Tadpole shrimp are a relatively new pest of Missouri rice, and they have also been found in northeast Arkansas rice fields. In Missouri, in 2008, at least 4,000 acres had tadpole shrimp present, and, of those infested, about 2,000 acres were economically impacted. Infestations also caused approximately 100 acres to be replanted.
Tadpole shrimp are pests of water-seeded rice fields. Sexually mature tadpole shrimp are found as early as 9–12 days after floods are established; therefore, rice plants have less than nine days to break the surface of the flood (i.e., the time at which rice is no longer vulnerable) before tadpole shrimp are large enough to uproot seedling rice. This is particularly important during periods of flooding.
Farmers who decide water-seeding is the only option may be planting in fields with mature tadpole shrimp that can damage rice. However, rice planted by dry-seeded methods should have periods of time when the field is dry and unable to support this aquatic critter. When the field is permanently flooded, plants should have an adequate root system to prevent stand loss. Therefore, tadpole shrimp are not pests in normal dry-seeded systems.
Once rice is no longer vulnerable to tadpole shrimp damage, tadpole shrimp may serve as a biological control agent for mosquitoes and/or weeds.
Lower seeding rates are more susceptible to tadpole shrimp damage than higher seeding rates. For example, losing 10 percent of a stand planted at 30 lbs/A is more detrimental than losing 10 percent of a stand planted at 90 lbs/A.
Distribution Survey Conducted To Pinpoint Pest
In 2010, a distribution survey was funded by the Missouri Rice Research and Merchandising Council to determine where this pest occurs in rice-producing areas of Missouri. After failed attempts to locate any in fields during the spring, we developed a different method for finding tadpole shrimp. After harvest, the top two inches of soil in an 8 inch x 8 inch area was collected. Low areas of the field were targeted because tadpole shrimp will congregate in areas where water remains on the field after it is drained. There was a distance of 50 to 100 feet between samples, and five samples per field were collected.
Tadpole shrimp eggs require a period of drying; therefore, the soil was dried completely. Then the soil was placed in a plastic container and spread to a depth no greater than one inch. Water was added as needed to containers to simulate a two-inch flood for 15 to 21 days. The containers were kept in an area that was warm and exposed to light so that the tadpole shrimp would hatch (room temperature should suffice). After two to three weeks, tadpole shrimp, if present, should be visible.
A bright light may aid in the detection of the tadpole shrimp. Additionally, the water may appear muddy, indicating their presence as well. When examining samples, many arthropods other than tadpole shrimp were recovered, so it is important for you to wait at least two weeks before reading samples to ensure proper identification. The longer these rice pests have to develop, the more likely that they will be identified correctly.
Options For Dealing With Infestations
While the methodology is effective for discovery of tadpole shrimp, current data are not sufficient to determine economic thresholds. However, if samples test positive for tadpole shrimp, the farmer should be aware of the risk associated with water-seeding the field and should consider drill-seeding that field.
If the farmer must water-seed, efforts should be made to seed the field as quickly as possible after the flood is established and should avoid planting low seeding rates.
Additionally, the field should be watched closely to ensure stand loss does not become problematic in the field. From an insecticide standpoint, although pyrethroids are effective at killing tadpole shrimp, this particular pest is not on the label.
Field-To-Field Transfer Theories
Preliminary results of the tadpole shrimp distribution study showed that they are widely distributed geographically; however, not every field within an area is infested. Most of the fields that were positive for tadpole shrimp are east of the Little River Drainage District.
Tadpole shrimp are transferred from field-to-field via floodwaters, wind, birds and other wildlife or via equipment that has been used in an infested field. They also may be transferred on the soles of boots when walking in an infested field, then walking in a non-infested field. The latter two methods of transfer could be avoided by adopting better sanitary practices when moving between rice fields.
Kelly V. Tindall is an Assistant Research Professor of Entomology with the University of Missouri. Contact firstname.lastname@example.org.