Implications of Larval Movements in Declining Butterflies: Investigating Methods of Tracking First Instar Larvae

DeRosa University Center, Ballroom

Several species of Speyeria butterfly are of conservation concern in California. Habitat loss and declines in Viola host plants are suggested as major contributors to this problem. One way these populations could be revitalized is through restoration of Viola host plants, but even with abundant host plant, larvae face challenges finding and moving from plant to plant in this system. At what density and arrangement should plants be planted to maximize larval survival? We are approaching this question by studying movements of first instar larvae, a critical time in the life cycle that requires locating an appropriate host after months of diapause. This project focuses on overcoming a challenge of working with very small larvae: efficiently collecting data. A desirable aspect of many contemporary motion analysis techniques includes using highly visible markers on the subject that are easily tracked for analysis. Unfortunately the first instar larvae studied here are too small for markers and many motion analysis programs are too expensive. Therefore, we tested performance of two markerless tracking applications (Tracker and Kinovea) that rely on pixel color differences from frame to frame, and compared the results with manual digitization. The best markerless method, Tracker, produced velocities with an average error of ~3%, indicating this is a viable alternative to digitizing frame-by-frame. Resulting estimates of larval movement using this method are a maximum of 1.77 meters per twelve hours, indicating that host plants should be located at distances less than this to insure larvae quickly find food in spring.