Man has inhabited the earth for about 1 million years, but was a hunter-gatherer for nearly 9,90,000 years, depending exclusively on the natural ecosystem. About 10,000 years ago, humans began to settle and cultivate crops. This required that the diverse natural vegetation be removed and usually replaced by few species of crops.
The loss of diversity and stability of the natural ecosystems also destroyed a major part of the natural fauna, while enabling a few species of insects, pathogens and weeds to survive and multiply in the new agro-ecosystems. These species began to compete with man for food, feed, forage, fibre, etc., and were, thus, described as pests.
A pest is any organism whose population increases to the extent that it starts causing annoyance, inconvenience or injury to man, his animals, plants and material possessions. A pest may be an insect, mite, nematode, bird, rodent, fungus, bacterium, virus, weed or any other organism.
The pest status of a species is determined by a number of criteria, viz., increase in the number of individuals, change in the type of damage, more pronounced damage due to conducive climatic conditions, changes in the methods of cultivation or harvesting, and fluctuations in the market value of the crop.
Pest infestations are sometimes referred to as epidemic or endemic. When the infestation occurs in a severe form in a region or locality at a particular season or time it is known as epidemic. If the infestation is a regular feature and confined mostly to a particular area or locality, it is referred to as endemic.
The outbreak of an animal pest or a disease or the spread of noxious weeds quiet often becomes a major concern of the forester, the farmer or public health custodian. Two major classes of pest outbreaks have been identified, viz., eruptive and gradient. The eruptive outbreak spreads out from local epicenters to cover large areas.
The eruptive outbreak is self- perpetuating, i.e., once initiated positive feedback processes operating at high population densities maintain the outbreak. On the other hand, gradient outbreak does not spread from local epicenters to cover large areas.
It is not self-driven and is entirely dependent on external environmental or internal genetic conditions. This type of outbreak arises and subsides as its driving forces change in time and space.
There are three categories of pests:
(i) Regular Pests:
These are generally found in abundance during a crop season, e.g. aphids, jassids and thrips.
(ii) Sporadic Pests:
These assume pest status occasionally in certain years and include locusts, grasshoppers, hairy caterpillars, crickets and cutworms.
(iii) Potential Pests:
These pests normally cause negligible damage but may become highly destructive resulting from some disturbance in the environment and the consequent increase in their number, e.g., armyworm on wheat.
Attainment of Pest Status:
A species may attain the status of a pest by the following ways:
1. Entry into a New Habitat:
A species may become pest by entering into a previously uncolonized habitat, e.g., introduction of San Jose scale and cottony cushion scale of citrus into India; gypsy moth and European corn borer into North America; and insect pests of Eucalyptus into New Zealand and South Africa.
2. Changes in Species Characteristics:
A species may assume the status of a pest due to certain changes in its characteristics that did not previously compete directly with man, e.g., the sudden appearance of previously unknown and only recently involved species of cynipid wasp on chestnut plants in Japan.
3. Change in Habitat:
The change in the habitat of the species or in the activities of man himself may increase his sensitivity to the species, i.e., increased urbanization in India has provided conditions conducive to breeding of Culex quinquefasciatus Say and filariasis has thereby spread to new areas.
4. Increase in Abundance:
A species whose interactions with man were previously negligible because of low numbers, may increase in abundance and assume the status of a pest. Such an increase may result from the greater supply of a limited resource, decreased severity or frequency of repressive environmental factors or by the simultaneous occurrence of both these changes.
The introduction of a susceptible and nutritious host plant into the environment of a species may lead to its multiplication and abundance, e.g., the Colorado potato beetle attained pest status with an increase in the crop area in North America.
Similarly, cotton jassid was not so serious a problem on the indigenous cotton, Gossypium arboreum in India, but with the introduction of American cotton, G. hirsutum, in the beginning of the century, this insect became the dominant pest of cotton.
It implies that the general evolutionary processes of nature (including man’s influence) provide situations for an organism to assume the status of a pest. It may not be possible for man to direct the evolutionary processes but the ecological events can be modified to his advantage. About 10,000 years ago, before man started living in communities and began to organize agriculture, the vegetation on earth and the animals it supported, presumably, lived more or less in a natural balance.
The supposition may be supported by the fact that the perennial forests today, untouched by man’s influence, exhibit a balanced ecosystem. It can be said that the plants and the animals in a stable ecosystem live and die fitting in the energy cycle like cogs of a wheel and no one species increases in numbers out of proportion to cause a threat to the others.
Due to geological upheavals or due to interference by man, however, conditions can change and in the process of adjustment to the new climatic and other conditions, many species might be unable to adjust themselves and, hence, become extinct; numerous examples are available in literature.
Another agency causing upheavals is man himself. To provide more and more food to the increasing population, man had to cut down forests and clear the savannas to bring more land under cultivation. In this process, he might have disturbed the balance of nature with the result that original fauna receded to whatever natural vegetation was left.
On cultivated lands, a faunal vacuum was created as new niches were formed in the newly established agro-ecosystems. Perhaps, the herbivora that survived were those which could do well on the crops grown or on the original vegetation that persisted.
In general, the destruction of vegetation and the cultivation of soil provided harsh conditions to those animal species, because they were exposed to weather in various developmental stages. However, some of the indigenous species that lived on weeds might have found the succulent and high yielding crops very attractive and consequently they multiplied at a fast rate.
Gradually, their population probably increased to the level of a regular pest. In time, they established their own less stable balance with the new environment of agro-ecosystems at a higher level of population which often rose above the economic threshold.
This level might have been reached every year or once every few years and the economic threshold varied according to the nature of the injury. For instance, a few dozen aphids might go unnoticed on one type of fruit or vegetable, whereas a single caterpillar boring into an apple may make it unfit for marketing.
While growing crops, man either practised monoculture-growing the same crop season after season-or mixed cropping, i.e., growing a number of crops in a locality in rotation with one another. These two practices presented entirely different situations. Under monoculture, the same crop was available in a season over large stretches of land.
The favourable food was well suited to the fast multiplication of a phytophagous animal feeding on it and, year after year, its population was built up. Naturally, such a species either synchronized its life-cycle with the host crop and did not require food throughout the year or was able to sustain itself in an off-season in lower numbers, on weeds or on other vegetation of the original ecosystem that was still available in uncultivated fields, along the hedges, in forest reserves, etc.
Under multiculture conditions, the sequence of cropping provided was such that for a given pest species, food was always available in greater or smaller quantity depending upon climatic and other conditions, being favourable or unfavourable to the development and multiplication of the species.
Invasive Pests in Agriculture:
Global trade has brought prosperity and benefits to societies around the world, much of which would be impossible without the exchange of goods. However, the movement of commodities through such trade has provided pathways for many insect species to spread and colonize new areas.
Alien species are non-native or exotic organisms which occur outside their natural adapted habitat. In contrast, invasive species is any organism that is outside of its native geographical range that may or has become injurious to animal or human health, the economy and/or natural environment.
In other words, alien species with potential to threaten ecosystem habitats or species, which establish a viable population in an area where it does not occur normally, is known as invasive species. The invasive insects, pathogens and weeds cause annual crop losses to the tune of US$ 58 billion.
It is well-known that in agriculture, crops have been introduced from far-off places, even from different continents. Many times pests were introduced from the homeland along with plant materials or with the household goods. In the early stages, the introductions were due to neglect and, in more recent times since the imposition of quarantine, due to accidents. These species were either known pests in the site of origin, or they were of no consequence there and became serious pests in the new country where there were few natural enemies.
San Jose scale, Quadraspidiotus perniciosus (Comstock), is an example of the former category, being introduced into southern Asia from the United States of America. The Japanese beetle, Popillia japonica Newman is of the latter type. It was quite a harmless insect in Japan but when it was introduced into the United States, it became a serious pest of many economic crops and plants.
Another example of invasive pest which became threat to wine industry is grape phylloxera, Daktulosphaira vitifoliae Fitch, which was introduced from USA in France in 1859. Introduction of boll weevil, Anthonomus grandis Boheman from Mexico at the beginning of the 20th century resulted in billions of dollars damage and almost complete eradication of the cotton crop in US.
The import of potatoes and associated packing material lead to invasion of Colorado potato beetle, Leptinotarsa decemlineata (Say) in Germany from USA in 1874, due to which the importation of potatoes was banned. The fire ants, Solenopsis invicta Buren were introduced in US in 1930-40s, as stowaways in cargo shipped from South America range. The estimated costs of control, medical treatment and damage to property due to this insect in US alone are greater than $6 billion annually.
The East Asian sawfly, Aproceros leucopodci Takeuchi, a pest of elm trees, was earlier detected in Poland and Hungry. It invaded into Italy in 2009 through the road traffic. The completely defoliated elms were observed in motorway parking areas where cars and trucks often stop in transit from central Europe to Italy.
In India, woolly apple aphid, Eriosoma lanigerum (Hausmann) was introduced during 18th century with imported rootstock from China. San Jose scale, Quadraspidiotus perniciosus (Comstock), a native of China reached India in 1911. Cottony cushion scale, Iceiya purchasi Maskell, was accidently introduced from Australia into India in 1921.
Potato tuber moth, Phthorimaea operculella (Zeller), was introduced with imported potatoes from Italy to India in 1937. During transit of coffee from Sri Lanka, coffee berry borer, Hypothenemus hampei Ferrari, was introduced in India in 1990. The polyphagous serpentine leaf miner, Liriomyza trifolii (Burgess), accidently invaded India in 1990-91 and caused infestation of several host plants like cucurbits, tomatoes, castor, ornamental plants, etc.
In an era of globalization, we cannot survive without the global trade, but the risk of invasive species can be limited by the implementation of various international/national agreements governing trade. One of the agreements is Sanitary and Phytosanitary Agreement, which deals directly with the trade related invasive species risk. Similarly, many countries have developed various organizations and quarantine programmes to check the introduction of invasive insects.
The preventive measures like quarantine, trade regulations, inspection, etc., have been identified as quite effective means to reduce introduction of invasive pests. However, more global coordination and cooperation is still required for effective implementation of various international agreements and national policies to cope up with the problem of invasive pest species.
Changing Status of Pests:
Agriculture in India started around 2000 BC during the period of Indus valley civilization in Sind and West Punjab, and spread to East Punjab, West U.P., Rajasthan and Madhya Pradesh. At around the same time, agriculture was also started in South and East Asia. The implements used during this period included plough, seed-drill, unwheeled cart, etc.
The only references regarding pest problems during this period pertain to wild birds and animals, which were scared away by use of terracotta sling-balls. The advent of Aryans brought the iron technology but there was still no indication of any pest problems. Only around 900 AD, locusts are listed along with birds and mammalian pests as causing damage to crops.
In ancient Indian literature, there are indications of a sound study of useful insects but the knowledge about harmful ones in the same period was expectedly poor. Although pest problems were quite serious, the knowledge about their nature and control was very inadequate. This situation continued almost till the end of the nineteenth century.
Traditionally, the crops were grown only during the monsoon period, and winter served as a closed period for crops as well as for pests in India. With no assured irrigation and little use of chemical fertilizers, the yields obtained were also low. However, even at that time a number of insect pests were known to cause serious damage to different crops.
Balfour in 1887 in his book Agricultural Pests of India and of Eastern and Southern Asia, reported many important groups of pests like cutworms, aphids, bugs, caterpillars and beetles damaging different field crops, viz., Leptocorisa and greenfly on rice; sugarcane borer, Diatraea saccharalis (Fabricius) and sugarcane bug on sugarcane, and pink bollworm and Helicoverpa on cotton.
Harold Maxwell Lefroy, who was appointed Entomologist to the Government of India in 1903, published a book on Indian Insect Pests in 1906, wherein he described pests injurious to cotton, rice, wheat and other crops. T.B. Fletcher, who succeeded Maxwell Lefroy as Imperial Entomologist (1913-1932), produced major treatises on cotton bollworms, sugarcane borers and stored grain pests.
Fletcher in 1920, at the Third Entomological Society Meeting presented a comprehensive list of several hundred species of insects associated with different crops in India. Of these, 45 species were reported as major pests on important crops. These included 8, 7, 7, 6, 6 and 1 species causing serious damage to paddy, cotton, oilseeds, pulses, vegetables and sugarcane, respectively.
Following the introduction of high yielding varieties (HYVs) and associated technology, there has been a tremendous increase in the number of insect pests damaging various crops, viz., pigeonpea (250), rice (225), sugarcane (215), cotton (170), etc. Many minor insects have started assuming serious proportions, whereas several pests have spread to new areas.
The incidence of brown planthopper, yellow stem borer and leaf folder has increased manifold with the cultivation of modern HYVs of rice. Gall midge, originally being confined to monsoon season, has, of late, become a pest in summer season also. The spread of rice cultivation into the non-traditional north-western states like Punjab and Haryana has brought up the problem of white-backed planthopper.
The continuous cropping of maize in northern parts of India has led to the emergence of shoot fly, Atherigona spp., as important pest in spring sown maize. Pyrilla perpusilla (Walker), a serious pest of sugarcane has developed a preference for hybrids of maize and sorghum. Sorghum midge, earhead bug and aphids are gaining importance in many sorghum growing regions.
Severe outbreaks of whitefly and American bollworm have been observed on cotton during the last decade. American bollworm appeared in serious form in parts of Andhra Pradesh and Tamil Nadu during 1987-88, where it caused about 66 per cent loss in yield of seed cotton.
The pest caused severe losses to cotton crop in Haryana, Punjab and Rajasthan during 1990s. Incidence of this pest on cotton has considerably declined following the introduction of Bt cotton. However, mealybug, Phenacoccus solenopsis Tinsley has started appearing in serious proportions.
A number of polyphagous pests have also attained serious status. Most dreaded of these is Helicouerpa armigera (Hubner), which has been recorded on more than 180 cultivated and uncultivated plant species, of which chickpea, pigeonpea, cotton, tomato and tobacco are considered to be the key source crops. Similarly, Spodoptera litura (Fabricius) has been recorded on more than 115 species of cultivated plants in India.
Insect pest problems in agriculture are probably as old as agriculture itself. However, under subsistence agriculture, the pest numbers were generally low as the productivity was poor. The insect pests were kept under check by cultural and mechanical practices developed by farmers largely through trial and error. Rapidly increasing population during the last century has necessitated intensification of agriculture, which has also resulted in severe outbreaks of insect pests in agricultural crops.
i. High Yielding Varieties:
When altering the genetic make-up of the crop plant to increase yield with little or no attention to pest attack, natural resistance may be lost or greatly reduced. Efforts to breed plants more palatable to human taste by elimination of such factors as bitterness and hairiness may also result in development of more susceptible varieties. Moreover, the fields planted to modern varieties, which are short and heavy tillering, develop a distinctly different microclimate conducive for proliferation of insect pests.
ii. Monoculture:
In agriculture, the natural plant community is removed, destroyed and usually replaced by a single crop species. Larger the area that is planted to a single crop, the greater the potential for pest problems. Generally, same crops are maintained in a given area year after year and this helps in survival and multiplication of the pest throughout the year. Large scale mono-culturing of cotton, paddy, sugarcane and vegetable crops in India in contiguous blocks has resulted in rapid increase in number and intensity of pest attacks.
iii. Nutrients:
Altering the nutrient level in the soil affects the concentration of nutrients in the host plant, which, in return, influence the pests that are feeding on the plant. The HYVs are nitrogen responsive and require extensive use of nitrogenous fertilizers to provide maximum yield. High levels of nitrogenous fertilizers significantly increase the incidence of most insect pests. Increase in soil moisture level due to frequent irrigations also helps in build-up of insect pests.
iv. Pesticides:
The advent of synthetic organic pesticides during 1940s enabled us to gain an upper hand in the struggle against insect pests. However, the large scale indiscriminate use and abuse of these insecticides has resulted in increased attacks of insect pests on many crops.
This may be due to development of insecticide resistance, resurgence of insect pests or outbreaks of secondary pests. The insecticides may kill natural enemies of target pest or potential pest. They may also alter crop physiology to make the plant more susceptible to attack by insects or they may even directly stimulate reproduction of surviving insects.
v. Cultural Practices:
New agronomic recommendations like closer spacings, sowing times, rotations, harvesting procedures, etc. are essentially meant for exploiting the full genetic potential of HYVs, but are simultaneously favourable to most of the insect pests. In cultivated fields, crop plant densities are carefully controlled to obtain the maximum population possible for optimal growth resulting in maximum economic yield. The new plant spacings often result in an ecological situation that encourages pest outbreaks.
vi. Climate Change:
The incidence of insect pests is likely to change as a result of global warming. Current estimates of changes in climate indicate an increase in global mean annual temperatures of 1°C by 2025 and 3°C by the end of next century. The doubling of CO2 is estimated to be attained between 2025 and 2070 depending on the level of emission of greenhouse gases.
Such changes are likely to result in expansion of geographical range of insect pests, increased overwintering and rapid population growth, changes in insect-plant and insect-natural enemy interactions, increased risk of invasion by migrant pests, changes in synchrony between insect pests and their crop hosts, and reduced effectiveness of crop protection technologies.