How to Manage Pests
UC Pest Management Guidelines
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.
Flooding and draining the field before planting will kill hatched tadpole shrimp through desication 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, unhatched shrimp 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
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 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.
UC IPM Pest Management Guidelines: Rice