Description of the Pest
Although they are crustaceans, tadpole shrimp resemble tadpoles in size, shape, color, and mobility. Tadpole shrimp have about 35 body segments, and all but the last six or seven have pairs of leaflike, gill-bearing appendages. A thin, olive-brown shield covers the front section of the body, and two long tails extend from the last segment. Two long, jointed appendages resembling antennae extend from below the well-developed chewing mandibles.
Adults deposit eggs singly on soil or on plants at the bottom of the field. They are highly resistant to drying and remain viable for several years in unflooded soil. Most of the eggs hatch 1 to 3 days after spring flooding of the rice fields, but hatching may continue for 1 to 2 weeks. The young develop rapidly by a series of molts and resemble the adults in less than 24 hours. They feed on a variety of small animals and plants commensurate with their size as they grow and molt. The somewhat transparent molt skins may be mistaken for dead shrimp. The first instar larva is about 0.2 inch (5 mm) long. Egg pouches are first noted when shrimp are about 0.63 inch (16 mm) long, and by 3 weeks old the shrimp may be 1 inch (2.5 cm) long. At maturity, tadpole shrimp are about 1.5 inches long. Masses of red-orange eggs occur in 7 to 10 days from hatching in two brood pouches on the appendages of the 11th segment at the lower margin of the shield.
Tadpole shrimp cause losses in seedling rice stands in two ways. First, they may chew off the coleoptile, roots, and leaves of the seedling, and uproot seedlings with their digging and feeding activity, all of which may kill the plants. Second, their digging activities associated with egg laying muddy the water, reducing light penetration and thereby slow the growth of the submerged seedlings. Tadpole shrimp cause no injury to rice once leaves have reached the water surface and roots are well established in the soil.
Management of tadpole shrimp involves rapid seeding of the field after flooding and monitoring twice within the first 2 weeks following flooding to determine the need for chemical treatment. As an alternative, some population reduction can be obtained by flooding and draining the field before flooding for seeding.
Most tadpole shrimp eggs hatch within 2 days after contact with water. The longer the time between hatching of the eggs and planting, the larger the size of the shrimp and the greater the potential for plant injury. Flood the field as fast as possible, and seed as soon as possible after flooding has been initiated. In very large fields that take more than a few days to flood, you may want to seed basins (checks) in sequence as they fill with water. Rice stands that have been reduced by shrimp feeding can be reseeded but generally a good stand of rice is difficult to establish in this manner.
Flooding and draining the field before planting will kill hatched tadpole shrimp through desiccation and are alternatives to chemical control. Do not drain the field until 4 to 5 days after initial flood so the maximum egg hatch can occur. The draining time will vary based on soil type and weather but should continue for at least 24 hours after all standing water is gone. Shrimp will gather in standing water in low areas and will reinfest the field if the drain period is too short. Reflooding may result in some shrimp from previously unhatched eggs, but they would be in noneconomic numbers and less likely to damage the older, firmer rooted seedlings. Any soil cultivation following the drain period may bring viable, unhatchedshrimp eggs to the soil surface for possible infestation upon reflooding, however.
A decision to drain must take into account possible negative aspects such as fertilizer loss, encouragement of weeds, or interruption of weed control procedures, interruption of pesticide holding requirements, and the economics of irrigation.
Organically Acceptable Methods
Draining, as described under cultural controls, and applications of copper sulfate are organically acceptable methods.
Check all your fields during seedling development, but give special attention to those that had significant populations of tadpole shrimp last year. Even though infestations may have been localized in previous years, monitor all quadrants of the field because the shrimp can occur well beyond previously observed boundaries. Masses of wind-blown cut leaves and floating seedlings along the levees combined with muddy water are good evidence of tadpole shrimp activity. Check for shrimp and floating shrimp molt skins to confirm the pest, because crayfish and seed midges cause similar damage. If the water is clear, you can usually see the shrimp on the bottom although they may be small. If the water is murky or muddy, you may still see the shrimp as they come to the surface but cool temperatures may slow their activity. A fish seine (1/8-inch mesh) pulled along the bottom of the field will reveal their presence or absence.
The action threshold is determined by counting numbers of seedlings at various stages of development. Take samples twice during seedling development: at 5 to 7 days after flooding is initiated and again 3 days to a week later. Use a metal or plastic cylinder or square with open ends that encloses 1 square foot and with sides higher than the water depth, to count the plant stand. Place it in the water gently so the area to be observed remains clear. If the water is murky because of wind and wave action, you may have to wait a day or two to allow it to clear. An alternative to waiting is to view the seedlings through a clear glass or plastic container or jar held within the sampling device. Hold the viewing device open end up and push it down into the cloudy water until you can see the bottom of the field and move the jar around to see the seedlings. Take 10 samples across the check on each sampling date. Take 20 samples if your checks are large. The treatment guidelines below are based on a seeding rate of 150 pound per acre.
Take the first sample 5 to 7 days after flooding begins. Treat if tadpole shrimp or their molted skins are present and less than 30 healthy seedlings per square foot are found, and there is damage due to shrimp feeding.
Take the second sample 8 to 14 days after flooding. An average of 25 or more healthy seedlings per square foot at this time should provide a good stand. Treat if less than 25 healthy seedlings are found and evidence of shrimp is present in the monitoring area. If 25 or more healthy seedlings are found but there is also evidence of tadpole shrimp activity, repeat the sampling procedure every 2 to 3 days until rice plants emerge above the water surface. Treat if the average number of seedlings drops below 25 plants.
If muddy water does not allow an adequate visual inspection of the plant stand after 8 days, treatment decisions must be based on the presence of shrimp and shed skins, and observations of chewed shoot tips or roots, or uprooted floating seedlings.
|Common name||Amount per acre||REI‡||PHI‡|
|(Example trade name)||(hours)||(days)|
|Not all registered pesticides are listed. The following are ranked with the pesticides having the greatest IPM value listed first—the most effective and least harmful to natural enemies, honey bees, and the environment are at the top of the table. When choosing a pesticide, consider information relating to air and water quality, resistance management, and the pesticide's properties and application timing. Always read the label of the product being used.|
|(Warrior II with Zeon)||1.6–2.56 oz||24||21|
|MODE-OF-ACTION GROUP NUMBER1: 3A|
|COMMENTS: Can be used safely when propanil products are being used for weed control. Do not release floodwater within 7 days of an application. See label for other restrictions.|
|B.||COPPER SULFATE PENTAHYDRATE#||5–10 lb||See label||See label|
|COMMENTS: Use smallest size crystal. The minimum rate is only effective if water is shallow and being held. Not all copper compounds are approved for use in organic production; be sure to check individual products.|
|(Sevin 4F)||1.5 qt||12||14|
|(Sevin XLR Plus)||1.5 qt||12||14|
|MODE-OF-ACTION GROUP NUMBER1: 1A|
|COMMENTS: Do not apply 15 days before or after the application of propanil. Rates for California only.|
|‡||Restricted entry interval (REI) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing. Preharvest interval (PHI) is the number of days from treatment to harvest. In some cases the REI exceeds the PHI. The longer of two intervals is the minimum time that must elapse before harvest.|
|1||Rotate chemicals with a different mode-of-action Group number, and do not use products with the same mode-of-action Group number more than twice per season to help prevent the development of resistance. For example, the organophosphates have a Group number of 1B; chemicals with a 1B Group number should be alternated with chemicals that have a Group number other than 1B. Mode-of-action Group numbers are assigned by IRAC (Insecticide Resistance Action Committee).|
|*||Permit required for county agricultural commissioner for purchase or use.|
|#||Acceptable for organically grown produce.|