Onion Seed Stalk Borer
Gynandrophthalma deficiens
📖 Description
Gynandrophthalma deficiens — the Onion Seed Stalk Borer — is one of the most damaging pests of onion seed crops in India, with economic significance centred on the Indian vegetable seed-production industry in Maharashtra (Nashik, Ahmednagar, Pune), Karnataka (Dharwad, Belagavi), and Gujarat (Junagadh, Amreli). 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 driven by private hybrid seed companies (Mahyco, Namdhari, Bejo Sheetal, VNR) and public sector ICAR-DOGR (Directorate of Onion and Garlic Research, Pune). The weevil is univoltine (one generation per year) and synchronised with the onion seed-stalk phase — adults emerge from overwintering sites in soil and crop debris in February–March, just as onion seed stalks begin to elongate. Females chew small cavities in the seed stalk and lay 30–60 eggs singly inside; larvae hatch in 5–7 days and immediately begin tunnelling upward through the hollow pith of the stalk, severing the vascular supply to the developing umbel. Each larva feeds for 18–25 days, then pupates inside the hollowed-out stalk. New adults emerge in April–May, feed briefly, then enter summer aestivation in soil. The economic impact is severe: a single larva can destroy an entire seed stalk (each stalk typically produces 800–1,500 seeds worth ₹5–15), and a heavy infestation can destroy 40–70% of seed stalks in a field. With hybrid onion seed valued at ₹8,000–25,000 per kg, even small infestations cause disproportionate economic loss. The pest is particularly damaging to hybrid seed production, where male-sterile lines are planted in specific ratios and any loss of seed stalks disrupts the crossing pattern. Management is best achieved through IPM — yellow sticky traps for monitoring, threshold-based spinosad or indoxacarb application (preserving natural enemies), crop rotation, deep summer ploughing, sanitation, and use of early-bolting trap crops.
⚠️ Symptoms to watch for
- Larvae bore INTO the hollow onion seed stalk, tunnelling upward through the pith and severing the vascular supply to the developing umbel.
- The first visible symptom is a sudden wilting and drooping of the seed stalk above the entry hole, often while the lower stalk still appears turgid.
- The umbel above the entry point fails to develop — seeds remain shrivelled or absent.
- A small entry hole (1–2 mm) is visible on the stalk, surrounded by a small pile of dark frass.
- Multiple larvae may attack a single stalk
- a heavy infestation can destroy 40–70% of seed stalks in a seed-production field, reducing seed yield by 50–80%.
- Damaged stalks often break off at the entry point during wind or harvest operations.
🩹 Treatment
- Apply lambda-cyhalothrin 5 EC at 0.5 ml/L when adults are first observed on seed stalks (typically February–March in rabi seed crops) — repeat at 10-day intervals through bloom.
- Apply indoxacarb 14.5 SC at 0.5 ml/L — translaminar activity reaches larvae inside the stalk.
- Apply spinosad 45 SC at 0.3 ml/L — IPM-compatible, low mammalian toxicity.
- Cut and destroy wilted seed stalks BELOW the entry hole — ensures larvae inside are killed and removed.
- Avoid planting seed crops adjacent to previous-year onion seed fields (the pest overwinters in crop debris).
🌿 Organic treatments
- Spinosad 45 SC — 0.3 ml/L
- derived from Saccharopolyspora spinosa
- OMRI-listed for organic seed production
- effective against adults and translaminar activity against larvae.
- Beauveria bassiana (1×10⁹ CFU/g) at 5 g/L — foliar spray on seed stalks
- entomopathogenic fungus
- effective under humid microclimate of the onion canopy.
- Neem seed kernel extract (NSKE) 5% — repels ovipositing adults
- azadirachtin also disrupts larval development inside the stalk.
- Mechanical removal — cut wilted seed stalks 5 cm below the entry hole
- collect and burn to kill larvae inside.
- Yellow sticky traps at 1 per 100 m² — capture adults during migration from overwintering sites.
- Sanitation — destroy all onion crop debris after harvest to eliminate overwintering sites.
🧪 Chemical treatments
- Lambda-cyhalothrin 5 EC — 0.5 ml/L
- contact + stomach pyrethroid
- fast adult knock-down
- 7-day pre-harvest interval for seed crop.
- Indoxacarb 14.5 SC (Avaunt) — 0.5 ml/L
- voltage-dependent sodium channel blocker
- translaminar activity reaches larvae inside stalk.
- Spinosad 45 SC (Tracer) — 0.3 ml/L
- GABA agonist
- IPM-compatible.
- Chlorantraniliprole 18.5 SC (Coragen) — 0.3 ml/L
- anthranilic diamide
- systemic activity protects stalk tissue from larval feeding.
- Deltamethrin 2.8 EC — 0.5 ml/L
- contact pyrethroid
- effective against adults on seed stalk surface.
- Avoid organophosphates (chlorpyrifos, quinalphos) — restricted in many Indian states
- rotate insecticides across IRAC mode-of-action classes.
🛡️ Prevention
- Practise crop rotation with non-Allium crops (cereals, legumes) for at least 2 years between onion seed crops — breaks the weevil's lifecycle.
- Destroy volunteer onion plants and crop debris within and around seed-production fields — these maintain pest populations between crops.
- Use yellow sticky traps at 1 per 100 m² from umbel emergence to monitor adult migration.
- Adjust planting dates to avoid peak adult emergence (February–March in Maharashtra/Karnataka) — late planting (December–January) reduces exposure.
- Plant trap crops of early-bolting onion varieties around the perimeter of seed-production fields — adults congregate on the trap crop and can be sprayed there, sparing the main crop.
- Plough fallow fields deeply in May–June to expose and kill soil-pupating larvae.
- Maintain 500 m isolation from previous year's onion seed crops.
- Use certified, pest-free seed — larvae inside seed stalks can contaminate harvested seed lots.
🪴 Affected plants
- Onion seed crops (Allium cepa var. cepa) grown for seed production — particularly the Indian seed-belt states of Maharashtra (Nashik, Ahmednagar, Pune), Karnataka (Dharwad, Belagavi), and Gujarat (Junagadh, Amreli).
- Also recorded on seed stalks of shallot (Allium cepa var. aggregatum) and garlic (Allium sativum) seed stalks.
- Common onion bulb crops (non-seed stalk) are not affected — the pest is a specialist of the flowering seed stalk.
🦋 Natural predators
- Anisopteromalus calandrae (pteromalid pupal parasitoid — also attacks stored-product pests; under evaluation by NBAIR Bangalore for biocontrol of onion seed stalk borer).
- Microctonus species (braconid adult parasitoid — under evaluation).
- Predatory bugs (Anthocoris nemoralis, Orius insidiosus) — attack eggs and young larvae on the seed stalk surface.
- Ground beetles (Carabidae — Pterostichus, Harpalus) — prey on larvae dropping to soil to pupate.
- Spiders (Thomisidae — crab spiders) — ambush adults on onion umbels.
- Natural parasitism rates of 20–40% are reported in unsprayed onion seed crops in Maharashtra and Karnataka, but commercial seed-production fields typically receive 4–6 insecticide applications that suppress natural enemies.
🌱 Host plants
- Onion seed crops (Allium cepa var. cepa — primary host).
- Also recorded on shallot (Allium cepa var. aggregatum) and garlic (Allium sativum) seed stalks.
- Common onion bulb crops (non-seed stalk phase) are NOT affected — the pest is a specialist of the flowering seed stalk.
🔄 Lifecycle
- Egg → Larva (3 instars) → Pupa (inside seed stalk) → Adult.
- Females lay 30–60 eggs singly inside cavities chewed into the seed stalk — eggs laid during stalk elongation (February–March in Maharashtra/Karnataka).
- Eggs hatch in 5–7 days.
- Larvae bore upward through the hollow pith for 18–25 days, moulting 3 times; each larva destroys one seed stalk completely.
- Mature larva pupates inside the hollowed-out stalk; pupation lasts 10–14 days.
- New adults emerge in April–May, feed briefly on remaining onion tissue, then enter summer aestivation in soil or under crop debris until the following February.
- Adults emerge when daytime temperatures exceed 20 °C and migrate to elongating onion seed stalks.
- Adult lifespan 8–11 months (including 7–9 months in aestivation).
- Total lifecycle ~13 months (univoltine — one generation per year).
📅 Season
- Adult migration to elongating onion seed stalks occurs in February–March (rabi seed crop in Maharashtra, Karnataka, Gujarat) when daytime temperatures exceed 20 °C.
- Oviposition peaks 7–14 days after stalk emergence.
- Larval development March–April.
- New adult emergence April–May — adults immediately enter aestivation in soil until next season's seed-stalk emergence.
- Single annual generation — univoltine.
🔎 Appearance
- Small, robust dark-brown weevil 6–8 mm long, covered with fine greyish scales that give a mottled appearance.
- Rostrum (snout) is moderately long, stout, curved downward, about one-third the body length.
- Antennae elbowed and clubbed, attached near the middle of the rostrum.
- Elytra have deep longitudinal striations.
- Legs dark brown with grey scales.
- Larvae are white, legless, C-shaped grubs reaching 8–10 mm with a small brown head capsule.
- Pupae are white, 6–7 mm, found inside the hollow onion seed stalk.
- Eggs are oval, yellowish, 0.6 mm, laid inside small cavities chewed into the seed stalk.
💥 Damage level
- Severe
🔁 Reproduction rate
- Slow
🗺️ Geographic regions
- Indian Subcontinent, South Asia, Middle East, Mediterranean.
- In India: Maharashtra (Nashik, Ahmednagar, Pune, Aurangabad), Karnataka (Dharwad, Belagavi, Haveri), Gujarat (Junagadh, Amreli, Rajkot), Madhya Pradesh (Indore, Mandsaur), Rajasthan (Kota, Jhalawar), Tamil Nadu (Dindigul, Krishnagiri).
- Pest native to South Asia and the Middle East — first reported as a serious pest of Indian onion seed crops in the 1990s with the expansion of hybrid seed production.
Have an Insect to Identify?
Upload or take a photo of an insect on your plant and our AI will identify it and recommend treatments.
📷 Diagnose from Image