Spatio-temporal patterns of mortality in Pemphigus populicaulis and P. populitransversus on cottonwoods

Galls of the aphid species Pemphigus populitransversus and P. populicaulis from a single cottonwood in Clinton, New York were censused for mortality approximately every ten days during the summer of 1978. P. populitransversus had significantly higher survival than P. populicaulis. Both species of aphid showed considerably elevated mortality later in the season. Mortality was not spatially random, but neighboring galls had a tendency to die at close to the same time. This tendency to spatially autocorrelated mortality varied from time to time, being more pronounced earlier in the season than later. Later in the season, galls on the outer portions of branches tended to have a higher mortality rate than galls closer to the main trunk of the tree. This effect was highly significant in P. populicaulis, but only suggestive in P. populitransversus.The proportion of dead galls in samples of the two species taken at other times and places was consistent with the data from the censuses. In addition, the potential of two common predators, Anthocoris sp. and Leucopis sp., for producing autocorrelated mortality in Pemphigus was assessed in these collections. There was some association of Leucopis with regions of higher mortality, but this was not the case in Anthocoris. Leucopis tended to be found in galls of P. populicaulis more frequently than in P. populitransversus unlike Anthocoris, suggesting that some of the differential mortality between P. populitransversus and P. populicaulis might be due to this dipteran predator.