Thrips: Biology, Behavior, and Agricultural Impact

Thrips: Overview and Impact

Order

Thysanoptera

Body Characteristics

  • Slender, elongated body measuring 1.5–2 mm in length.
  • Undergoes modified hemimetabolic (remetabolic) development.

Wings

  • Two similar strap-like pairs of wings, fringed with long, movable setae.

Mouthparts

  • Asymmetric piercing-sucking mouthparts.

Feeding Habits

  • Some species are predatory, feeding on mites and small insects.
  • Many are phytophagous, consuming plants, pollen, nectar, fungi, or algae.

Economic Importance

  • Some species, such as flower thrips and onion thrips, are of high economic significance as they act as vectors for plant viruses.
Thrips: Biology, Behavior, and Agricultural Impact
Fig. Thrips

Thorax Characteristics

Name derived from their distinctive “fringed wings.”

Tarsi

  • Composed of 1 or, more commonly, 2 segments.
  • Claws are present only during the larval stages.
  • Adult tarsi have an eversible, glandular vesicle located at the apex.

Development

  • Modified hemimetabolic (remetabolic) development.

 

ALSO READ ABOUT: INTERNAL ANATOMY OF INSECTS

 

Head and Feeding Mechanism

Thrips are significant pests due to their feeding habits and role as virus vectors. They cause direct damage by feeding on parenchyma cells and laying eggs in plant tissue, leading to symptoms like silvering, necrotic spots, and distorted growth. Additionally, they spread diseases like TSWV, causing severe agricultural and economic impacts. Effective management strategies are essential to control thrips populations and protect crops.

The mouthparts and feeding mechanism of thrips (Thysanoptera) highlight their asymmetrical piercing-sucking adaptations.

1. Head Capsule

The protective outer covering of the head that encases the mouthparts and associated structures.

2. Mouth Cone

The cone-shaped structure containing the piercing and feeding apparatus.

3. Mandibles and Maxillary Styletsi

  • Mandibles: Asymmetrical, with one being reduced or absent. They are used for piercing plant tissues.
  • Maxillary Stylets: Paired needle-like structures that interlock to form two channels:
    • Food Canal: Used for sucking plant sap or cellular contents.
    • Salivary Canal: Delivers saliva into plant tissues.

4. Salivary and Food Canal

The dual-channel system within the maxillary stylets for ingestion and secretion.

5. Hypopharynx

A structure that plays a role in feeding by helping to form the salivary canal.

6. Glossae

Lobes at the base of the hypopharynx, possibly aiding in feeding.

7. Salivarium

A chamber connected to the salivary ducts, where saliva is temporarily stored before being injected into plant tissues.

8. Precibarium and Cibarium

  • Precibarium: The anterior region of the alimentary canal.
  • Cibarium: The suction pump-like cavity behind the mouthparts that aids in drawing fluids into the food canal.

9. Salivary Glands and Ducts

Produce and transport saliva, which contains enzymes that facilitate the breakdown of plant tissues for easier ingestion.

10. Esophagus

The tube that connects the mouthparts to the digestive system.

Feeding Behavior and Damage

  • Thrips larvae, specifically Frankliniella occidentalis (Western Flower Thrips), cause damage by puncturing and draining the contents of peripheral leaf cells.
  • Their short mandibles and maxillae limit their feeding to the outer layers of plant tissue, resulting in characteristic superficial damage.
  • This feeding behavior exhausts the cells, leading to visible symptoms like silvery streaks and reduced plant vigor.

Typical Damage Caused by Thrips

Thrips: Biology, Behavior, and Agricultural Impact
Fig. Thrips impact on Plant.

Punctual Brightening/Silvering

  • Cause: Thrips use their asymmetrical mouthparts to puncture individual plant cells and suck out the contents, including chlorophyll and other cell fluids.
  • Appearance: Small, shiny, or silvery patches on the leaves.
  • Common Locations: Upper surface of leaves.

Necrotic Spots

  • Cause: Damage to plant cells leading to cell death (necrosis).
  • Appearance: Small, dead, brown or black areas on the leaves or flowers.
  • Impact: Reduces photosynthetic capacity and aesthetic value.

Leaf and Flower Distortion

  • Cause: Disruption of normal growth of plant tissues.
  • Appearance: Curling, crinkling, or misshapen growth of leaves and flowers.
  • Impact: Reduces aesthetic and economic value, affects photosynthesis and reproduction.

Thrips as Virus Vectors

  • Thrips feed on parenchyma cells, causing direct damage without producing honeydew.
  • Egg laying by Terebrantia thrips can further damage plant tissues.
  • Thrips, particularly Frankliniella occidentalis, are important vectors of plant viruses like Tomato Spotted Wilt Virus (TSWV), which can devastate crops.

 

Infection and Transmission of INSV in Frankliniella occidentalis

Life Stages and Virus Acquisition

The complex interaction between the virus and its vector during ontogenesis emphasizes how different barriers in the thrips’ body are overcome, enabling the virus to reach the salivary glands and be transmitted to plants. This process is critical for understanding the role of F. occidentalis as a vector of Impatiens Necrotic Spot Virus (INSV) and for developing strategies to manage its spread.

  1. Larva I

The virus is acquired when the larvae feed on infected plant material. The virus enters the gut through the mouthparts and travels into the digestive system.

2. Larva II

The virus continues to replicate and progress through barriers, such as the midgut and epithelial cells.

 

Barriers to Virus Movement

  1. Gut Barriers

The virus must bypass structures like the glycocalyx, peritrophic membrane, and midgut epithelium to reach the hemocoel (body cavity).

2. Hemocoel

The virus circulates within the hemocoel and infects various tissues.

Virus Replication and Tissue Infection

Thrips can only transmit plant viruses like Tospoviruses if they acquire the virus during their first larval stage (L1). If the virus is ingested during later developmental stages, it is excreted via the Malpighian tubules and does not lead to transmission. This unique characteristic underscores the importance of targeting thrips control efforts early in their life cycle to prevent the spread of devastating plant viruses.

  • The virus may invade Malpighian tubules and other tissues, ensuring it spreads systemically within the thrips’ body.
  • The brain and nervous system might regulate viral transmission or behavior related to feeding.

Salivary Gland Infection

  • During the prepupal and pupal stages, the virus infects the salivary glands, a critical step for successful transmission to plants.
  • Once the salivary glands are infected, the virus is primed for transmission.

Adult Stage and Transmission

  • Adults, having developed from infected pupae, transmit the virus to healthy plants during feeding. The virus is introduced into plant tissues through the salivary canal.
  • The cycle begins again as the virus establishes infection in the plant.

 

Virus Acquisition and Transmission in Thrips

1.Virus Acquisition During the First Larval Stage

  • First Larval Stage (L1)

Thrips go through two larval stages (L1 and L2) before becoming adults. The first larval stage (L1) is the only time thrips can acquire and retain the virus in a way that allows them to transmit it later in life.

 

  • Why Only L1?

During the first larval stage, the thrips’ gut is thought to be more permeable to virus particles, allowing the virus to cross the gut barrier and establish an infection. The virus can then replicate and spread to other tissues, including the salivary glands, which is essential for transmission.

2.Virus Particles Taken Up in Later Stages

  • Later Developmental Stages (L2, Prepupa, Pupa, Adult)

If thrips are exposed to the virus during these stages, they may ingest the virus, but it will not establish a systemic infection.

  • Excretion via Malpighian Tubules

The Malpighian tubules are excretory organs in insects, similar to kidneys in vertebrates. When virus particles are ingested during later stages, they are unable to cross the gut barrier and are instead excreted through the Malpighian tubules. As a result, the virus is not retained or transmitted by the thrips.

3. Why This Mechanism Matters

  • Efficient Virus Transmission

Thrips can only transmit viruses if they acquire them during the first larval stage. This is because the virus must be replicated and spread to the salivary glands, which only occurs if the virus is acquired early in the thrips’ development.

  • Implications for Disease Spread

Plants infected with Tospoviruses are most likely to spread the disease if thrips larvae (L1) feed on them. Adult thrips that did not acquire the virus as larvae cannot transmit it, even if they feed on infected plants later.

4. Practical Implications for Thrips Management

  • Early Intervention

Controlling thrips populations during the early larval stages is crucial to prevent virus acquisition and transmission.

  • Monitoring

Farmers and growers should monitor thrips infestations, especially during the early growth stages of crops, to minimize the risk of virus spread.

  • Barrier Methods

Using physical barriers or insecticides to protect young plants from thrips feeding can reduce the likelihood of virus transmission.

Developmental Cycle of Thrips

Thrips: Biology, Behavior, and Agricultural Impact
Fig. Thrips life cycle

Developmental Stages

Egg

  • Thrips lay their eggs inside plant tissues, protecting them from environmental hazards and predators.
  • This concealed placement makes it difficult to target eggs using chemical or biological control methods.

1st Instar Nymph

  • After hatching, the first instar nymph begins feeding on plant tissues, causing initial damage.
  • Nymphs are small, lack wings, and remain on the plant, making them somewhat accessible for control.

2nd Instar Nymph

  • The second instar continues feeding more aggressively, causing significant damage to leaves, flowers, and fruits.
  • Both instar stages can also acquire and transmit plant viruses, exacerbating their impact.

Prepupa

  • The prepupa is a transitional, non-feeding stage where the nymph begins developing into the pupal stage.
  • Prepupae often drop into the soil or plant debris, making them harder to target with foliar applications of pesticides.

Pupa

  • The pupal stage occurs in the soil or protected areas, where thrips are inactive and not feeding.
  • This stage is resistant to many control measures since the thrips are shielded in the environment.

Adult

  • Adults emerge from the pupal stage and are fully winged.
  • They feed on plant tissues, lay eggs, and serve as primary vectors for plant viruses. Their mobility makes them difficult to control.

Importance of Thrips Control

Challenges in Control

  • Eggs and pupae are protected in plant tissues or the soil, making them inaccessible to most foliar treatments.
  • Adults are highly mobile and can rapidly infest new plants.

Integrated Pest Management (IPM)

  • Targeting nymphs with biological controls (e.g., predators like predatory mites) or insecticides is often more effective.
  • Soil treatments or environmental modifications can help disrupt pupal development.
  • Monitoring and trapping adults can prevent further infestation and reduce egg-laying.

Thrips Control: Challenges with Chemical Methods

Using chemical insecticides to control thrips is particularly challenging due to several factors:

  1. Complex Life Cycle

Thrips have a specific developmental cycle that makes it difficult to target all stages effectively with chemicals.

2. Hidden Behavior

Thrips often hide in hard-to-reach areas such as flowers and leaf axils, reducing the effectiveness of chemical treatments.

3.Protected Eggs

Thrips lay their eggs inside plant tissue, where they are shielded from direct contact with insecticides.

4. Soil-Dwelling Stages

Late second instar larvae (L2), prepupae, and pupae reside in the soil, making them less accessible to chemical applications.

5. Rapid Reproduction

Thrips reproduce quickly, often requiring multiple rounds of insecticide spraying to manage their populations effectively.

6. Insecticide Resistance

Thrips have developed resistance to many commonly used insecticides, further complicating control efforts.

7. Impact on Biological Control

Frequent chemical spraying can harm beneficial organisms used for biological control of other pests in greenhouse environments, reducing their effectiveness.

 

Thrips Control: Chemical Options

Several chemical insecticides are used to control thrips, each with distinct active ingredients and modes of action:

  1. Spinosyne
  • Active Ingredient (AI):Spinosad, derived from the fungus Saccharopolyspora spinosa.
  • Products:Spin Tor, Conserve.
  • Mode of Action:Acts as a neurotoxin, disrupting the nervous system of thrips.

2. Neem

  • Active Ingredient (AI)

Azadirachtin (e.g., NeemAzal).

  • Mode of Action

Inhibits development, affecting the growth and reproduction of thrips.

3. Abamectine

  • Source

Derived from the bacterium Streptomyces avermitilis.

  • Mode of Action

Functions as a neurotoxin, targeting synapses in the nervous system.

4. Pyrethrine

  • Source

Extracted from Chrysanthemum (e.g., Spruzit).

  • Mode of Action

Acts as an axonic neurotoxin, disrupting nerve signal transmission.

5. Neonicotinoids

  • Example

Thiacloprid, often used in combination with carbamates (e.g., Lizetan: Thiacloprid + Methiocarb).

  • Mode of Action

Functions as a neurotoxin at synapses, affecting the nervous system.

 

Biological Control: Using Predatory Antagonists

Biological control of thrips involves the release of natural predators that specifically target thrips stages present on plants. Key predators include:

1.Predatory Bugs

  • Example:Orius species (e.g., minute pirate bugs).
  • Role:These bugs actively hunt and feed on thrips, helping to reduce their populations.

2. Predatory Mites:

  • Example:Amblyseius
  • Role:These mites prey on thrips, particularly targeting the larvae and adult stages found on plant surfaces.

Important Thrips Species: Frankliniella occidentalis (Western Flower Thrips)

Characteristics

  • Extremely polyphagous (feeds on a wide variety of plants).
  • Thermophilic (thrives in warm conditions).
  • Widely distributed, including across Europe.

Impact on Agriculture

  • Highly significant in protected cultivation (e.g., greenhouses).
  • A key pest on many vegetable and ornamental crops, causing growth disorders (Wuchsstörungen) and necrotic damage (Nekrosen).

Role as a Virus Vector

  • An important vector of plant virus diseases, particularly the Tomato Spotted Wilt Virus (TSWV).
  • TSWV causes substantial damage to crop, making Frankliniella occidentalisa major concern for growers.

Important Species

Kakothrips robustus (Pea Thrips)

  • Polyphagous pest that commonly affects legumes, including sweet pea (Lathyrus odoratus).

Thrips tabaci (Onion Thrips)

  • Exhibits worldwide distribution and is highly polyphagous.
  • It causes significant damage to crop such as onions, tobacco, cucumber, and ornamentals.
  • Serves as an important vector for plant viruses, including:
  • TSWV:Tomato Spotted Wilt Virus
  • INSV:Impatiens Necrotic Spot Virus
  • IYSV:Iris Yellow Spot Virus.

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