Eco-friendly Pest Control: A Biological Approach
Application of agrochemicals in the modern agricultural practice is important for the control of a large number of insects, pathogens and weeds to improve crop yields and to ensure food quality and safety. In a study it has been estimated that among the total crop losses due to pests, weeds alone causes losses to the tune of 28% followed by diseases (25%) and insects (23%) (Fig.1). Intensive and extensive use of pesticides is an inevitable response to protect losses in yield. It is now well recognized that an excessive and indiscriminate use of pesticides can lead to many problems, such as, environmental contamination, ecological imbalances, pest resistance and resurgence, outbreak of secondary pests and human health risk. Considering these problems, eco-friendly approaches to control insect-pest-diseases are gaining importance worldwide even by the farmers. Since the beginning of agriculture, indeed long before then, plants co-evolved with pest and with their natural enemies as well.
As plants developed inherent protective mechanisms against pests they were helped by numerous partners in the ecosystem for example:
- Beneficial insects that predate crop pests and mites either by chewing them up or sucking out their juices
- Beneficial parasites and parasitoids which commandeer pests for habitat and/or foods
- Disease-causing microorganism including fungi, bacteria, viruses, protozoa and nematodes that fatally sicken insects
- Insects that feed on weeds
- Beneficial fungi and bacteria on root surface that block attack by disease organisms.
By integrating these natural strategies, pest can be managed in a way that is healthier for the environment and eliminates many of the problems associated with agrochemicals use. The major advantages of eco-friendly pest control through biological means are target specificity, comparatively cheaper with no or very few side effects as compared synthetic chemical pesticides; lack or residues on the marketed products and lack of phytotoxicity on plants. Bio control agents are unlikely from the angle of development of resurgence and secondary pest out break; generally no waiting periods are required between application(s) and harvesting crop; release of natural enemies much more comfortable to spray personnel than applying chemicals and last but not the least biological control agents are very effective in many inaccessible areas.
The use of natural enemies to manage pests is biological control. The natural enemies may be a predator, parasite or disease causing microganisms that will attack the insect-pests.
Biological control is form of enhancing natural defenses to achieve a desired target. It usually involves raising and releasing one insect to have it attack another almost like a “living insecticide”. Usually, few important terms viz., ‘predator’, ‘parasite’ and ‘pathogen’ are highly connected to this aspect. Predators are large, free-living and consume a number of preys during their life cycle. They are not generally specific to a particular species for their feeding. Examples are Lady bird beetle, green lacewing, rove beetle, spiders etc. Insect predators fall into one of two groups depending on their mouthparts. Most species have “cutting and chewing mouthparts.”
These predators typically eat most of their prey by chewing. Other predators have “piercing-sucking” mouthparts, to suck the prey’s body-fluids. Insects at all life stages can be attacked by one or more predatory species. Praying mantids, both the immature (i.e., nymph) and adult stages, feed on caterpillars, green lacewings (grubs) and lady bird beetles (both grubs and adult stage) on aphids and other soft-bodies insects like jassids, leafhoppers, whitefly, mealy bugs, small caterpillars, insect eggs etc. larvae of hover flies (syrphid fly) on aphids and soft-bodied insects, ants on insect eggs and small insects. A major group on non-insect predators are predacious throughout their lives.
Parasitoids, unlike predator, complete their development in a single host whereas the adults are free-living. They generally attack the host larger than their body size. The parasitoids that are having high host searching ability and high largest and stage specificity can be used as a good biocontrol agent. Trichogramma sp are the widely used egg parasitoid, similarly, bracon spp., Cotesia spp., Pediospilus pupivora is the example of pupal parasitoid. Insect parasitism can be again classified as simple parasitism where only one parasitoid attack on the host irrespective of the number of eggs laid; Super parasitism when many individuals of the same species of the parasitoid attack a single host; multiparasitism where different species of parasitoids feed on the same host at a time; and hyperparasitism observed when a parasitoid is attacked by another parasitoid.
Insect pathogens (Entomopathogens) are the microscopic parasitic organisms that invade the host’s body and cause disease to the insects. These includes bacteria (Bacillus thuringiensis, Bacillus papillae), Virus (Nuclear Polyhedrosis Virus, Granulosis Virus), fungi (Beauveria bassiana, Metarhizium anisopliae, Varticillum lacanii, Nomuraea rileyi) (fig.6) nematode (Steinernema spp). Pathogens gain entry into the insect’s body through two main routes: directly through the insect’s “skin” or orally when the insect eats contaminated plant parts. Fungi invade through the “skin” from a spore that adheres on the host’s body. Eventually, the host becomes filled with a growing mass of “hyphe” that turns the host “fuzzy” and “rigid”. According to reberts (1981) and Samson et al. (1988) most of the netomopathogenic fungal disease development involves the following nine steps:
- Attachment of the infective units like conidia or zoospore to the insect epicuticle.
- Germination of the infection unit on the cuticle.
- Penetration of the cuticle either directly by germ tubes or by infection pags from appresoria.
- Multiplication of the yeast phase-hyphal bodies in the haemocoel.
- Death of the host.
- Growth in the mycelial phase with invasion of virtually all host organs.
- Penetration of the hyphae from the interior through the cuticle to the exterior of the insect.
- Production of infective units of the exterior of the insect.
- Omission or failure of these fungi at first four steps will produce low virulent fungi through they might have high toxin biosynthetic capability.
Bacteria and viruses enter through the host’s digestive system after the host has eaten contaminated food materials. Once inside the host body, these pathogens rapidly multiply, and eventually liquefy the host’s internal organs. Due to their microscopic size, pathogens are most often noted for the disease they cause and the changes in the insect’s body after infection. Insects at all life-stages can have one or more associated diseases.
Some of the flowering plants that attract natural enemies are: Fennel (Foeniculum vulgare), family, Sunflower (Helianthus spp) of Compositae family, legumes like Alfelfa (Medicago sativa), Sweet clover (Melilotus offcinalis) and Mustard (Brassica spp.) of brassicaceae family. These crops attract the natural enemies and there by their populations are increased in the main field. So, Iconservation of these ‘insectary plants’ that lure the beneficial insects are now-a-days advocating by plant protection scientists. Thus, one of the general approaches to ecological pest management is to conserve these ‘beneficial’ by adopting practices that are friendly to natural enemies. Some of the practice that enhance natural enemy effectiveness are summarized below:
- Diverse sources of nourishment to natural enemies may be provided by planting mixed annual flowers which can provide nectar and pollen. A permanent bed of perennials may be maintained and planting ‘banker plants’ in the farm to provide shelter for beneficial insects. Also ground covers/cover crop and/or crop residues may be maintained to supply over-wintering sites for natural enemies.
- Application of sugar-water or protein sprays to attract/maintain natural enemies.
- Diversification of crop planting using intercropping, mixed cropping, relay cropping etc.
- Altering harvest and/or cultivation practice to maintain ‘refuge strips’ for natural enemies.
- Planting flower/using cultivars that provide pollen and nectar sources.
- Providing shelters or avoiding destruction of nests of social wasps.
- Avoid to use broad spectrum pesticides that are detrimental to the natural enemies.
- Control measures may be implemented only when it is economically and environmentally justified as some plants can withstand considerable amount of injury from pests before yield is affected.
Eco-friendly pest management is the use of natural enemies usually called ‘beneficial’ to reduce, prevent or delay outbreaks of insects, nematodes, weeds or plant diseases. These ‘beneficials’ can be effectively exploited through identification, understanding their individual biological cycles and changing farm management to enhance their population. The aim is to hold a target pest below economically damaging level but not to eliminate it completely, since decimating the population also removes a critical food resource for the natural enemies that depend on it. Classical biological control is the importation and release of beneficial insects against exotic pests, and conservation biological control is the conservation of augmented natural enemies and the ones that are already present in and around their field. The future looks forward to the conservation of natural enemies.
- Robert, D.W. 1981. Toxins of entomopathogenic fungi. In: H.D. Burges (ed.) Microbial Control Of Pest And Plant Diseases 1970-1980. Academic press, London, pp. 441-464.
- Samson RA, Evans HC and Letg, JP 1988. Atlas of entomopathogenic fungi. Springer, Berlin Heidelberg, New York.