Assessing sensitivity of Quino checkerspot butterfly larvae to two common herbicides used for habitat management
Native plant cover is being threatened and displaced in Southern California, mainly by highly invasive exotic forbs and the grasses. In Marron Valley, San Diego County, exotic annuals have displaced many native grassland species, including those animals use for food and other resources. The Quino Checkerspot butterfly Euphydryas editha quino (Behr) utilizes as its larval host several native annual forbs that are found at relatively very low abundances in Marron Valley. Field studies in Marron Valley, showed promising effects of herbicide application for reducing exotic plant growth and improving habitat for food plants of Quino caterpillars. Therefore, this study was designed to evaluate effects of commonly used herbicides (Fusilade II®, Transline®) and application surfactant on Quino larval development, survival, and pupal weights. The study was designed to assess both direct and indirect effects of herbicide exposure, by comparing larval growth, proportions of larvae pupating, and pupal weights. Performance was compared among larvae that had experienced direct contact with herbicides, surfactant, or water, and that were fed food plants that were exposed to herbicides, surfactant, or water. Two groups of post-diapause larvae were obtained from a captive rearing facility. Approximately 600 larvae were treated and measured through post-diapause instars towards pupation.
Initially larvae grew well and, while there were large differences in weights between the two groups of larvae, there was no significant difference in larval weights among treatments in either group. However, as larvae neared pupation sizes, they stopped feeding and re-entered another diapause instar, which also happens in nature. Although larvae did not pupate, and we could not assess effects of herbicide treatments on pupal weights, results from this experiment suggested that there were no direct or indirect effects on growth of post-diapause Quino checkerspot larvae from exposure to Fusilade or Transline and/or surfactant. That there was no indication of gross toxicity, and that post-diapause larval growth did not appear to be reduced by herbicide exposure is encouraging. Further studies when larvae are available will provide a more complete assessment of potential herbicide effects on this species’ developmental and reproductive biology. Studies such as this are key to successful conservation management.