Localized outbreak of a willow leaf beetle: plant vigor or natural enemies?

The possible roles of plant quality (vigor) and natural enemies in the deve lopment of a localized outbreak of the leaf beetle Galerucella lineola (Col eoptera: Chrysomelidae) in a stand of Salix cinerea were investigated. Cage d and uncaged larvae on six bushes in the outbreak area were compared with caged and uncaged larvae on six bushes in an adjacent nonoutbreak area in t erms of performance. Ln 1997, when the studies were performed, the natural density of the insect (beetles plus eggs) was six times higher in the outbr eak area compared with the nonoutbreak area. Even though the vigor (measure d as shoot length) of bushes in the outbreak was 72% higher than that of hu shes in the nonoutbreak area, we found no difference between areas in the p erformance (survival, developmental time, pupal. weight) of caged larvae or in the willingness of caged females to lay eggs. Among larvae exposed to n atural enemies, the disappearance rate was significantly higher in the nono utbreak area. The density of generalist predators was significantly higher in the nonoutbreak than in the outbreak area. We conclude that differences in plant quality, despite the observed difference in plant vigor, could not explain the observed difference in beetle density between areas. Lower pre dation pressure in the outbreak area could, however, not be excluded as a p ossible reason for the higher density of leaf beetles in this area.