The frequent use of toxic chemicals for the control of pests on the crop plant has been increased tremendously during the last few decades

 NIA, Tandojam.

Nov 15 - 28, 2004





It is a well known fact that insects have been a problem to the farmers ever since agriculture has been started. About 8,000 insects species are injurious to crops of which about 200 are serious pests. Approximately, one third of the world's food supply is being destroyed by pests, while, millions of people are starving and it is fear that developing countries like Pakistan, where there is a shortage of food may never reach their full agricultural potential, until pests depredations are inhibited. Today, with changes in crop production to achieve high yield, pests' species are often favored, particularly by the development of extensive acreages of crop monocultures.

The frequent use of toxic chemicals for the control of pests on the crop plant has been increased tremendously during the last few decades. It was not realized at first that how polluted the environment would become and how these chemicals would become accumulated in the soils. This realization came only when pest with a greater number of severity was evidenced as a result of the death of their natural enemies, so the pest management emerged as a dominant concept by the recommendations of entomologists/scientists that pest should be controlled by integrating the use of biological agents along with the use of insecticides. Based on this concept, Bartlett 1956 coined the term integrated pest control, which was defined as the blending of biological control agents with chemical control measures. Later on in 1961, Geier & Clark advocated the integrated use of all available resources/techniques for the control of insects and not confining only to the biological and chemical methods of control. According to the expert panel of the Food and Agricultural Organization (FAO), the pest management may be defined as, "A system that in the context of the associated environment and the population dynamics of pest species, utilizes all suitable techniques and methods in as compatible manner as possible and to maintain the pest population at levels below those causing economic injury".

Aim and objective: The ultimate aim and objective of scientific pest management is to maintain a low level of pest population, which would not only maintain the damage lower than the economic injury level but also support the growth and survival of its natural enemies. For that reason, the broad-spectrum insecticides should not be used because they often have the effect of eliminating the pest as well as natural enemies. Philosophy of pest management is to maintain the population of a potential pest at a sub-threshold level than to eradicate it. This philosophy is based on the observation that every plant can withstand a level of pest population without showing loss in yield or vigor. To understand these concepts more clearly, quantitative measurements are undertaken which define clearly the degree of damage and allowable damage. The studies include: (a) Economic injury level (EIL): it is the lowest pest population density that will cause economic damage. It is the level at which damage can no longer be tolerated and, therefore, at that point or before reaching that level, it is desirable to initiate control operation. (b) Economic threshold level (ETL): it is the pest density at which control measures should be applied to prevent an increasing pest population from reaching the economic injury level. Control measures are taken at this stage, so that the pest does not exceed the economic injury level. (c) General equilibrium position (GEP): it's the average population density of a pest over a long period of time unaffected by the temporary interventions of pest control. Comparing it with economic injury level, it may be understood that EIL may be at any level from well below to well above the general equilibrium position (GEP). GEP touches EIL & ETL, approximately 2 to 5 years, for many insect pests species due to injurious effect of pesticides or due to favorable weather conditions; such insects are called occasional pests. Sometimes, the control measures are required frequently to bring down the GEP well below the EIL & ETL, such insects are called regular pest. We can determine the EIL, and know when to apply control measures. There is a need to emphasize costs and benefits. In most pest control activities, the benefits are usually not known, because these cannot be measured, hence the cost of prevention becomes the cost of production.

The judicious use of insecticides should be the philosophy of pest management. It is estimated that, generally, 10% of the insecticide applied, reaches the target pest and the rest contaminates environment or cause rnortality of the non target or even useful species; pest resurgence, outbreaks of secondary pests and resistance in pest population are other problems. Many insecticides are highly toxic to vertebrates as well as invertebrates and their use has caused adverse effects on some wild life population, honeybees and other pollinators. Some insects that were of minor importance have attained the status of major pests. Indiscriminate sprays of pesticides have promoted the development of resistance strains of Helicoverpa, whitely and aphids, in the past these pests were not important in this country. Pesticides residues in soil may alter the species composition of the animals living there and may slowdown the process of soil formation and reduce soil fertility. Wrong pesticides have been applied to edible crops, vegetables and fruits even close to harvest time, several of these insecticides have been proved to cause mutations, cancer and birth defects. Studies have shown that pesticides residues are present in meat and milk of livestock and even in the poultry eggs.



In Pakistan, sole reliance on chemicals for pest control has created several problems. Introduction of IRRI varieties of rice seem to have encouraged stem borers and leafhoppers, and delta pine cotton attracted severe infestation of sucking pests and bollworms. The continuous presence of sugarcane in the field due to extensive ratooning and delayed harvests to supply sugar factories up to May have intensified borers damage, moreover, the maize borer is much more severe on the hybrid corn varieties than the local varieties.

According to a conservative estimate, almost 70% of the insecticides are being used on cotton crop. The number of spray applications have risen up to nine in some cases, so heavy outbreaks of previously insignificant pests like mites, Jassid, whitely, aphids, thrips and bollworms complex are being experienced, perhaps a consequence of widespread use of pesticides on cotton killing natural enemies of the pests. An example of collapse of insecticidal control program in Nicaragua should serve as an eye opener for us where due to poor planning and complete dependence on insecticidal control, the growers had to apply 30 or more sprays for getting a good crop. This made cotton an unprofitable crop. Cotton growers in Peru and Egypt faced a similar situation. The reversion to integrated control brought the situation under check.

METHODS OF PEST CONTROL: The various methods of pest control, which are integrated in pest management practices, include the following:

1- LEGISLATIVE METHODS: Restriction upon people and their manufactured products, legislation for compulsory plant and animal quarantines, and regulatory measures play an important role in limiting the distribution of potential insect pests and the disease they may carry. Different legislations are foreign and domestic quarantines to prevent the introduction of new pests from abroad, to prevent the spread of established pests within the country, legislation for notified campaigns of control against pests, and legislation to prevent adulteration and mishandling of pesticides.

2- CULTURAL METHODS: Cultural methods of insects control comprise the regular farm operations so performed as to destroy the insects or to prevent them from causing injury. For the achievement of cultural control, the various agricultural practices can be grouped under the following heads: I- Tilling and cultivating the soil. II- The use of clean seed. III- Regulating irrigation. IV- The use of resistance varieties. V- Manuring and stimulating plant growth. VI- Clean culture. VII- Cropping scheme and trap crops. VIII- Pruning and thinning. IX- The times of sowing and harvesting. X- Destruction of plant residues.

3- MECHANICAL METHODS: These include destruction and obstruction of pests by hand, machine or barrier, these control measure are: I- Hand picking. II- Use of hand nets and bag nets. III- Beating and hooking. IV- Sieving and winnowing. V- Mechanical exclusion bands around the trees, screening the windows, doors and ventilators of house, wrapping individual fruits, trenching the field. VI- The use of mechanical traps cricket traps, house fly traps, light traps, air suction traps, electric traps.

4- PHYSICAL METHODS: The use of physical factors of the environment like temperature, light, humidity: I- Application of heat super heating of empty godowns, exposing infested grains to sun, steaming woolen clothes. II- Application of cold refrigeration at 5- 10oC of all eatable products including dry fruits. III- Manipulation of moisture draining away of stagnant water, reducing the moisture contents of grain below 8%.

5- BIO-ECOLOGICAL PRACTICES: It is the natural resistance of plant types to pests due to their biochemical qualities: I- Non preference and preference comprise a group of host characters and insect responses that lead away from or to the acceptance of a host for shelter, food and oviposition or for all three. II- Antibiosis. It indicates adverse effect on insect development, survival, size, fecundity and life history by a resistant host. III- A plant or host as compared with another is said to be tolerant when it has the ability to grow, reproduce or repair injury while supporting a pest population equal to that of a susceptible host.

Resistance in the plants may be contributed by multiple factors: physical, chemical and physiological. Hence, these characteristics are heritable through hybridization. With pure line selection, the desired factors can be intensified in plants. Efforts have been made to determine the chemical, physical or other factors that contribute to resistance. Through a known gene source, the characters are transmitted to hybrids and then stabilized for their performance. The entomologist and the plant breeder can start work together. After obtaining the seed, further progeny can be segregated be selfing or by crossing with other variety. The usual procedures can then be followed relevant to the crop concerned. A biochemist can join the team later on in order to determine the basis of resistance. At that stage, it become easier for the breeder/entomologist to screen the various progeny lines and eliminate the susceptible or the undesirable one.

6- GENETIC METHODS: It includes propagation and release of sterile and genetically incompatible individuals in a pest population. We can say it an autocidal control. The employment of an insect to destroy its own kinds, or bring about the self-destruction of the species. The male sterilization technique has been successfully used for insect control since mid 1950 on the screwworm fly and fruit fly. It involves the artificial rearing and releasing of sterilized males, which are otherwise physically healthy to compete with natural males in the filed. The females are thus deprived of the opportunity to produce fertilized and viable eggs. Consequently, a certain proportion of the expected population does not appear and pest is suppressed in numbers and eventually may even be eradicated. The sterility principle can be applied in two ways: I- By the direct application of radiations II- By using chemicals known as chemosterilants, which are fed in baits. Regarding radiations, X-rays sterilization effect on insects had been observed as early as 1916, it was found that pupae were irradiated with X-rays within 2 days of adult emergence, a dosage of 2500 R sterilized the males, but a dosage of 5000 R was required to sterilize the females. After the advent of availability of man-made isotopes, it was become much easier and convenient to irradiate insects with gamma rays. The isotopes most commonly used as a gamma source are cobalt-60 with a half-life of 53 years, and cesium-137, with a half-life of 30 years. There is little difference between X-rays and gamma rays in their biological effectiveness in treating insects. The advantage of gamma rays from cobalt-60 sources is that much larger volumes of material can be irradiated at one time and cobalt sources are more economical to operate than X-rays equipment. Chemosterilants: Insect's chemosterilants are chemicals capable of causing sexual sterility in insects to prevent reproduction. About 600 compounds have been screened and more than 300 have shown promise as chemosterilants, the effective groups are alkylting agents, antimetabolites, antibiotics, alkaloids and organotin compound.

7- BIOLOGICAL CONTROL: It is use of parasites predators and other microbial agents, these are: I- Parasitoid. An insect that lives in its development stages in or on another insect to which it kills after completing its own feeding. They include in the insect orders, Lepidoptera, Hymenoptera, and Strepsiptera. II- Predator: A free living organism that feeds on other, commonly smaller living organism (prey), the prey is killed and eaten, these are in the insect orders Odonata, Hemiptera, Neuroptera, Lepidoptera, Diptera, Coleoptera, spider, mites. Agents of biological control: Predators and parasites of pests are grouped under: Vertebrates, Nematheminthes, Arthropods, protozoa and Micro- organisms. Microbial control is pest population management through disease causing microorganisms these are: fungi- Entomophtora, bacteria- bacillus, and virus- polyhedrosis. Management practices for conservation and enhancement of natural enemies are: I- preservation of inactive stages. II- Avoidance of harmful cultural practices. III- Maintenance of diverse alternate host. IV- Natural and artificial food supplements and shelter. V- Control of honeydew feeding ants. VI- Protection from pesticides. Biological control has advantages: Control agents are selective; ecosystem is less affected, no resistance problems, less dangerous to human than pesticides. Procedure for bio control is selection, handling, rearing and release of biocontrol agents.



8- CHEMICAL CONTROL: Use of toxic chemicals and sterilants as well as semiochemicals including attractants, repellents, antifeedants, growth regulators and hormones etc. Insecticides can be grouped according to their made of action: I- Stomach poisons, including systemic poisons. II- Inert dusts or abrasive compounds. III- Contact poisons. IV- Fumigants. V- Chemicals having other types of actions.

Chemosterilants such as apholate, tepa, metepa are fed to the insects along with food and through their physiological action, they cause sterility and ultimately the pest population declines. The chemicals, which deliver behavioral messages in insects, are termed as semiochemicals, these are active in minute dose and are fancy tools for pest management. Insect behavior is modified with regard to search for shelter, oviposition sites or searching the mates to decrease population levels. Inter specific semiochemicals which provide adaptive advantage to the producer are known as allomones, while, those which provide adaptive advantage to the receiver are known as kairomones. The intra specific semiochemicals are termed as pheromones, which include sex attractants. Pheromones are exocrine secretions, which elicit reactions in individuals of the same species like sex attraction, alarm or aggregations: pink bollworm- gossyplure, cotton boll weevil-grandlure, oriental fruit fly-methyl eugenol, melon fruit fly-cuelure, silk worm-bomby col, gypsy moth- dispalure. Repellents elicit avoidance and thus lead the insect pests to move away from the source. Mosquitoes' dimethyl phthalate, flies, fleas' deet, mites benzyl benzoate. Insect's hormones are the compounds, which are physiologically active at low concentration. These are transported through haemolymph or through neural axons. They are released from I- Neurosecretary cells such as brain hormone. II- Specialized endocrine glands such as juvenile hormone, secreted in carpora allata, and molting hormone ecdysone from prothoracic glands. These compounds are known to control insect activities like molting, metamorphosis, reproduction, metabolism and water loss. In pest management, the juvenile hormone is being used for the control of insects by hampering their normal growth and development. They produce morphogenetic and gonadotrophic effects: mosquito-farnesol, housefly-methoprene. Insects enzymes can be used indirectly for insect's control, for example chitinase sprayed in combination with insecticides makes the chitin of insect weak and gives a far better control than that with insecticide alone. The anti-feedants are chemicals, which inhibit the feeding of insects due to their presence on the natural food; death in the end is due to starvation: triazines, organotins, carbamates-baygon. Anti-metabolites: inhibit the utilization of body metabolites. When introduced into the biological system, prevent normal function of nutrients in food.

All these insect pests management approaches are needed to be integrated and implemented effectively. Bringing people to an understanding of pest management is the best to deal with insect pest problems. Effective communication and practical demonstration are the key to successful public understanding and acceptance.