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  1. Naturally occurring - part of biological occurrence
    e.g. locust and stick insect plagues.
  2. Pests arising from man's increasing demands
    We want higher standards and have less tolerance. Blotches on fruit are no longer acceptable, ants in kitchens or bush flies are not acceptable.
  3. Pests arising from man's alteration to the ecosystem
    1. Plant and animal breeding - higher yielding crops and livestock can have lower resistance to insect attack. E.g. merino sheep have wrinkled skin for greater fleece yield but these skin folds are condusive to attack by sheep blow fly.
    2. Introduction of new species
      • 50% of pests in Australia have been introduced from overseas. These species do not have to contend with the predators and parasites from their native origins.
      • Native species attack introduced crops and livestock.
        e.g. bogong moth larvae on cotton
    3. Resource concentration and ecosystem simplification
      Less diversified ecosystems have less stability
      • Storage e.g. grain silos provide a haven for grain beetles
      • Increases and concentration of human population and cities provide easier transport for disease vectors leading to increased possibility of epidemics
      • Monoculture of agricultural crops

      Scattered, nonapparent plants are now easier for insects to find and are in a simplified ecosystem.

      Large food supply and few populations limitations can cause rapid population growths of pests.

      (i)  Increased food supply

      Comparison of virgin land versus wheat field.
        No of species Density (individuals/sqm)
      Virgin 330 200
      Wheat 140 350

      i.e. there is a decrease in the number of insect species in the wheat field but the number of individuals living in the field has greatly increased. This effect is commonly seen in the rise of native pests on introduced crops, e.g. cluster caterpillar in Ord river cotton field and wireworms in crops.

      (ii)  Reduction of predators and parasites

      "Clean farming" with zero tolerance of weeds and debris destroys over wintering sites and alternative food sources such as pollen and nectar in flowering weeds, thus limiting the populations of predators and parasites.

      (iii)  Plant vigour

      Crops are irrigated, pruned, fertilised etc further increasing the food supply for pests

    4. Use of insecticides - getting rid of one pest may lead to rise of another.
      e.g. DDT was successfully used against codling moths in apple orchards however, it also selectively killed a predatory mite which had kept a leaf eating apple mite population in check, leading to the rise of the phytophagous mite as a pest.



This is a complex interaction dependent upon the part(s) of the plant attacked,  the intensity attack, growth stage of plant and environmental factors at the time.

  1. Part of plant attacked
    1. Fruit, tubers, grain etc - (yield forming organs) direct injury
    2. Injury to non yield forming organs - usually stems, roots etc - injury is indirect. Harvest is affected by slowing of growth and reduced photosynthetic ability of crop.
  2. Intensity of injury
    • Greater density of pest leads to greater degree of injury.
    • Stage of development of pest is also important. E.g. Lepidoptera do  damage in larval form only. In particular, noctuid  larvae consume 85%  of its total larval food in the last larval instar.
  3. Growth stage of plant
    1. Seedlings - very susceptible - total crop can be lost
    2. Young plants - tolerant - fast vigorous growth can compensate for pest injury
    3. Stage of formation of yield forming organ ( flowers, developing fruit, tubers etc) - susceptible. Significant loss of harvest and quality.
    4. Plants close to harvest -less significant if damage is to non yield forming organs.
  4. Environmental conditions
    Plants under stress (drought, frost, nutrient deficiency) are less able to withstand injury.


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