Dead hearts in basmati: separating stem borer from leaf folder

Two different pests are present; do not merge them.

• Yellow stem borer (Scirpophaga incertulas): 6% dead hearts in patches, a history of white ears in the same field, and 12 moths per trap. Confirm by pulling a dead heart - it comes away easily, has a hole and frass at the base, and splitting the tiller shows a cream-yellow larva with a dark head and no stripes.
• Rice leaf folder (Cnaphalocrocis medinalis): 12% of leaves folded, scraped whitish and containing frass. Leaf folder does not cause dead hearts.

Thresholds. Stem borer action threshold at tillering is about 5% dead hearts; leaf folder about 10% damaged leaves. Both are marginally over, which justifies one well-targeted action, not a calendar of sprays.

Why the chlorpyriphos spray failed against stem borer.

1. The larva feeds inside the tiller; a contact/ingestion spray cannot reach it once it has bored in.
2. Chlorpyriphos is not systemic, and at 500 L/ha the spray does not wet the base of the tillers where the larvae enter.
3. It was applied after dead hearts had already appeared, i.e. after the damage window; the effective timing is around egg hatch, roughly 10–12 days after a moth peak, or at booting to prevent white ears.
4. It is broad-spectrum and kills the Trichogramma and Telenomus parasitoids that suppress borer populations.

Old dead hearts stay dead. Only newly formed dead hearts after treatment tell you whether the next intervention worked.

1. Weather first. Granules or sprays applied 12–24 hours before heavy rain are largely lost to run-off and dilution. With heavy rain forecast on 8 September, move the application to 9–10 September, once the rain has passed and the field still holds 2–3 cm of water.

2. Product choice. At this stage a granular formulation is the right tool for stem borer: cartap hydrochloride 4 G, fipronil 0.3 G or chlorantraniliprole 0.4 G, broadcast uniformly into 2–3 cm standing water and hold the water for 2–3 days without draining. If the leaf folder is also above threshold, a directed spray of chlorantraniliprole 18.5 SC or flubendiamide 39.5 SC covers both - but do not stack a granule and a full insecticide spray in the same week without evidence that both are needed.

3. Do not mix urea with the insecticide granules. Urea is hygroscopic, the two materials have very different prill sizes and segregate during broadcasting, so neither the insecticide nor the nitrogen is spread uniformly. The operator cannot calibrate the dose, control concentrates in patches, and both efficacy and crop safety suffer. Apply urea separately as a top dressing. The labour saved is not worth the loss of dose accuracy and the risk of uneven nitrogen.

4. Follow-up. Re-scout after 7 days, count only new dead hearts, and record product, dose, water level and date.

Stem borer damage changes character with crop stage: dead hearts at tillering, white ears at flowering, and the head-bearing tiller is fixed at booting. So protection must be in place before booting, not after white ears appear.

Plan.

• Keep the pheromone traps running. A fresh moth peak about 10–14 days before expected booting signals the need for one spray at boot-leaf stage (chlorantraniliprole, flubendiamide, or a granule where water management allows) to protect against white ears.
• Spray in the evening, not during morning anthesis, to avoid interfering with flowering and to reduce harm to pollinators.
• Once white ears are visible, that yield is already lost. Count them and use the figure for next season's planning instead of chasing them with late sprays.
• Check the label pre-harvest interval for whatever is used at booting and make sure the PHI falls before the expected harvest date; if it does not, choose a different product or skip the spray.
• Record dead-heart %, white-ear %, trap counts and every product used. Next year the trap data - not the calendar - should set the date of the first granular application.