Walnut Blue Butterfly
Chaetoprocta odata
Chewing Insects Moderate Risk 28–34 mm wingspan (adult); 20–25 mm (mature larva)
📖 Description
Chaetoprocta odata is a Himalayan endemic lycaenid butterfly whose larvae specialise on walnut foliage. The species is the most important lepidopteran defoliator of Indian walnut in the temperate belt of J&K, Himachal Pradesh, and Uttarakhand, where walnut is grown on approximately 90,000 ha producing 35,000 tonnes annually. Outbreaks are cyclical, with peak damage reported in 2007, 2014, and 2022 — typically following mild winters that enhance larval survival. The butterfly-ant mutualism is ecologically notable: larvae possess a dorsal nectary organ that secretes honeydew for attendant ants, which in turn defend the larvae from parasitoid wasps and lacewing larvae. This mutualism can make biological control challenging.
⚠️ Symptoms to watch for
- Larvae feed on walnut leaves, leaving characteristic 'skeletonised' patches where only the lower epidermis and veins remain
- severe infestations can defoliate entire branches by mid-summer
- reduced kernel fill in heavily defoliated trees
- presence of ants tending the lycaenid larvae on leaves (mutualistic association — ants protect larvae from parasitoids in exchange for honeydew secretions).
🩹 Treatment
- Hand-pick larvae from young trees (practical for nurseries and smallholdings) during the first two weeks after hatch.
- Spray Bacillus thuringiensis var. kurstaki (Btk) at 1 g/L when larvae are in early instars (L1–L3).
- Apply lambda-cyhalothrin 5 EC at 0.5 ml/L only if defoliation exceeds 30 % of canopy — avoid broad-spectrum sprays which kill the ant attendants and parasitoids that naturally regulate populations.
🌿 Organic treatments
- Bacillus thuringiensis var. kurstaki (1 g/L) at first-instar hatch.
- Neem oil (1 % Azadirachtin) at 5 ml/L.
- Encourage ant attendance (Tapinoma and Lasius species) by maintaining ground vegetation — ants can suppress parasitoid wasps but also deter generalist predators, so the net benefit is context-dependent.
- Release Trichogramma chilonis at 50,000/ha against the egg stage.
🧪 Chemical treatments
- Lambda-cyhalothrin 5 EC (0.5 ml/L), emamectin benzoate 5 SG (0.3 g/L), or chlorantraniliprole 18.5 SC (0.3 ml/L).
- Apply at early-instar stage for best efficacy.
- Do not spray during bloom to protect pollinators.
🛡️ Prevention
- Prune lower branches to improve airflow and reduce oviposition sites.
- Remove ground vegetation that harbours ants in late winter to disrupt the ant-butterfly mutualism.
- Plant walnut cultivars with thicker leaves ('Franquette', 'Hartley') which are less preferred for oviposition.
🪴 Affected plants
- Walnut (Juglans regia) is the sole host.
- All Indian walnut cultivars are attacked
- thin-shelled 'Kashmiri Burmese' and 'Lake English' are preferred over hard-shelled local types.
🦋 Natural predators
- Birds (Parus major — great tit, Aegithalos concinnus — black-throated tit) are the most important predators of larvae.
- Generalist predators include spiders (Thomisidae), lacewings (Chrysoperla), and predatory bugs (Anthocoridae).
- Parasitoids include Cotesia spp. (Braconidae) and Compsilura concinnata (Tachinidae), though ant attendance reduces their effectiveness.
🌱 Host plants
- Juglans regia (walnut — sole host).
- Not recorded on Juglans nigra (black walnut) which is rare in India.
🔄 Lifecycle
- Egg (7–10 days, laid singly on walnut leaf buds) → Larva (25–35 days, 4 instars, attended by ants) → Pupa (8–12 months, overwinters in leaf litter or under bark) → Adult (15–25 days, May–June flight period).
- Univoltine (one generation per year).
📅 Season
- Adult flight: May–June (post-bloom).
- Oviposition: June.
- Larval feeding: June–August.
- Pupation: August onward.
- Overwintering pupal stage: September–May.
🔎 Appearance
- Adult males are vivid metallic blue above with narrow dark borders; females are larger with broader dark borders and brownish-blue suffusion.
- Underside is pale grey-brown with a row of submarginal black spots on both wings and a thin white fringe.
- Larvae are flattened, woodlouse-shaped, green with a darker green dorsal stripe, pale lateral stripes, and a small retractable head; mature larvae reach 22 mm.
💥 Damage level
- Moderate
🔁 Reproduction rate
- Moderate
🗺️ Geographic regions
- Indian Himalayan walnut belt (Kashmir, Himachal Pradesh, Uttarakhand)
- also reported from Pakistan (Azad Kashmir), Nepal, and Bhutan at elevations of 1,500–2,800 m.
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