Locoweeds, Astragalus mollissimus Torr. and Oxytropis sericea Nut., are native rangeland weeds that have caused significant damage to livestock in the western United States. Astragalus mollissimus and Oxytropis sericea contain the indolizidine alkaloid, swainsonine, which causes locoweed poisoning. Once consumed by vertebrates, swainsonine can cause the condition known as locoism, which has severe consequences including depression, abortion and death. Several native insects have been observed feeding on locoweed roots and stems. One of the most common, Cleonidius trivittatus (Say), the four-lined locoweed weevil, is being studied to determine if herbivory influences swainsonine content. Young seedlings of A. mollissimus and O. sericea were collected in Union County, New Mexico and grown in a greenhouse under drip irrigation. Swainsonine will be extracted from field plants and greenhouse plants experiencing different levels of herbivory damage using cation exchange chromatography and analyzed using liquid chromatography-mass spectrometry. Cleonidius trivittatus were collected in stems and roots of Oxytropis plants collected in Larimer County, Colorado and in stems and roots of Astragalus plants collected in Union County, New Mexico. Eggs and larvae were placed in an artificial locoweed diet to determine optimum conditions for rearing adults. The reared adults will be used in choice and no choice experiments with plants having variable swainsonine levels. The results of these experiments will add to the knowledge of biological control as an option for managing locoweed.