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Density of adults (A) and nymphs (N) of Euceraphis betulae, Betulaphis brevipilosa and Callipterinella tuberculata aphids on green and yellowing silver birch leaves on 31 July 2008. Only trees with yellow leaves were analysed. Filled bars represent green leaves and open bars represent yellow leaves. ***indicates statistical significance at p < 0.001 level.
