Cassava Shoot Fly
Silba pandicornis
📖 Description
Silba pandicornis — the Cassava Shoot Fly — is a lonchaeid dipteran pest of cassava in India, with economic significance centred on the cassava belts of Tamil Nadu (Salem, Namakkal, Dharmapuri, Erode, Cuddalore — India’s largest cassava-growing state with ~1.5 lakh ha), Kerala (Thiruvananthapuram, Kollam, Pathanamthitta, Alappuzha), Andhra Pradesh (Visakhapatnam, Srikakulam, East Godavari), and Northeast India (Nagaland, Mizoram, Meghalaya). India produces ~5 million tonnes of cassava from ~2.5 lakh ha (much of it processed into sago and tapioca — Salem is India’s ‘sago capital’ with ~750 sago factories). The pest is native to Southeast Asia but has spread throughout tropical Asia — it was first reported in India in the 1990s and has since become established as a major pest of Indian cassava, particularly during the kharif cropping season (June–October). The pest’s biology makes it particularly damaging: (1) eggs are laid on the underside of cassava leaves — inconspicuous and difficult to detect; (2) larvae are concealed inside the terminal shoot — protected from contact insecticides; (3) pupation can occur inside the shoot or in soil — protected from foliar insecticides; and (4) wide host range (all Manihot species and other Euphorbiaceae) — survives on wild cassava and Jatropha weeds on field margins between cassava crops. The economic impact is significant: heavy infestations can lose 20–40% of terminal shoots over a single season, reducing tuber yield by 15–30% — for a typical Tamil Nadu cassava grower cultivating 1 ha of cassava (yield 25–30 t/ha at ₹3,000–4,000 per tonne), a severe S. pandicornis infestation can wipe out ₹15,000–25,000 of annual income. Damage is especially severe in newly planted cassava (first 3–4 months after planting), where terminal shoot loss can stunt the plant permanently — affected plants produce multiple lateral branches that compete for resources, reducing tuber bulking. Management is best achieved through IPM combining tolerant varieties (Sree Jaya, Sree Vijaya — CTCRI-released varieties with reduced oviposition preference), early planting (escaping peak adult emergence), close spacing (90 cm × 90 cm reduces adult oviposition), intercropping with cowpea or groundnut (providing habitat for natural enemies), windbreaks (reducing adult migration), NSKE 5% foliar sprays during peak oviposition, mechanical removal of wilted terminals, and crop rotation. The Central Tuber Crops Research Institute (ICAR-CTCRI, Thiruvananthapuram) is the primary research institution for cassava IPM in India and maintains a network of field stations for testing tolerant varieties and biocontrol agents.
⚠️ Symptoms to watch for
- Larvae bore INTO the terminal shoot of cassava, tunnelling downward through the apical meristem and killing the growing point. The first visible symptom is a characteristic 'deadheart' — the central shoot wilts, turns yellow, and dies while the lower leaves remain turgid. The killed shoot easily pulls out from the leaf whorl, revealing frass and the feeding larva inside. Multiple lateral buds below the killed terminal subsequently sprout, giving the plant a bushy, multi-leader habit that is undesirable for cassava (the plant fails to develop the single straight stem required for tuber production). Heavily infested cassava fields lose 20–40% of terminal shoots over a single season, reducing tuber yield by 15–30%. Damage is especially severe in newly planted cassava (first 3–4 months after planting), where terminal shoot loss can stunt the plant permanently.
🩹 Treatment
- Apply carbofuran 3G at 5 g per plant (basal application) in young cassava plantings — systemic uptake protects terminal shoots for 30–45 days (NB: carbofuran is restricted-use in India — apply only with proper PPE and observe 30-day waiting period). Apply imidacloprid 17.8 SL at 0.3 ml/L — systemic; effective against larvae inside shoots. Apply thiamethoxam 25 WG at 0.4 g/L — systemic. Apply lambda-cyhalothrin 5 EC at 0.5 ml/L — effective against adults and neonate larvae before shoot entry. Apply spinosad 45 SC at 0.3 ml/L — translaminar activity. Cut and destroy wilted terminal shoots BELOW the larval entry point — ensures larvae inside are removed. Plant cassava varieties with tolerance to shoot fly — e.g., Sree Jaya, Sree Vijaya (CTCRI-released varieties).
🌿 Organic treatments
- Bacillus thuringiensis var. israelensis (Bti) — 1 kg/ha in 500 L water; effective against neonate larvae before shoot entry. Neem seed kernel extract (NSKE) 5% — repels ovipositing adults; azadirachtin disrupts larval development. Beauveria bassiana (1×10⁹ CFU/g) at 5 g/L — entomopathogenic fungus; effective under humid microclimate. Mechanical removal — cut wilted terminals 5–10 cm below the entry hole; collect and burn to kill larvae inside. Yellow pan traps with soapy water at 1 per 100 sq m — capture adults during migration. Sanitation — destroy all cassava crop debris after harvest to eliminate overwintering pupae. Plant cassava varieties with tolerance to shoot fly — Sree Jaya, Sree Vijaya (CTCRI-released varieties). Plant windbreaks around cassava fields — flies are weak fliers and windbreaks reduce migration. Intercropping with cowpea or groundnut — reduces shoot fly infestation by providing habitat for natural enemies.
🧪 Chemical treatments
- Carbofuran 3G (Furadan) — 5 g per young plant (basal application, incorporated into soil); systemic uptake protects terminals for 30–45 days; RESTRICTED-USE in India — wear proper PPE, observe 30-day waiting period. Imidacloprid 17.8 SL (Confidor) — 0.3 ml/L; neonicotinoid; systemic; effective against larvae inside shoots; 7-day PHI. Thiamethoxam 25 WG (Actara) — 0.4 g/L; neonicotinoid; systemic. Lambda-cyhalothrin 5 EC — 0.5 ml/L; contact + stomach pyrethroid; effective against adults and neonate larvae. Spinosad 45 SC (Tracer) — 0.3 ml/L; GABA agonist; translaminar. Indoxacarb 14.5 SC (Avaunt) — 0.5 ml/L; voltage-dependent sodium channel blocker; effective against larvae that have just entered the shoot. Avoid organophosphates (chlorpyrifos, quinalphos) — restricted in many Indian states; rotate IRAC groups to prevent resistance.
🛡️ Prevention
- Practise 1-year crop rotation with non-cassava crops — breaks the shoot fly lifecycle. Destroy volunteer cassava plants and Manihot weeds within and around fields — these maintain pest populations between crops. Plant early in the season (June–July for Tamil Nadu kharif crop) — escapes peak adult emergence. Plant tolerant varieties (Sree Jaya, Sree Vijaya — CTCRI-released varieties with reduced oviposition preference). Adjust planting density — close spacing (90 cm × 90 cm) reduces adult oviposition by providing shade that deters ovipositing females. Plant windbreaks (maize, sorghum, gliricidia) around cassava fields — flies are weak fliers and windbreaks reduce migration. Intercrop with cowpea or groundnut — provides habitat for natural enemies and reduces shoot fly infestation by 20–40%. Apply NSKE 5% as a foliar spray on cassava terminals during peak oviposition (August–September in Tamil Nadu). Cut and destroy wilted terminal shoots BELOW the larval entry hole — ensures larvae inside are removed and reduces next-generation populations. Plough fallow fields deeply in May–June to expose and kill soil-pupating larvae. Maintain 500 m isolation from previous year's cassava fields.
🪴 Affected plants
- Cassava (Manihot esculenta — primary host). Also on wild cassava (Manihot glaziovii, M. carthaginensis) and other Euphorbiaceae — Jatropha curcas, Jatropha gossypifolia, Croton bonplandianum. In India, the pest is most damaging in cassava belts of Tamil Nadu (Salem, Namakkal, Dharmapuri, Erode, Cuddalore — India's largest cassava-growing state), Kerala (Thiruvananthapuram, Kollam, Pathanamthitta, Alappuzha), Andhra Pradesh (Visakhapatnam, Srikakulam, East Godavari), and Nagaland, Mizoram, Meghalaya (Northeast India).
🦋 Natural predators
- Trichogramma chilonis (egg parasitoid wasp — commercially reared by NBAIR Bangalore; release rate 50,000/ha). Telenomus remus (egg parasitoid wasp — also effective against Spodoptera eggs on cassava). Eucelatoria species (tachinid larval parasitoid — attacks larvae inside shoots). Apanteles species (larval braconid parasitoid). Coccinella septempunctata (7-spot ladybird — predator of eggs and neonate larvae on leaves). Chrysoperla zastrowi sillemi (green lacewing — predator). Spiders (Thomisidae — crab spiders) — ambush adults on cassava leaves. Predatory ants (Oecophylla smaragdina — weaver ant; effective predator on cassava in Kerala and Northeast India). Natural enemies are most effective in unsprayed cassava crops and in intercropped systems (cassava + cowpea or groundnut) — these provide habitat for natural enemies and reduce shoot fly infestation by 20–40%.
🌱 Host plants
- Cassava (Manihot esculenta — primary host). Wild cassava (Manihot glaziovii, M. carthaginensis). Other Euphorbiaceae — Jatropha curcas (physic nut), Jatropha gossypifolia (Bellyache bush), Croton bonplandianum (ban tulsi).
🔄 Lifecycle
- Egg → Larva (3 instars, shoot-boring) → Pupa (in shoot or in soil) → Adult.
- Females lay 50–100 eggs singly or in small clusters on the underside of cassava leaves.
- Eggs hatch in 2–4 days.
- Larvae bore into the terminal shoot, tunnelling downward through the apical meristem for 10–14 days, moulting 3 times; each larva destroys one terminal shoot completely.
- Mature larva pupates inside the hollowed-out shoot or drops to the soil and pupates in leaf litter; pupation lasts 8–12 days.
- New adults emerge, mate within 24 hours, and females begin oviposition 2–3 days later.
- Adult lifespan 15–25 days (females) or 10–15 days (males).
- Total cycle 20–30 days at 28 °C; 8–12 overlapping generations per year in Indian cassava-growing belts.
- Pest survives hot dry summer (April–May) as diapausing pupae in soil and on wild cassava and Jatropha weeds on field margins.
📅 Season
- Peak activity August–October (kharif cassava crop in Tamil Nadu, Kerala, Andhra Pradesh — southwest monsoon period with high humidity and temperature 25–32 °C, ideal for fly reproduction).
- Lower activity November–March (cool dry winter — adult activity reduced).
- No activity April–May (hot dry summer — temperatures exceed 38 °C, suppressing reproduction; pest survives as diapausing pupae in soil).
- Secondary peak June–July (early kharif crop — adult emergence from diapausing pupae with first monsoon rains).
🔎 Appearance
- Small, robust, shining black lonchaeid fly 4–5 mm long.
- Wings are clear with characteristic lonchaeid venation — a distinctive anal cell with a long pointed extension.
- Head is large with bright red compound eyes; antennae are short, 3-segmented, with a feathery arista.
- Thorax is shining black with two pairs of dorso-central bristles.
- Abdomen is shining black, metallic-blue in some lights.
- Legs are black.
- Larvae are white to pale yellow maggots reaching 6–8 mm, lacking a distinct head capsule (typical of dipteran larvae); posterior spiracles are characteristic 'tripartite' shape (used for species identification).
- Pupae are golden-brown, 4–5 mm, found inside the cassava shoot or in soil beneath.
- Eggs are white, 0.8 mm, laid singly or in small clusters on the underside of cassava leaves.
💥 Damage level
- Moderate
🔁 Reproduction rate
- Fast
🗺️ Geographic regions
- Indian Subcontinent
- South Asia
- Southeast Asia. In India: Tamil Nadu (Salem
- Namakkal
- Dharmapuri
- Erode
- Cuddalore
- Villupuram)
- Kerala (Thiruvananthapuram
- Kollam
- Pathanamthitta
- Alappuzha
- Kottayam)
- Andhra Pradesh (Visakhapatnam
- Srikakulam
- East Godavari
- West Godavari)
- Telangana (Khammam
- Nalgonda)
- Nagaland
- Mizoram
- Meghalaya
- Assam (Karbi Anglong). Pest native to Southeast Asia — first reported in India in the 1990s; has since spread to all major cassava-growing belts.
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