What is Entomology?

• Entomology comes from the Greek roots entomo (insect) + logos (to study). It is the branch of zoology that studies insects — their classification, morphology, physiology, ecology, behavior, economic importance and control.
• What is a pest? A pest is any organism that causes economic loss or damage to human interests (crops, stored products, structures, health, livestock, etc.). Pests include weeds, insects, fungi, bacteria, viruses, nematodes, rodents and birds.

Relative contribution to crop losses (as given)

• Weeds: 33%
• Diseases: 26%
• Insects: 20%
• Rodents: 6–8%
• Birds: 1–2%
• Others: 1–3%

Despite control measures, pests still destroy about 35% of all crops worldwide annually.

Economic concepts used in pest management

• Economic Threshold Level (ETL): The ETL is “the density of the pest population at which control measures should be applied” — that is, the pest density that warrants action to prevent the population from reaching the Economic Injury Level. In other words, it’s the action point: when monitoring shows pest numbers have reached ETL, you apply control measures.
• Economic Injury Level (EIL): The EIL is “the lowest population density that will cause economic damage” or more operationally, “the pest population density at which the incremental damage equals the cost of control.” It defines the level of pest-caused loss at which treatment costs are justified by the damage avoided.
• Relationship: ETL is set below EIL so that when action is taken at ETL the population is prevented from rising to EIL (where losses exceed control costs).

General characteristics of insects

• Kingdom/Phylum/Class context: insects are arthropods (Phylum Arthropoda) and belong to class Hexapoda.
• Body plan: three distinct regions — Head, Thorax, Abdomen.
• Legs: three pairs of jointed (working) legs located on the thorax (total six legs).
• Wings: typically two pairs of wings (some groups have one pair or are wingless).
• Antennae: one pair of antennae on the head.
• Metamorphosis: insects undergo either complete or incomplete metamorphosis (see next section).

These characters together are diagnostic for most insects and separate them from other arthropods.

Metamorphosis (developmental change)

Metamorphosis = radical changes in morphology during development. Two major types:

(i) Incomplete metamorphosis (Hemimetabolous)

• Stages: Egg → Nymph → Adult
• Nymphs generally resemble small adults but lack fully developed wings and reproductive organs; they usually undergo a series of moults, gradually becoming adult.
• Typical orders: many Hemiptera (bugs, whitefly immatures often described as nymph-like), Orthoptera (grasshoppers, locusts).

(ii) Complete metamorphosis (Holometabolous)

• Stages: Egg → Larva → Pupa → Adult
• Larva looks very different from the adult, usually a feeding stage specialized for growth; pupal stage is a non-feeding transformation stage where larval tissues are reorganized into the adult form.
• Typical orders: Lepidoptera (butterflies, moths), Coleoptera (beetles), Diptera (flies), Hymenoptera (bees, wasps).

Insect morphology — the integument (body wall)

The study of insect structure and function is insect morphology. The insect body wall or integument has three main layers:

• Cuticle (outermost) — non-cellular, non-living layer secreted by the hypodermis (epidermis).
  • Composition (as provided): chitin (25–60%) and protein (25–40%).
  • Function: protection, support, barrier to water loss; often sclerotized (hardened) in places to form tough plates (sclerites).
  • Subdivisions:
    • Epicuticle — the outermost, non-chitinous protein-rich layer; thin, water-resistant surface layers.
    • Procuticle — beneath epicuticle; contains chitin and protein; may be further divided into exocuticle (more sclerotized) and endocuticle (less sclerotized).
  • Important cuticular proteins/terms:
    • Arthropodin — term used for main structural proteins of insect cuticle.
    • Sclerotin — a tanned (cross-linked) protein that is water-insoluble and gives hardness where needed (sclerotization).
    • Resilin — an elastic protein that imparts flexibility, especially in wing hinges, leg joints and other elastic regions.

• Epidermis (Hypodermis) — a cellular layer beneath the cuticle; its main function is secretion of the cuticle. Epidermal cells synthesize chitin and cuticular proteins and regulate moulting.
• Basement membrane — innermost, non-cellular layer on which the epidermis rests; muscles attach to the inner surface of the basement membrane, connecting the integument to internal structures.

Moulting (ecdysis) — how insects grow

Because the cuticle is rigid and non-living, insects moult (shed old cuticle) to grow. Moulting generally involves two major processes:

• Apolysis — the initial phase: separation of the old cuticle from the underlying epidermal (hypodermal) cells. During apolysis the epidermis becomes active, secretes moulting fluid into the space, and begins synthesizing materials for the new cuticle beneath the old one.
• Ecdysis — the final phase: the actual shedding (casting off) of the old cuticle. After the new cuticle is formed beneath and the old cuticle is split, the insect wriggles free of the old cuticle (ecdysis). The new cuticle then expands (by intake of air or water), and subsequently hardens and sclerotizes.

Moulting is hormonally controlled (e.g., ecdysteroids stimulate apolysis and ecdysis; juvenile hormone levels influence the type of moult and whether metamorphosis proceeds).

Types of larva (common forms and examples)

Different insect orders have distinctive larval (immature) forms. Types listed with examples:

• Nymph: Description: immature stages in insects with incomplete metamorphosis; often similar to adults but smaller, wingless or with wing pads. Examples: immatures of bugs (Hemiptera), whiteflies, aphids, jassids; also grasshoppers and locusts (Orthoptera).
• Caterpillar: Description: the larval form of Lepidoptera, typically soft-bodied, chewing mouthparts, often with prolegs. Examples: larvae of butterflies, moths, bollworms, borers, loopers, semiloopers.
• Grub: Description: fleshy, C-shaped larvae with well-developed head capsule and thoracic legs (depending on group); commonly used for many beetle larvae. Examples: larvae of beetles, weevils (Coleoptera); also larvae of some Hymenoptera (bees, wasps, braconids) are sometimes called grubs.
• Maggot: Description: legless, soft-bodied larva lacking obvious thoracic legs; typical of many Diptera. Examples: larvae of house flies, shoot flies, midges (Diptera).