NPM-123: Disturbance in Agroecosystems: when insects become pests

Based on the economic importance, insects are classified into

• Harmful insects: insects that feed or damage the objects of human interest e.g. American bollworm, paddy stem borer, white fly, locust etc.
• Beneficial insects: insects that benefit humans by their products or actions for eg honey bees, lac insects etc.
• Neither harmful nor beneficial insects: insects whose functions related to human interests are not known e.g. fire flies

What is a pest?

• Pests: This term is referred to insect pests in general but technically – any organism that hampers the interests of humans economically and or aesthetically or health wise, then they are termed as pests.
• Economic Threshold level (ETL): When insects increase in number to a limit beyond which they can cause economic damage to the crop is called Economic threshold level. This is the boundary of minimum economic loss
• Economic Injury Level (EIL): this is the lowest population density of an insect that can cause economic injury.

Why insects become pests?

• When introduced in new environments where their natural enemies are not present i.e. import of pests along with new crops
• Indiscriminate use of pesticides (pest resistance and resurgence)
• In the absence of natural enemies
• Large scale Monoculture and cultivation of single variety over large stretches
• Environment favourable to the pest but unfavourable to the natural enemies
• Loss of competition due to destructive elimination of non-target pests
• Pest adaptation to selection pressure

Insects are the largest living beings on earth. They constitute about 75 % of the living beings and originated 30 crore years back where as human evolution started 10 lakh years back. During the period even Jurassic animals were extinct but insects persisted because of the

• Ability to withstand a wide variety of conditions: Their bodies are build to withstand harsh weather conditions and also quickly adopt to the changing situations.
• Body construction: they have different shape, size and abilities in their life cycle. for e.g most of the insects have four states in their life as egg, larva, pupae and adult. Each stage has different feeding habit and different lifestyle. In some larvae crawl and feed on plants while adult have six legs, wings and fly all over. Compound eyes which can view a quick moments very efficiently and always stable while on the move due to the 6 legs
• Short lifecycles: Most of the insect life is less than 60 days therefore they multiply faster.
• High fecundity: Each time an insect may lay up to 500 eggs on average. Even if we assume 30% hatch and grow to reach the damaging state their population build up would be very high.
• Flying nature: As the adult have wings they can easily fly away to new places in search of food and that takes the pest every where.

Given these strengths, by now insects might have been covering the entire earth with several layers but for the

• Natural ecological Balance: There is always an ecological balance between different species in nature, pests and non pests, within insects harmful and beneficial, within harmful different kinds of insects etc. these gets distorted either by environmental changes or human action.
• the ingenuity of the farmers: Over centuries of in the evolution of agriculture farmers managed to keep the ecological balance by adapting suitable cropping patterns and crop production practices.

All these got disturbed due to the current models of agriculture which are characterized by

• • Reducing biodiversity: Monocultures of crops and varieties is one of the major factor for increasing pest problem. Vast areas under single crop and that single variety attracts more insects and gives them opportunity to continue their lifecycles without a break. Cotton after cotton, brinjal after brinjal, large area under paddy all are responsible for the high pest incidences in those crops.
  • Reducing Soil Health: The distortions in nutrient use due to chemical fertiliser use, reducing soil fertility cause distortions in pest problems. for eg. high use of nitrogenous fertilisers will result in increase in sucking pests like brown plant hopper in paddy or aphids/jassids/whiteflies in cotton or chillies. Soils also become sick due to changes in electrical conductivity or pH for eg. salinity increase result in more of grass weeds we see in paddy fields, soils also harbour pathogenic fungi/bacteria and result in soil borne disease like wilt in redgram.
  • High use of chemicals which kill natural enemies: High use of agrochemicals also results in killing of natural enemies and non target insects which cause ecological disturbance. pest resistance, pest revival are all a result of such pesticide induced problems read more
  • Resurgence and Resistance caused by selection pressure: Even if insects/disease causing microorganisms or weeds  do not have resistance for a particular chemical initially repeated use builds selection pressure and induce mutation to adapt to the pressure resulting in resistance read more
  • Favourable conditions: Certain favorable conditions also increases the chances of the insect or disease causing organisms or weeds. for eg if the planting is done closely in paddy and light do not reach the bottom of the plants, darkness breeds brown plant hopper (BPH), high humidity due to cloudiness or continuous rainfall or dew/fog may result in increase of diseases.
  • Climate Change: Changes in weather conditions in short run and long run also have resulted in pest distortion. It is observed that high temperatures during last week of may reduce the hatching in helicoverpa (American bollworm) or Spodoptera (tobacco caterpillar) read more