Onion Thrips (Seed Crop Race)
Thrips tabaci (Seed-Crop Biotypes)
📖 Description
Thrips tabaci — the Onion Thrips — is the most damaging insect pest of onion seed crops in India, with economic significance centred on the seed-production industry in Maharashtra (Nashik, Pune, Ahmednagar), Gujarat (Junagadh, Amreli), Karnataka (Dharwad), Madhya Pradesh (Mandsaur), and Rajasthan (Kota). India is the world’s second-largest onion producer (~28 million tonnes from ~1.4 million ha) and onion seed production is a ₹600–800 crore industry. The direct damage from thrips feeding (silvery streaks, leaf curling, stalk distortion) is economically tolerable, but the indirect damage through transmission of Iris Yellow Spot Virus (IYSV, genus Tospovirus, family Tospoviridae) is devastating — IYSV can cause 50–80% seed yield loss in infected seed crops and is non-persistently transmitted, meaning a single thrips can acquire and transmit the virus within minutes of feeding on an infected plant. The seed-crop race of T. tabaci differs from the bulb-crop race in three important ways: (1) higher IYSV acquisition efficiency — seed-crop biotypes acquire the virus at rates of 60–80% compared to 20–40% in bulb-crop biotypes; (2) different insecticide resistance profile — seed-crop biotypes have documented resistance to lambda-cyhalothrin, chlorpyrifos, and imidacloprid in Maharashtra, while bulb-crop biotypes remain susceptible; and (3) higher reproductive rate on seed stalks — seed-crop biotypes complete a generation in 14–18 days on seed stalks compared to 20–25 days on bulb leaves. The pest is parthenogenetic (males are rare) — females lay 40–80 eggs singly inside leaf tissue; eggs hatch in 4–6 days; nymphs feed for 8–12 days; pupation occurs in soil or leaf litter for 3–5 days; adults live 20–30 days and produce 2–3 generations during the seed-crop bloom period (February–May). The combination of concealed eggs (inside leaf tissue), pupation in soil, parthenogenetic reproduction, and high reproductive rate makes this pest extremely difficult to control — most seed growers apply 6–10 insecticide sprays per season, driving resistance evolution. The recommended IPM approach uses IYSV-free planting material, 1 km isolation from bulb crops, blue sticky trap monitoring, threshold-based spinetoram application, and rotation across IRAC mode-of-action groups to preserve susceptibility.
⚠️ Symptoms to watch for
- Adults and nymphs rasp the leaf surface and seed stalk with their single mandibular stylet, sucking up the exuded cell sap.
- Feeding causes silvery-white streaks and patches on leaves and seed stalks, giving them a 'scorched' appearance.
- Heavy infestation causes leaf curling, stalk distortion, and premature senescence.
- The greater economic impact is the transmission of Iris Yellow Spot Virus (IYSV) — symptoms include characteristic diamond-shaped chlorotic lesions on seed stalks, often with a green 'thumbprint' in the centre of each lesion.
- Heavily infected seed stalks fail to produce seed
- seed yield losses of 50–80% are common in IYSV-infected seed crops, and the virus is non-persistently transmitted so even low thrips populations can cause high infection rates.
🩹 Treatment
- Apply spinetoram 11.7 SC at 0.5 ml/L — the most effective thrips insecticide in India
- translaminar activity reaches larvae inside leaf folds.
- Apply cyantraniliprole 10 OD at 1.5 ml/L — systemic diamide
- long residual activity.
- Apply imidacloprid 17.8 SL at 0.3 ml/L — systemic
- effective against nymphs.
- Apply fipronil 5 SC at 1 ml/L — contact + systemic
- effective but use with caution (toxic to bees — avoid during bloom).
- Apply azadirachtin 1% at 2 ml/L — antifeedant
- IPM-compatible.
- Use blue sticky traps at 4–8 per ha to monitor adult thrips migration.
- Rotate insecticide mode-of-action (IRAC groups) to prevent resistance — T. tabaci has documented resistance to organophosphates, pyrethroids, and neonicotinoids in Indian onion seed crops.
🌿 Organic treatments
- Azadirachtin 1% (NeemAzal) — 2 ml/L
- antifeedant and growth disruptor
- OMRI-listed.
- Spinosad 45 SC — 0.3 ml/L
- derived from Saccharopolyspora spinosa
- effective against adults and larvae
- OMRI-listed.
- Beauveria bassiana (1×10⁹ CFU/g) at 5 g/L — entomopathogenic fungus
- effective under humid microclimate.
- Lecanicillium lecanii (1×10⁹ CFU/g) at 5 g/L — entomopathogenic fungus specific to thrips and whiteflies.
- Blue sticky traps at 8–12 per ha — mass-trap adults
- blue is more attractive to thrips than yellow.
- Reflective mulch (silver-coloured polythene) — repels thrips from seedling stage.
- Intercropping with carrot (umbel flowering) — attracts predatory bugs and hoverflies.
- Avoid overhead irrigation (thrips thrive in humid microclimate)
- use drip irrigation.
🧪 Chemical treatments
- Spinetoram 11.7 SC (Radiant) — 0.5 ml/L
- nicotinic acetylcholine receptor allosteric activator
- the most effective thrips insecticide in India
- translaminar activity
- 7-day PHI.
- Cyantraniliprole 10 OD (Benevia) — 1.5 ml/L
- anthranilic diamide
- systemic activity
- long residual (14–21 days).
- Imidacloprid 17.8 SL — 0.3 ml/L
- neonicotinoid
- systemic
- effective against nymphs.
- Fipronil 5 SC — 1 ml/L
- GABA-gated chloride channel blocker
- contact + systemic
- TOXIC TO BEES — avoid during bloom.
- Abamectin 1.9 EC — 0.5 ml/L
- glutamate-gated chloride channel activator
- translaminar.
- Diafenthiuron 50 WP — 1 g/L
- mitochondrial ATP synthase inhibitor
- effective against all stages.
- Avoid organophosphates (chlorpyrifos, quinalphos) — T. tabaci has documented resistance
- rotate IRAC groups to prevent resistance.
🛡️ Prevention
- Use IYSV-free certified planting material — the virus is the primary economic damage, and thrips only vector the virus from infected source plants.
- Practise 2-year crop rotation with non-Allium crops — breaks the thrips lifecycle and the virus reservoir.
- Destroy volunteer onion plants and Allium weeds within and around seed-production fields — these maintain both thrips and IYSV between crops.
- Use reflective silver-coloured mulch — repels thrips from seedling establishment.
- Plant windbreaks (maize, sorghum) around seed fields — thrips are weak fliers and windbreaks reduce migration.
- Use blue sticky traps at 8–12 per ha from crop emergence to monitor adult thrips density.
- Avoid planting seed crops adjacent to bulb crops — bulb crops are the primary IYSV reservoir.
- Apply threshold-based insecticide sprays (10 thrips per plant) rather than calendar sprays — preserves natural enemies and slows resistance.
- Remove and destroy IYSV-infected plants immediately — rogue out symptomatic plants to prevent spread.
- Maintain 1 km isolation from commercial bulb crops — this is the single most effective IYSV prevention measure.
🪴 Affected plants
- Onion seed crops (Allium cepa var. cepa — primary host, especially the seed-stalk phase during February–May).
- Also on garlic (Allium sativum), shallot (Allium cepa var. aggregatum), leek (Allium porrum), chives (Allium schoenoprasum), and ornamental Allium species.
- Seed-crop biotypes differ from bulb-crop biotypes in their higher IYSV acquisition and transmission efficiency — a single thrips can transmit IYSV to 30–50 plants during its 20–30 day adult lifespan.
🦋 Natural predators
- Amblyseius swirskii (predatory mite — the most effective biological control agent for thrips; under mass-rearing by NBAIR Bangalore).
- Orius insidiosus (minute pirate bug — predatory anthocorid; attacks all thrips stages).
- Coccinella septempunctata (ladybird beetle — predator of thrips nymphs on leaves).
- Chrysoperla zastrowi sillemi (green lacewing — predator of thrips nymphs).
- Frankliniella occidentalis (western flower thrips — competitor species; lower populations of T. tabaci where F. occidentalis dominates).
- Predatory thrips (Aeolothrips intermedius — feeds on T. tabaci nymphs).
- Natural enemies are most effective in unsprayed fields — commercial seed-production fields typically receive 6–10 insecticide applications per season that suppress natural enemies to ineffective levels.
🌱 Host plants
- Onion (Allium cepa var. cepa — primary host, especially the seed-stalk phase).
- Garlic (Allium sativum), shallot (Allium cepa var. aggregatum), leek (Allium porrum), chives (Allium schoenoprasum).
- Ornamental Allium species.
- Some cucurbit and brassica weeds serve as secondary hosts.
- Seed-crop biotypes have strong preference for flowering Allium seed stalks over bulb-crop leaves.
🔄 Lifecycle
- Egg → Nymph (2 instars) → Pre-pupa → Pupa (in soil) → Adult.
- Females lay 40–80 eggs singly inside leaf tissue or seed stalk — eggs are concealed and not visible to the naked eye.
- Eggs hatch in 4–6 days.
- Nymphs feed on leaf/stalk surface for 8–12 days, moulting twice.
- Pre-pupal and pupal stages occur in soil or leaf litter, lasting 3–5 days — pupae do not feed.
- Adults live 20–30 days; females begin oviposition 2–3 days after emergence.
- Parthenogenetic reproduction (males rare).
- Total cycle 18–25 days on seed crops (faster than on bulb crops due to higher nutritional quality of seed stalk tissue); 4–6 overlapping generations per year in Maharashtra/Karnataka seed-production belts.
- Overwintering occurs as adults in crop debris or as nymphs on volunteer Allium plants.
📅 Season
- Peak activity February–May (rabi seed crop bloom period in Maharashtra, Karnataka, Gujarat) when daytime temperatures exceed 25 °C.
- Lower activity June–September (southwest monsoon — heavy rain suppresses thrips populations).
- Secondary peak October–November (kharif seed crop in Tamil Nadu, Andhra Pradesh).
- IYSV symptoms appear 10–14 days after thrips-mediated transmission — peak IYSV symptom expression occurs in March–April on rabi seed crops.
🔎 Appearance
- Minute, slender, yellowish-brown thrips 0.8–1.2 mm long with feathery fringed wings.
- Females are pale yellow to brown; males are rare (populations are predominantly parthenogenetic).
- Nymphs are smaller, pale yellow or whitish, wingless.
- The seed-crop race is morphologically indistinguishable from common onion thrips but differs in host preference, pesticide resistance profile, and vectoring ability for Iris Yellow Spot Virus (IYSV) — the most damaging virus of onion seed crops.
- Eggs are minute (0.2 mm), kidney-shaped, inserted into leaf tissue and invisible to the naked eye.
💥 Damage level
- Severe
🔁 Reproduction rate
- Fast
🗺️ Geographic regions
- Indian Subcontinent, South Asia, cosmopolitan.
- In India: Maharashtra (Nashik, Pune, Ahmednagar, Aurangabad), Gujarat (Junagadh, Amreli, Rajkot), Karnataka (Dharwad, Belagavi, Haveri), Madhya Pradesh (Mandsaur, Indore, Ujjain), Rajasthan (Kota, Jhalawar, Bundi), Tamil Nadu (Dindigul, Krishnagiri), Andhra Pradesh (Kurnool, Anantapur).
- Pest is cosmopolitan — found in all onion-growing regions of the world
- first reported as a major pest of Indian onion seed crops in the 1990s with the expansion of hybrid seed production.
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