Elephant Foot Yam Hornworm

📖 Description

The elephant foot yam hornworm, Theretra oldenlandiae (sometimes misidentified as Theretra pinastrina or Theretra silhetensis in older Indian literature), is the major lepidopteran defoliator of elephant foot yam (Amorphophallus paeoniifolius) in India. Adult sphinx moths emerge at dusk, mate, and females lay 200–500 eggs singly on the undersides of elephant foot yam leaves. The pale green larvae hatch in 4–6 days and pass through 5 instars over 18–25 days, growing from 5 mm to 80–90 mm in length. The larvae are voracious defoliators — a single larva can consume 4–6 full elephant foot yam leaves during its development, and a heavily infested plant (5–10 larvae) can be completely stripped of leaf area within 5–7 days. The defoliation is most damaging during the tuber-bulking phase (July–September), when the plant needs maximum leaf area to fill the underground corm — yield losses of 20–40% have been reported from Kerala and Tamil Nadu plantings. The hornworm is found across all major elephant foot yam growing regions of India, with peak infestations during the southwest monsoon (June–September) when ambient humidity and temperature are ideal for larval development. Despite its size and conspicuousness, the larva is surprisingly easy to overlook — its green or brown coloration blends with the foliage, and it feeds at night and rests motionless along the midrib during the day. The large black or green droppings (frass) that fall on lower leaves and the ground are often the first sign of infestation. The pest is well-controlled by hand-picking in smallholder plantings, but commercial growers typically rely on Btk sprays, NPV application, and judicious use of emamectin benzoate or chlorantraniliprole. In the major elephant foot yam growing states of Kerala (Thrissur, Palakkad, Malappuram), Tamil Nadu (Salem, Erode, Dharmapuri), Andhra Pradesh (East Godavari, West Godavari), Odisha, and West Bengal, annual losses to hornworm are estimated at ₹80–120 crore.