Tadpole Shrimp

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 6 or 7 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 soil surface. Eggs are highly resistant to drying and remain viable for several years in unflooded soil. Only eggs in the top 0.5 inch of the soil hatch, and most 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 adults in less than 24 hours. They feed on a variety of small animals and plants corresponding to their size as they grow and molt. Once the tadpole shrimp are easily visible without magnification aid, they can feed on germinating rice seeds. The somewhat transparent molt skins may be mistaken for dead shrimp.

At maturity, tadpole shrimp are about 1.5 inches long. Seven to 10 days after hatching, masses of red-orange eggs are visible in two brood pouches on the appendages of the 11th segment at the lower margin of the shield. After 30 days of flooding, most tadpole shrimp will have completed their cycle and their numbers will decrease until they disappear. If the field remains flooded, no more eggs will hatch until next year.

Tadpole shrimp cause losses in seedling rice stands in two ways. First, they may chew off the coleoptile and roots of the seedling, and uproot seedlings with their digging and feeding activity, all of which may kill the plants. Germinating seeds are very susceptible to tadpole shrimp damage; once the spike is present and roots are anchored in the soil, tadpole shrimp may not cause direct injury to rice, but they might still dislodge seedlings.

Second, their digging activities associated with egg laying and food foraging muddy the water, reducing light penetration and thereby slowing the growth of the submerged seedlings. Tadpole shrimp don’t reduce stands once leaves have reached the water surface and roots are well established in the soil.

Management of tadpole shrimp involves prompt seeding of the field after flooding and monitoring frequently within the first 2 weeks following seeding to determine the need for pesticide application. As an alternative, some reduction of numbers can be obtained by flooding and draining the field before flooding for seeding, such as when a stale seedbed is implemented.

Most tadpole shrimp eggs hatch within 2 days of contact with water. The more time between hatching of the eggs and seeding, 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 is an alternative to chemical control. Do not drain the field until 4 to 5 days after initial flood to maximize the number of eggs that hatch. 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 for seeding may result in some shrimp hatching from previously unhatched eggs, but in lower numbers. Any soil cultivation following the drain period may bring viable, unhatched shrimp eggs to the soil surface for possible infestation upon reflooding.

Draining soon after seeding to encourage stand establishment (Leather’s method) can eliminate hatched shrimp from the field. However, shrimp can survive in puddles if drying is not complete and reinfest the field when it is reflooded.

A decision to drain carries with it possible negative aspects such as fertilizer loss, encouragement of weed germination, or interruption of weed control procedures and of pesticide holding requirements, and the economics of irrigation.

Drain fields, as described under cultural controls, and apply copper sulfate in a certified organic crop.

Check all fields during seed germination and seedling development, but give special attention to those that had significant numbers of tadpole shrimp last year. Even if infestations were 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 that tadpole shrimp damage has occurred.

Check for tadpole shrimp frequently after seeding. Tadpole shrimp develop fast and may appear suddenly in fields that seemed free of them. In clear water, shrimp can usually be seen resting on the soil, although they may be small and difficult to see. If the water is murky or muddy, shrimp may still be seen as they come to the surface, but cool temperatures may slow their activity. A fish seine (one-eighth-inch mesh) pulled along the soil surface will reveal their presence or absence. When flooding times are long, check for tadpole shrimp presence after the field is flooded before seeding.

When monitoring for tadpole shrimp, determine the number of healthy seedlings per square foot. For example, 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 or wave action, allow a day or two for the water 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.

Count the number of seedlings found in the 1-square-foot sampling area and record the results. Take 10 samples across the check to capture variability. Note the presence of shrimp and their relative abundance.

An average of 25 plants per square foot are required to maximize yields. When the stand falls below 12 plants per square foot, significant yield reductions can occur.

During seed germination soon after seeding, tadpole shrimp presence warrants a pesticide application. Once seedlings have a well-developed spike and anchored roots, consider pesticide application if less than 25 healthy seedlings are found per square foot and evidence of shrimp is present in the monitoring area. If 25 or more healthy seedlings are found per square foot 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. Consider a pesticide application if the average number of seedlings drops below 25 plants per square foot.

If muddy water does not allow an adequate visual inspection of the plant stand, 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.