Senna Caterpillar

📖 Description

Senna caterpillar, Catopsilia pyranthe — commonly known as the ‘mottled emigrant’ butterfly — is the major lepidopteran defoliator of senna (Cassia angustifolia, Indian senna) cultivation in India. Adult butterflies emerge in the morning, mate, and females lay 200–400 eggs singly on the undersides of young senna leaves. The velvety green larvae hatch in 3–4 days and pass through 5 instars over 14–21 days, growing from 1.5 mm to 35–45 mm in length. The first two instars feed on the underside of leaflets, leaving a ‘window-pane’ skeleton; older larvae consume entire leaflets, leaving only the midrib. A single larva can consume 4–6 full senna leaves during its development, and a heavily infested plant can show 40–70% defoliation within 7–14 days. The defoliation is particularly damaging because senna is grown specifically for its leaflets (and pods), which are the source of sennoside — the active laxative compound used in Ayurvedic, Unani, and modern pharmaceutical preparations. Sennoside is synthesised in the leaf tissue, so leaf-area loss directly translates to sennoside-yield loss. Even lightly damaged leaves are downgraded at market — premium export-grade senna leaves must be undamaged, and damaged leaves command only 30–50% of the export-grade price. India supplies approximately 80% of global senna demand, and the senna trade is worth ~₹250 crore annually, of which 60–70% is exported to the US, EU, and Japan. Yield losses of 30–50% in leaflet yield and 15–30% in sennoside content have been reported from Rajasthan and Gujarat, with annual economic losses estimated at ₹40–60 crore. Outbreaks are most frequent during the cool dry post-monsoon (October–April), which coincides with the main senna harvest season. The pest is well-controlled by hand-picking in smallholder plantings (larvae and chrysalises are conspicuous), but commercial growers typically rely on Btk sprays (effective on young larvae and residue-free), NPV application, and judicious use of reduced-risk insecticides immediately after each harvest (to allow 14–21-day pre-harvest intervals before the next harvest). The senna harvest cycle — leaves are picked every 30–45 days during the 5–6 month harvest season — makes pesticide timing particularly challenging, as sprays made just before a harvest would violate pre-harvest intervals and lead to export rejection. Integrated management combines hand-picking, Btk sprays, NPV application, Trichogramma egg parasitoid releases, trap crops of Cassia fistula around the perimeter, and rigorous crop rotation. CAZRI (Central Arid Zone Research Institute, Jodhpur) has developed IPM packages for senna caterpillar that have been adopted by senna growers in Rajasthan and Gujarat. In the major senna growing states of Rajasthan (Jodhpur, Bikaner, Barmer, Jaisalmer, Pali), Gujarat (Banaskantha, Kutch, Mehsana), and Tamil Nadu (Tirunelveli, Madurai), C. pyranthe is the single most damaging insect pest of senna, alongside the senna leaf miner and the senna pod borer.