Past pyrethroids: alternative management approaches for tadpole shrimp in rice

Tadpole shrimp (Triops longicaudatus) are an early-season pest in California rice. Soon after flooding, eggs hatch and growing shrimp are soon large enough to damage germinating rice seedlings. Currently, pyrethroid insecticides are heavily relied upon for management, as they are in many cropping systems, because of their efficacy and low cost. However, contamination of surface waters is a concern, as is insecticide resistance. We evaluated alternative management strategies that could be used to manage tadpole shrimp. We tested a number of materials using 1 m² metal ring plots and natural shrimp populations. This included several timings of applications to mimic an early, more proactive application along with a later, rescue application as well as reduced rates of a number of insecticides. In addition, we used large 9.29 m² square metal plots to evaluate if a fish released for mosquito control, mosquitofish (Gambusia affinis), could suppress shrimp populations. We found that a number of materials could suppress shrimp populations, while unregistered materials did not provide sufficient control. Mosquitofish also proved able to suppress shrimp populations, although extremely high shrimp populations could escape predation and shrimp could outgrow risk to predators. These results could help a field crop industry move beyond pyrethroids, which will be important to address an issues of environmental contamination or regulatory changes as well as changes in susceptibility.