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Environmental Factors

Environmental factors affect insect populations. A limiting factor exceeds the tolerance level for a species, e.g. low winter temperature in ticks. Determining a limiting factor usually requires lab studies as there are many factors at work in the field.

WEATHER

Important in the distribution of insects -daily conditions more important than averages.

A. Light

B. Temperature

Body temperature of insects governs rate of growth. Since insects have limited control over body temperature, outside temperature is important.

Most insect activity and reproduction occurs between 15 -35o C but each species usually has an optimum temperature.

Note that:

C. Wind

D. Moisture

E. Interaction between temperature and moisture

A = region of most rapid development
B = region of favourable development
C = region of retarded development
D = region of no development

It can be seen that development is adversely affected at both temperature extremes but development is favoured at higher relative humidities.

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HABITAT

Where a particular species lives - forest, savannah etc or terrestrial aquatic etc

The type of soil in a habitat influences an insect's distribution and abundance and is easily disturbed by agriculture, e.g. Irrigation changes moisture and subsequently, the type of pest in a crop. Chemicals in soil affect plant growth and therefore the dependant insects. See food quality.

Niche = physical location and function of a species at a given time. If two species occupy the same niche, they will compete and one will displace the other (see competition).

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FOOD

Absolute shortage

Effective shortage

Potential food exists but is unavailable for consumption

Food quality

Nutrient deficiencies. Mite numbers, egg production and longevity are directly related to N2 content in leaves.

Development time in mites is indirectly related to the N2 content of leaves.

Plant variety - cultivated varieties supported greater populations of aphids than wild beans. [One female aphid placed on wild beans produced a population of 37 after 14 days. However, when placed on each of 19 bean varieties, a single female gave populations of between 286 - 1037]

Different foods produced different development and survival rates in moth (Ephestia) larvae

and similarly for Thrips:

Note that although the addition of pollen, (high in protein), may or may affect survival, it greatly enhances egg prduction.

Different foods can produce different castes of insect. Eg. Exclusive feeding of royal jelly to bee larvae produces queens rather than workers.

HOW DO INSECTS OVERCOME FOOD PROBLEMS?

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OTHER ORGANISMS

Intraspecific - between individuals of the same species.

This involves population density - scarcity and crowding

At low density males and females may not meet eg sheep keds. These wingless flies move from sheep to sheep by sheep contact. Ked population will die out id there are less than one per sheep.

At high density there is competition between individuals. This leads to reduced reproductive rate, survival and longevity.

Interspecific - between individuals of other species.

Competition

Involves niche overlap whereby the needs of 2 or more different species for a resource coincide. This leads to competitive displacement. Eg. Two species of flour beetle (Tribolium confuse and T. cutaneum) were placed in flour at 29 oC and counted every 30 days for 3 years.

T. casteneum will displace T. confusum under these conditions.

However, if the temperature is < 29 oC, T. confusum will dominate.

Natural enemies

These may be

Predators

1. Prey population density effects predator response: Functional response of predator.

2. Prey population effects predator population:
Numerical response of predator.

Predator numbers increase by births and immigration

3. Interaction of prey and predator/predator

Note that predator numbers will be lower than prey numbers and that there is a time lapse for the predator response.

With extended time there is an extended cycle of responses of both prey and predator.

THE CULPRITS

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