Pestipedia

Under-leaf Thrips

**Under-leaf Thrips** (Order Thysanoptera) are a large group of minute, slender insects named for the fringe of hairs surrounding their narrow wings. They are major agricultural and ornamental pests, often preferring to feed on the protected undersides of leaves. The conflict is **cell destruction and virus transmission**: they feed using a rasping/sucking action, puncturing epidermal cells and sucking up the contents, leaving characteristic silvered or bronzed patches, and transmitting devastating plant viruses (e.g., **Tomato Spotted Wilt Virus – TSWV**).

Taxonomy and Classification

Thrips belong to the Order Thysanoptera. They undergo a unique metamorphosis involving quiescent (non-feeding) pupal-like stages in the soil or protected crevices.

Physical Description

Adults are minute, 0.5\text{ mm} to 3\text{ mm} long.

  • **Adult (Key ID):** Very small, slender, cylindrical body; usually yellow, brown, or black; wings have a distinctive fringe of long hairs.
  • **Damage ID (Key):** Leaves show **silvering, streaking, or bronzing**; presence of minute, dark fecal droplets (key distinguishing symptom); distorted or scarred flowers and fruit.
  • **Vector Status:** The most destructive conflict is virus transmission.
  • **Conflict:** Agricultural, Greenhouse, Ornamental.

Distribution and Habitat

Cosmopolitan, found worldwide (e.g., **Western Flower Thrips** – *Frankliniella occidentalis*). Habitat is the sheltered parts of plants: leaf undersides, flower petals, and growing points.

Behavior and Conflict

The conflict is their feeding method and mobility.

  • **Stippling:** Feeding causes the silvering effect as the air-filled remnants of destroyed cells reflect light.
  • **Protection:** Their preferred location on the leaf undersides or deep in flower buds makes them difficult to reach with contact insecticides.

Management and Prevention

Management is **Integrated Pest Management (IPM) with Sanitation**.

  • **Biological Control (Key):**
    • Release of specialist predatory mites (*Amblyseius* spp.) or minute pirate bugs (*Orius* spp.).
  • **Chemical Control:**
    • Use of systemic or targeted contact insecticides, often requiring rotation to manage high levels of insecticide resistance.
  • **Monitoring:**
    • Use of blue or yellow sticky traps to detect early populations.

    • Conservation and Research

    Research is intensely focused on understanding the molecular mechanisms of TSWV transmission and managing high levels of insecticide resistance in greenhouse populations.