population variation of a specialist versus a generalist aphid sharing the same host plant in field

Aphids in unsprayed canola (brassica napus linnaeus) fields in isfahan province (central iran) were sampled for two growing seasons, 2011-2013. sampling unit was a whole plant and 20 plants were sampled weekly. in the laboratory, heat-extracting and sub-sampling techniques were used to estimate the density of aphids. to determine the relationship between population growth rate of the aphids and degree-days,linear regressions were done between log of aphid density and accumulated degree-days. homogeneity tests were done using pairwise comparisons between slopes. aphids' preference for upper (10-15 cm upper part of stem) and lower (the rest of stem) parts of the plant was analyzed using student’s t-test. aphid fauna included: cabbage aphid [ca; brevicoryne brassicae (l.)], green peach aphid [gpa; myzus persicae (sulzer)], and turnip aphid [ta; lipaphis erysimi (kaltenbach)]. the gpa developed small population in comparison with ca and ta and was occasionally found. the population density of ca and gpa tended to show two peaks and, during flower initiation, population began to decrease. the average population growth rates of gpa and ca were 0.001 and 0.003, respectively. homogeneity tests indicated that, at all sites and years, gpa showed reduced growth rate compared to ca. on average, 36 and 64% of ca and gpa populations were found on the lower parts of plants, respectively. this indicated that ca preferred upper part while gpa preferred lower part of the plants. the obtained results could be used to make a contribution to systematize the field monitoring of predominant aphids in canola crop.