The Spatial Pattern of the Pea Aphid in Alfalfa Fields

Abstract

The terminal 6 inches of a vigorously growing, nonblooming alfalfa stem were used as the sampling unit in a study of the spatial pattern of the pea aphid, Macrosiphum pisi (Harris). Samples of 100 stems each were taken in four New York alfalfa fields at various intervals throughout the 1961 growing season, and the number of aphids per stem was counted. The counts of the total number of aphids and the counts of the total number less the first instar nymphs were fitted to the Poisson and negative binomial series. There was a very low probability of a greater chi-square when the samples taken in the spring and in the late summer were fitted to the Poisson series. The samples taken in midsummer showed a very low population of pea aphids and a close agreement to the Poisson series. as indicated by the very nearly equal mean and variance of each sample. The spring- and late-summer samples, and a midnight sample, generally gave good fits to the negative binomial series, although there were a few that did not fit the contagious distribution very well. When one or two stems with unusually high numbers of aphids were omitted from some of these samples. there was a decided improvement of the fit to the negative binomial. Extremes in daily weather conditions and the presence of parasitized and alate aphids had little apparent effect on the spatial pattern form.