Grassland birds are declining faster than birds in any other biome likely due to land conversion, intensification of agriculture practices, and increased inputs of pesticides. The endangered species, Streaked Horned Lark, is a ground-nesting grassland bird that utilizes agriculture land to forage and breed. Willamette Valley houses the largest populations of Streaked Horned Larks within their range. Agriculture is the predominant land use type in Willamette Valley, making it crucial habitat for the persistence of the species. Though the primary threat to the larks is scarcity of large, open habitat, pesticide use may further reduce available suitable habitat. Pesticides can impact the larks through either direct or indirect means. Herbicides can decrease host plants for insects, impacting the crucial food sources for young. While insecticides negatively impact insect abundance directly. Imidacloprid, a neonicotinoid insecticide, is the most toxic pesticide used in Willamette Valley. Ingestion of treated seeds can intoxicate and even kill small passerine species with just a single dose (LD50) of 41 mg/kg bw. In addition, imidacloprid is among very few chemicals that has robust research on the indirect effects to birds including decreasing crucial insects for 15 bird species, 6 of which were granivorous partially reliant on insects like Streaked Horned Larks. The altered trophic cascade resulted in declines of all 15 species. Metribuzin, 2,4-D, Bentazon, and Simazine upper LD50 is within the moderately toxic range. Atrazine, Dicamba, Diuron, Dimethenamid-p, Glyphosate, and Metolachlor-s are slightly toxic to practically non-toxic. Due to limitations of available literature and data, pesticide indirect risk couldn’t be quantified. Though indirect impacts are discussed when literature was present, risk was ranked only by the direct effects of pesticides. More studies must be conducted on agrochemical used in Willamette Valley to obtain a more holistic review of the risk posed to Streaked Horned Larks.
Cascadia Prairie-Oak Partnership
Providing coordination for the technical prairie community in the Willamette Valley, Puget Trough, Georgia Basin (WPG) ecoregion and beyond.