Biological Control of Agricultural Insect Pests

Abstract

Pests are highly responsible for heavy crop losses and reduced food supplies, poorer quality of agricultural products, economic hardship for growers and processor. Generally, chemical control methods are practiced for their control which is neither always economical nor effective and may have associated unwanted health, safety and environmental risks. However, to meet the challenge of feeding to the ever increasing human population, an efficient, economical and environment friendly disease control methods are requisites. In this regard, biological control may be an effective means of reducing or mitigating the pests and pest effects through the use of natural enemies. Biological control is an environmentally sound which involves the use of beneficial microorganism to control plant pathogens and diseases they cause. Therefore, in this chapter we will provide a comprehensive account of this environmental friendly approach for effectively management of plant diseases. This chapter will also accentuate the development of biological control agents for practical applications and the underlying mechanism. The contents in the chapter will be beneficial and advantageous to all those working in academia or industry related to crop protection.

1. Introduction

1.1 Definition

Since the beginning of agriculture, farmers had to compete with the harmful organisms called “Pests”. These organisms are not only responsible for potential loss of revenue due to heavy crop damage and reduced food supplies but also significantly damage the machinery, equipment and property as well. They are prevented, destroyed, repelled or mitigated using different types of pesticides such as insecticides, herbicides, rodenticides and fungicides. Such chemical control methods are neither always economical nor effective and are generally associated with unwanted health, safety and environmental risks.

Therefore, in the recent decades, elevated awareness on the impacts of pesticide use on the human health and environment has fostered to decrease the reliance on chemical controls. Furthermore, the development of pesticide resistance is also indicative for the need of change for pests management with emphasis on the human health and environment. In this regard, biologically based technologies could be more convectional to solve the urgent needs in pest management. Biological control or biocontrol is particularly the use of animals, fungi, or other microbes to feed upon, parasitize or interfere with a targeted pest species. When the chemical pesticides were not appropriate for controlling the specific pest, the use of biological control came as a practical solution to the pest problem. The biological control method is innovative and sustainable way to control pests. This method leaves no chemical residues and has no harmful impact on the humans or other organisms. If the method is successfully implemented following introduction, it may provide a permanent control with favaourable cost–benefit ratio. A commonly accepted definition of biological control is:

The use of living organisms to suppress the population of a specific pest organism, making it less abundant or less damaging than it would otherwise be [1].

The organisms used to feed on, parasitize, or otherwise interfere with targeted pests are called as the biocontrol agents. Table 1 below summarizes some of the different types of biological control agents.

Biocontrol agents	Examples
Predators	Ladybugs, Dragonflies, Lacewings, Pirate Bugs, Rove and Ground Beetles, Aphid midge, Centipedes
Parasitoids	Ichneumonid wasps, Braconid wasps, Chalcid wasps, Tachinid flies
Nematodes	Heterorhabditidae spp., Mermithidae spp., Rhabditidae spp., Steinernematidae spp.
Bacteria	Bacillus thuringiensis, Bacillus popillae
Viruses	Cytoplasmic polyhedrosis (CPV), Granulosis (GV), Entomopox viruses (EPN)
Fungi	Metarhizium anisopliae, Beauveria bassiana, Trichoderma viride

Table 1.

Different types of biological control agents.

Source: Homes et al. [2].

Though the biological agents controls the pest populations with the use of natural predators and minimizes their impact on economic and environmental practices, however, this control method can offer a few distinct advantages as well as disadvantages.

1.2 Advantageous and disadvantages of biological control over chemical/other control methods

Entomologists consider biological control as an option when the widespread and repetitive usage of chemicals develops resistance in insects. It is a great deal of concern in the recent years.

Extensive application of chemicals is the primary reason for undesirable ecological side effects. Agrochemicals cause many environmental adverse consequences. They may contaminate the groundwater; some may enter the food-chains as a consequence producing a threat to the health of human and other organisms. Besides, the spraying of pesticides is also unsafe for the user. Frequent use of chemical causes inefficacy and is an important part of agricultural enterprise cost [3, 4].

Besides the known adverse consequences of pesticides use, it is not possible to uproot chemicals from the pest management. However, pest mortality caused by chemicals should be added/included to natural causes of death rather than considering the death as substitutive. And here the importance of natural enemies becomes more comprehensible, as the eradication of one pest species could lead to enormous increase in other pest numbers [4].

Therefore, the increasing concern of adversities associated with pesticide use is resulting in a more environmental friendly and sustainable agriculture. Several acceptable regulations are imposed with chemical restrictions or bans, especially in the developed nations. And here natural control method is gaining more attention for pest management at the better prices in market. Such factors hopefully create a favorable condition for the widespread of biological control methods [5].

1.2.1 Advantages

Biological control has several advantages as a pest control method, especially when it is compared with insecticides. The most pivot benefit is that such control methods are environmental friendly and do not add any pollutant into the environment. Kok [6] stated that biological control method may be applied whenever required as it does not pollute the environment.

Another most important advantage of this method is its selectivity i.e., specific pest target strategy. Unlike the chemical/other control method, this method controls the pest meant to target and do not harm the other species or plant. Therefore, danger of damage to non target plant species is restricted. Weeden and Shelton [7] have confirmed that this natural control method do not arise any new complication, like conventional pesticides. However, the side effects may not be totally excluded, though it rarely appears [8]. When discussing the balance of agricultural ecosystems, selectivity plays a vital role because a great damage to non target species can lead to the restriction of natural enemies’ populations. Once the natural enemies are introduced into the environment, they reduce the target pest population and sustain their own population as well. Therefore, after initial introduction very little effort is required to keep the system in balance. How successfully a Biological control agent (BCA) can be deployed in an agricultural ecosystem, so as not to damage non target pests, depends on appropriate host specificity tests which determine the potential host range [6].

Another interesting advantage of biological control method is the ability to self-perpetuate. According to Kok (1999), biological control agents will increase in their number and spread. They are self-propagating, dispersing, and self-perpetuating too. This becomes important in relation to the economic feasibility of biological control [9].

An important advantage of biological control method is that the pest is unable or very slow to develop resistance [7]. It is generally not possible for a target pest to develop mechanisms of defense when attacked by a natural enemy [10]. Some examples of defense mechanisms that could develop by pests are escape behavior and repellent chemicals. However, as Van Emden [8] states that “we know of no cases where previously successful biological control has failed because of selection for resistance”.

The adopted and established biological control methods in a specific area can be kept in a place for longer time than the chemical or other control methods which require repetitive application [6]. Hence, biological control methods are also cost effective as they are needed to be applied only once. The effectiveness of biological control methods is higher than the other control methods and is based on self-perpetuation and self-propagation as mentioned earlier. A small number of biocontrol agents can grow to very high densities and provide continuous control of a pest over a large area. When the cost of deployment of biological control agent is considered in contrast to pesticide applications, biological control is generally less expensive than the chemical control [11]. The financial benefit of biological control is greatest in cases when there is no other option. Another interesting point regarding the cost efficiency of this method is that the yield benefit of biological control is probably less than yield achieved by agrochemicals, but the primary cost of biological control agent is generally lower than chemical pesticides [9].

1.2.2 Disadvantages

There are several cases where we also find the breakdown of successfully implemented biological control programmes. The story of the Cane toad introduction in Australia and Harmonia axyridis introduced across continental Europe as a biological control agent is the best example. In 1935 canes toads were introduced in Australia as a biological control method against the Greyback cane beetle damaging the sugarcane crops. However, the management went horribly wrong because the life history and ecology of cane toads were not fully considered before its introduction and the cane toads today have become an invasive pest in Australia. Similarly, H. axyridis (Coleoptera: Coccinellidae), the harlequin ladybird, was from Sible Hedingham, Essex, England, in 2004 [12] and introduced in the areas of continental Europe as a biological control agent against aphids. The individuals dispersed and became invasive from Europe to Britain due to their excellent dispersal abilities as well as some anthropogenic activities also [13, 14].

Therefore, there are also serious disadvantages that restrict the popularity and use of biological control agents to the growers promoting the chemical use. The most important disadvantage is the probability of revenue stability. Reichelderfer [9] well mentioned that the biological control agents are highly proned to environmental conditions rather than the chemical control. As a consequence the pest population is highly fluctuated. And this is a challenge for the growers in relation to the product quality, to the crop yield and obviously to the price of product on market. Moreover, if the annual harvest of a crop is not stable, it will affect grower’s income stability.

Another important disadvantage is the incompatibility with conventional pesticides by the growers. Growers are characterized by lack of patience and chemical control is one of the quick fix to any pest population [6]. Hence, the growers prefer the potent pesticides rather than the biological control method which is a slow process and requires lot of time and patience producing long term effect. Van Emden [8] states the limitation of biological control over subsequent use of pesticides, “where biological control agents are being used against one pest, it is clearly difficult to continue using insecticides against other pests on the same crop or other disease vectors in the same area. This may make the use of biological control impossible”. The slow action of biological control lacks the immediacy of chemical control [6]. This method only reduces the number of pest population; it does not completely wipe out the pest as the chemical control methods. The pests are present in intolerable populations. And the pesticides cannot be used as it will destroy the biological control system. Therefore, shifting to the use of biological control from chemical control is unattractive for growers [8].

Due to the reliability of natural enemies on environmental conditions biological control is often unpredictable. It is well reported in the biological control of whitefly in glasshouses, that a sudden change in weather or a period of extreme hot or cold may lead to a breakdown of the system” [8]. The introduction of natural enemies in a new environment needs to carry out extensive research work to achieve the desirable results against climatic constraints.

Another disadvantage of biological control is that it does not exterminate the pest. As Weeden and Shelton [7] points out that: the general aim of biological control is to depress the pest population below the Economic Injury Level (EIL): i.e., where the costs of the control measures start to exceed those of the extra revenue. When this method is used to control the pests in fresh fruits and vegetable, where certain quality standards are demanded by the consumers, the incomplete pest control is not desirable. And damage of product appearance is, therefore, not acceptable by the growers [9].

Selectivity is a major advantage of biological control method, however, it could also be disadvantageous. Since, natural enemies are species specific, the other pests which are not affected could cause damage, so that the benefit of bio control technique could be extremely eliminated. Reichelderfer [9] has stated that when several insect species of the same general type are potential pests of the crop, the economic efficiency of biological control technique is extremely restricted.

Though this method of control is cost effective, a lot is expensed for its successful implementation in the environmental system. Butt [3] mentions that the lack of infrastructure which facilitates transfer of new technologies and research knowledge to the growers is a major inhibitor factor to its commercial perspective. It is not easy and sometimes expensive too for implementing biological control in field because it requires high qualified scientific staff [7]. The growers generally choose the easy applications of pesticides. There is also relatively less investment in biological control research in compare with chemical pesticides.

Variability in the production batches is also one of the significant disadvantage. “The variation and changes in behaviour of natural enemies that can be caused by rearing conditions are manifold” [15]. This variation arises due to lack of appropriate rearing procedures and often leads to incompatibility. The application of appropriate rearing procedures and the production of high quality biological control agents ultimately increase the cost production of natural enemies. Due to this cause, quality measures in mass rearing are often not applied by companies and consequently production of good quality natural enemies becomes challenging [15].

Even though the biological control method is environmentally safe as it provides less risk of residues in food chain, there are risks associated with disruption of biological control agents in the natural food chain. Kok [6] has reported that “biological control is most suited for exotic pest that are not closely related to indigenous beneficial species”. Thus, the natural enemies must be exotic species too. Lenteren [15] has mentioned few negative effects for the import of natural enemies and many countries deal with risk issues concerning a release of a new natural enemy. However, none of the biological control agent alone provides a completely satisfactory solution to crop pest control problem [4].

Table 2 summarizes the advantages and disadvantages of biological control over chemical/other control methods:

Advantages	Disadvantages
Specific to a particular pest	Can sometimes fail in its specificity
Self-sustaining system	a slow process
Cheap after startup	Expensive at startup
Works most of the time	Does not completely destroy a pest
Higher recycling potential	Requires expert supervision
Capability to withstand polluted water	Often unpredictable
Low toxicity level
Slow development of resistance

Table 2.

Advantages and disadvantages of biological control over chemical/other control methods.

The use of biological agents to control pest population has long history. Biological control has been in practice since ancient times, however, they were not scientifically validated. During ancient times, Chinese, observed that ants were effective predators of many citrus pests. They multiplied and increased the populations of ants and took their nests from surrounding habitats and placed them into their orchards. Thus, the use of natural enemies to reduce the impacts of pests is just a modern adaptation of the original ideas from the history. In this chapter we will discuss the theories of biological control and examine their approaches and applications in the modern pest management.

2. Theories of biological control

2.1 Biological control success and its correlation with the geographical area

2.1.1 Island theory-Islands vs. continental areas

67% of biological control successes which occurred on islands later occurred in continental areas.

2.1.2 Tropical areas vs. temperate areas

All the successes in tropical areas were strengthened by the fact that initial biological control successes occurred on tropical islands. Success is the consequence of latitude and is reinforced on a physiological basis that is explained comparing the Heterodynamous insects and Homodynamous insects as below:

1. Heterodynamous insects

   1. are unable to continuously reproduce throughout year.

   2. generally have disconnected generations, i.e., all their stages may not be present at any one time.

   3. require conjunction between natural enemy and pest.

2. Homodynamous insects

   1. are capable to reproduce throughout year.

   2. have synchronized or connected generations and all their stages may be present at any one time.

   3. are easiest species to control with biological method.

2.2 Introduction strategies

2.2.1 Pros and Cons of multiple introductions

1. Pemberton & Willard [16] gave the theory that multiple parasitism was destructive. Natural enemy species may be pulled down by superior natural enemies when in competition.

2. The above theory of Pemberton & Willard was disapproved by H. S. Smith who stated that biotic potential is solely an indicator for the success of a parasitoid and competition between parasitoids for the same host will subsequently result to increased host mortality rather than natural enemy individually.

3. Multiple parasitism could be detrimental under two theoretical situations:

   1. Parasitization of host (with overlapping generations) giving rise to disconnected generations because of the eliminated host stage.

   2. Parasitization of host by incapable parasitoid that may lead to reduced intraspecific competition between individuals of the host species at high densities.

4. Advantages of multiple parasitism.

   1. Enhanced effective biological control may be achieved over complete geographical range of the host.

   2. Increased death of the host with a single natural enemy alone.

   3. Higher chances of introduced natural enemy to utilize other hosts when primary host population is low.

   4. Invasion on all host stages (sequence theory).

Biological control could be controlled by adding and subtracting predators.

Larval ectoparasitoids (e.g., Diglyphus spp.) and larval-pupal endoparasitoids (e.g., Chrysocharis oscinidis, Ganaspidium utilis) may interfere with each other when present in a cropping system. Larval ectoparasitoids which parasitize the leafminers, also indirectly harm the living endoparasitoid larva, already present in the leafminers, thus, leading to the death of the endoparasitoid. However, the endoparasitoids do not parasitize the leafminers with ectoparasitoids because if they do so, the parasitized hosts will not pupate, so the endoparasitoids can complete their life cycles. Therefore, numerous species may be needed to biologically control the leafminers in various crop systems.

2.2.2 Single introductions vs. multiple introductions

1. Turnbull and Chant [17] again gave the theory that single species introduction is best for biological control.

2. To gain a desired level and effective biological control result of 2nd or 3rd natural enemy species are valuable.

3. The lack of predictive theory is a major problem to implement biological control as the outcome of introduced natural enemy, with no prior history of classical biological control efforts, may not be assumed or predicted for new exotic species.

2.2.3 The “Sequence Theory”

1. Howard and Fiske [18] more elaborated the above theories. The authors stated that biological control would be more efficient if only one developmental stage of the pest species is attacked. Biological control should be achieved through a variety of natural enemies attacking several developmental stages of the host, making a sequence of natural enemies for satisfactory control.

2.2.4 Time factor with respect to expected results from introduced biological control agent

1. Curtis P. Clausen (1951) highlighted the time factor theory for introduction of biological control agents. He stated that an effective natural enemy might be expected to show evidence of control at the point of release within a period of 3 host generations or 3 years and concluded the following:

   1. A fully effective natural enemy is always easily and quickly established.

   2. If failure occurs, it indicates that the control will not be fully effective even after establishment is achieved.

   3. Further, the colonization of exotic species, to be used as biological control agent, may be discontinued after 3 years, if it fails to establish.

2.2.5 Hyperparasites elimination prior to parasitoid introduction

Hyperparasitism can significantly decline the ability of parasite to control the host. Therefore, their elimination prior to the parasitoid introduction must be of prime consideration in introduction phase of biological control agent.

Geographic races should be neglected.

3. Approaches to biological control

The use of biological control suppresses the pest populations, making them less damaging than they would be. They play an important role in limiting the densities of potential pest and include natural enemies such as predators, parasitoids, and pathogens. There are three general approaches to biological control; importation, augmentation and conservation of natural enemies (Figure 1). These have been accepted as an effective, environmentally non-degrading, technically appropriate, economically viable and socially acceptable method of pest management. Each of these techniques can be used either alone or in combination in a biological control program.

Figure 1.

Three general approaches to biological control.

3.1 Importation

Importation of natural enemies, is also called as classical biological control. It refers to the planned introduction of an exotic biological control agent for permanent establishment and long-term pest control to an area that is invaded by pest. Its target is to restore the balance between pest and natural enemy populations in the area invaded by pest without its natural enemies [1].

The import of pests either accidentally, or in some cases, intentionally in any countries where they are not native is continuous. However, due to a lack of natural enemies to suppress their populations, these introduced organisms of exotic origin may become pests, In these cases, importation of natural enemies can be highly effective [19]. Following the identification of the country of origin of the imported pest, a search may be conducted to explore a promising natural enemy. If the natural enemies are identified, their potential impact on the pest in the native country may be evaluated and imported into the new country for further study. Natural enemies are imported into the country under permit by the concerned authorities. The introduced natural enemies are first placed in quarantine for one or more generations to ensure the accidental importation of undesirable species (diseases, hyperparasitoids etc.). Further permits are required from concerned authorities for shipping to different states and field release.

The alfalfa weevil, Hypera postica (Gyllenhall) is a native of Europe. The species was introduced and detected in several countries. The first introduction was detected in the US in Utah in 1904. Biological control of this pest is best example of a successful program using importation of natural enemies [20].

3.2 Augmentation

Manipulation of natural enemies for enhancing the effectiveness of biological control is termed as augmentation. This can be adopted by one, or both, of the two general methods as given below:

1. mass production and periodic colonization; or.

2. Genetic enhancement of natural enemies

The first one mass production and periodic colonization is most commonly used. The natural enemies are produced in insectaries, and thereafter, released either inoculatively or inundatively. Augmentation is used where populations of a natural enemy are not present or cannot respond quickly enough to the pest population. This approach, therefore, does not provide permanent solution for the suppression of pests, as the outbreak of pest may occur with importation or conservation methods.

The use of the parasitoid wasp, Encarsia formosa Gahan, to suppress populations of the whitefly, Trialeurodes vaporariorum is one of the best example of the inoculative release method [21, 22]. The whiteflies are a global pest of vegetable and floriculture crops that is very difficult to manage with pesticides. Immediate release of E. Formosa following detection of the first whitefly on the crop effectively prevent the populations from developing to damaging levels. The releases should be made in context of an integrated crop management program taking into account the low tolerance of the parasitoids to pesticides.

3.3 Conservation

When we are to introduce any biological control attempt, conservation of natural enemies is key element for successful effectiveness. The factors which may limit the effectiveness of natural enemy must be identified, further modifying them to enhance the effectiveness.

This approach may be adapted by two ways as follows:

1. reduce the factors which interfere with natural enemies or

2. provide the resources that natural enemies need in their environment

Several factors are responsible for reducing the effectiveness of a natural enemy. Pesticide applications may directly kill natural enemies or have indirect effects through crop reduction in the numbers or availability of hosts. Cultural practices such as tillage or burning of crop debris may be detrimental for natural enemies by killing them or reducing their population by destroying the habitat. In orchards, frequent tillage may generate dust deposits on leaves, killing small predators and parasites and further, increasing certain insect and mite pests.

A study revealing the biological control of California red scale, Aonidiella aurantii natural (Maskell), suggests that the control may be achieved through periodic washing of citrus tree foliage that increases the parasitoids efficiency [23]. Some host plant effects such as chemical defenses which are harmful to natural enemies but the pest on the host plant is best adapted to it, also reduces the effectiveness of biological control. There are some pests that are able to sequester toxic components of their host, and use them as defense against their own enemies. In such cases also the effectiveness of biological control is reduced. Some cases like physical characteristics of the host plant such as leaf hairiness, may reduce the ability of the natural enemy to find and attack hosts.

Therefore, conservation ensures that the ecological requirements of the natural enemy are reached out in the cropping environment. To be effective, natural enemies may need access to; alternate hosts, adult food resources, overwintering habitats, constant food supply, and appropriate microclimates [24]. In a study reported by Doutt and Nakata [25] Anagrus epos Girault, is the fundamental parasitoid of the grape leafhopper, Erythroneura elegantula. A substitute is needed in grape vineyards for overwintering. This host, another leafhopper, overwinters on blackberry foliage in riparian areas, at some distance from the vineyards. Thus, during spring season the occurrence of early colonization by the parasitoids is often observed in the vineyards near to the natural blackberry. This forms the healthier and preferable biological control. Wilson et al. [26] have reported that the French prune trees also harbor another overwintering host. Their plantation in the upwind of the vineyards will effectively conserve A. epos.

4. Current applications of biological control

Biological control is an interesting science. This control method is constantly incorporating and introducing new knowledge and techniques. This part will deal with different ways by which efficient biological control may be adapted to meet the current pest management challenges.

4.1 Modern approaches in augmentation of natural enemies

Augmentation generally involves mass-production and periodic colonization of natural enemies. This has imparted to its commercial development. Recently, there are hundreds of commercially available biological control products for pest invertebrates, vertebrates, weeds, and plant pathogens.

The practice of augmentation not only differs for importation and conservation or in making a change in an agro ecosystem to improve its efficacy. Rather, this approach seeks to adapt natural enemies to fit into existing systems.

Inundative release of Trichogramma wasps is an excellent example of an augmentative practice is successfully adapted in agricultural systems. These are minute endoparasitoids Their eggs are released on the crops timed to the presence of pest eggs. Trichogramma is highly efficient biological control agent and most widely augmented species of natural enemy. Worldwide, over 32 million ha of agricultural crops and forests are treated annually with Trichogramma spp. in 19 countries, mostly in China and republics of the former Soviet Union [27].

Developed countries such as China, generally follow a simple, low labour cost, innovative technology for agricultural production and pest management systems. They highly use the Trichogramma spp. for the management Chilo spp., populations in sugarcane. The natural enemies are inundatively released and are protected from rain and predators inside emergence packets. Their eggs commercially reared in insectaries are wrapped in sections of leaves and slipped by hand over blades of sugarcane. Trichogramma is mostly produced in localized areas of China.

Implementation of biological control in western countries have to face socio-economics issues for its implementation [28]. Current in large-scale production agricultural systems, some incentives are there on the efficiency and economy of scale. Large industries have developed around the application of agrichemicals, including application equipment manufacturing, distribution and sales, as well as application services. Therefore, biological control products have to compete strongly with pesticides, they should be as effective as pesticides and they should have the capacity to be applied quickly on a large scale with conventional application equipment. So it is expected that the biological agents must have many characteristics same as pesticides.

In Western country such as Europe, commercial marketing of three products utilizing the European native, Trichogramma brassicae Bezdenko, to suppress the European corn borer, Ostrina nubilalis Hübner, in corn fields was almost possible following two decades of intensive research [29]. Annual application of these products in approximately 7,000 ha, 150 ha, and 15,000 ha is carried out in Switzerland and Germany, Austria and France respectively. All the three products are manufactured in plastic or paper packets for safeguarding the wasps against weather extremes and predation until their application in the field.

Trichogramma products are mostly manually applied to crop fields. With the exception of Trichocaps, which may be disseminated either by hand or by aircraft using conventional application equipment. Their packets are walnut-shaped cardboard capsules (2 cm. diam.) and contain approximately 500 parasitized Mediterranean flour moth, Ephestia kuehniella Zwolfer, eggs [30]. Developing Trichogramma inside capsules are induced into an overwintering (diapause) state in the insectaries. These are then stored in refrigerated conditions for nine months without loss of quality. By this system, production of Trichogramma product during winter months may be possible. The product may then be distributed to growers when needed in the summer.

When the refrigerated Trichogramma is removed from cold storage, it will start its development inside the capsules and begin emergence approximately 100°C. It is required to control this ‘reactivation’ process for uniform emergence of Trichogramma, at different developmental stages, in the fields. The companies only make planning and preparation of the product for its application. The growers are only responsible for applying the product to crop fields.

4.2 Landscape ecology and the conservation of natural enemies

The land disturbance studies and its effects on insect community dynamics as well as the emergence of the discipline of landscape ecology have imparted the way to think about the conservation of natural enemies. Since last 20 years, ecologists have recognized the central role of disturbance in the structuring of ecological communities [21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32]. Among the various ecosystems, the terrestrial ecosystems is highly disturbed one and this ecosystem experiences one disturbance event every several years (e.g. fire in grasslands), however, in agricultural ecosystems multiple events occur in each growing season (plowing, planting, nutrient and pesticide applications, cultivation and harvest) and their outcomes may be anticipated from an ecological point of view [33]. Highly disturbed systems exhibit reduced species diversity and short food chains, resulting to well adaptation of the species (i.e. pests) which have only few natural enemies to reduce their populations. Therefore, the role of additional disturbance events, such as pesticide applications, is needed to be initiated that controls the initial negative symptom, may also precipitate its reoccurrence.

Due to increasing reliance on mechanization and pesticides, diversity in farmlands has rapidly disappeared and the impacts on natural enemies must be studied and understood [34]. Increased habitat fragmentation, isolation and decreased landscape structural complexity destabilizes the biotic interactions which serve to regulate natural ecosystems [35, 36]. Therefore, this current systems of crop production (mechanization and use of pesticides) shape the physical structure of our agricultural landscapes [37].

The goal of an ecological approach to conservation biological control is just to modify the intensity and frequency of disturbance to a point where the natural enemies can effectively function. This requires its occurrence in field, farm and larger landscape-levels. Few modifications of tillage intensity and frequency (reduced tillage or no-tillage) in fields leave behind increased plant residue on the soil surface and have a positive influence on the predators (ground beetles and spiders) as well. Similarly, intercropping may be also modified, changing the microclimate of crop fields will make them more favorable for the parasitoids [38].

When taking at the farm level, the presence and distribution of non-crop habitats can be dangerous for natural enemy survival. Eriborus terebrans (Gravenhorst) is a wasp which parasitizes European corn borer larvae. They grow at moderate temperatures and require a source of sugar (nectar of flowering plants or aphid honeydew). But they are unable to met these demands in a conventionally managed corn field. Therefore, wasps seek more sheltered locations in wooded fence rows and woodlots where they find reduced temperatures, higher relative humidity and abundant sources of adult food. Besides, they also parasitize European corn borer larvae in corn field edges near their habitats at two to three times the rate of those in field interiors (up to 40%) [39]. Thus, the current research is creating natural enemy resource habitats and examining the potential of modifying corn production systems to increase natural control of European corn borers. Intercrops, strip crops, as well as modification of grass waterways, shelterbelts, buffer and riparian zones are some of the promising techniques in this regard.

Now at the landscape-level, the physical structure of agricultural production systems can have an impact on the pest and natural enemy diversity and abundance. Ryszkowski et al. [34] has reported in his study in the mosaic landscapes that natural enemies are highly dependent on refuge habitats than are pests and the greater abundance of these refuges in the mosaic landscapes resulted in their higher diversity, abundance and ability to respond to prey numbers. Further studies have also revealed enhanced parasitism of true armyworm, Pseudaletia unipuncta (Haworth), in structurally-complex versus simple agricultural landscapes. The parasitism in the complex sites was three times higher than in the simple sites (13.1% versus 3.4%). This differences was attributed to the abundant population of one wasp species, the braconid, Meterous communis (Cresson) in the complex sites.

Earlier, conservation was endeavored with introduction of one species at a time, concentrating to fulfill the needs of natural enemy in a particular system. Though it is a useful approach, now it seems possible that basic ecological theory could inform the design and management of landscapes to conserve and enhance the effectiveness of entire communities of natural enemies.

5. Conclusion

Biocontrol is a progressive and environment friendly way to control the pest. It leaves behind no chemical residues that may have a harmful impact on humans or other organisms. Importation, augmentation and conservation of natural enemies form the three basic approaches to biological control. Specified techniques underlying these approaches are developing at constant and are modified to meet the switching requirements of pest management. Modifications and improvements in rearing and application techniques and genetic advancement of natural enemies have increased the effectiveness of biological control agents. Further, application of new ecological theory is transforming the research need at conservation of natural enemies. For its successful implementation with full potentiality, continued refinement and adaptation of approaches and applications are necessary. Additional burden from the consumers and the expanding organic market requirements for biological control, come up with advantageous conditions for future development of the biological control agents in agriculture.

Conflict of interest

The authors declare no conflict of interest.