Categories of Insect Pests and Diseases: A Comprehensive Guide (2024)

The term “pest” is derived from the French word “Peste” and the Latin term “Pestis,” both of which mean a contagious disease or plague.

Pests include various organisms such as insects, nematodes, mites, snails, slugs, as well as vertebrates like rats and birds.


A pest is any animal that is harmful, destructive, or troublesome to humans or their interests.

Pests are organisms that occur in large numbers and come into conflict with human welfare, convenience, and profits.

A pest is an organism that causes significant harm to humans or their property or has the potential to do so (Woods, 1976).

Pests are organisms that burden the human population by causing

(i) damage to crops, forests, and ornamental plants,

(ii) annoyance, injury, and even death to humans and domesticated animals,

(iii) the destruction or devaluation of stored products.


1) General Equilibrium Position (GEP): GEP is the average population density of a pest over a long period of time. It represents the point around which the pest population tends to fluctuate due to biotic and abiotic factors in the absence of permanent environmental changes.

Imagine it as the comfort zone for the pest population. It’s like the average number of pests we usually have over a long time, just like how you feel most comfortable in your favorite chair. The GEP is a state of balance where the pest population remains stable without any significant changes.

In essence, the GEP is like a cozy spot where pests tend to congregate, with their numbers remaining neither too high nor too low, and everything remaining steady and predictable. It’s the level at which pests typically exist without causing any significant issues.

2) Economic Threshold Level (ETL): It’s also called the Action Threshold and is the pest population density at which it is necessary to implement control measures to prevent the population from reaching a point where it could cause significant damage.

The Economic Threshold Level can be thought of as a ‘wake-up call’ for pest control. When the pest population reaches the threshold, it serves as an alarm, indicating that we need to take action. This is similar to waking up to an alarm in the morning, motivating us to address the pest problem before it becomes bothersome.

3) Economic Injury Level (EIL): EIL is the lowest population density that will cause economic damage to crops and other resources.

Consider the EIL as the pest-related ‘danger zone’. This threshold represents the minimum population density at which pests begin to cause economic harm, like entering a danger zone where the impact of pests becomes noticeable. It’s like the red zone, urging us to take immediate action to prevent significant losses.

4) Damage Boundary (DB): DB is the lowest level of damage that can be measured. ETL is always less than EIL and helps provide enough time for implementing control measures before the pest population reaches an economically damaging level.

Imagine the DB as an “early warning system” for pest control. This system acts as a highly sensitive gauge, capable of detecting the slightest level of damage. Visualize it as a detective tool that notifies us when pests are about to cause high damage.


Pests can be categorized into 3 types. They are enlisted below and explained:

1) Based on Occurrence

2) Based on level of infestation

3) According to EIL, GEP and DB


1. Regular pests:
- Definition: Regular pests are those that are frequently found on crops and have a close association with specific plants. They have a consistent presence and can cause recurring damage to the crops they infest.
- Characteristics: These pests have adapted to a particular crop’s ecosystem and life cycle. They may have multiple generations in a single cropping season, leading to continuous infestations.
- Examples: Rice stem borer and Brinjal fruit borer.

2. Occasional pests:
- Definition: Occasional pests are those that infrequently occur and are not closely associated with specific crops. They might attack crops sporadically and are not predictable in their occurrence.
- Characteristics: These pests may not have a well-established population in a particular crop’s environment. Their infestations can vary significantly from year to year.
- Examples: Caseworm on rice and Mango stem borer.

3. Seasonal pests:
- Definition: Seasonal pests are pests that appear during a particular season each year. They often coincide with specific climatic conditions or the growth stages of their host plants.
- Characteristics: These pests have a distinct seasonal pattern, and their occurrence is more predictable compared to occasional pests.
- Examples: Red hairy caterpillar on groundnut and Mango hoppers.

4. Persistent pests:
- Definition: Persistent pests are those that remain on a crop throughout the year and are challenging to control. Unlike other pests that have seasonal occurrence, persistent pests are consistently found, and their population can be relatively stable over time.
- Characteristics: These pests have adapted well to the crop’s environment and can survive and reproduce continuously, making their management more difficult.
- Examples: Chilli thrips and mealy bug.

5. Sporadic pests:
- Definition: Sporadic pests are those that occur in isolated localities and during specific periods. They might have irregular outbreaks and are not consistently present in a given area.
- Characteristics: These pests might be influenced by localized factors, leading to their sporadic occurrence.
- Examples: Coconut slug caterpillar.

Are sporadic pests and occasional pests the same?

No, sporadic pests and occasional pests are not the same. Sporadic pests are characterized by their isolated occurrences in specific localities during certain periods, while occasional pests infrequently attack crops without a consistent association with specific plants or locations. Both types of pests have irregular occurrences, but they differ in their patterns and the factors that influence their presence. Understanding these differences is crucial for implementing effective pest management strategies tailored to each pest category.


1. Pest Epidemic:
- Definition: A pest epidemic occurs when there is a sudden outbreak of a pest in a severe form over a large area or region, typically at a specific time.
- Characteristics: During a pest epidemic, the pest population grows rapidly and reaches high densities, causing significant damage to crops or plants.
- Example: An outbreak of the Brown Planthopper (BPH) in the Tanjore region or the Red Hairy Caterpillar (RHC) in the Madurai or Pollachi regions could be considered as pest epidemics.

2. Endemic Pest:
- Definition: An endemic pest refers to the occurrence of a pest at a low and relatively constant level in specific pockets or localized areas.
- Characteristics: Endemic pests are present throughout the year but in limited numbers and are confined to certain regions or locations.
- Example: The Rice Gall Midge in the Madurai region or Mango Hoppers in Periyakulam, occurring consistently but not in outbreak proportions, would be considered endemic pests


Key pests: These pests are the most destructive, causing significant damage. The GEP (General Economic Population) always exceeds the EIL (Economic Injury Level). Although human intervention may temporarily reduce the population below the EIL, it quickly rebounds, often necessitating repeated interventions like sprays to minimize damage. Key pests are persistent, requiring environmental changes to bring the GEP below the EIL. Examples include the Cotton bollworm and Diamond backmoth.

Major pests: These pests frequently surpass the EIL. Timely and repeated sprays can prevent economic damage caused by these pests, such as the Cotton jassid and Rice stem borer.

Minor pests: The population of these pests rarely exceeds the EIL and fluctuates around the ETL (Economic Threshold Level). However, they are easily controlled with available measures, typically requiring a single application of insecticides to prevent economic damage ranging from 5–10%.

Potential pests: Normally, these pests do not cause economic damage. However, any disruption in the ecosystem may lead them to become economically damaging.

Sporadic pests: These pests have a GEP below the EIL, but can appear in large numbers under favorable conditions, crossing the DB and EIL multiple times. Effective management strategies are necessary to control them. They are influenced by abiotic conditions and only a residual population survives after the favorable season ends. Examples include White grub, hairy caterpillars, cutworms, and grasshoppers.


The main causes of pest outbreaks are human activities that disrupt the natural balance of ecosystems. Here are some interventions and their reasons for pest outbreaks:

1. Deforestation and cultivation:
- Pests that originally fed on forest trees are forced to feed on crops.
- Forests typically have a higher biomass per unit area compared to agricultural land.
- Changes in weather patterns also affect insect development.

2. Destruction of natural enemies:
- Excessive use of insecticides kills natural enemies of pests.
- This disrupts the natural control mechanism, leading to pest outbreaks.

3. Intensive and extensive cultivation:
- Monoculture (intensive cultivation) encourages pest multiplication.
- Extensive cultivation of susceptible crop varieties over large areas reduces competition for food, allowing pests to multiply rapidly. For example, stem borers in rice and sugarcane.

4. Introduction of new varieties and crops:
- Varieties with favorable physiological and morphological characteristics can promote insect multiplication.
- Certain crop varieties create ideal conditions for specific pests to thrive. For example, succulent, dwarf rice varieties favor leaf folder infestations, and Cambodia cotton favors stem weevil and spotted bollworm.

5. Improved agronomic practices:
- Increased use of nitrogen fertilizer can lead to higher incidence of leaf folder on rice.
- Closer planting can result in increased populations of pests like brown planthopper (BPH) and leaf folder.
- The use of granular insecticides can have phytotoxic effects on rice plants.

6. Introduction of pests to new environments:
- Pests can multiply rapidly in new areas where their natural enemies are absent.
- For example, the apple wooly aphid Eriosoma lanigerum multiplied quickly due to the absence of its parasitic enemy Aphelinus mali.


Plant disease, also known as phytopathology, is a pathological condition or abnormality that affects plants, disrupting their normal physiological functions and leading to observable symptoms

Plant diseases are classified into two types:
1) Infectious Plant Diseases
2) Noninfectious or Physiological Disorders


1. Diseases caused by fungi:
- Fungal diseases are one of the most common types of plant diseases. Fungi are microscopic organisms that can infect various parts of plants, such as leaves, stems, roots, and fruits.
- Symptoms: Fungal diseases can result in symptoms like wilting, yellowing, spots on leaves, moldy growth, and rotting.
- Examples: Powdery mildew, rust, and damping-off are some common fungal diseases affecting plants.

2. Diseases caused by bacteria:
- Bacterial diseases in plants are caused by microscopic bacteria that invade the plant’s tissues and disrupt their normal functions.
- Symptoms: Bacterial diseases may lead to wilting, leaf spots, necrosis, and cankers on stems or leaves.
- Examples: Fire blight in apples and bacterial spot in tomatoes are examples of bacterial diseases affecting plants.

3. Diseases caused by parasitic higher plants:
- Parasitic higher plants are plants that attach themselves to host plants and draw nutrients from them, causing damage.
- Symptoms: These parasitic plants can weaken their host plants, leading to stunted growth and reduced productivity.
- Examples: Dodder and mistletoe are examples of parasitic higher plants that cause diseases in other plants.

4. Diseases caused by viruses:
- Viral diseases are caused by tiny infectious particles called viruses, which can only replicate inside living cells.
- Symptoms: Viral diseases can cause a wide range of symptoms, including mosaic patterns on leaves, stunted growth, and deformities in plant structures.
- Examples: Tobacco mosaic virus (TMV) and Tomato yellow leaf curl virus (TYLCV) are common viral diseases affecting plants.

5. Diseases caused by nematodes:
- Nematodes are tiny, worm-like organisms that can be plant parasites and cause damage to plant roots.
- Symptoms: Infected plants may exhibit stunted growth, wilting, and nutrient deficiencies due to root damage.
- Examples: Root-knot nematodes and cyst nematodes are common plant parasites causing diseases in plants.


1. Nutrient deficiencies:
- Nutrient deficiencies occur when plants do not receive an adequate supply of essential nutrients required for their proper growth and development.
- Symptoms: Plants suffering from nutrient deficiencies may exhibit yellowing or discoloration of leaves, stunted growth, and other abnormal physical characteristics.
- Examples: Iron deficiency can cause chlorosis (yellowing) of leaves, while nitrogen deficiency may lead to overall stunted growth.

2. Mineral toxicities:
- Mineral toxicities happen when plants accumulate excessive amounts of certain minerals, which can be harmful to their normal physiological processes.
- Symptoms: Plants experiencing mineral toxicities may display leaf browning, necrosis, and even plant death in severe cases.
- Examples: Excessive accumulation of copper or zinc in the soil can lead to mineral toxicities in plants.

3. Lack or excess of soil moisture:
- Soil moisture imbalance affects plant water uptake, leading to either drought stress or waterlogging conditions.
- Symptoms: Drought-stressed plants may have wilting leaves and yellowing, while waterlogged plants may show root rot and yellowing leaves.
- Examples: Prolonged periods of drought or heavy rainfall can cause soil moisture imbalances.

4. Too low or too high temperature:
- Extreme temperature conditions can negatively impact plant growth and development.
- Symptoms: Plants exposed to low temperatures may experience frost damage and wilting, while those subjected to high temperatures may suffer from heat stress and leaf scorching.
- Examples: Cold-sensitive plants may be affected by frost during winter, while heat-sensitive plants may struggle during heatwaves.

5. Lack or excess of light:
- Inadequate or excessive light can affect photosynthesis and overall plant health.
- Symptoms: Plants in low light conditions may have elongated stems and pale leaves, while those exposed to excessive light may develop sunburned or bleached leaves.
- Examples: Indoor plants positioned away from windows may experience insufficient light, while plants in direct sunlight without adequate shade may face light stress.

6. Lack of oxygen:
- Oxygen deficiency in the root zone can lead to root rot and suffocation of plant roots.
- Symptoms: Plants may show wilting, yellowing leaves, and reduced growth due to lack of oxygen in the root system.
- Examples: Waterlogged soils can restrict oxygen availability to plant roots.

7. Air pollution:
- Air pollution, especially certain gases and particulates, can have detrimental effects on plant health.
- Symptoms: Leaves may have necrotic spots or stippling due to exposure to pollutants in the air.
- Examples: High levels of sulfur dioxide or ozone in the air can damage plant tissues.

8. Soil acidity or alkalinity (pH):
- Extremes in soil pH can affect nutrient availability to plants, causing nutrient deficiencies or toxicities.
- Symptoms: Plants suffering from pH imbalances may show nutrient-related symptoms, such as yellowing leaves or stunted growth.
- Examples: Acidic soils with low pH may have limited nutrient uptake, while highly alkaline soils with high pH can lead to micronutrient deficiencies.

Categories of Insect Pests and Diseases: A Comprehensive Guide (2024)
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