Comments on the dynamics of insect population assemblages and sampling plans for aphids in commercial alpine yarrow fields

Achillea collina Becker ex Rchb. (yarrow) is a medicinal plant grown for commercial purposes in southern Switzerland and northern Italy. The economically most relevant population assemblage is organized around the phloem feeding aphids Macrosiphoniella millefolii (De Geer), Aphis spiraecola Patch and Coloradoa achilleae Hille Ris Lambers. The study is a contribution to explaining the dynamics of this assemblage in two Alpine fields, on the basis of multitrophic population interactions. The method consisted in stratifying the field and taking random samples of yarrow stems and leaves, aphid numbers, and numbers of adult coccinellids and mummified aphids throughout the growing period. There were no significant differences in aphid densities between the strata (α = 0.05; F = 1.561; P = 0.138). Once the photoperiod reaches 14 h, a time window opens for stem elongation and reproductive development that creates suitable conditions for the aphids. There are indications for an influence of the plant and a possibly limited effect of natural enemies on aphid infestations. The coefficients b (1.920, 1.917) and a (3.911, 14.539) of Taylor's power law indicate aggregated distributions of M. millefolii and A. spiraecola. To obtain a reliable density estimate, 100 or 400 plants should be sampled for M. millefolii and A. spiraecola, respectively. An enumerative sequential sampling plan appears to be more efficient than the enumerative fixed sample size sampling plan.