Evaluating the effects of a grass-specific herbicide on oviposition and larval survivorship of the silvery blue butterfly, Glaucopsyche lygdamus columbia, in an Oregon prairie

In western Oregon and Washington, over 95% of the historic prairie-oak habitat has been lost or degraded. Many remnant shortgrass swards are threatened by exotic tall grasses and selective herbicides are a cost-effective tool used to improve habitat for native plants and animals. However, the benefits of improved habitat must be weighed against any detrimental costs to non-target species. In previous laboratory trials, grass-specifc herbicides decreased butterfly larval survivorship and shortened development time. At a site in Oregon's Willamette Valley, I tested for an effect of an early spring application of fluazifop-p-butyl (Fusilade) and a nonionic surfactant (Nufilm) on adult behavior, oviposition, and larval survivorship of a common Lycaenid, the silvery blue butterfly (Glaucopsyche lygdamus columbia). In contrast with a preivous field study, there was no difference in the amount of time that butterflies spent in herbicide and control plots. Additionally, total number of eggs and larvae on lupine (Lupinus oreganus) host plants was similar between plot types. I found that in the field, factors such as lupine flowering stalk phenology and larva-ant associations are more important than herbicide treatment for predicting differences in egg loads and larval survivorship.