Toxicity of Malathion and Roundup® to the San Diego Fairy Shrimp

The purpose of this study was to measure the toxicity of malathion and glyphosate (the active ingredient in Roundup®, to San Diego fairy shrimp (Branchinecta sandiegonensis). The endemic San Diego fairy shrimp has been listed as endangered due to habitat loss and degradation of southern California vernal pools. The risk to fairy shrimp from the potential impact of herbicides and insecticides sprayed near pools has not been assessed. The LC50 for malathion was 24.5 mg/L (estimated active ingredient in Dexol Home Insect Killer). Based on possible environmental concentrations of malathion, it is unlikely to pose a threat to individual San Diego fairy shrimp. The LC50 for glyphosate was 11.8 μg/L (estimated active ingredient in Monsanto Ready-To-Use Weed & Grass Killer). Environmental concentrations of glyphosate are usually well below concentrations that show acute toxicity to other crustaceans, but assuming a 1/20 safety margin as recommended by the U.S. Environmental Protection Agency, they may be above safe concentrations estimated from toxicity data measured here for B. sandiegonensis. However, concentrations in pools are not known, so caution is recommended when making management decisions about these pesticides. We recommend that spraying be avoided in pool watersheds in the rainy season, and that future testing be done on different chemicals, other organisms, and possible sublethal effects on shrimp.