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Aquaculture Division Home
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Insect Management
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Adult |
Larva |
Description
The adult is about 3 mm (1/8 inch) long, brown with olive gray areas and with dark (almost black) areas on the thorax and elytra (wing covers). The beak is short and stout and about as long as the thorax. Eggs are pearly white, 1/32 inch in length, and cylindrical with rounded ends. Eggs are inserted into submerged leaf sheaths. The larvae (sometimes called root maggots) are white, legless, attain a length of about 8 mm (1/4 inch) and have a brown head. Each abdominal segment (2 through 7) has a pair of dorsal hooks on small projections. The dorsal hooks are probably used for locomotion, but are modified spiracles (regulated openings) specifically used to obtain oxygen from plant roots.
Distribution
Lissorhoptrus otyzophilus is found in the US, Canada, Mexico, and Central and South America. It is not native to CA, but was found there in 1959. Only females are found in CA and reproduction occurs parthenogenetically. Female weevils, presumably from CA, were found in Japan in 1976 and in 1 0 years spread over all of the islands. Korea and northern China (recently) have also become infested with rice water weevils.
Life Cycle
Adults enter diapause and overwinter in accumulated leaf litter around trees, in the base of bunch grasses, and in other sheltered places. The indirect flight muscles degenerate while overwintering. Spring temperatures determine the rate of muscle regeneration. Adults fly from overwintering sites and begin feeding on host plants. Adult feeding scars can be found on rice that has just emerged. Oviposition (egg laying) is stimulated by the presence of submerged plants. After a short time in flooded host plants (as short a 5 to 7 days) flight muscles begin to degenerate. Eggs are deposited in the leaf sheath below the water surface. The eggs hatch in 4 to 9 days (dependent on temperature) and the larvae may feed in the leaf sheath for a short time, chew an exit hole, sink to the soil surface, and burrow into the mud. Small larvae can be found inside roots, while larger larvae feed on the roots. Larvae complete development in about 4 weeks. Pupation occurs in an oval, water-tight cocoon attached to a root. The pupal stage lasts 7 to 1 0 days. The time from egg to adult takes from 35 to 40 days in AR, LA, MS, MO, TX and up to 75 days during the cooler periods in CA. In AR most first generation adults feed on rice leaves in July, August, and Sept. and begin to fly to overwintering sites in Sept. A small percentage of first generation adults will deposit eggs in rice, giving a partial second generation. In LA two complete generations and a partial third are present each year. TX, MS, MO, and CA have generations similar to that in AR.
Injury and Symptoms
Adults feed on the leaf blade surface causing narrow longitudinal scars that parallel the midvein of the leaf. Leaf scarring can be heavy but rarely even the heaviest scarring will result in economic damage. Larvae feed in and on the roots of rice plants. Feeding reduces root volume and can result in decreased ability of plants to acquire, translocate and utilize available nutrients. Damaged plants most often will not show any symptoms unless root damage (pruning) is severe. Severely damaged plants become yellow and stunted, and will have delayed maturity and reduced yield. Occasionally root pruning will be so severe plants cannot remain anchored in the soil and when disturbed will float on the water surface.
Scouting
Two methods of scouting are currently available - leaf scar counts and larval counts; in the near future a trap for monitoring adults will be available.
Drill-seeded rice - leaf scar counts. Leaf scars are a result of adults feeding on rice leaves. Begin scouting and using leaf scar counts within 4 to 7 days after flooding. Examine only the youngest leaf for feeding scars on plants at least 6 feed from levee furrows (barrow pit or bar pit) and avoid areas of thin stands. At each stop inspect 40 plants and record the number of plants with a scar on the youngest leaf. Make the decision to stop scouting or continue scouting by using the accompanying Table 1. If a decision cannot be made after a reasonable number of stops, rescout the field in 3 to 5 days. Do not use the leaf scar method on drill-seeded rice for more than 2 weeks after permanent flood. When the percentage of new leaves with feeding scars in a field exceeds approximately 60%, between 85 and 90% of the time the number of larvae per core sample will reach or exceed 1 0 larvae per core (the old treatment threshold).
Water-seeded rice. Begin scouting and using leaf scar counts when plants emerge above the permanent flood. Research on leaf scars in water-seeded rice is progressing and no firm recommendations are available. However, when the percentage of new leaves with feeding scars exceeds 50%, insecticide intervention would be recommended. The reasoning for this statement is not to let rice water weevil infestations get too large on very small rice plants. The leaf scar method can be used in water-seeded rice for up to 2 weeks after permanent flood or longer if an insecticide application was made during the initial 2 weeks after permanent flood.
Larval scouting. Scouting for larvae is the same for drill or water-seeded rice. Plants and soil surrounding roots can be used to determine the infestation level of rice water weevils. The size of the soil sample needs to be 4 inches in diameter and 3 to 4 inches deep if soil is from a silt loam and 2 to 3 inches deep if soil is from a heavy clay. Place the core sample in a bucket with a 40-mesh screened bottom. Wash the soil from the plants by thoroughly stirring the plants in the water. Push the bucket up and down vigorously in the water several times. Some force is needed to dislodge larvae from the roots. Most larvae will float on the water surface. The number and size of larvae can be used to predict how much damage will occur. If small or medium sized larvae are found along with large larvae and the total is 1 0 or above, the field may suffer more damage. If most of the larvae are large (1/4 inch) and root pruning is evident but plants do not appear stressed, the field may not suffer any extra damage. Although no research is presently available to confirm the benefit, application of extra nitrogen (30-40 lb/acre) at mid season may help the plants recover from damage.
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University of Arkansas • Division of Agriculture |
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