Temporal and spatial dynamics of Empoasca fabae (Harris) (Homoptera: Cicadellidae) in alfalfa

We describe the dynamics of potato leafhopper, Empoasca fabae (Harris) (Homoptera: Cicadellidae), populations in a 4-ha alfalfa field over 2 yr. Population growth and spatial structure were strongly influenced by days after cutting. Capture of E. fabae by suction traps above the boundary layer along with sex ratios of in-field populations suggested that immigrants contributed to population growth throughout the second and third alfalfa growth cycles. Initial sex ratios were strongly female biased (1995, 80%; 1996, 90%), with the degree of bias decreasing and approaching a 1:1 ratio through the third growth cycles. A higher proportion of the population was located in the edge relative to the interior plots in three of four alfalfa growth cycles. Spatial correlation between females and males was initially low, but increased as density increased; this correlation also decreased immediately after alfalfa harvest, and significantly increased over time after harvest. These data suggest that dynamic in-field spatial organization exists for E. fabae. Although the entire field was colonized, we hypothesize an edge-biased colonization process, initiated by females for at least the second growth cycle in the northeastern United States, followed by density-dependent movement away from crowded areas of declining host quality.