University of California Agriculture and Natural Resources researchers reported on Apr. 6 that certain populations of tadpole shrimp in California rice fields have developed resistance to the commonly used insecticide lambda-cyhalothrin, a type of pyrethroid. The findings are based on laboratory bioassays assessing the susceptibility of various shrimp populations to different concentrations of the chemical.
The issue is important for rice growers who have long relied on pyrethroids due to their effectiveness and low cost. However, repeated use has led to regulatory concerns about environmental impacts and contributed to localized resistance among pest populations. “Pyrethroids have a risk of off-target movement and aquatic invertebrate toxicity, so there has been a risk of regulatory action or additional reporting requirements should pyrethroids move out of fields and into surface waters,” according to the report.
The study found significant variation in how different tadpole shrimp populations responded to lambda-cyhalothrin. Some groups remained highly susceptible, while others showed high levels of resistance—even at concentrations above recommended label rates. For example, Colusa 1 and Sutter populations did not reach more than 50% mortality even with doses much higher than standard application rates.
The research suggests that resistance develops locally because tadpole shrimp do not move far between fields; thus, frequent applications within individual fields increase the risk for resistant strains emerging there. “When pyrethroids work, they work very well… When pyrethroids do not work because of resistance, this can be dramatic,” the authors said.
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