Effects of Population Density on Long‐Distance Dispersal in the Goldenrod Beetle Trirhabda Virgata

Recent theoretical and empirical studies suggest that an animals's long—distance dispersal behavior can have a major influence on its population dynamics. In this study I examined the role of dispersal in the population dynamics of a goldenrod leaf beetle, Trirhabda virgata. I measured the long—distance emigration rates of this beetle in the field under different population densities. I manipulated densities of Trirhabda larvae early in the season to create crowded, heavily defoliated host patches and uncrowded, lightly defoliated patches. Newly emerged, teneral beetles were added to these two density treatments and their long—distance emigration rates were measured. Beetles flew from host patches that had high densities of adults and heavy damage caused by larvae. No flight was detected from low—density patches with little defoliation. Beetles that had developed in these crowded and uncrowded patches were placed on lush, undefoliated plants as teneral adults. Their emigration rates did not differ. In a final experiment, female beetles were placed on plants with or without mates. The plants were one of two types, either heavily defoliated or lush. On lush plants, females without mates showed a stronger tendency to leave than females that had access to mates. On defoliated plants, females left at high rates whether or not mates were present. These experiments indicate that long—distance emigration is triggered by conditions experienced during the teneral phase. Heavy host defoliation inflicted by high population densities appears to induce flight. This density—dependent emigration should reduce local population pressure and result in the spread of local outbreaks across a larger area. The influence of dispersal on population dynamics will vary depending on the pattern of beetle densities across the region.