Liquorice Leaf Weevil

📖 Description

Liquorice leaf weevil, Myllocerus undecimpustulatus (often referred to as the ‘cotton grey weevil’ or ‘sub-auricular weevil’ in Indian agriculture), is a major polyphagous pest that has emerged as a serious defoliator of liquorice (Glycyrrhiza glabra) cultivation in India. Adult weevils emerge from the soil in May–July, mate, and females lay 200–400 eggs in clusters of 5–15 in the soil at the base of liquorice plants. The creamy-white C-shaped legless grubs that hatch in 7–10 days feed on the small roots of the liquorice plant for 30–45 days, moulting 4 times, then pupate in earthen cells 5–10 cm deep in the soil. Adult weevils emerge 10–14 days later and climb up the plant at dusk to feed on the leaves, chewing characteristic ragged or notched margins. Adult lifespan is 60–90 days; total lifecycle is 60–80 days at 25–28 °C, allowing 3–4 overlapping generations per year. The weevil is particularly damaging because both adults and larvae cause economic damage — adult weevils defoliate the leaves, reducing photosynthesis and the synthesis of glycyrrhizin (the active medicinal compound, synthesised in leaves before being translocated to the roots), while the soil-dwelling larvae feed directly on the liquorice roots — the very tissue that is the harvested medicinal product. Heavy infestations can defoliate 30–50% of a liquorice planting within 2–3 weeks, reducing both herbage yield and root glycyrrhizin content. Larval root-feeding causes yellowing, wilting, and stunting of plants, and can reduce root yield by 20–40% in heavy infestations. Yield losses of 20–30% in root yield and 15–25% in glycyrrhizin content have been reported from Haldwani (Uttarakhand) and Bikaner (Rajasthan), with annual economic losses estimated at ₹20–30 crore. The pest is highly polyphagous, attacking cotton, soybean, groundnut, sunflower, sesame, pulses, okra, and many other crops — making it one of the most widespread weevil pests in Indian agriculture. Outbreaks are most frequent during the warm dry summer (April–June) when adult weevils emerge from the soil and feed actively on newly-emerged liquorice foliage. The pest is difficult to control with contact insecticides because the larvae are protected in the soil and the adults are nocturnal feeders that hide in plant debris during the day — systemic insecticides (imidacloprid, thiamethoxam) that penetrate the plant tissue and reach both adults via plant sap and larvae via root uptake are most effective. Integrated management combines systemic insecticides at planting, neem oil foliar sprays during the adult-feeding phase, biological soil treatments (Metarhizium anisopliae, Heterorhabditis indica nematodes), neem cake basal manure, hand-picking of adult weevils at dusk, and crop rotation with mustard (Brassica juncea) to break the pest cycle. CIMAP (Central Institute of Medicinal and Aromatic Plants, Lucknow) has developed IPM packages for liquorice leaf weevil that have been adopted by medicinal-plant growers in Uttarakhand, Rajasthan, and Punjab. In the major liquorice growing states of Uttarakhand (Haldwani, Nainital, Pantnagar), Rajasthan (Bikaner, Jaisalmer, Barmer), Punjab (Ferozepur, Bathinda), and Haryana (Hisar, Sirsa), M. undecimpustulatus is one of the top 3 insect pests of liquorice, alongside the liquorice aphid (Aphis craccivora) and the liquorice root-knot nematode (Meloidogyne incognita). Because liquorice is a slow-growing perennial that takes 3–4 years to produce a harvestable root crop, pest management must be sustained over many seasons — making biological soil treatments (which establish long-term in the soil) particularly valuable for this crop.