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Nguyen HU, Phan QN, Dang HT, Son TTN, Nguyen HN, Nguyen THT. 2021. Morphology, biology, and damage behaviors of nutsedge borer (Bactra venosana Zeller) in Tra Vinh Province, Vietnam. Biodiversitas 23: 733-741. Nutsedge borer, Bactra venosana Zeller (Lepidoptera: Tortricidae), is a common pest of sedge in Tra Vinh Province. To control this species, its morphology, biology, and behaviors were investigated in Tra Vinh Province. The study was conducted with the aim of determining the information related to the Bactra venosana, as a basis for developing a strategy to manage this species by means of safety solutions. The study was carried out by surveying the behavior of nutsedge borer in sedge fields and determining their morphological and biological characteristics under laboratory conditions. The results showed that the eggs were 1.06 ± 0.17 mm (in length), 0.84 ± 0.16 (in width), oval, and white after turning pale yellow. Larvae progressed through 5 instars with body length and width of 1.20 ± 0.18 mm to 11.3 ± 1.23 mm and from 0.25 ± 0.05 to 1.05 ± 0.25 mm, respectively. Newly hatched larvae were dark yellow, and pupae were pale green. The male pupa was 7.75 ± 0.68 mm long, while the female was 8.46 ± 0.93 mm long, light green, and dark brown about to mature. The male adult of B. venosana was usually gray-brown and 7.70 ± 0.60 mm in body length, whereas the female adult was silver-gray, 8.41 ± 0.86 mm. The life cycle passed 35.9 ± 2.55 days in laboratory conditions. Under field conditions, eggs were laid scattered on the surface of the host plants. Larvae bored inside the stems of sedge. Newly damaged sedge plants had no symptoms at first whose bodies got soft, turned from light green to pale yellow, and gradually wilted and died.