100 wasps per second across Africa from the aeroplane, in addition to many releases from the ground. Over the next 7-8 years, mealybug problem was effectively eliminated in 30 countries. Dr. Hans R. Herren was awarded the 1995 World Food Prize for developing and implementing the world’s largest biological control project.
Technique # 2. Conservation:
Conservation of biological control agents refers to modification of the environment or existing practices to protect and enhance specific enemies or other organisms to reduce the effects of pests. In contrast to classical biological control and augmentative releases, conservation does not involve collection and rearing of natural enemies for release. Rather, this approach involves creating conditions under which natural enemies are able to enhance their efficacy as bio-control agents. Conservation is often the least emphasized of the three major approaches to biological control.
However, it can constitute the most important approach because of following two reasons:
(i) The practices conducive to natural enemy conservation influence the success of native, imported and periodically released natural enemies,
(ii) Conservation of natural enemies enables farmers to utilize beneficial species that exist in the agro-ecosystem.
Thus, it serves to strengthen farmers’ appreciation of biological control and their understanding of the natural enemies’ ecology in relation to the crop production systems.
Agro-ecosystems are characterized by many agricultural practices and designs that have the potential to enhance the functional biodiversity of natural enemies and many others that negatively affect it. In general, there seem to be reluctance on the part of farmers to modify their production practices to accommodate or promote natural enemies. This may be because many a time, such practices conflict with conventional crop production or protection methods.
The farmers are also not fully convinced about the potential of natural enemies for pest suppression. Most importantly, there is often very little knowledge about the environmental factors that limit or encourage the effectiveness of natural enemies. Conservation requires detailed knowledge of the natural enemy’s phenology and resource requirements.
Moreover, knowledge of interactions of various management practices, both agronomic as well as pest management techniques, with natural enemy populations is essential. At present, such information is available only for a few species and systems. The most important strategies to conserve natural enemies within an agro-ecosystem are cultural control by habitat manipulation and rational use of pesticides.
Conservation means the avoidance of measures that destroy natural enemies and the use of measures that increase their longevity and reproduction or the attractiveness of an area to natural enemies.
The conservation and enhancement of natural enemies are as follows:
(a) Preservation of Inactive Stages:
This is most critical when there is a small reservoir of natural enemies outside the cropped area. Pupae of Epipyrops are found in large numbers on the trashes of sugarcane leaves at the time of harvesting. If these are not burnt but left around the harvested fields, the adults emerge to augment the supply of natural enemies in the pre-monsoon season against Pyrilla perpusilla (Walker) on the young crop of sugarcane.
(b) Avoidance of Harmful Cultural Practices:
Cultural practices like ploughing, mowing or burning can be harmful to natural enemies. For example, burning of sugarcane trash destroys the resting stages of Epipyrops. Such practices can be modified to avoid harmful effects.
(c) Maintenance of Diversity:
The concept “more the diversity more is the stability”, holds true here also, since diverse system may provide alternate hosts as source of food, overwintering sites, refuges and so on. Moreover, natural enemies have evolved in natural diverse communities than agro-ecosystem so these are expected to perform in diverse systems, e.g., mixed cropping, intercropping, etc.
(d) Natural Food, Artificial Food Supplements and Shelters:
Many parasitoids and predators require foods frequently not available in monocultures. The availability of predatory mite, Euseius hibisci (Chant) in California, was related to the availability of pollen, even when prey mites were absent. In irrigated desert-areas of California, artificial honeydew and pollen in the form of food sprays induced early opposition of the antlion, Chrysoperla spp. and Coccinellids, in the alfalfa fields. In North California, marked reduction of tobacco hornworms was achieved by predacious wasps, following erections of nesting shelters near the fields.
(e) Control of Honeydew Feeding Ants:
These ants make biological control of honeydew producing insects like scales, mealy bugs and aphids difficult because of interference with natural enemies.
(f) Protection from Pesticides:
Almost all kinds of pesticides and their formulations have adverse effects on natural enemies. The most striking case of interference is the long standing biological control by the Vedalia beetle, Rodolia cardinalis (Mulsant) of cottony cushion scale, Icerya purchasi Maskell, on citrus in California by the indiscriminate use of DDT in 1946 and 1947. Vedalia beetles were killed, while the cottony cushion scales were not killed, resulting is serious outbreaks of the pest.
Later, adjustment of spray treatments allowed Vedalia beetles to regain control in subsequent years. Similarly, several predaceous mites are killed by the pesticides meant for the control of phytophagous mites. The solution lies in the use of relatively resistant strains of predators, selective use of pesticides and manipulation of prey and predator population through certain management practices.
Technique # 3. Augmentation:
Augmentative bio-control is focused on enhancing the numbers and/or activity of natural enemies in agro-ecosystems. This strategy involves mass multiplication and periodic release of natural enemies so that they may multiply during the growing season. The processes of augmentation or inoculation with natural enemies occur where natural enemies are absent or where they exist at levels that are ineffective for pest control.
The aim is to establish a natural enemy population that is able to suppress pest numbers below economically damaging levels until harvest. The rationale for this approach to biological control is that many effective natural enemies cannot survive on a year-round basis on a crop. However, they are able to survive and affect control at certain times of the year, hence the need for annual releases.
The critical difference between this approach and that of classical biological control is, therefore, that it is not long term. Ecologically, the general equilibrium of the pest population is not altered as in case of classical biological control. The releases have to be repeated periodically and only temporary suppression of the pest is achieved.
Following issues must be considered before releasing natural enemies for augmentation programmes:
i. The cost and the anticipated benefits of using bio-control relative to alternative tactics (e.g., pesticides), must be weighed carefully.
ii. The overall crop production system should be considered; releases may not be of any use if crop production practices are inimical to the use of natural enemies.
iii. The information on the most appropriate species or strain, most effective means of application (e.g., method of release, number of releases per unit area or per pest, evaluation of efficacy, etc.) must be available.
More than 125 species of natural enemies are commercially available at global level for augmentative biological control. This form of control is applied in the open field in crops that are attacked by only a few pest species, and it is particularly popular in greenhouse crops, where the whole spectrum of pests can be managed by a suite of natural enemies.
When compared with chemical control, augmentative control does not cause any phytotoxic effects on young plants, premature abortion of flowers and fruits does not occur, release of natural enemies takes less time, several key pests can be controlled only by natural enemies, and there is no safety or re-entry period after release of natural enemies, this allows continuous harvesting without danger to the health of greenhouse personnel.
Augmentation includes all activities designed to increase numbers or effect of existing natural enemies. These objectives may be achieved by releasing additional numbers of a natural enemy into a system or modifying the environment in such a way as to promote greater numbers or effectiveness.
These releases differ from introduction of imported natural enemies in that these have to be repeated periodically. Further, they result only in temporary suppression of the pest rather than permanent lowering of the general equilibrium position as in introductions. The periodic releases may be either inoculative or inundative.
(a) Inoculative releases may be made as infrequently as once a year to re-establish a species of natural enemy which is periodically killed out in an area by unfavourable conditions during part of the year, but operates very effectively the rest of the year. Here control is expected from the progeny and subsequent generations, not from the release itself.
(b) Inundative releases involve mass culture and release of natural enemies to suppress the pest population directly as in the case of conventional insecticides. These are most economical against pests that have only one or at the most a few discrete generations every year.
Massive releases have been attempted in several programmes involving natural enemies like Trichogramma spp., a tiny wasp that parasitizes insect eggs, and general predators like green lace wings, Chrysoperla carnea Stephens and ladybird beetles, Hippodamia convergens (Guerin-Meneville).
Augmentation may also result from environmental manipulations to increase the effectiveness of natural enemies. This may be achieved by providing alternate nutrients, nesting habitats, overwintering sites, etc. The possibilities for increasing the population of predaceous arthropods through habitat management are many. Small changes in agricultural practices can cause great increases in key predators by affecting the availability of alternate non-economic prey or other foods. Some of the weeds can also be excellent habitats for predators.