Citronella Stem Borer
Chilo partellus
📖 Description
Citronella stem borer, Chilo partellus — commonly known as the ‘spotted stem borer’ — is the major stem-boring pest of citronella grass (Cymbopogon nardus and C. winterianus) plantations in India. Originally a major pest of sorghum and maize, C. partellus has emerged as a serious pest of citronella in the last 2–3 decades as aromatic-grass cultivation has expanded in Assam and Karnataka. Adult moths emerge at dusk, mate, and females lay 200–400 eggs in flat, overlapping scale-like masses of 20–100 on the undersides of citronella leaves and leaf sheaths. The pale-yellow larvae hatch in 4–6 days and initially feed on the leaf tissue outside the stem, then bore into the stem through the leaf sheath, tunnelling downward through the internodes. The larvae are creamy-white with a brown head capsule and conspicuous dark spots (setae-bearing tubercles) along the body — the ‘spotted’ appearance that gives the species its common name. Larvae feed inside the stem for 20–35 days, moulting 5–6 times, then pupate inside the stem or in the leaf sheath. Adult moths emerge 7–10 days later. Total lifecycle is 35–50 days at 25–28 °C, allowing 4–6 overlapping generations per year. The most diagnostic field sign is the ‘deadheart’ symptom — the central leaf whorl dries up and turns brown as the stem-borer larvae sever the vascular tissue. Heavy infestations can destroy 20–40% of tillers in a citronella planting, reducing both herbage yield and essential-oil content. Boreholes at the base of the plant also provide entry points for fungal pathogens (Fusarium, Colletotrichum) that cause secondary stem rots. Yield losses of 15–30% in herbage and 10–25% in essential-oil content have been reported from Assam and Karnataka, with annual economic losses estimated at ₹40–60 crore. The pest is difficult to control with contact insecticides because the larvae are protected inside the stem — systemic and translaminar products (carbofuran, chlorantraniliprole, emamectin benzoate) that penetrate the stem tissue are most effective. Integrated management combines Btk sprays (effective on young larvae before they enter the stem), Trichogramma egg parasitoid releases, Cotesia flavipes larval parasitoid releases, the push-pull strategy (Napier grass trap crop + Desmodium intercrop), hand-collection and burning of ‘deadheart’ tillers, and judicious use of systemic insecticides. CIMAP (Central Institute of Medicinal and Aromatic Plants, Lucknow) has developed IPM packages for citronella stem borer that have been adopted by aromatic-grass growers in Assam, Karnataka, and Andhra Pradesh.
⚠️ Symptoms to watch for
- Larvae hatch from eggs and bore into the citronella stem through the leaf sheath, tunnelling downward through the internodes. The central leaf whorl dries up and turns brown ('deadheart' symptom) — the most diagnostic field sign. Infested tillers may also show 'chaffy' panicles (if flowering) and lodging. Heavy infestations can destroy 20–40% of tillers in a citronella planting, reducing both herbage yield and essential-oil content (oil yield is reduced because the stem-borer damages the vascular tissue that transports oil-bearing leaf exudates). Boreholes at the base of the plant also provide entry points for fungal pathogens (Fusarium, Colletotrichum) that cause secondary stem rots. Yield losses of 15–30% in herbage and 10–25% in essential-oil content have been reported from Assam and Karnataka.
🩹 Treatment
- Apply Bacillus thuringiensis kurstaki (Btk) at 1 g/L — bacterial endotoxin specifically targets young stem-borer larvae before they enter the stem. Spray carbofuran 3 G at 10 kg/ha as a soil incorporation — systemic uptake controls larvae inside the stem. Apply neem oil (1% azadirachtin) at 5 ml/L with sticker — disrupts larval development. Release Trichogramma chilonis egg parasitoid at 50,000 per ha — parasitises stem-borer eggs before they hatch. Apply chlorantraniliprole 18.5 SC at 0.3 ml/L — translaminar and systemic, reaches larvae inside the stem. Install pheromone traps for adult males at 4 per acre — species-specific C. partellus pheromone is commercially available. Cut and burn infested tillers ('deadhearts') to prevent larvae from completing development.
🌿 Organic treatments
- Bacillus thuringiensis kurstaki (Btk) — 1 g/L; spray at dusk on the leaf whorls; targets young larvae before they enter the stem. Neem oil (1% azadirachtin) at 5 ml/L with 0.5 ml/L non-ionic sticker — larval growth disruptor. Trichogramma chilonis egg parasitoid — 50,000 per ha, released at 7-day intervals during adult flight. Cotesia flavipes braconid wasp — larval parasitoid; commercially available for augmentative release; the female wasp oviposits inside stem-borer larvae through the borehole. Beauveria bassiana at 5 g/L — entomopathogenic fungus; can penetrate the stem and infect larvae inside. Metarhizium anisopliae at 5 g/L. Pheromone traps at 4 per acre — captures adult males; reduces mating. Hand-collection and burning of 'deadheart' tillers — kills the larva inside. Push-pull strategy: plant Napier grass (Pennisetum purpureum) as a trap crop around the perimeter — C. partellus oviposits preferentially on Napier grass but larvae fail to develop; plant Desmodium uncinatum as an intercrop to repel ovipositing females.
🧪 Chemical treatments
- Carbofuran 3 G — 10 kg/ha as a soil incorporation at planting; systemic (restricted use due to toxicity). Chlorantraniliprole 18.5 SC (Coragen) — 0.3 ml/L; ryanodine receptor modulator, translaminar and systemic, 14-day PHI. Emamectin benzoate 5 SG — 0.3 g/L; chloride channel activator, translaminar, 7-day PHI. Indoxacarb 14.5 SC — 0.5 ml/L; sodium channel blocker, 7-day PHI. Spinosad 45 SC — 0.3 ml/L; spinosyns, 3-day PHI. Lambda-cyhalothrin 5 EC — 0.5 ml/L; broad-spectrum pyrethroid, 7-day PHI. No chlorpyrifos, no monocrotophos — banned on aromatic grasses. Note: contact insecticides are largely ineffective against stem-borer larvae because the larvae are protected inside the stem — systemic and translaminar products (carbofuran, chlorantraniliprole, emamectin benzoate) are most effective.
🛡️ Prevention
- Plant only pest-free certified slips from registered nurseries (CIMAP Lucknow, IIHR Bangalore). Inspect plantings weekly for 'deadheart' symptoms during the vegetative stage (May–September). Cut and burn infested tillers ('deadhearts') at the base — kills the larva inside and prevents adult emergence. Plough deeply after harvest to expose and kill pupae in the stubble. Avoid planting citronella near sorghum, maize, or sugarcane — these are major C. partellus reservoirs. Apply neem cake (250 kg/ha) as basal manure — systemic azadirachtin uptake confers moderate resistance. Use the push-pull strategy: plant Napier grass as a trap crop around the perimeter and Desmodium uncinatum as an intercrop. Plant trap crops of marigold (Tagetes erecta) around the perimeter. Install pheromone traps at 4 per acre from April onward — early detection of adult moth flight allows timely intervention. Rotate citronella with non-host crops (legumes, oilseeds) every 3–4 years to break the pest cycle.
🪴 Affected plants
- Citronella grass (Cymbopogon nardus, C. winterianus) — primary economic host in India. Also a major pest of sorghum (Sorghum bicolor — the most important economic host), maize (Zea mays), pearl millet (Pennisetum glaucum), sugarcane, and other Cymbopogon species (lemongrass, palmarosa). On citronella, all commercial Indian cultivars are susceptible, including 'Java Citronella' (C. winterianus — the main cultivar) and 'Ceylon Citronella' (C. nardus).
🦋 Natural predators
- Trichogramma chilonis egg parasitoid — commercially available. Cotesia flavipes braconid wasp — gregarious larval parasitoid; the female wasp oviposits inside stem-borer larvae through the borehole; lays 20–40 eggs per host. Sturmiopsis inferens tachinid fly — larval parasitoid. Green lacewing Chrysoperla zastrowi — predator of eggs and young larvae. Spined soldier bug Podisus maculiventris — predator of late-instar larvae. Predatory ground beetles (Carabidae) — predators of larvae and pupae in the stubble. NPV — natural epizootic pathogen. Beauveria bassiana and Metarhizium anisopliae — entomopathogenic fungi that can penetrate the stem.
🌱 Host plants
- Major economic hosts: citronella grass (Cymbopogon nardus, C. winterianus), sorghum (Sorghum bicolor — the most important economic host), maize (Zea mays), pearl millet (Pennisetum glaucum), sugarcane, and other Cymbopogon species (lemongrass, palmarosa). On citronella, all commercial Indian cultivars are susceptible.
🔄 Lifecycle
- Egg → Larva (5–6 instars) → Pupa → Adult.
- Females lay 200–400 eggs in flat, overlapping scale-like masses of 20–100 on the undersides of leaves and leaf sheaths.
- Eggs hatch in 4–6 days.
- Larvae initially feed on leaf tissue, then bore into the stem through the leaf sheath, tunnelling downward through the internodes.
- Larvae feed inside the stem for 20–35 days, moulting 5–6 times.
- Pupation occurs inside the stem or in the leaf sheath, lasting 7–10 days.
- Adults emerge at dusk, mate within 24 hours, and females begin oviposition 2–3 days later.
- Adult lifespan 5–10 days.
- Total cycle 35–50 days at 25–28 °C; 4–6 overlapping generations per year.
- Larvae can enter diapause inside the stubble during unfavourable conditions (cold winter, hot dry summer) — surviving for up to 6 months and emerging as adults when conditions improve.
📅 Season
- Year-round in tropical citronella growing regions, but outbreaks peak during the warm humid monsoon and post-monsoon (June–October) when citronella is in active vegetative phase and conditions are ideal for larval development (25–30 °C, >70% humidity).
- Lowest incidence during the cool dry winter (December–February) when larvae enter diapause in the stubble.
🔎 Appearance
- Adult moths have a 20–28 mm wingspan, with straw-yellow to pale brown forewings marked by darker scale tufts and irregular transverse lines; hindwings are pearly white.
- Body 10–13 mm long, slender, straw-yellow.
- Larvae (stem borers) are creamy-white with a brown head capsule and conspicuous dark spots (setae-bearing tubercles) along the body — the 'spotted' appearance that gives the species its common name 'spotted stem borer'.
- Larvae reach 20–25 mm when fully grown.
- Pupae are dark brown, 12–15 mm, found inside the stem or in the leaf sheath.
- Eggs are laid in flat, overlapping scale-like masses of 20–100 on the undersides of leaves and leaf sheaths.
💥 Damage level
- Moderate to Severe
🔁 Reproduction rate
- Fast
🗺️ Geographic regions
- Indian Subcontinent
- South Asia
- Southeast Asia
- East Asia
- Africa. In India: Assam (Jorhat
- Dibrugarh
- Tinsukia)
- Karnataka (Coorg
- Mysore
- Hassan)
- Andhra Pradesh (Anantapur
- Kurnool)
- Tamil Nadu (Salem
- Dharmapuri)
- Kerala (Palakkad
- Malappuram)
- Maharashtra
- Gujarat
- and the sorghum and maize belts of Maharashtra (Solapur
- Aurangabad)
- Karnataka (Dharwad
- Belgaum)
- Andhra Pradesh
- and Telangana.
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