Figure 1.
Population dynamics of Phratora vulgatissima in plantations (upper four) and natural stands (lower four).
The four examples from willow plantations represent outbreak dynamics (fluctuations between low and high densities), whereas the examples from natural stands represent fluctuations around relatively stable high (upper two) and low (bottom two) mean densities. Density is measured as the average number of adult beetles per 35 cm willow shoots in the spring.
Figure 2.
Regression models between population variability and mean density of P. vulgatissima.
Models are fitted to the relationships between population variability (PV) and mean density in willow plantations (upper figure) and natural willow stands (lower figure).
Figure 3.
Correlations between P. vulgatissima and natural enemies.
Each data point represent the mean density of the three Heteropteran predators Orthotylus marginalis, Closterotomus fulvomaculatus and Anthocoris nemorum added together, and the leaf beetle Phratora vulgatissima over seven years (1999–2005) in willow plantations (top figure) and natural willow stands (bottom figure). There was a significant negative correlation in the natural habitat (r = −0.46, P = 0.019, n = 20), but not in plantations (r = –0.06, P = 0.40, n = 20).