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Common name: Rice Water Weevil Scientific name: Lissorhoptrus oryzophilus Kuschel
Adults are grayish-brown weevils (1/8 inch long) with a dark brown V-shaped mark on their back. Adults fly into rice fields and begin feeding on the leaves of rice plants. Male and female RWW adults mate, then females deposit eggs in the leaf sheath at or below the waterline. White larvae emerge from eggs in about one week and develop through four instars while feeding on rice plant roots. Larvae pupate in cocoons that are covered with a compact layer of mud and are attached to rice roots. The life cycle from egg to adult takes about one month.
Damage caused by adult: Adult rice water weevils feed on the upper surface of rice leaves, leaving narrow longitudinal scars that parallel the leaf midrib. Adult feeding can kill plants when large numbers of weevils attack very young rice, but this is rare and is usually localized along the field borders.
Damage caused by larva: Most economic damage is caused by larvae feeding in or on rice roots. This feeding, root pruning, can reduce the number of tillers and may interfere with nutrient uptake by plants. Damage to roots ultimately can result in yield losses by decreasing panicle densities, numbers of grains per panicle, and grain weights. Infested stands are often thin in appearance and are more susceptible to lodging. At harvest, plants from heavily infested fields will be shorter than normal and have lower yields. Yield losses tend to be higher in water-seeded rice fields. This is because these fields are usually flooded at an earlier stage of plant growth and thus are susceptible to oviposition and infestation by larvae earlier in their growth cycle. Young rice plants are more susceptible to yield losses than are older plants with more established root systems (Picture 4).
Facts: Application of the permanent flood triggers females to lay eggs in rice leaf sheaths. Economic damage caused by larval root pruning reduces the number of tillers produced by the damaged plant.
What should you look for: Elongate feeding scars caused by adult feeding. Infested stands may be thin in appearance and contain yellow plants.
How you can manage rice water weevils: Avoid late planting of rice. Delay the application of permanent flood (but not much past tillering stage of growth). Apply a registered pesticide to control rice water weevil adults at the correct time. Adults must be controlled before egg-laying occurs. Consider using a registered insecticide seed treatment if it is compatible with your production practices. Sample larvae by taking core samples.
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Adult rice water weevil
Rice water weevil pruned roots
Adult rice water weevil leaf scarring
Rice water weevil larvae
