Sweet Potato Hornworm
Agrius convolvuli
📖 Description
The Sweet Potato Hornworm (Agrius convolvuli, also called the Convolvulus Hawkmoth) is the largest and most conspicuous lepidopteran defoliator of sweet potato in India. The species is widespread across the entire Old World tropics and subtropics, and is one of the most powerful and agile flying insects in the world — adults are strong fliers capable of sustained migratory flights covering hundreds of kilometres, allowing rapid colonisation of new areas when conditions become favourable. The adult moth is also an important pollinator of long-tubed flowers (including jasmine, honeysuckle, and petunia) and is one of the few insects capable of hovering flight (similar to hummingbirds) — making it a familiar sight in Indian gardens at dusk. The larva (hornworm) is one of the largest caterpillars found in Indian agriculture, reaching 90 mm in length and consuming 5–10 g of leaf tissue per day in the final instar. Despite the larva’s large size and conspicuous appearance, the species is generally not a major economic pest in commercial sweet potato production — populations are typically low because of strong natural enemy pressure (especially from Trichogramma egg parasitoids) and the larva’s exposed feeding behaviour makes it vulnerable to predation. However, in nursery beds and in organic smallholder fields where natural enemy pressure has been disrupted by broad-spectrum insecticides, the pest can cause complete defoliation within a week. The Central Tuber Crops Research Institute (CTCRI, Thiruvananthapuram) recommends an integrated approach combining hand-picking, Btk application, and conservation of natural enemies — broad-spectrum insecticides should be avoided because they disrupt the natural enemy complex that typically keeps the pest under biological control.
⚠️ Symptoms to watch for
- Sudden defoliation of sweet potato vines — the diagnostic symptom.
- The large larva consumes entire leaves (often 5–10 per day) and can defoliate a young plant in 2–3 days.
- Heavy infestation (5+ larvae per plant) can completely strip a field within a week, killing young plants and severely stunting mature ones.
- Damage is most severe in the 30–60 day age class when vines are still establishing
- defoliation at this stage can reduce storage root yield by 30–60 %.
- The pest is a particular problem in nursery beds where dense planting allows rapid population buildup.
🩹 Treatment
- Mechanical: hand-pick and destroy larvae — the single most effective control in small fields because the large larva is conspicuous.
- Cultural: deep ploughing after harvest to expose and kill overwintering pupae in the soil.
- Chemical: spray carbaryl 50 WP at 2 g/L or chlorpyrifos 20 EC at 2 ml/L against young larvae (before the 4th instar — older larvae are resistant)
- apply Bacillus thuringiensis var. kurstaki at 1 kg/ha on young larvae.
- Apply neem oil 5 % between chemical sprays to deter oviposition.
🌿 Organic treatments
- Hand-picking and destruction of larvae (the single most effective organic control — best done at dawn when larvae are feeding actively on the upper leaves)
- application of Bacillus thuringiensis var. kurstaki at 1 kg/ha on young larvae (highly effective against early instars, less so against later instars)
- spraying neem oil 5 % + soap 0.1 % emulsion at 7-day intervals (anti-feedant and growth disruptor)
- release of Trichogramma chilonis egg parasitoids at 50,000/ha
- deep ploughing after harvest to expose overwintering pupae to predators and desiccation
- trap cropping with Ipomoea weeds around field margins to divert oviposition.
🧪 Chemical treatments
- Carbaryl 50 WP @ 2 g/L, Chlorpyrifos 20 EC @ 2 ml/L, Lambda-cyhalothrin 5 EC @ 0.5 ml/L, Emamectin benzoate 5 SG @ 0.4 g/L, Spinosad 45 SC @ 0.5 ml/L.
- Apply against young larvae (before 4th instar)
- older larvae are highly resistant to most insecticides.
- Pre-harvest interval 14–21 days.
🛡️ Prevention
- Deep ploughing after harvest to expose and kill overwintering pupae in the soil
- hand-pick and destroy larvae at the first sign of damage
- maintain clean field margins by removing Ipomoea weeds that serve as alternative hosts
- plant early-maturing sweet potato varieties that escape peak moth activity
- rotate sweet potato with non-host crops (cereals, legumes) for 2 years
- install light traps (one per 5 ha) at night to capture adult moths during the peak flight period (June–September)
- conserve natural enemies by avoiding broad-spectrum insecticides.
🪴 Affected plants
- Sweet potato (Ipomoea batatas — primary economic host), water spinach (Ipomoea aquatica), morning glory (Ipomoea spp.), and other Convolvulaceae.
- The larva also occasionally feeds on bindweed (Convolvulus arvensis) and other Convolvulaceae weeds.
🦋 Natural predators
- Trichogramma chilonis (egg parasitoid — 30–50 % natural parasitism in unsprayed fields, the most important natural enemy)
- Telenomus sp. (egg parasitoid)
- Apanteles sp. (larval braconid parasitoid)
- predatory pentatomid bugs (Eocanthecona furcellata)
- predatory ants (Oecophylla smaragdina)
- wasps (Vespidae — capture larvae to provision nests)
- birds (drongos, cuckoos, bee-eaters)
- and the entomopathogenic fungi Metarhizium anisopliae and Beauveria bassiana that naturally infect soil-dwelling pupae.
🌱 Host plants
- Ipomoea batatas (sweet potato — primary economic host)
- Ipomoea aquatica (water spinach)
- Ipomoea nil (morning glory)
- Ipomoea purpurea (common morning glory)
- Ipomoea pes-tigridis (tiger-foot morning glory)
- Ipomoea cairica (railway creeper)
- Convolvulus arvensis (field bindweed)
- Calystegia sepium (hedge bindweed).
🔄 Lifecycle
- Adult female lays 200–500 small, spherical, pale-green eggs singly on the underside of sweet potato leaves.
- Egg stage 4–7 days.
- Larva passes through 5 instars over 14–25 days; the larva is the damaging stage, defoliating vines.
- Pupation in the soil at 5–15 cm depth; pupa is dark brown, smooth, with a characteristic 'handle' (loop of the developing proboscis) projecting forward — pupal stage 14–28 days (overwinters in this stage in subtropical tracts).
- Adult longevity 10–20 days (strong flier, migratory).
- Total lifecycle 35–55 days; 4–6 overlapping generations per year.
📅 Season
- Peak infestation during the active growing season (June–November in north India; August–January in south India).
- Population low during the cool winter (December–February) and the hot dry season (April–June).
- Adults are strong migrants and can rapidly colonise new areas when conditions become favourable.
🔎 Appearance
- Adult is a large, powerful hawk moth (Sphingidae) with greyish-brown forewings bearing intricate darker markings and pinkish-grey hindwings with dark bands; the body is robust with characteristic pink and black bands on the abdomen.
- The proboscis is exceptionally long (80–100 mm), allowing the moth to feed from deep-throated flowers while hovering.
- Larva is a large, stout, bright green or brownish 'hornworm' with a distinctive posterior horn (hence the common name) and seven diagonal white stripes on each side of the body; the larva is the damaging stage, defoliating sweet potato vines.
💥 Damage level
- Moderate
🔁 Reproduction rate
- Medium
🗺️ Geographic regions
- Cosmopolitan in tropical, subtropical, and warm temperate regions of the Old World
- in India — all states and union territories.
- Strong flier with documented migratory flights covering >500 km
- occasionally recorded as a stray in Europe and North America.
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