Principles of Plant Science - Entomology Notes

Applied Entomology, Plant Pathology, and Weed Science

  • CPP202 covers applied entomology (insect control), plant pathology (disease control), and weed science (weed control).

Factors Affecting Crop Growth

  • Physical Factors: Soil (temperature, moisture, fertility) and climate (rainfall, humidity, solar radiation, photoperiod).

  • Biological Factors: Pests (arthropods, rodents) and pathogens (fungi, viruses, bacteria, nematodes).

Economic Importance of Insects

  • Insects compete with humans for resources; locusts (Schistocerca gregaria, Locusta migratoria) are major pests.

  • Swarms can consume tons of vegetation daily, impacting crops like cacao, cowpea, and maize.

  • Examples of pests include cocoa mirids (Sahlbergella singularis, Distantiella theobroma), cowpea pests (aphids, Ootheca mutabilis, Megalurothrips sjostedti, heteropteran species), and fall armyworm (Spodoptera frugiperda).

  • Stored product pests: Sitophilus zeamias, S. oryzae, Sitotroga cerealella, Callosobruchus maculatus, Dermestes maculatus, and Necrobia rufipes.

Insects of Public Health Importance

  • Insects directly affect humans through bites, stings, or indirectly by transmitting diseases.

  • Examples: Mango fly, mosquitoes (Anopheles gambiae - malaria, Aedes aegypti - yellow fever, Culex quinquefasciatus - elephantiasis), tsetse flies (sleeping sickness), blackflies (river blindness), cockroaches (amoebiasis, salmonellosis).

  • Insects can transmit plant diseases, such as groundnut rosette virus (GRV) by Aphis craccivora.

Beneficial Insects

  • Insects provide food, raw materials (silk from Bombyx mori, Anaphe venata), and assist in pest and weed control.

  • Entomophagy: Winged termites (Macrotermes bellicosus), grasshoppers, crickets, larvae of silkworm, and grubs of beetle species serve as food.

  • Natural enemies/biological control agents: Apoanagyrus lopezi (controls cassava mealybug), Cotesia vestalis (controls diamondback moth), Novius cardinalis (controls cottony cushion scale), Neochetina bruchi and N. eichorniae (control water hyacinth).

  • Insects contribute to nutrient recycling and are used in scientific studies (Drosophila melanogaster) and pollination.

  • Forensic entomology: Arthropod succession helps estimate post-mortem interval (PMI).

Nature of Insect Injury and Damage

  • Injury: Effect of insect activity on host physiology.

  • Damage: Measurable loss of host utility.

  • Damage boundary (Db): Point where injury results in damage.

    • DirectDirect injury: Consumption of plant parts, piercing and sucking, oviposition stings, bites and stings on animals.

    • IndirectIndirect injury: Vectors of pathogens, causing allergic reactions, contamination.

Symptoms of Insect Attack on Plants

  • Leaves: Skeletonization, leaf mining, shot holing.

  • Stems/branches: Boring, gall formation, galleries, stem girdling.

  • Fruits/tubers: Tunnels, surface feeding, mining.

  • Flowers: Flower abortion.

Plant Reaction to Injury

  • Damage curve: Tolerance phase (x1), compensation phase (x2), overcompensation phase (x2a), phase of linearity (x3), phase of desensitization (x4), inherent impunity (x5).

Factors Determining Intensity of Damage

  • Stage of plant attacked: Seedlings are susceptible.

  • Part of plant attacked: Yield-forming organs are critical.

  • Type of injury: Boring, girdling, defoliation.

  • Pest population: Direct relationship.

  • Environmental factors: Wet season allows better compensation.

Phylum Arthropoda

  • Joint-feet animals: Includes Insecta, Crustacea, Arachnida, Chilopoda, Diplopoda.

Characteristics of Arthropoda

  • Bilateral symmetry, chitinous exoskeleton, jointed appendages, open circulatory system.

  • Excretion via Malpighian tubules or coxal/green glands, ventral nerve cord.

  • Bisexual, reduced coelom.

Biological Success of Insects

  • Flight, adaptability, external skeleton, sensory system, small size, rapid reproduction.

Insect Life Cycle

  • Life cycle: Sequence of morphological stages.

  • Life history: Quantitative events associated with life cycle (varies among members of same species).

  • Larva vs. nymph: Differ in mouthparts and habitat.

  • Pupa: Non-feeding stage with tissue breakdown and adult formation.

  • Seasonal cycle: Number of generations per year.

  • Homodynamic vs. Heterodynamic: Continuous vs. Dormancy.

  • Voltinism: Frequency of life cycle completion (Univoltine, Bivoltine, Multivoltine, Merovoltine/Semivoltine).

Types of Insect Life Cycle

  • Paurometabolous: Egg to nymph to adult (cockroach, termite, grasshopper).

  • Hemimetabolous: Aquatic nymphs (naiads) (dragonflies).

  • Holometabolous: Complete metamorphosis (butterfly, true flies).