Unit4 - Subjective Questions

Definition:
Insect Ecology is the branch of entomology that deals with the study of the interrelationship between insects and their environment. It involves understanding how insects interact with the physical (abiotic) and biological (biotic) factors of their surroundings.

Components of the Environment:
The environment consists of two main components:

1. Abiotic Factors (Physical):

   • Climatic factors: Temperature, Humidity, Rainfall, Light, Wind, and Atmospheric pressure.
   • Edaphic factors: Soil type, soil moisture, and soil reaction (pH).
   • Topographic factors: Altitude, slope, and exposure.

2. Biotic Factors (Biological):

   • Food (Nutrition): Availability and quality of host plants.
   • Competition: Inter-specific and Intra-specific competition for resources.
   • Natural Enemies: Predators, Parasitoids, and Pathogens.
   • Symbiotic relationships: Mutualism and Commensalism.

Effect of Temperature:
Temperature is the most critical physical factor affecting insects because they are poikilothermic (cold-blooded).

1. Metabolism & Development: The rate of metabolism and development increases with temperature up to a certain optimum level. Below the threshold, development stops; above the lethal limit, death occurs.
2. Zones of Temperature:
   • Fatal Zone (Low/High): Temperatures where death occurs instantly or after short exposure.
   • Zone of Inactivity: Too cold or too hot for activity, leading to dormancy.
   • Zone of Effective Temperature: The range in which development takes place.
   • Optimum Temperature: The temperature at which development is fastest and survival is highest.

Thermal Constant:
The total amount of heat required for an insect to complete its development is considered a constant.

The relationship is expressed by the formula:

• = Thermal constant (Degree-days)
• = Duration of development (in days)
• = Average prevailing temperature
• = Threshold temperature of development (developmental zero)

This concept helps in predicting pest outbreaks based on weather data.

Insects adapt to unfavorable environmental conditions through various forms of dormancy:

1. Hibernation (Winter Dormancy):

   • Cause: Occurs in response to low temperatures (winter).
   • Function: To survive freezing conditions and food scarcity.
   • Example: Many beetles and mosquitoes overwinter in sheltered places.

2. Aestivation (Summer Dormancy):

   • Cause: Occurs in response to high temperatures and dry conditions (summer).
   • Function: To prevent desiccation (water loss).
   • Example: Some larvae of lepidoptera suspend activity during hot summer months.

3. Diapause:

   • Definition: A physiological state of arrested development that is hormonally controlled and often initiated by environmental cues (like photoperiod) before the adverse conditions actually arrive.
   • Nature: It is obligatory in some species and facultative in others. It involves metabolic reduction.
   • Key Difference: Unlike simple quiescence (which ends immediately when conditions improve), diapause requires a specific stimulus to break the dormancy even if conditions become favorable.

Influence of Humidity:

• Water Balance: Insects must maintain a water balance to prevent desiccation. High humidity usually favors survival, while low humidity causes water loss through the cuticle and spiracles.
• Development: Optimum humidity is required for egg hatching and molting. For example, locusts require specific soil moisture for egg development.
• Fungal Diseases: High humidity encourages the spread of entomopathogenic fungi (e.g., Beauveria bassiana), which can naturally control insect populations.

Influence of Rainfall:

• Physical Mortality: Heavy rains can physically wash away small insects like aphids, jassids, and eggs of various pests.
• Soil Insects: Rainfall influences the emergence of soil-dwelling insects (e.g., Red hairy caterpillar emergence is triggered by the first monsoon rains).
• Host Plant Growth: Rain promotes vegetative growth of crops, providing abundant food, which triggers pest multiplication.

Photoperiodism:
It is the physiological reaction of organisms to the length of day or night (light and dark periods). In insects, the photoperiod acts as a signal or token stimulus to predict seasonal changes.

Importance:

• It regulates Diapause (initiation and termination).
• It influences migration and seasonal forms (polymorphism).

Types based on response:

1. Short-Day Species: Diapause is induced by short day lengths (typically signaling approaching winter). Example: The Mulberry silkworm (Bombyx mori) bivoltine race.
2. Long-Day Species: Diapause is induced by long day lengths (signaling approaching summer/drought).
3. Day-Neutral Species: These insects are unaffected by the length of daylight and development continues irrespective of photoperiod, provided other factors like temperature are favorable.

Food as a Biotic Factor:
Food is the most vital biotic factor determining the survival, longevity, reproduction, and abundance of insects. The nutritional quality of the host plant directly affects the fecundity (egg-laying capacity) of the pest.

Classification based on Food Range (Host specificity):

1. Monophagous:

   • Insects that feed on a single species of plant or plants within a single genus.
   • Example: Mango stem borer (Batocera rufomaculata) or Rice stem borer.

2. Oligophagous:

   • Insects that feed on plants belonging to a few related genera or a specific family.
   • Example: Diamondback moth (Plutella xylostella) feeds on plants of the family Cruciferae (Cabbage, Cauliflower).

3. Polyphagous:

   • Insects that feed on a wide range of plants belonging to different families.
   • Example: Locusts, Termites, and the Gram pod borer (Helicoverpa armigera).

Competition:
The struggle among organisms for essential resources (food, space, mates, shelter) that are in limited supply.

1. Intraspecific Competition:

   • Definition: Competition between individuals of the same species.
   • Dynamics: Occurs when population density is high. It leads to self-regulation of the population.
   • Effects:
     • Cannibalism (e.g., Helicoverpa larvae eating each other).
     • Reduction in size and fecundity.
     • Migration (e.g., Locust phase transformation from Solitary to Gregarious due to crowding).

2. Interspecific Competition:

   • Definition: Competition between individuals of different species.
   • Dynamics: Occurs when two different species share the same ecological niche.
   • Competitive Exclusion Principle: Two species competing for the exact same resources cannot stably coexist; one will outcompete and displace the other.
   • Example: Competition between the fruit fly Dacus dorsalis and Dacus zonatus for mango fruits. Often, parasitoids compete for the same host larva.

Predators:

1. Size: Generally larger than their prey.
2. Consumption: An individual predator kills and consumes many prey individuals during its lifetime to complete development.
3. Independence: They are free-living organisms throughout their life cycle.
4. Stage: Both adults and immature stages (nymphs/larvae) can be predatory.
5. Example: Ladybird beetles, Praying Mantis, Spiders.

Parasitoids:

1. Size: Generally smaller than their host.
2. Consumption: Completes its larval development on or inside a single host individual, eventually killing it.
3. Dependence: The larval stage is parasitic and attached to the host, while the adult is free-living.
4. Host Specificity: Usually more host-specific than predators.
5. Example: Trichogramma spp. (egg parasitoid), Bracon spp. (larval parasitoid).

Definition of Pest:
A pest is any organism that causes economic loss, physical damage, or annoyance to humans, their crops, livestock, or property. From an agricultural perspective, an insect is considered a pest when its population density crosses the Economic Threshold Level (ETL) and causes economic damage.

Parameters/Criteria:

1. Population Density: The insect numbers must be high enough to cause injury.
2. Economic Damage: The injury caused must result in a measurable loss of yield or quality that translates to financial loss.
3. GEL (General Equilibrium Level): For a pest, the average population density (GEL) is usually closer to the economic injury level compared to non-pests.
4. Interaction: It must compete with humans for resources (crops, stored grains).

Based on how frequently they occur, pests are classified into the following categories:

1. Regular Pests:

   • Occur frequently on the crop and are closely associated with it.
   • Example: Aphids on cotton, Thrips on chillies.

2. Occasional Pests:

   • Occur infrequently and do not cause damage every season. They become pests only under specific favorable environmental conditions.
   • Example: Caseworm on rice.

3. Seasonal Pests:

   • Occur during a specific season of the year consistently.
   • Example: Red Hairy Caterpillar (occurs during Kharif/Monsoon), Mango hoppers (during flowering season).

4. Persistent Pests:

   • Occur on the crop throughout the year and are difficult to control.
   • Example: Scales and mealybugs on fruit crops.

Based on the intensity of the outbreak and the area covered:

1. Epidemic Pests:

   • Occur in severe form in a large area rapidly at a specific time.
   • Example: Locust swarms, Fall Armyworm outbreaks.

2. Endemic Pests:

   • Confined to a particular locality or geographical area and occur regularly in low to moderate numbers.
   • Example: Rice Gall midge in certain districts, Woolly apple aphid in hilly regions.

3. Sporadic Pests:

   • Occur in isolated localities here and there in a scattered manner.
   • Example: White grubs, Coconut slug caterpillar.

These concepts are fundamental to pest management and ecology:

1. General Equilibrium Level (GEL):

   • The average population density of an insect over a long period of time, unaffected by temporary interventions (like pesticide sprays). The population fluctuates around this mean level due to natural biotic and abiotic factors.

2. Economic Threshold (ET) / Action Threshold:

   • The population density at which control measures should be initiated to prevent an increasing pest population from reaching the Economic Injury Level.
   • At this stage, the cost of control is less than the potential loss. It serves as a warning signal.

3. Economic Injury Level (EIL):

   • The lowest population density that will cause economic damage.
   • At this point, the cost of control measures equals the value of the yield loss saved.
   • Relationship: for manageable pests. If , the insect is a severe key pest.

1. Key Pests:

• Definition: These are the most severe and damaging pests of a crop.
• Characteristics:
  • Their General Equilibrium Level (GEL) is always above the Economic Injury Level (EIL).
  • They cause significant damage every year unless managed.
  • Environment is usually favorable for them.
• Example: Cotton bollworms, Diamondback moth on cabbage.

2. Potential Pests:

• Definition: Insects that are not currently pests but have the potential to become pests if the ecosystem changes.
• Characteristics:
  • Their GEL is currently well below the EIL.
  • They typically cause no economic damage under normal conditions.
  • They may become pests if natural enemies are killed (secondary pest outbreak) or if a resistant crop variety is replaced by a susceptible one.
• Example: Certain mites or minor leaf feeders often become potential pests after broad-spectrum insecticide use.

Dispersal and Migration:

• Passive Dispersal: Most small and weak-flying insects rely on wind currents for long-distance transport. This is known as anemochory.
  • Example: Aphids, Whiteflies, and first-instar larvae of scale insects (crawlers) are carried miles by wind.
• Migration: Strong fliers like Locusts use wind currents to assist their migratory flights, covering hundreds of kilometers.

Physical Impact:

• Strong winds can mechanically damage insects or dislodge them from host plants, interfering with feeding and mating.
• It increases the rate of evaporation, leading to desiccation of the insect body.

Pheromone Communication:

• Wind direction and velocity are crucial for insects that use pheromones for mating. The wind carries the scent plume from the female to the male.

Symbiosis refers to a close and long-term biological interaction between two different biological organisms.

1. Mutualism ():

   • Both species benefit from the association.
   • Example 1 (Insect-Insect): Ants and Aphids. Aphids provide honeydew (sugar) to ants, while ants provide protection to aphids from predators.
   • Example 2 (Insect-Microbe): Termites and protozoa (Trichonympha). The protozoa live in the termite gut and digest cellulose, which the termite cannot digest alone.

2. Commensalism ():

   • One species benefits while the other is neither helped nor harmed.
   • Example: Phoresy. Some mites attach themselves to beetles or flies for transportation to a new food source without feeding on the carrier.

3. Parasitism ():

   • One organism (parasite) benefits at the expense of the other (host).
   • Example: Parasitoid wasps laying eggs in caterpillars.

Pests damage plants in specific ways based on their mouthparts and feeding strategies:

1. Chewing Pests (Defoliators/Borers):

   • Possess chewing and biting mouthparts.
   • Leaf eaters: Consume leaf tissue (e.g., Grasshoppers, Caterpillars).
   • Borers: Bore into stems, fruits, or roots (e.g., Stem borers, Fruit flies).

2. Sucking Pests:

   • Possess piercing and sucking mouthparts.
   • They suck cell sap, causing yellowing, curling, or drying of leaves.
   • They often transmit viral diseases.
   • Examples: Aphids, Jassids, Whiteflies, Thrips.

3. Mining Pests:

   • Larvae feed between the upper and lower epidermis of the leaf, creating tunnels or mines.
   • Example: Leaf miners.

4. Gall Makers:

   • Cause abnormal growth (galls) on plant tissues due to enzymatic secretions.
   • Example: Gall midges, Gall wasps.

Biotic Potential (Reproductive Potential):

• It is the inherent ability of an organism to reproduce and survive under ideal environmental conditions.
• Insects generally have a high biotic potential due to:
  • Short life cycle.
  • High fecundity (egg-laying capacity).
  • Quick adaptation.

Environmental Resistance:

• These are the sum of all the factors in the environment that limit the population growth and prevent it from realizing its full biotic potential.
• Factors:
  • Abiotic: Adverse temperature, rain, wind, lack of space.
  • Biotic: Limited food supply, predators, diseases, competition.

Balance of Life:
The interaction between Biotic Potential (pushing population up) and Environmental Resistance (pushing population down) determines the actual population density observed in nature.

Ecosystem:
An ecosystem is a self-sustaining structural and functional unit of the biosphere consisting of a community of living organisms (biotic) interacting with each other and their non-living physical environment (abiotic).

Agro-ecosystem:
It is a man-made ecosystem simplified for agricultural production.

Structure:

1. Producers (Autotrophs): The crop plants (and weeds) that synthesize food via photosynthesis.
2. Consumers (Heterotrophs):
   • Primary Consumers (Herbivores): The insect pests that feed on the crops.
   • Secondary Consumers (Carnivores): Predators and parasitoids that feed on the pests.
3. Decomposers (Saprotrophs): Fungi, bacteria, and scavengers that break down dead organic matter and recycle nutrients.
4. Abiotic Environment: Soil nutrients, water, fertilizers, and pesticides applied by humans.

Direct Pests:

• Definition: Insects that feed directly on the part of the plant that is harvested or utilized by humans.
• Impact: Even a small population can cause significant economic loss because the quality or quantity of the marketable product is directly reduced.
• Example: Fruit borers attacking fruits, Bollworms attacking cotton bolls, Pod borers in pulses.

Indirect Pests:

• Definition: Insects that feed on parts of the plant other than the harvested portion (usually leaves, roots, or stems).
• Impact: The plant can often tolerate a certain level of damage (compensation) before yield is affected. Loss occurs due to reduced photosynthetic area or reduced vigor.
• Example: Aphids feeding on leaves of a fruit tree, Stem borers in sugarcane (where sugar recovery is affected indirectly by stalk damage), Leaf folders in rice.

Based on their geographical origin, pests are classified into:

1. Indigenous Pests:

   • Definition: Pests that are native to the specific geographical area where they are causing damage.
   • Characteristics: They have evolved in the local ecosystem and usually have a complex of natural enemies that keep their populations in check (except during outbreaks).
   • Example: Rice stem borers and Grasshoppers in India.

2. Exotic (Alien/Introduced) Pests:

   • Definition: Pests that have been introduced (accidentally or intentionally) from one country or region into another where they were not previously present.
   • Characteristics:
     • They often become severe pests because they arrive without their specific natural enemies from their native land.
     • They find abundant food and favorable climate.
   • Examples:
     • Parthenium beetle (introduced to control the weed).
     • Cottony Cushion Scale (Icerya purchasi) introduced into India/USA.
     • Fall Armyworm (Spodoptera frugiperda) - recent invasive pest in Asia/Africa originating from the Americas.