Population Variation of a Specialist versus a Gener alist Aphid Sharing the Same Host Plant in Field

Abstract

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 t he laboratory, heat-extraction and sub-sampling techniques were used to estimate the d ensity 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 comparis ons 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 f ound. The population density of CA and GPA tended to show two peaks, and during flo wer initiation, population began to decrease. The average population growth rates of GP A 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. T his 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 monito ring of predominant aphids in canola crop. Keywords: Brevicoryne brassicae , Canola, Degree-days, Myzus persicae , Population growth rate.